A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

(Final Draft)

Water table change between

1973 Mar andBillabong 74 Dec

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers

Butian Wang, Shahbaz Khan, Natalie O’Connell

CSIRO Land and Water, Griffith Laboratory

Version: 30/10/2003

A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

(Final Draft)

Butian Wang, Shahbaz Khan, Natalie O’Connell

CSIRO Land and Water, Griffith Laboratory

Version: 30/10/2003

A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Executive summary

Environmental degradation associated with shallow saline watertables is a major threat to the sustainability of agricultural industry throughout the Murray-Darling Basin. Located in the western part of the Murray Valley of NSW, the Wakool Irrigation District has experienced a history of water table rise, including likely contributions from widespread flooding. The community is interested in scientific evidence quantifying the impact of flooding on the shallow groundwater, in order to target management actions to control water table rise and salinity in this area.

This study estimates the spatial and temporal impact of flooding on shallow groundwater for the Wakool Irrigation District through an extensive GIS analysis based on a large amount of piezometric data monitored over many years.

By compiling the piezometric data into a GIS database and analyzing the data in a GIS application, we are able to quantify the net recharge caused by flooding and to visualize the spatial extent of the impact of flooding on the shallow water table reflected by water table change.

The results show that flooding has a significant impact on the shallow groundwater. The floods during the record wet period of 1973-75 caused a net recharge of around 116x103 ML (0.52ML/ha in average) at the stage when water table rise reached its maximum value around December 1975.

Apart from the magnitude of flooding, the amount of the net recharge caused by a single flood event is also related to the initial water table before the flood, which affects the shallow groundwater storage capacity. The higher the initial water table is, the less the shallow groundwater storage capacity will be, and consequently there will be less room for the net recharge, as shown during the 1973-75 floods.

More frequent flooding such as the one experienced in 1981, whose recurrence interval is estimated as around 1 in 10 years, could result in 42.68x103 ML or an average of 0.19ML/ha net recharge at the stage around maximum water table mound, given the initial average water table depth being at 4.28m.

There are strong connections between the local rainfall, flood, and water table change, suggesting that the floods happened in this area are normally due to both upstream and local rainfall. The major flood recharge areas within the Wakool area are mainly located along the Edward – Niemur River system. The groundwater recession following a flooding is affected by a number of factors, such as the initial water table depth, the climate conditions, the management actions, and etc.

Key words : flood, GIS, groundwater, water table, Wakool, Murray.

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Table of Contents

Executive summary ············································································································································ i

Table of Content ··················································································································································ii

List of Tables ························································································································································ iii

List of Figures ······················································································································································iv

List of Figures in Appendix A ····················································································································vi

1. Introduction ······················································································································································1

2. Classification of floods ······························································································································2

3. Historical flood ················································································································································3

4. Data collection for this study ················································································································4

5. Data processing ···············································································································································5

6. Spatial extent and magnitude of water table response to the 1973-75 floods ··········6 6.1 The nature of 1973-75 floods reflected by the spatial extent of water table rise ····6 6.2 The water table rise process and water table change spatial extent ·······························7 6.3 Comparison between water table change extent and the 1956 flood extent ··············7

7. Quantifying impact of 73-75 floods on shallow groundwater ·········································17 7.1 Quantifying areas of different water table change ······························································17 7.2 Quantifying the impact on shallow groundwater storage ················································19

8. Quantifying impact of more frequent floods on shallow groundwater ····················23 8.1 The 1981 flood ······································································································································23 8.2 The 1992 flood ······································································································································25 8.3 Change in shallow groundwater for the whole data period ············································26

9. Summary and conclusions ····················································································································28

Acknowledgement ·············································································································································29

Reference ·······························································································································································29

Appendix A. Spatial Distribution of Piezometers with Data and Interpolated Water Table ·········································································································································································30

- ii - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

List of Tables

Table 1. Largest recorded floods (in terms of river height). ····························································3

Table 2. Summary of piezometric data collected. ··················································································4

Table 3. Summary of temporal and spatial distribution of piezometers with data available for each monitoring round. ·························································································5

Table 4. Area (%) of different water table rise during the 1973-75 floods.···························18

Table 5. Area (%) of different water table depth during the 1973-75 floods.·······················19

Table 6. Groundwater recession three months later following maximum water table mound during the 1973-75 floods. ···························································································20

Table 7. Net recharge when water table mound reached maximum for each major flood during 1973-75.·····················································································································20

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List of Figures

Figure 1. Location of the Wakool Irrigation District and major irrigation areas in south NSW. ·································································································································1

Figure 2. Waterways around the MIL area and the inundated area around Wakool in the 1956 flood.·········································································································4

Figure 3. Water table depth in March 1973 in the Wakool area.···································8

Figure 4. Spatial extent of water table change between March 1973 and June 1973.···9

Figure 5. Spatial extent of water table change between March 1973 and September 1973.·····························································································9

Figure 6. Spatial extent of water table change between March 1973 and December 1973. ··························································································································10

Figure 7. Spatial extent of water table change between March 1973 and March 1974.·············································································································10

Figure 8. Spatial extent of water table change between March 1973 and June 1974.·············································································································11

Figure 9. Spatial extent of water table change between March 1973 and September 1974.···························································································11

Figure 10. Water table change between March 1973 and September 1974 and in comparison with the inundated area of 1956 flood in the Wakool area.······12

Figure 11. Water table rise > 0.5m area in September 1974 as compared with Water table depth in March 1973 in the Wakool area.·································12

Figure 12. Water table rise > 1m area in September 1974 as compared with Water table depth in March 1973 in the Wakool area. ···········································13

Figure 13. Spatial extent of water table change between March 1973 and December 1974. ···························································································13

Figure 14. Spatial extent of water table change between March 1973 and March 1975.·············································································································14

Figure 15. Spatial extent of water table change between March 1973 and June 1975.·············································································································14

Figure 16. Spatial extent of water table change between March 1973 and September 1975.···························································································15

- iv - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Figure 17. Spatial extent of water table change between March 1973 and December 1975. ···························································································15

Figure 18. Water table change between March 1973 and December 1975 and in comparison with the inundated area of 1956 flood in the Wakool are. ··········16

Figure 19. Water table rise > 0.5m area in December 1975 as compared with Water table depth in March 1973 in the Wakool area. ············································16

Figure 20. Water table rise > 1m area in December 1975 as compared with Water table depth in March 1973 in the Wakool area. ············································17

Figure 21. Average water table change (spatial average) during 1973-75 floods as compared with water table in March 1973. ·····························································18

Figure 22. Groundwater recession process following the maximum water table mound of the 1975 flood. ·······································································································21

Figure 23. Net groundwater storage change as compared to water table in March 1973. ··································································································································22

Figure 24. Water table depth in February 1981 in the Wakool area.·······································23

Figure 25. Water table depth in August 1981 in the Wakool area. ··········································24

Figure 26. Water table change between February 1981 and August 1981.··························24

Figure 27. Groundwater recession process following the maximum water table mound of the 1981 flood. ·······································································································25

Figure 28. Water table change in March 2001 as compared with water table in March 1973. ··································································································································26

Figure 29. Change in shallow groundwater storage and average water table depth over the data period for the whole Wakool area. ··········································27

- v - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

List of Figures in Appendix A

Figure A1. Water table in the Wakool area in December 1963 and piezometers with data available for that time from which the water table is generated.··············································· 31

Figure A2. Water table in the Wakool area in March 1964 and piezometers with data available for that time from which the water table is generated.································································ 31

Figure A3. Water table in the Wakool area in June 1964 and piezometers with data available for that time from which the water table is generated.································································ 32

Figure A4. Water table in the Wakool area in September 1964 and piezometers with data available for that time from which the water table is generated.··············································· 32

Figure A5. Water table in the Wakool area in December 1964 and piezometers with data available for that time from which the water table is generated.··············································· 33

Figure A6. Water table in the Wakool area in March 1965 and piezometers with data available for that time from which the water table is generated.································································ 33

Figure A7. Water table in the Wakool area in June 1965 and piezometers with data available for that time from which the water table is generated.································································ 34

Figure A8. Water table in the Wakool area in September 1965 and piezometers with data available for that time from which the water table is generated.··············································· 34

Figure A9. Water table in the Wakool area in December 1965 and piezometers with data available for that time from which the water table is generated.··············································· 35

Figure A10. Water table in the Wakool area in March 1966 and piezometers with data available for that time from which the water table is generated.································································ 35

Figure A11. Water table in the Wakool area in June 1966 and piezometers with data available for that time from which the water table is generated.································································ 36

Figure A12. Water table in the Wakool area in September 1966 and piezometers with data available for that time from which the water table is generated.··············································· 36

Figure A13. Water table in the Wakool area in December 1966 and piezometers with data available for that time from which the water table is generated.··············································· 37

Figure A14. Water table in the Wakool area in March 1967 and piezometers with data available for that time from which the water table is generated.································································ 37

Figure A15. Water table in the Wakool area in September 1967 and piezometers with data available for that time from which the water table is generated.··············································· 38

