The Water Resources of the Waikouaiti River
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The Water Resources of the Waikouaiti River © Copyright for this publication is held by the Otago Regional Council. This publication may be reproduced in whole or in part provided the source is fully and clearly acknowledged. ISBN 1-877265-73-X Published December 2008 The Water Resources of the Waikouaiti River i Foreword Rivers and streams are a major feature of any Otago landscape. The clean waters provide a rich ecological environment, serve rural and urban communities, and act as a tourist attraction. In many parts of the region, surface waters are also vital for irrigation water, which enables or enhances primary production during the dry summer months. This report on the water resources of the Waikouaiti River is one of an occasional series describing Otago river catchments. The Waikouaiti River in north-eastern Otago drains both high and low producing grasslands. Although surface water is not used extensively for irrigation at this stage, it is essential to fully understand the natural hydrology of an area before effective and efficient sustainable management decisions can be made for the future. This report has drawn together historical hydrological information, along with a recent series of low flow gaugings to help in that decision-making process. The Water Resources of the Waikouaiti River The Water Resources of the Waikouaiti River ii The Water Resources of the Waikouaiti River The Water Resources of the Waikouaiti River iii Executive summary The Waikouaiti River Catchment in eastern Otago covers an area of 425 km2, but the river yields little water under normal conditions due to the low and sporadic rainfall. The predominant land use within the catchment is sheep and beef farming followed by areas of native vegetation, and some areas of dairying in the lower catchment. Landforms range from flat floodplains close to sea level, to plateau areas, steeplands and dissected hills inland with a maximum elevation of 700 m. The river supports a variety of fish, both trout and a range of threatened native species. Consumptive use of river water is limited with four water takes in the catchment totalling 0.129 m3/s as well as the undefined takes associated with mining at Macraes Flat. Rainfall across the catchment is distributed relatively evenly throughout the year. Inland areas experience larger totals in December from summer thunderstorms and slightly lower rainfall occurs in late autumn/winter in some areas. A large variation in monthly rainfall totals between years indicates the sporadic and unreliable nature of rainfall in the catchment. Average annual rainfall varies from 500 mm at Macraes Flat to over 1100 mm in the Silverpeaks. The river has two branches of relatively different character. The north branch has a mean flow of 1.96 m3/s, with much higher flows in the winter season than the irrigation season. The south branch has a mean flow of 0.850 m3/s and little difference between irrigation season and winter flow regimes. While it has a catchment area less than a third of the north branch, the south branch flows at a similar rate to the north branch under normal conditions due to the higher rainfall. Normal monthly flows are highest in the winter months and lowest in autumn. Specific discharge (the flow per unit catchment area) during both normal and low-flow conditions is lower in the north branch, reflecting the lower rainfall and lower relief. The two branches converge approximately 8 km upstream of the outlet and the lower 5 km of the river through the Karitane estuary are under tidal influence. Both branches produce large floods at irregular intervals, with some years having two or more large floods and some none. The Water Resources of the Waikouaiti River The Water Resources of the Waikouaiti River iv The Water Resources of the Waikouaiti River The Water Resources of the Waikouaiti River v Table of contents 1. Introduction .............................................................................................................1 2. Catchment description.............................................................................................2 2.1 Area...........................................................................................................2 2.2 Relief and geology ....................................................................................3 2.3 Soils...........................................................................................................3 2.4 Vegetation cover/land use .........................................................................3 2.5 Regional Plan values.................................................................................4 2.6 Water quality.............................................................................................5 2.7 Water takes /Present water use..................................................................6 2.7.1 Water allocation...........................................................................7 3. Rainfall ....................................................................................................................9 3.1.1 Annual rainfall distribution..........................................................9 4. Hydrology..............................................................................................................13 4.1 Flow statistics..........................................................................................14 4.1.1 Annual flow distribution............................................................15 4.1.2 Flow duration curves .................................................................18 4.2 Flooding ..................................................................................................20 4.2.1 Flood flows – frequency estimates ............................................21 4.3 Low flow discharges ...............................................................................23 4.3.1 7-day MALF in the north and south branches...........................23 4.3.2 7-day MALF in the main stem...................................................23 4.4 Specific discharge from sub-catchments during summer conditions......24 5. Conclusion.............................................................................................................26 6. References .............................................................................................................27 Appendix A Details of flood flow estimation methods.....................................................28 Appendix B Details of the low flow analysis methods .....................................................30 The Water Resources of the Waikouaiti River The Water Resources of the Waikouaiti River vi List of figures Figure 1.1 Location of the Waikouaiti Catchment in eastern Otago............................... 1 Figure 2.1 The Waikouaiti River Catchment .................................................................. 2 Figure 2.2 Consented surface water takes in the Waikouaiti Catchment........................ 8 Figure 3.1 Median annual rainfall over the Waikouaiti Catchment (from growOtago) 10 Figure 3.2 Monthly rainfall trends for the Waikouaiti Catchment. See note on August totals............................................................................................................. 11 Figure 3.3 Rainfall and river flow monitoring sites in and around the Waikouaiti Catchment.................................................................................................... 12 Figure 4.1 Monthly flows in the Waikouaiti River (north branch) at Bucklands Crossing (1991-1999). The average used is the median.............................. 16 Figure 4.2 Monthly flows in the Waikouaiti River (south branch) at Lawsons (1991- 2007). The average used is the median........................................................ 16 Figure 4.3 Monthly flows in the Waikouaiti River (north branch) at Cloverdowns (1977-1987). The average used is the median............................................. 17 Figure 4.4 Flow duration curves for the Waikouaiti River........................................... 18 Figure 4.5 Flow duration curve for the Waikouaiti River (south branch) during irrigation season (September-April) and winter season (May - August)..... 19 Figure 4.6 Flow duration curve for the Waikouaiti River (north branch) during the irrigation season (September - April) and winter season (May - August)... 19 Figure 4.7 Flood waters blocking SH1 ......................................................................... 20 Figure 4.8 Flood waters inundating houses in Karitane................................................ 20 Figure 4.9 Waikouaiti flood plain ................................................................................. 21 Figure 4.10 Annual maximum floods with GEV and P3 distributions fitted ................. 22 Figure 4.11 Specific discharge under low flow conditions 1/3/07 ................................. 25 List of tables Table 2.1 Ecosystem values of the Waikouaiti River.................................................... 4 Table 2.2 Water quality trends in the Waikouaiti River. Statistically significant (p>0.2) trends are depicted in blue for improving water quality and red for declining water quality .................................................................................. 5 Table 2.3 Consumptive water takes in the Waikouaiti Catchment................................ 6 Table 4.1 Glossary of hydrological terms...................................................................