Figure A16. Water table in the Wakool area in December 1967 and piezometers with data available for that time from which the water table is generated.··············································· 38

Figure A17. Water table in the Wakool area in March 1968 and piezometers with data available for that time from which the water table is generated.································································ 39

Figure A18. Water table in the Wakool area in June 1968 and piezometers with data available for that time from which the water table is generated.································································ 39

Figure A19. Water table in the Wakool area in September 1968 and piezometers with data available for that time from which the water table is generated.··············································· 40

- vi - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Figure A20. Water table in the Wakool area in December 1968 and piezometers with data available for that time from which the water table is generated.··············································· 40

Figure A21. Water table in the Wakool area in March 1969 and piezometers with data available for that time from which the water table is generated.································································ 41

Figure A22. Water table in the Wakool area in June 1969 and piezometers with data available for that time from which the water table is generated.································································ 41

Figure A23. Water table in the Wakool area in September 1969 and piezometers with data available for that time from which the water table is generated.··············································· 42

Figure A24. Water table in the Wakool area in December 1969 and piezometers with data available for that time from which the water table is generated.··············································· 42

Figure A25. Water table in the Wakool area in March 1970 and piezometers with data available for that time from which the water table is generated.································································ 43

Figure A26. Water table in the Wakool area in June 1970 and piezometers with data available for that time from which the water table is generated.································································ 43

Figure A27. Water table in the Wakool area in September 1970 and piezometers with data available for that time from which the water table is generated.··············································· 44

Figure A28. Water table in the Wakool area in December 1970 and piezometers with data available for that time from which the water table is generated.··············································· 44

Figure A29. Water table in the Wakool area in March 1971 and piezometers with data available for that time from which the water table is generated.································································ 45

Figure A30. Water table in the Wakool area in June 1971 and piezometers with data available for that time from which the water table is generated.································································ 45

Figure A31. Water table in the Wakool area in September 1971 and piezometers with data available for that time from which the water table is generated.··············································· 46

Figure A32. Water table in the Wakool area in December 1971 and piezometers with data available for that time from which the water table is generated.··············································· 46

Figure A33. Water table in the Wakool area in March 1972 and piezometers with data available for that time from which the water table is generated.································································ 47

Figure A34. Water table in the Wakool area in June 1972 and piezometers with data available for that time from which the water table is generated.································································ 47

Figure A35. Water table in the Wakool area in September 1972 and piezometers with data available for that time from which the water table is generated.··············································· 48

Figure A36. Water table in the Wakool area in December 1972 and piezometers with data available for that time from which the water table is generated.··············································· 48

Figure A37. Water table in the Wakool area in March 1973 and piezometers with data available for that time from which the water table is generated.································································ 49

Figure A38. Water table in the Wakool area in June 1973 and piezometers with data available for that time from which the water table is generated.································································ 49

Figure A39. Water table in the Wakool area in September 1973 and piezometers with data available for that time from which the water table is generated.··············································· 50

- vii - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Figure A40. Water table in the Wakool area in December 1973 and piezometers with data available for that time from which the water table is generated.··············································· 50

Figure A41. Water table in the Wakool area in March 1974 and piezometers with data available for that time from which the water table is generated.································································ 51

Figure A42. Water table in the Wakool area in June 1974 and piezometers with data available for that time from which the water table is generated.································································ 51

Figure A43. Water table in the Wakool area in September 1974 and piezometers with data available for that time from which the water table is generated.··············································· 52

Figure A44. Water table in the Wakool area in December 1974 and piezometers with data available for that time from which the water table is generated.··············································· 52

Figure A45. Water table in the Wakool area in March 1975 and piezometers with data available for that time from which the water table is generated.································································ 53

Figure A46. Water table in the Wakool area in June 1975 and piezometers with data available for that time from which the water table is generated.································································ 53

Figure A47. Water table in the Wakool area in September 1975 and piezometers with data available for that time from which the water table is generated.··············································· 54

Figure A48. Water table in the Wakool area in December 1975 and piezometers with data available for that time from which the water table is generated.··············································· 54

Figure A49. Water table in the Wakool area in March 1976 and piezometers with data available for that time from which the water table is generated.································································ 55

Figure A50. Water table in the Wakool area in June 1976 and piezometers with data available for that time from which the water table is generated.································································ 55

Figure A51. Water table in the Wakool area in September 1976 and piezometers with data available for that time from which the water table is generated.··············································· 56

Figure A52. Water table in the Wakool area in March 1977 and piezometers with data available for that time from which the water table is generated.································································ 56

Figure A53. Water table in the Wakool area in June 1977 and piezometers with data available for that time from which the water table is generated.································································ 57

Figure A54. Water table in the Wakool area in September 1977 and piezometers with data available for that time from which the water table is generated.··············································· 57

Figure A55. Water table in the Wakool area in December 1977 and piezometers with data available for that time from which the water table is generated.··············································· 58

Figure A56. Water table in the Wakool area in March 1978 and piezometers with data available for that time from which the water table is generated.································································ 58

Figure A57. Water table in the Wakool area in September 1978 and piezometers with data available for that time from which the water table is generated.··············································· 59

Figure A58. Water table in the Wakool area in February 1979 and piezometers with data available for that time from which the water table is generated.··············································· 59

Figure A59. Water table in the Wakool area in August 1979 and piezometers with data available for that time from which the water table is generated.··············································· 60

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Figure A60. Water table in the Wakool area in January 1980 and piezometers with data available for that time from which the water table is generated.··············································· 60

Figure A61. Water table in the Wakool area in August 1980 and piezometers with data available for that time from which the water table is generated.··············································· 61

Figure A62. Water table in the Wakool area in February 1981 and piezometers with data available for that time from which the water table is generated.··············································· 61

Figure A63. Water table in the Wakool area in August 1981 and piezometers with data available for that time from which the water table is generated.··············································· 62

Figure A64. Water table in the Wakool area in February 1982 and piezometers with data available for that time from which the water table is generated.··············································· 62

Figure A65. Water table in the Wakool area in April 1982 and piezometers with data available for that time from which the water table is generated.································································ 63

Figure A66. Water table in the Wakool area in May 1982 and piezometers with data available for that time from which the water table is generated.································································ 63

Figure A67. Water table in the Wakool area in August 1982 and piezometers with data available for that time from which the water table is generated.··············································· 64

Figure A68. Water table in the Wakool area in September 1982 and piezometers with data available for that time from which the water table is generated.··············································· 64

Figure A69. Water table in the Wakool area in November 1982 and piezometers with data available for that time from which the water table is generated.··············································· 65

Figure A70. Water table in the Wakool area in December 1982 and piezometers with data available for that time from which the water table is generated.··············································· 65

Figure A71. Water table in the Wakool area in February 1983 and piezometers with data available for that time from which the water table is generated.··············································· 66

Figure A72. Water table in the Wakool area in May 1983 and piezometers with data available for that time from which the water table is generated.································································ 66

Figure A73. Water table in the Wakool area in August 1983 and piezometers with data available for that time from which the water table is generated.··············································· 67

Figure A74. Water table in the Wakool area in November 1983 and piezometers with data available for that time from which the water table is generated.··············································· 67

Figure A75. Water table in the Wakool area in February 1984 and piezometers with data available for that time from which the water table is generated.··············································· 68

Figure A76. Water table in the Wakool area in May 1984 and piezometers with data available for that time from which the water table is generated.································································ 68

Figure A77. Water table in the Wakool area in June 1984 and piezometers with data available for that time from which the water table is generated.································································ 69

Figure A78. Water table in the Wakool area in July 1984 and piezometers with data available for that time from which the water table is generated.································································ 69

Figure A79. Water table in the Wakool area in February 1985 and piezometers with data available for that time from which the water table is generated.··············································· 70

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Figure A80. Water table in the Wakool area in July 1985 and piezometers with data available for that time from which the water table is generated.································································ 70

Figure A81. Water table in the Wakool area in February 1986 and piezometers with data available for that time from which the water table is generated.··············································· 71

Figure A82. Water table in the Wakool area in July 1986 and piezometers with data available for that time from which the water table is generated.································································ 71

Figure A83. Water table in the Wakool area in February 1987 and piezometers with data available for that time from which the water table is generated.··············································· 72

Figure A84. Water table in the Wakool area in July 1987 and piezometers with data available for that time from which the water table is generated.································································ 72

Figure A85. Water table in the Wakool area in February 1988 and piezometers with data available for that time from which the water table is generated.··············································· 73

Figure A86. Water table in the Wakool area in July 1988 and piezometers with data available for that time from which the water table is generated.································································ 73

Figure A87. Water table in the Wakool area in February 1989 and piezometers with data available for that time from which the water table is generated.··············································· 74

Figure A88. Water table in the Wakool area in July 1989 and piezometers with data available for that time from which the water table is generated.································································ 74

Figure A89. Water table in the Wakool area in February 1990 and piezometers with data available for that time from which the water table is generated.··············································· 75

Figure A90. Water table in the Wakool area in July 1990 and piezometers with data available for that time from which the water table is generated.································································ 75

Figure A91. Water table in the Wakool area in February 1991 and piezometers with data available for that time from which the water table is generated.··············································· 76

Figure A92. Water table in the Wakool area in July 1991 and piezometers with data available for that time from which the water table is generated.································································ 76

Figure A93. Water table in the Wakool area in February 1992 and piezometers with data available for that time from which the water table is generated.··············································· 77

Figure A94. Water table in the Wakool area in July 1992 and piezometers with data available for that time from which the water table is generated.································································ 77

Figure A95. Water table in the Wakool area in February 1993 and piezometers with data available for that time from which the water table is generated.··············································· 78

Figure A96. Water table in the Wakool area in July 1993 and piezometers with data available for that time from which the water table is generated.································································ 78

Figure A97. Water table in the Wakool area in February 1994 and piezometers with data available for that time from which the water table is generated.··············································· 79

Figure A98. Water table in the Wakool area in July 1994 and piezometers with data available for that time from which the water table is generated.································································ 79

Figure A99. Water table in the Wakool area in February 1995 and piezometers with data available for that time from which the water table is generated.··············································· 80

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Figure A100. Water table in the Wakool area in July 1995 and piezometers with data available for that time from which the water table is generated.································································ 80

Figure A101. Water table in the Wakool area in February 1996 and piezometers with data available for that time from which the water table is generated.··············································· 81

Figure A102. Water table in the Wakool area in July 1996 and piezometers with data available for that time from which the water table is generated.································································ 81

Figure A103. Water table in the Wakool area in March 1997 and piezometers with data available for that time from which the water table is generated.································································ 82

Figure A104. Water table in the Wakool area in August 1997 and pie zometers with data available for that time from which the water table is generated.··············································· 82

Figure A105. Water table in the Wakool area in March 1998 and piezometers with data available for that time from which the water table is generated.································································ 83

Figure A106. Water table in the Wakool area in August 1998 and piezometers with data available for that time from which the water table is generated.··············································· 83

Figure A107. Water table in the Wakool area in March 1999 and piezometers with data available for that time from which the water table is generated.································································ 84

Figure A108. Water table in the Wakool area in August 1999 and piezometers with data available for that time from which the water table is generated.··············································· 84

Figure A109. Water table in the Wakool area in March 2000 and piezometers with data available for that time from which the water table is generated.································································ 85

Figure A110. Water table in the Wakool area in August 2000 and piezometers with data available for that time from which the water table is generated.··············································· 85

Figure A111. Water table in the Wakool area in March 2001 and piezometers with data available for that time from which the water table is generated.································································ 86

- xi - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

1. Introduction

The Wakool Irrigation District (WID) is one of the four irrigation districts of the Murray Irrigation Area. The Murray Irrigation Area is one of the largest irrigation areas in the well- known Murray Darling Basin (MDB) and is located in the south NSW, just north of the Murray River across NSW – Victoria border (Dept. of National Development, 1953). Most part of the Murray Land and Water Management Plan (LWMP) area belong to Murray Irrigation Area (Fig. 1).

The total area of the Wakool area is around 230,000 hectares. The climate in this area can be described as arid to semi-arid with a mean annual rainfall at Moulamein Post Office being 356mm (Jan-Dec, 1889 –1998 data).

Elevation in this area ranges from 60m in the northwest to 84m in the southeast. The average ground slope is approximately 1:5,000 from the southeast towards the northwest. Charac- terized by its geographical location in the lower MDB, its flatness and flood plain nature, and the highly variable climatic conditions, this area is prone to frequent flooding (see Fig. 1).

Irrigation in the Wakool area expanded significantly in both area and water use from the 1970’s to the 1980’s and stabilized since then (WCLWMP, 2001). The shallow water table has been rising with the increased area under irrigation and subsequently agricultural sustainability has been threatened by land salinity resulting from the overall water table rise in this area. Murrumbidgee Irrigation Area

Griffith Hay

Coleambally Irrigation Area

Wakool Wakool

Deniliquin Finley Murray Land and Water Management Plan Area

Shepparton

Murray Darling Basin

Figure 1. Location of the Wakool Irrigation District and major irrigation areas in south NSW. (Most of the Murray LWMP area is Murray Irrigation Area under Murray Irrigation Ltd.)

- 1 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

To control water table rise and salinity problem, management actions have been taken, such as the Wakool Tullakool Sub Surface Drainage Scheme (WTSSDS), which is a drainage system with a combination of groundwater pumping, subsurface drainage and evaporation basins (Water Conservation and Irrigation Commission, 1975). To justify the cost and effectiveness of the scheme, it is necessary to estimate the recharge to the shallow groundwater from various sources.

The initial purpose of this study is to quantify the impact of flooding on shallow water table in the Wakool area and to identify the spatial extent of the impact, and consequently to improve knowledge and understanding of the regional groundwater dynamics and to provide evidence for justification of land and water management in this area.

This report summarizes the results from a range of GIS analysis in quantifying the shallow groundwater response to major flood events in the Wakool area.

2. Classification of floods

According to its cause/source, floods in this area can be classified as the following: a). Flood caused by local rainfall; b). Flood caused by rainfall in the upstream catchment area; c). Flood caused by water release from the upstream storages; d). Combination of the above. a). Flood caused by local rainfall. The amount of groundwater recharged from this type of flood depends on rainfall intensity, extent, duration, and the antecedent soil water conditions, as well as the soil type. In terms of the groundwater recharge, this type of flood has the following characteristics as compared with other types of flood: · It has the shortest path for rainwater to reach water table; · Among the groundwater recharge sources, it has the best water quality, mainly due to a less evaporation and dissolution of minerals along its path as compared with the water coming from other areas. b). Flood caused by rainfall in the upstream catchment area and discharged downstream. Depending on the flood magnitude, this type of flood can be further classified as: · Confined within the riverbanks. In this case, the interaction between the floodwater and the groundwater is similar to any other channel system. · Overflowed the riverbanks and caused inundation of the adjacent areas. c). Flood caused by water released from upstream storages. This can happen when the upstream storages reache their maximum limit and there is a need to release excess water to the downstream due to dam safety reasons. Then the situation would be similar to b) but it would be less likely that the water would overflow riverbanks and inundate adjacent areas. d). Combination of the above. The majority of floods most likely belong to this category. This type of flood has the potential to cause a serious flood disaster.

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3. Historical flood

Table 1 shows the year and magnitude order of the worst five flood events recorded at some major locations in the Murray Darling Basin since the 19th century. From Table 1 it can be seen that for the same order of flood magnitude the year in which the flood happened varies at different locations, indicating the variations in spatial/temporal rainfall patterns causing these floods.

Table 1. Largest recorded floods (in terms of river height). (source: Mussared, 1997) Order of floods and year happened River Location 2nd 3rd 4th 5th Largest Largest Largest Largest Largest Murray Albury 1870 1917 1975 1974 1931 Murray Mildura 1870 1956 1931 1917 1975 Murray Morgan 1956 1870 1931 1917 1974 Darling Bourke 1864 1890 1976 1974 1950

For example, according to Table 1, the 1956 flood is the largest recorded flood event at Morgan for the lower Murray River reach in South Australia, with the estimated recurrence interval of 1 in 160 to 180 years (SA Government, 1989), while flood happened in the same year is ranked as the second largest recorded at Mildura upstream of Morgan. The 1956 flood is not among the top five worst floods at further upstream locations such as Albury listed in Table 1, indicating that heavy rainfalls in the downstream area worsened the 1956 flood in the lower part of Murray Darling Basin significantly.

The 1956 flood extent data for the Wakool area obtained from DLWC Deniliquin office is shown in Fig. 2, which indicates around 50% of the total area was inundated at some stage during the flood.

Since the 1956 flood, the chances of flooding have been reduced for the lower Murray River reach due to development of irrigation and building of dams and reservoirs in the upstream area. For example, a previous study shows that at Chowilla of the Murray River (around SA/NSW border area) the probability in a year for getting a flood event with floodwater exceeding 100000ML/day would be 32% under natural condition rather than 9% under current regulated condition (Mussared, 1997).

The Edward River, which is branched from the Murray River near Mathoura and joined at around Deniliquin by several other waterways branched from the Murray River, is the major waterway carrying surface water from the upstream into the Wakool area. The Edward River is divided into two major waterway systems around the Wakool area when it passes Deniliquin, one can be described as the Edward – Niemur River system flowing through the central and northern part of the Wakool area and the other can be described as the Wakool River system flowing around the southern and south-western boundary of Wakool (see Fig. 2). Under normal major flood conditions floodwater passing through Deniliquin is approximately evenly distributed into these two systems (Water Resources Commission – NSW, 1981).

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4. Data collection for this study

Information about streamflow gauging stations and some streamflow data were collected for an initial assessment for their applicability in this study. It is found that the available streamflow data does not have a good control of the streamflow flowing into and out of the Wakool area and thus can hardly be used in this study.

Billabong N Moulamein Ck Waterways around Murray LWMP Area # and 1956 Flood Extent in Wakool Area W E Niemur

S Edward River Yanko Wakool Ck River Billabong Ck Jerilderie #

River Murray # Wakool River Deniliquin Wakool # River Finley Berrigan Town # # # Barham Road Bunnaloo River, creek # # Mathoura Murray Tocumwal# River Drain # Barooga Murray

Boundary of Murray LWMP area Mulwala# Boundary of Wakool 10 0 10 20 30 Area inundated in # River Moama Kilometres 1956 flood Figure 2. Waterways around the Murray Land and Water Management Plan area and the inundated area around Wakool in the 1956 flood.

The data collection and analysis in this study was mainly concentrated on the piezometric data collected by Murray Irrigation Ltd (MIL). The shallow groundwater table has been monitored and recorded since 1963. Before 1978, the readings were normally taken in the months of Mar, Jun, Sep, and Dec of each year. After 1978, the piezometric readings are taken twice a year for most of the years, one around Feb and one around Aug.

The number and spatial distribution of piezometers with readings recorded at each of the reading months vary from time to time. The piezometric data collection is summarized in Table 2.

Table 2. Summary of piezometric data collected. Total number of monitored piezometers: 1480 Total data period: 1963 Dec ~ 2001 Mar Total number of years (up to 2001 Mar) covered by data: 38 Number of months in which readings were taken: 112 Maximum number of readings taken for a single piezo: 106 Maximum number of readings taken for a piezo in a year: 7 (in 1982) Frequency of readings per year for most of the period: Quarterly or biannually Average number of readings taken per piezo over the 33.9 whole data period (1963 Dec ~ 2001 Mar): Number of piezos with no data (or data not available): 4.3%

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5. Data processing

The piezometric data was converted to a ArcView GIS database so that spatial analysis of the water table change can be carried out.

In order to interpolate reliable water table surfaces at given dates from the GIS database for the whole Wakool area, the spatial distribution of piezometers with data available for each of the dates need to well cover the whole Wakool area. The piezometric data are further assessed for their temporal and spatial distribution. The number of piezometers with data available for each of the monitoring times and the description of their spatial coverage are listed in Table 3.

Table 3. Summary of temporal and spatial distribution of piezometers with data available for each monitoring round. (Continued from left) (Continued from left) Spatial Date Count 1973-Mar 339 full 1984-Feb 853 full Coverage 1973-Jun 217 full 1984-May 455 partial 1963-Dec 45 partial 1973-Sep 195 full 1984-Jun 122 partial 1964-Mar 53 partial 1973-Dec 302 full 1984-Jul 869 full 1964-Jun 53 partial 1974-Mar 358 full 1985-Feb 871 full 1964-Sep 73 partial 1974-Jun 297 full 1985-Jul 860 full 1964-Dec 92 partial 1974-Sep 269 full 1986-Feb 885 full 1965-Mar 97 partial 1974-Dec 303 full 1986-Jul 878 full 1965-Jun 113 partial 1975-Mar 417 full 1987-Feb 750 partial 1965-Sep 114 partial 1975-Jun 450 full 1987-Jul 997 full 1965-Dec 116 partial 1975-Sep 426 full 1988-Feb 884 partial 1966-Mar 118 partial 1975-Dec 356 full 1988-Jul 1128 full 1966-Jun 125 partial 1976-Mar 448 full 1989-Feb 966 partial 1966-Sep 125 partial 1976-Jun 453 full 1989-Jul 1167 full 1966-Dec 150 partial 1976-Sep 455 full 1990-Feb 992 partial 1967-Mar 148 partial 1977-Mar 427 full 1990-Jul 1187 full 1967-Jun 141 partial 1977-Jun 445 full 1991-Feb 885 partial 1967-Sep 136 partial 1977-Sep 104 partial 1991-Jul 871 partial 1967-Dec 161 partial 1977-Dec 382 full 1992-Feb 916 partial 1968-Mar 148 partial 1978-Mar 450 full 1992-Jul 910 partial 1968-Jun 69 partial 1978-Sep 431 full 1993-Feb 889 partial 1968-Sep 160 partial 1979-Feb 482 full 1993-Jul 904 partial 1968-Dec 161 partial 1979-Aug 510 full 1994-Feb 891 partial 1969-Mar 168 partial 1980-Jan 544 full 1994-Jul 909 partial 1969-Jun 198 partial 1980-Aug 636 full 1995-Feb 895 partial 1969-Sep 247 partial 1981-Feb 645 full 1995-Jul 861 partial 1969-Dec 309 full 1981-Aug 521 full 1996-Feb 125 partial 1970-Mar 282 full 1982-Feb 712 full 1996-Jul 750 partial 1970-Jun 302 full 1982-Apr 92 partial 1997-Mar 387 partial 1970-Sep 301 full 1982-May 424 partial 1997-Aug 377 partial 1970-Dec 287 full 1982-Aug 804 full 1998-Mar 348 partial 1971-Mar 306 full 1982-Sep 98 partial 1998-Aug 389 partial 1971-Jun 317 full 1982-Nov 451 partial 1999-Mar 385 partial 1971-Sep 326 full 1982-Dec 105 partial 1999-Aug 389 partial 1971-Dec 314 full 1983-Feb 823 full 2000-Mar 385 partial 1972-Mar 347 full 1983-May 469 partial 2000-Aug 507 full 1972-Jun 339 full 1983-Aug 826 full 2001-Mar 505 full 1972-Sep 340 full 1983-Nov 123 partial 1972-Dec 340 full

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(Field note: Date – months in which readings were taken; Count – total number of piezometers with data available for that reading round; Spatial Coverage – description of if the piezometers with data cover the whole Wakool area or not, full – if they do, partial – if they do not.)

Maps showing spatial distributions of the piezometers with data available at each of the dates listed in Table 3 and surfaces representing the water table depth generated from those data using the ArcView GIS Spatial Analyst extension are presented in Appendix A.

Those water table surfaces generated from the piezometers with data available covering the whole Wakool area are used in the flood impact analysis.

6. Spatial extent and magnitude of water table response to the 1973-75 floods

The 1973 – 75 is the wettest period within the data period (1963 –2001) with major floods happened during this period ranked in the top five floods ever recorded at major locations in the Murray Darling Basin (see Table 1). There were 10 flood peaks occurred in the Murray Irrigation Area during the 1973-75 extremely wet period (Bogoda et al. 1995)

Water table in March 1973 (see Fig. 3) was selected as the reference water table as there was no significant water table change before that time and from that time onwards water table started to change significantly due to the flooding (see Fig. A23 - A45 in Appendix A).

Water tables after March 1973 were then compared with that in March 1973. From changes in the spatial extent of water table depth during the floods (see Fig. A37 – A49 in Appendix A), it is shown that there have been significantly change in water table in each of the three years of 1973-75. The maximum water table mound happened around December in each of the years. The floods happened in 1974 and 1975 were much worse than that in 1973.

GIS techniques, such as spatial analysis and 3D analysis, were applied to identify and visualize the spatial extent of water table change during the floods. Fig. 4-20 show the spatial extent of water table change at different stages during the floods, that is, the spatial extent of the difference between the water table in March 1973 and that in other respective months.

6.1. The nature of 1973-75 floods reflected by the spatial extent of water table rise

Both the spatial extent of water table rise area identified through GIS analysis and the rainfall data at Moulamein Post Office (refer to Table 4-5 and Fig. 23) suggest that the floods were caused by large-scale heavy rainfalls. The floodwater came from both upstream discharges and local rainfalls. This is indicated by that a large portion (82%) of the Wakool area had showed a water table rise between 0~3m from the March 1973 level in a short period of time to June 1973, instead of showing water table rise starting from a relatively small portion of areas along the waterways and in the low lying areas as it would if the floodwater were only from upstream area, suggesting that local rainfall at the early stage of the wet period was the major cause of the initial large extent water table rise.

The spatial extent of water table change at different times and the rainfall data at Moulamein Post Office also suggest that during 1973–75 at least three large flood events happened around the Wakool area, one during September 1973 ~ January 1974, one around June 1974 –

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December 1974. Following the 1973-74 floods, another big flood event happened around December 1975 in this region (see also Table 1). Several large flood events happened in a relatively short period of time is a major characteristic of the 1973-75 floods and a reflection of the highly variable climate affecting this region.

6.2. The water table rise process and water table change spatial extent

As the floodwater from both local rainfall and upstream discharge continued recharging shallow groundwater, high water table area started to spread from the original high water table area in all directions but more extensively towards the northwest as shown in Fig A37 - A49 in Appendix A.

With floodwater from both local rainfall and upstream discharge filling up relatively low lying areas and causing inundation, water table in these areas rose much more greater than that in other areas and started to form new high water table spots (see Fig. 4 – 20 and Fig. A37-A49 in Appendix A).

Fig. 4 – 20 also indicate that, for the 1973 flood, the floodwater formed in the upstream area initially came from the Edward River and then flowed into the Niemur River, inundating low lying areas and forming high water table rise spots along the Niemur River. As the flood continued worsening, the Wakool River was overflown between June 1973 and September 1973 forming another high water table rise spot in the south-west border of the Wakool area. The spatial pattern of water table change for the 1974 flood was similar to that of 1973.

The spatial distribution of the high water table rise areas during the 1975 flood were mainly distributed along the Niemur River, which was different from that during 1973-74 floods and suggested that the flood coming into Wakool area have a larger proportion from the Niemur - Edward Rivers than that from the Wakool River, as compared with the 73-74 floods. It was estimated that 35% of the flood passing Deniliquin flowed into the Wakool River system in November during the 1975 flood instead of 50% in other major flood events prior to 1975 due to the effect of flood mitigation engineering work built between the 1974 flood and the 1975 flood, (Water Resources Commission – NSW, 1981).

Fig. 10 shows the water table change contour and the high water table rise spots where water table rose by more than 4m when the water table rise reached its maximum around September - December 1974 during the 1973-74 floods. At that stage water table in 96% of the Wakool area rose by more than 0.5m and water table in 75% of the area rose by more than 1m, as compared with the March 1973 water table (see Fig. 10-12 and Table 4-5).

For the 1975 flood there was no significant difference in water table change areas at its maximum water table mound around December 1975, as compared with the areas of water table change around September 1974 for the 1974 flood (see Fig. 19-20 and Table 4-5). However, the high water table rise areas were distributed more along the Niemur – Edward River system than along the Wakool River system, which is consistent with the estimation of floodwater distribution between the two major waterway systems for the 1975 flood (Water Resources Commission – NSW, 1981).

6.3. Comparison between water table change extent and the 1956 flood extent

- 7 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Compared with the 1956 flood (see Fig. 2 and Fig. 10), the spatial extent of high water table rise areas during the 73-75 floods were approximately consistent with the inundated areas in the 1956 flood, suggesting that apart from the amount of floodwater the topography and the nature of waterways are the dominant factors affecting the spatial extent of flooding and subsequently the spatial extent of water table rise due to flooding. That is, water table rise due to the 73-74 floods was more dramatic in the same areas inundated in the 1956 flood than that in non-inundated area in the 1956 flood, with all the spots where water table rose more than 4m were in the 1956 inundated areas.

For the 1975 flood, most of the high water table rise areas were within the 1956 flood inundation zoon, but mainly along the Niemur River as a larger portion of floodwater came from the Niemur – Edward River system than did other floods (Fig. 17).

Water table depth in: 73Mar

Billabong

Ck

River, Ck Main Rd Niemur Edward Drainage

River River Water Table Depth (m) Wakool < 0.5 0.5 - 1 1 - 1 .5 1 .5 - 2 2 - 2 .5 River 2 .5 - 3 Murray 3 - 3 .5 3 .5 - 4 River 4 - 4 .5 4 .5 - 5 5 - 5 .5 5 0 5 10 15 20 > 5 .5 N Kilometers Figure 3. Water table depth in March 1973 in the Wakool area.

- 8 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between

1973 Mar andBillabong 73 Jun

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 4. Spatial extent of water table change between March 1973 and June 1973.

Water table change between

1973 Mar andBillabong 73 Sep

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 5. Spatial extent of water table change between March 1973 and September 1973.

- 9 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between

1973 Mar andBillabong 73 Dec

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 6. Spatial extent of water table change between March 1973 and December 1973.

Water table change between

1973 Mar andBillabong 74 Mar

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 7. Spatial extent of water table change between March 1973 and March 1974.

- 10 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between

1973 Mar andBillabong 74 Jun

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 8. Spatial extent of water table change between March 1973 and June 1974.

Water table change between

1973 Mar andBillabong 74 Sep

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 9. Spatial extent of water table change between March 1973 and September 1974.

- 11 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Boundary of inundated area in 1956 flood. WT change contour (interval = 0.5m)

WT Change Maximum WT rise Up (m) < -5.5 spot (WT rise > 4m) -5.5 - -5.0 -5.0 - -4.5 -4.5 - -4.0 -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 WT -0.5 - 0.0 0.0 - 0.5 0.5 - 1.0 1.0 – 1.5 5 0 5 10 15 20 1.5 – 2.0 > 2.0 Down Kilometres N Figure 10. Water table change between March 1973 and September 1974 and in comparison with the inundated area of 1956 flood in the Wakool area (see also Fig. 2).

Percentage of area where WT rise >0.5m : 96%

WT change contour (interval=0.5m) River, Ck Main Rd Drainage Maximum WT rise spot (WT rise > 4m)

WT Change WT rise > 0.5m WT rise £ 0.5m

5 0 5 10 15 20 N Kilometers Figure 11. Water table rise > 0.5m area in September 1974 as compared with Water table depth in March 1973 in the Wakool area.

- 12 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Percentage of area where WT rise >1m : 75%

WT change contour (interval=0.5m) River, Ck Main Rd Drainage Maximum WT rise spot (WT rise > 4m)

WT rise > 1m

5 0 5 10 15 20 N Kilometers Figure 12. Water table rise > 1m area in September 1974 as compared with Water table depth in March 1973 in the Wakool area.

Water table change between

1973 Mar andBillabong 74 Dec

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 13. Spatial extent of water table change between March 1973 and December 1974.

- 13 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between

1973 Mar andBillabong 75 Mar

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 14. Spatial extent of water table change between March 1973 and March 1975.

Water table change between

1973 Mar andBillabong 75 Jun

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 15. Spatial extent of water table change between March 1973 and June 1975.

- 14 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between

1973 Mar andBillabong 75 Sep

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 16. Spatial extent of water table change between March 1973 and September 1975.

Water table change between

1973 Mar andBillabong 75 Dec

Ck

River, Ck Main Rd Niemur Edward WT Change Drainage Up (m) Maximum WT rise < -5.5 River -5.5 - -5.0 River spot (WT rise > 4m) -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 17. Spatial extent of water table change between March 1973 and December 1975.

- 15 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table change between 1973 Mar and 75 Dec Boundary of inundated area in 1956 flood. WT change contour (interval = 0.5m)

WT Change Maximum WT rise Up (m) < -5.5 spot (WT rise > 4m) -5.5 - -5.0 -5.0 - -4.5 -4.5 - -4.0 -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 WT -0.5 - 0.0 0.0 - 0.5 0.5 - 1.0 1.0 – 1.5 1.5 – 2.0 > 2.0 5 0 5 10 15 20 Down N Kilometers Figure 18. Water table change between March 1973 and December 1975 and in comparison with the inundated area of 1956 flood in the Wakool area (see also Fig. 2).

Percentage of area where WT rise >0.5m : 93% Billabong

Ck WT change contour (interval=0.5m) River, Ck Main Rd Niemur Edward Drainage Maximum WT rise River River spot (WT rise > 4m)

Wakool

River Murray

River WT rise > 0.5m

5 0 5 10 15 20 N Kilometers Figure 19. Water table rise > 0.5m area in December 1975 as compared with Water table depth in March 1973 in the Wakool area.

- 16 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Percentage of area where WT rise >1m : 74% Billabong

Ck WT change contour (interval=0.5m) River, Ck Main Rd Niemur Edward Drainage Maximum WT rise River River spot (WT rise > 4m)

Wakool

River Murray

River WT rise > 1m

5 0 5 10 15 20 N Kilometers Figure 20. Water table rise > 1m area in December 1975 as compared with Water table depth in March 1973 in the Wakool area.

7. Quantifying impact of 73-75 floods on shallow groundwater

7.1. Quantifying areas of different water table change

Results from the GIS analysis showed that water table at the start of the floods was already relatively shallow, with water table depths in 70% of the Wakool area being less than 6m and with an average water table depth being 4.86m.

By the time the 1973-74 floods reached its maximum impact on the shallow groundwater around September - December 1974, the average water table rose by 1.58m from the March 1973 level, the average water table depth was reduced to 3.28m. Fig. 21 shows changes in the average water table rise and average water table depth at different stages during the floods. With the 1975 flood followed, the average water table depth was further reduced to 3.13m (see also Fig. A48 in Appendix A).

- 17 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Average Water Table Change over Whole Wakool Area during 73-75 Floods as Compared with Water Table in Mar 1973 (Note: water table rise is expressed in negative value) -2.0 0.00 AvgWT Rise (m) -1.8 0.50 -1.73 AvgWT depth (m) -1.58 -1.6 -1.54 1.00 Average water table -1.42 (Spatial average) -1.4 1.50

-1.22 depth (m) -1.2 2.00 -1.05 -1.07 -1.0 -1.09 2.50

rise (m) -0.8 3.28 3.13 3.00 3.44 -0.72 -0.75 3.64 -0.6 3.81 3.32 3.50 (Spatial average) -0.33 Average water table -0.4 3.77 4.00 4.11 3.79 4.14 -0.2 4.50 4.53 4.86 0.0 5.00 73Jun 74Jun 75Jun 73Mar 74Mar 75Mar 73Sep 73Dec 74Sep 74Dec 75Sep 75Dec

Time

Figure 21. Average water table change (spatial average) during 1973-75 floods as compared with water table in March 1973.

Areas of different water table changes derived from the GIS analysis are summarized in Table. 4 and Table. 5. For the spatial extent of these areas, please see relevant figures in the previous section and Appendix A. (Note: the total Wakool area used in this study is 223.3x103ha, which is calculated by GIS application based on the Wakool boundary data obtained from MIL.)

Table 4. Area (%) of different water table rise during the 1973-75 floods. Year and Month 73Mar 73Jun 73Sep 73Dec 74Mar 74Jun 74Sep 74Dec 75Mar 75Jun 75Sep 75Dec Total WT rise 0.0% 82.0% 94.0% 98.0% 95.3% 98.3% 99.3% 98.0% 94.5% 95.1% 95.2% 98.8% area 0~0.5M 0.0% 57.0% 41.8% 18.1% 20.5% 5.9% 3.2% 6.7% 12.7% 13.6% 12.5% 5.3% 0.5~1M 0.0% 15.6% 25.9% 39.2% 47.6% 25.7% 21.2% 18.6% 21.0% 26.1% 29.6% 19.3%

1~1.5M 0.0% 4.7% 13.6% 18.7% 22.1% 27.8% 32.1% 21.0% 23.2% 30.2% 27.2% 17.7% 1.5~2M 0.0% 2.5% 6.7% 10.6% 3.9% 18.6% 20.3% 25.1% 20.1% 15.8% 14.6% 19.8% 2~2.5M 0.0% 1.4% 3.3% 5.5% 0.9% 10.2% 11.0% 12.5% 11.8% 7.9% 7.1% 17.3% 2.5~3M 0.0% 0.6% 1.7% 3.3% 0.2% 4.9% 5.3% 6.9% 4.8% 1.3% 3.1% 8.1% 3~3.5M 0.0% 0.1% 0.7% 1.7% 0.0% 2.6% 2.9% 3.5% 0.7% 0.1% 1.0% 6.1% table rise range 3.5~4M 0.0% 0.0% 0.2% 0.8% 0.0% 1.6% 1.8% 2.1% 0.1% 0.1% 0.2% 3.3%

Area (%) of different water >4m 0.0% 0.0% 0.0% 0.1% 0.0% 1.1% 1.5% 1.5% 0.1% 0.0% 0.0% 1.9% >0.0m 0.0% 82.0% 94.0% 98.0% 95.3% 98.3% 99.3% 98.0% 94.5% 95.1% 95.2% 98.8%

>0.5m 0.0% 25.1% 52.1% 79.9% 74.8% 92.4% 96.2% 91.3% 81.8% 81.5% 82.7% 93.5% >1.0m 0.0% 9.4% 26.2% 40.7% 27.1% 66.7% 74.9% 72.7% 60.8% 55.4% 53.1% 74.2% >1.5m 0.0% 4.7% 12.6% 22.0% 5.1% 39.0% 42.8% 51.6% 37.6% 25.3% 25.9% 56.5% >2.0m 0.0% 2.2% 5.9% 11.4% 1.1% 20.3% 22.5% 26.5% 17.4% 9.4% 11.4% 36.7% >2.5m 0.0% 0.7% 2.6% 5.9% 0.2% 10.1% 11.5% 14.0% 5.7% 1.6% 4.3% 19.4% >3.0m 0.0% 0.1% 0.9% 2.6% 0.0% 5.2% 6.3% 7.1% 0.9% 0.2% 1.2% 11.3% >3.5m 0.0% 0.0% 0.2% 0.9% 0.0% 2.6% 3.4% 3.6% 0.2% 0.1% 0.2% 5.2% table rise magnitude >4.0m 0.0% 0.0% 0.0% 0.1% 0.0% 1.1% 1.5% 1.5% 0.1% 0.0% 0.0% 1.9% Area (%) of different water >5.0m 0.0% 0.0% 0.0% 0.0% 0.0% 0.2% 0.1% 0.0% 0.0% 0.0% 0.0% 0.1%

- 18 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Table 5. Area (%) of different water table depth during the 1973-75 floods. Year and Month 73Mar 73Jun 73Sep 73Dec 74Mar 74Jun 74Sep 74Dec 75Mar 75Jun 75Sep 75Dec 0~0.5m 0.0% 0.0% 0.0% 0.1% 0.0% 0.2% 0.4% 0.0% 0.0% 0.0% 0.0% 0.0% 0.5~1m 0.0% 0.7% 0.6% 1.7% 0.4% 7.2% 8.5% 0.5% 0.1% 0.1% 0.4% 1.0% 1~1.5m 1.3% 4.3% 7.3% 12.0% 3.8% 13.8% 11.4% 9.6% 2.0% 3.8% 4.4% 11.8% 1.5~2m 5.5% 9.0% 12.1% 10.3% 12.7% 7.5% 10.1% 18.6% 12.6% 11.2% 10.5% 20.2% 2~2.5m 8.2% 7.4% 7.0% 7.6% 8.1% 6.8% 7.4% 13.7% 12.5% 10.0% 11.2% 12.5% 2.5~3m 6.3% 5.5% 5.9% 7.1% 5.7% 7.4% 8.7% 13.1% 14.6% 10.4% 11.7% 13.3%

3~3.5m 5.4% 6.9% 5.8% 9.0% 5.9% 12.6% 8.9% 8.4% 14.3% 14.1% 13.4% 10.6%

ent water table depth 3.5~4m 5.1% 3.4% 9.0% 11.0% 14.2% 8.7% 10.0% 6.3% 9.1% 10.1% 10.1% 4.6% range 4~5m 17.0% 21.6% 21.2% 13.4% 18.0% 13.9% 11.9% 9.7% 12.3% 15.3% 12.2% 8.4% 5~6m 21.3% 16.5% 9.9% 9.4% 12.4% 9.2% 14.4% 9.7% 13.4% 13.8% 12.4% 7.1% 6~7m 15.2% 11.7% 10.4% 10.2% 12.8% 8.5% 7.3% 8.2% 6.7% 8.6% 11.4% 9.1% 7~8m 13.4% 11.1% 9.3% 7.4% 4.8% 3.6% 0.9% 2.0% 2.2% 2.3% 2.0% 1.2%

Area (%) of differ 8~9m 1.2% 1.2% 1.0% 0.6% 0.9% 0.5% 0.1% 0.2% 0.3% 0.3% 0.2% 0.1% 9~10m 0.1% 0.5% 0.4% 0.2% 0.1% 0.0% 0.0% 0.0% 0.0% 0.1% 0.1% 0.0% Total 99.9% 99.8% 99.8% 99.9% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% < 0.5m 0.0% 0.0% 0.0% 0.1% 0.0% 0.2% 0.4% 0.0% 0.0% 0.0% 0.0% 0.0% < 1m 0.0% 0.7% 0.6% 1.8% 0.4% 7.4% 8.9% 0.5% 0.1% 0.1% 0.4% 1.0% < 1.5m 1.3% 5.0% 7.9% 13.8% 4.3% 21.3% 20.3% 10.1% 2.1% 3.9% 4.8% 12.8% < 2m 6.8% 14.0% 20.0% 24.0% 17.0% 28.8% 30.4% 28.7% 14.7% 15.1% 15.3% 33.0% < 2.5m 15.0% 21.4% 26.9% 31.6% 25.1% 35.6% 37.8% 42.4% 27.2% 25.1% 26.5% 45.5% < 3m 21.3% 26.9% 32.9% 38.7% 30.8% 43.0% 46.5% 55.4% 41.8% 35.5% 38.2% 58.9% < 3.5m 26.7% 33.8% 38.7% 47.7% 36.7% 55.6% 55.4% 63.8% 56.1% 49.6% 51.6% 69.5% < 4m 31.8% 37.2% 47.7% 58.7% 50.9% 64.3% 65.4% 70.1% 65.2% 59.6% 61.7% 74.0% < 5m 48.8% 58.8% 68.8% 72.1% 68.9% 78.1% 77.3% 79.8% 77.5% 75.0% 73.9% 82.4% < 6m 70.0% 75.4% 78.7% 81.5% 81.2% 87.4% 91.7% 89.6% 90.8% 88.8% 86.3% 89.5% < 7m 85.2% 87.0% 89.1% 91.7% 94.1% 95.9% 99.0% 97.8% 97.5% 97.3% 97.7% 98.7% < 8m 98.6% 98.1% 98.4% 99.1% 98.9% 99.5% 99.9% 99.7% 99.7% 99.6% 99.7% 99.9%

Area (%) of different water table depth < 9m 99.8% 99.4% 99.4% 99.8% 99.8% 100.0% 100.0% 100.0% 100.0% 99.9% 99.9% 100.0% < 10m 99.9% 99.8% 99.8% 99.9% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0%

7.2. Quantifying the impact on shallow groundwater storage

GIS techniques were applied to quantify the net shallow groundwater storage change as compared with the water table level in March 1973. The volumes between the water table surfaces were calculated in the GIS application. Then an appropriate average specific yield (Sy) was selected for the estimation of the net equivalent water volume.

Based on the soil types in the profile recorded in the bore logs for the south part of the Wakool area and the estimated Sy for a certain soil type, it is estimated that for the soil types in the Wakool area, the average Sy over the whole Wakool area is approximately between 0.03 and 0.05. Considering the soil types in the rest of Wakool area tend to be clayey and heavier (Smith et al., 1943), 0.03 was used for the estimation.

The estimated net groundwater storage change from the March 1973 level over the whole Wakool area is presented in Fig. 23, along with the monthly rainfall recorded for the same period at Moulamein Post Office.

The results (see Fig. 23) show that:

- 19 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

· There is a significant connection between the local rainfall and the water table change, suggesting that the local rainfall have contributed a significant portion to the floods in the Wakool area. · The first maximum water table mound appeared around December 1973, after that water table start to decline. With the 1974 flood followed, the water table rose again and reached the maximum water table mound around September – December in 1974. · The maximum increase in the net shallow groundwater storage caused by the flooding by the end of 1975 is around 116x103 ML (equivalent to an average net recharge of 0.52ML/ha or an average water table rise of 1.73m from the March 1973 level). · After water table rise reached its maximum extent, there was around 19 ~ 28x103 ML of the groundwater discharged in the following three months for each of the large flood events during 1973-75. The amount discharged appeared to be related to the water table depth, that is, the higher the water table the larger the amount discharged (Table 6), suggesting that a higher water table created a higher hydraulic gradient which would accelerate groundwater discharging and a higher water table would also increase evaporation from the groundwater. · The groundwater recession after flooding slowed down gradually and subject to the weather conditions and the management actions following the flooding. A typical groundwater recession process is shown in Fig. 22, which shows groundwater recession following the maximum water table mound of the 1975 flood from December 1975 to March 1978, just before the 1978 flood, during which there appeared no significant events affecting the water table. · Apart from flood magnitude, the amount of floodwater recharged to the groundwater is also related to the groundwater storage capacity. The higher the water table is, the less the groundwater storage capacity will be. Table 6 and Table 7 shows that at the early stage of the wet period, as water table was relatively low, there was 70.1x103 ML recharged to the groundwater by the 1973 flood at the stage of maximum water table mound. When the next flood came, as water table was already high, the maximum recharge due to 1974 flood reduced to around 55.2x103 ML. Similarly, the 1975 flood resulted in a further reduced net recharge of 44.2x103 ML.

Table 6. Groundwater recession three months later following maximum water table mound during the 1973-75 floods. GW storage GW storage Date of maximum Average WT depth GW storage change (from change 3 mths 3 WT mound (m) reduced by (10 ML) 73Mar) (103ML) later (103ML) 73Dec 3.81 -70.10 -50.51 19.59 74Dec 3.28 -105.72 -81.94 23.78 75Dec 3.13 -116.08 -88.49 27.59

Table 7. Net recharge when water table mound reached maximum for each major flood during 1973-75. Initial average WT Average WT depth Net recharge Date of maximum Change in average depth before flood at max. WT mound caused by the flood WT mound WT depth (m) (m) (m) (103ML)/(ML/ha) 73Dec 4.86 3.81 1.05 70.10/ 0.31 74Dec 4.11 3.28 0.83 55.2/0.25 75Dec 3.79 3.13 0.66 44.2/0.20

- 20 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Groundwater Recession Following the 1975 Flood

-140.00 1.00

-120.00 1.50 Average water table depth (m) Groundwater storage ML) 3 -100.00 Avg WT depth 2.00

-80.00 2.50

-60.00 3.00

-40.00 3.50 from 73Mar level (10 -20.00 4.00 Net groundwater storage change

0.00 4.50 76Jul 77Jul 76Jan 76Jun 76Oct 77Jan 77Jun 77Oct 78Jan 76Feb 76Mar 76Apr 77Feb 77Mar 77Apr 78Feb 78Mar 75Dec 76Sep 76Nov 76Dec 77Sep 77Nov 77Dec 76May 76Aug 77May 77Aug Date

Figure 22. Groundwater recession process following the maximum water table mound of the 1975 flood. (Negative storage change means increase in groundwater storage from March 1973 level.)

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Net Groundwater Storage Change From Mar 1973 For the period of Mar 1973 - Jul 1977 Note: (1). Monthly rainfall is at Moulamein (Moulamein Post Office). (2). Negative storage change means increase in groundwater storage.

-200 2.3 4 0 3.9 3.4 3.4 4.4 4.6 5.5 5.1 5.1 5.4 7.2 12.4 15.7 15.6 16.2 18.1 20.2 20.7 23.2 23.8 25.4 26.2 27.2 27.2 31 32.3 32.4 33.8 35.6 36.3 39.2 40.6 43 43.1 45 46.1 45.8

52.6 50 56.1 63.4 66.6 66.9 72.8 76.2 78.6 87.3 -150 89.5 95 99.8

103 100 113.8 114.9 122.4 Monthly Rainfall (mm)

ML) -116.1 3 150 -103.4 -105.7 -95.5 -100 180.8 -88.5 -81.9 200 -77.4 -73.3 -70.1 -71.8 -69.0

-57.7 -55.3 250 -48.0 -50.5 -50 Storage Change (10 300 -22.4

350 0 0

400 73-01 73-02 73-03 73-04 73-05 73-06 73-07 73-08 73-09 73-10 73-11 73-12 74-01 74-02 74-03 74-04 74-05 74-06 74-07 74-08 74-09 74-10 74-11 74-12 75-01 75-02 75-03 75-04 75-05 75-06 75-07 75-08 75-09 75-10 75-11 75-12 76-01 76-02 76-03 76-04 76-05 76-06 76-07 76-08 76-09 76-10 76-11 76-12 77-01 77-02 77-03 77-04 77-05 77-06 77-07

Year And Month Figure 23. Net groundwater storage change as compared to water table in March 1973.

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8. Quantifying impact of more frequent floods on shallow groundwater.

8.1. The 1981 flood

Based on the weather conditions represented by the rainfall data at Moulamein Post Office, the frequency of the flood in 1981 is roughly in the order of 1 in 10 years. In 1981, there was around 63% of the annual rainfall (519mm, Jan – Dec) fell between February and July of that year (329.4mm), 75% of which fell in March (93.2mm), June (77mm) and July (77.8mm). Fig. 24 and Fig. 25 show water table in February 1981 and August 1981 respectively. The water table change between them is shown in Fig. 26.

Referenced to the water table at February 1981 level with an average water table depth being 4.28m (Fig. 24), by the time in August 1981, the net groundwater storage increased by around 42.68x103 ML (an average of 0.19ML/ha) and the average water table depth reduced to 3.64m, estimated in the same way as mentioned previously (Fig.24-26).

Water table depth in: 81Feb

River, Ck Main Rd Drainage

Water Table Depth (m) < 0.5 0.5 - 1 1 - 1 . 5 1 .5 - 2 2 - 2 . 5 2 .5 - 3 3 - 3 . 5 3 .5 - 4 4 - 4 . 5 4 .5 - 5 5 - 5 . 5 5 0 5 10 15 20 > 5 .5 N Kilometers Figure 24. Water table depth in February 1981 in the Wakool area.

- 23 - A GIS Approach to Quantify Impact of Flooding on Shallow Groundwater Levels in the Wakool Irrigation District

Water table depth in: 81Aug

River, Ck Main Rd Drainage

Water Table Depth (m) < 0.5 0.5 - 1 1 - 1 . 5 1 .5 - 2 2 - 2 . 5 2 .5 - 3 3 - 3 . 5 3 .5 - 4 4 - 4 . 5 4 .5 - 5 5 - 5 . 5 5 0 5 10 15 20 > 5 .5 N Kilometers Figure 25. Water table depth in August 1981 in the Wakool area.

River, Ck Main Rd Drainage Contour of WT change (interval = 0.5m) WT Change Up (m) < -5.5 Maximum WT rise -5.5 - -5.0 spot (WT rise > 3m) -5.0 - -4.5 -4.5 - -4.0 -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 WT -0.5 - 0.0 0.0 - 0.5 0.5 - 1.0 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 26. Water table change between February 1981 and August 1981.

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By the time in February 1982, the water table had not returned to the level of a year ago, as the net groundwater storage was still 26.93x103 ML greater than that in February 1981. It took around one and half years from August 1981 for the water table to return to the February 1981 level under a very dry condition in 1982, with only 140.8mm rainfall for that year (Jan- Dec) and being the second driest year recorded at Moulamein Post Office (1889-1999 data).

Compared with the water table in February 1981, by August 1981 the area where water table rose more than 2m was around 4% of the total Wakool area and the area where water table rose more than 0.5m was around 48% (see Fig. 26).

By February 1982, the area where water table was still 0.5m higher than that in February 1981 reduced to 35% and the area where water table was still 2m higher than that in February 1981 reduced to less than 1% of the total Wakool area.

The recession in groundwater storage following the 1981 flood was almost linear with time before the next flood event in 1983 under the dry conditions experienced in 1982, as shown in Fig. 27.

Groundwater Change around 1981 Flood

-90.00 3.40 -80.00 GW storage change 3.60 ML) Average WT depth (m) 3 -70.00 Avg WT depth 3.80 -60.00 4.00 -50.00 4.20 -40.00 4.40 -30.00 -20.00 4.60 -10.00 4.80 GW storage change (10 (referenced to Mar 1973 WT) 0.00 5.00 Oct-80 Jun-81 Oct-81 Jun-82 Oct-82 Feb-81 Apr-81 Feb-82 Apr-82 Feb-83 Dec-80 Dec-81 Dec-82 Aug-80 Aug-81 Aug-82 Time

Figure 27. Groundwater recession process following the maximum water table mound of the 1981 flood. (Negative storage change means increase in groundwater storage from March 1973 level.)

8.2. The 1992 flood

Based on the rainfall data at Moulamein Post Office, the frequency and magnitude of the 1992 flood is similar to that of 1981 flood. Around 56% (293.8mm) of the annual rainfall (528.4mm, Jan-Dec) in 1992 fell in the summer months from October to December. Due to lack of the piezometric data around 1992, the impact of 1992 flood was not estimated. However, it can be expected that the initial impact would be similar to that of 1981 flood.

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As 1992 and 1993 were both relatively wet years, water table change following the 1992 flood would be different from that of the 1981 flood.

8.3. Change in shallow groundwater for the whole data period.

All the available piezometric data are processed and groundwater changes referenced to the March 1973 level are calculated. Groundwater changes at those times for which water table can be derived from the piezometric data for the whole Wakool area are shown in Fig. 29 (see also Table 3 and Appendix A).

Before the 1973 flood, the water table fluctuated up and down not far away from the March 1973 level. After the 1973-75 wet period, water table rose considerably and has showed no sign of returning to the March 1973 level, except the unknown period of the 1990s due to lack of data.

By March 2001, the average water table was 0.82m higher than that of March 1973 with corresponding net groundwater storage of 54.93x103 ML (an average of 0.25ML/ha) greater than that of March 1973. However, for most of the area within the WTSSDS boundary, water table was below the March 1973 level due to the drainage effect of the scheme (see Fig. 28).

Billabong River, Ck Main Rd Ck Drainage WTSSDS boundary

Niemur Edward WT Change Up (m) < -5.5 River -5.5 - -5.0 River -5.0 - -4.5 -4.5 - -4.0 Wakool -4.0 - -3.5 -3.5 - -3.0 -3.0 - -2.5 -2.5 - -2.0 -2.0 - -1.5 -1.5 - -1.0 -1.0 - -0.5 River WT -0.5 - 0.0 Murray 0.0 - 0.5

0.5 - 1.0 River 1.0 – 1.5 1.5 – 2.0 > 2.0 Down 5 0 5 10 15 20 N Kilometers Figure 28. Water table change in March 2001 as compared with water table in March 1973.

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Change in Net Shallow Groundwater Storage and Average WT Depth as Compared with 1973 March Water Table

Note: Negative groundwater storage change indicates water table rise.) -140.00 3.00 3.28 3.32 3.13 3.44 3.54 3.64 3.64 3.77 3.70 3.79 3.80 3.81 3.83 3.88 4.00 4.01 4.03 4.01

-120.00 4.04 116.08 4.09 4.10 - 4.10 4.14 4.13 4.11

4.15 4.17 4.00 4.22 4.24 4.24 4.24 4.11 4.26 4.27 4.28 4.31 4.33 4.31 4.33 105.72 - 4.53 4.63 103.36 - 4.70 4.79 4.81 4.81 4.83 4.86 4.86 4.88 4.87

-100.00 4.91 4.92 4.93 ML) 4.98 95.45 - Period without adequate piezometric

3 5.00 88.49 - data to cover whole Wakool area. Net storage Average WT depth (m) 81.94 81.46 - - AvgWT depth

-80.00 77.38 - 73.35 - 71.84 - 70.88 70.10 - - 68.97

- 6.00 65.71 - 57.65 - 56.87 57.13

-60.00 - - 55.32 - 54.93 - 51.67 51.02 - 50.51 50.83 - - - 50.00 - 48.67 48.04 - - 47.26 - 46.00 - 7.00 42.79 - 41.85 41.44 41.56 - - - 40.43 39.65 - - 38.78 - 36.89 36.58 -

-40.00 - 35.73 35.43 - -

22.39 8.00 - -20.00 15.66 - 10.72 Net groundwater storage Change (10 - 4.44 - 3.46 3.13 - 2.20 - WT Rise - 9.00 0.00 0.00 0.37 0.27 1.29 WT Down 3.09 3.75 4.60 8.06

20.00 10.00 69 70 70 71 71 72 72 73 73 74 74 75 75 76 76 77 77 78 78 79 79 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87 88 88 89 89 90 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97 98 98 99 99 00 00 ------Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Jun Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec

Time

Figure 29. Change in shallow groundwater storage and average water table depth over the data period for the whole Wakool area.

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9. Summary and conclusions.

By compiling piezometric data into a GIS database and analyzing the data in a GIS application, we are able to quantify net recharge caused by flooding and to visualize the spatial extent of the impact of flooding on shallow water table reflected by water table change.

Restricted by the available data, the quantification of flood impact is mainly carried out for those years with sufficient piezometric data available.

The results show that flooding has a significant impact on shallow groundwater. The floods during the record wet period of 1973-75 caused a net recharge of around 116x103 ML (0.52ML/ha in average) at the stage when the water table rise reached its maximum value around December 1975. In a big flood event, such as experienced during 1973-75, recharge from other sources other than flood may be negligible.

Apart from the magnitude of flooding, the amount of net recharge caused by a single flood event is also related to the initial water table before the flood, which affects shallow groundwater storage capacity. The higher the initial water table is, the less shallow groundwater storage capacity will be, and consequently there will be less room for net recharge, as shown during the 1973-75 floods.

More frequent flooding such as the one experienced in 1981, whose recurrence interval is estimated as around 1 in 10 years, could result in 42.68x103 ML or an average of 0.19ML/ha net recharge at the stage around maximum water table mound, given the initial average water table depth being at 4.28m.

The major flood recharge areas within the Wakool area are mainly located along the Edward – Niemur river system.

Groundwater recession following a flooding is affected by a number of factors, such as initial water table depth, climate conditions, management actions and etc.

The average specific yield (Sy) is a critical parameter in estimating the net recharge and is very difficult to determine accurately. Sy=0.03 was used in this study based on the borelog analysis in the Wakool area and the experience in this area.

There are strong connections between local rainfall, flood, and water table change, suggesting that the floods happened in this area are normally due to both upstream and local rainfall.

By the time of March 2001, the shallow groundwater storage and average water table depth has not returned to the March 1973 level. The net groundwater storage in March 2001 is still 54.93x103 ML (an average of 0.25ML/ha) higher than that in March 1973, equivalent to 0.82m in average water table rise. However, the spatial distribution of water table depth has changed significantly, with water table in most areas within the WTSSDS boundary dropped below the March 1973 level and showing a sign that high water table area has been shifting towards the northwest.

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Acknowledgement

This study is done in collaboration with Murray Irrigation Ltd (MIL) under funding from Rice CRC. The piezometric data and base GIS layers (roads, rivers, Murray LWMP area and Wakool boundaries) used in this study are supplied by MIL. The GIS data about the 1956 flood extent and some streamflow data are obtained from DLWC Deniliquin office.

The authors acknowledge feedback and suggestions from Mr. Ary van der Lely, a prominent hydrogeologist in the NSW Riverina region.

Reference:

Bogoda, K. R., Kulatunga, N. and Hehir, K. (1995). Overview of Hydrogeology and Assessment of Subsurface Drainage Option for Watertable Control in Wakool. DLWC, Deniliquin, NSW.

Dept. of National Development (1953). Resources and Development of the Murray Valley, Volume Two - Maps. Dept. of National Development. Printed by McLaren & Co. Pty. Ltd., Melbourne.

Mussared, D. (1997). Living on Floodplains. CRC for Freshwater Ecology, the Murray Darling Basin Commission.

Smith, R., Herriot, R. I., and Johnston, E. J. (1943). The Soil and Land-Use Survey of the Wakool Irrigation District, New South Wales. Australian Council for Scientific and Industrial Research, Bulletin No. 162, Melbourne.

South Australia Government (1989). Murray Valley Management Review -Final Report. Published by the Dept. of Environment and Planning, SA.

Wakool LWMP Working Group (2001). Wakool Community’s Land & Water Management Plan. Wakool LWMP Working Group, Wakool, NSW.

Water Conservation and Irrigation Commission (1975). An Intergrated Scheme for Control of Groundwater and Salinity in Tullakool Irrigation Area and Wakool Irrigation District, N.S.W.. Water Conservation and Irrigation Commission.

Water Resources Commission - NSW (1981). Guidelines for Edward and Wakool Rivers Flood Plain Development, Deniliquin to Moama-Moulamwin Railway. Water Resources Commission – NSW, Sydney.

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