Weather and Climate (2000) 20: 29-46 29 THE 1997-99 AND HISTORIC DROUGHTS IN NORTH OTAGO Zhijia (Harry) He Otago Regional Council, Dunedin ABSTRACT INTRODUCTION The 1997-99 drought was considered a Droughts have received increasing major event in North Otago, with record attention in New Zealand recently due to the breaking high temperatures, low rainfall and impact of the 1997-99 drought on the economy low river flows. The driest area was centred of the country. Historic droughts in North in the coastal downlands, including Oamaru, Otago have been analysed previously. Borlase the middle and lower catchment of the (1976) applied Herbst's drought evaluation Kakanui and Waianakarua River, and the technique to evaluate rainfall data in the lower catchment of the Shag River. North Otago Downlands based on the Oamaru Herbst's drought evaluation technique rainfall record for the period of 1867-1974 (Herbst et al. 1966) was applied to 132 years (Figure 1). In total, 58 droughts of varying of monthly rainfall record at Oamaru to re- severity were identified. evaluate historic drought events with 75 Several techniques were used by the Otago droughts identified. The 1997-99 drought was Catchment Board and Regional Water Board ranked by the Herbst's technique as the 3rd (1983) to evaluate the Oamaru rainfall record most severe event in North Otago in the past for the period of 1867 — 1982. These techniques 132 years and the 2"d most severe drought include the Herbst's method, the method of during the 1980's and 1990's. The drought has deciles, the technique of cumulative departure been particularly lengthy but not particularly from the mean and a mathematical filter intense. analysis. A total of 65 separate drought events Low flow was significantly more persistent during this period were identified using in the Kakanui River during the 1997-99 Herbst's method. The most severe drought drought compared with other events in the occurred between 1963 and 1968 with a 1980's and 1990's. However, the extreme low duration of 53 months. Significant 10-11 and flows were not as low as those of extended 6-7 year rainfall cycles were demonstrated by droughts. Low flows were generally more the mathematical filter analysis, which were sustained in the Kakanui River catchment similar to cyclic patterns occurring in rainfall than in the Shag River catchment. records throughout New Zealand. However, There was generally no strong relationship different methods usually give different between monthly rainfall in North Otago and results even though they are used on the same the El Nino Southern Oscillation. However, record. Bee (1984), using outputs based on there appeared to be a general negative Herbst's method of the Otago Catchment relationship between spring SOI and summer Board and Regional Water Board, compared rainfall, with low rainfall relatively more likely the rainfall method of Herbst and the water during La Nina events. During El Nino events, balance method of Coulter (1973) and the possibilities of receiving either high or low concluded that Herbst's method generally rainfall were similar, but El Ni -no springs were overestimated the frequency of drought in slightly more likely to bring high rainfall to North Otago and identified longer droughts North Otago than springs of other episodes. than the water balance method. Otago 30 Historic Droughts in North Otago Regional Council (1991) used a three year 1997-99 drought with historic events. The moving average technique to demonstrate the method was chosen over other commonly used variability of rainfall within the Shag River techniques in order to be able to compare the catchment but concluded that no cyclic rainfall results with previous studies. The spatial and pattern existed to allow predictions to be temporal distributions of rainfall during the made. 1997-99 event were also briefly examined. The relationship between El Nino / La Nina Streamflow was assessed to identify the and precipitation and streamflow values is impact of droughts on rivers. Low flow well documented in New Zealand. For regimes of the Kakanui River and Shag River example, research has shown that El Nino were compared (Figure 1). Relationship generally brings a phase of more westerly and between ENSO phenomenon and rainfall southwesterlies, and makes it drier in the variations at Oamaru and Palmerston were north and east, while La Nina promotes investigated, particularly between El Nifio easterly and northeasterly winds, with Southern Oscillation phases in spring and conversely wetter conditions in the north and rainfall variation during the following east, and drier western and southern regions summer. (Basher, 1998). A good correlation has been identified between below average rainfall in THE STUDY AREA the lower North Island and the La Nina phase (Gordon, 1986). A general positive relationship The study area extends from the Waitaki between summer rainfall in Christchurch and River in the north to the Pleasant River in the SOI was also found (Basher, 1998). south and includes the catchments of the Connection between spring (September- Kakanui, Waianakarua and Shag Rivers. November) SOI and summer (December- Figure 1 shows the study area, while Table 1 February) Clutha lake inflows (location of lists rainfall and current surface water Clutha lakes: S Lat 44°45', E Long 168°50') has monitoring stations within the study area. been identified by NIWA (Moss eta]., 1994 and The Kakanui River has a total catchment McKerchar, 1994). The studies indicated that area of 894 km2, which consists mostly of river low values of summer mean inflows are valley and rolling hills or downlands of less relatively unlikely when a moderate spring El than 600 m elevation. About a quarter of the Nino is experienced, but are relatively area is mountainous, reaching heights of some common when a moderate La Nina spring 1,640 m. Flow at Clifton Falls Bridge is proceeds. deemed as natural flow, since there is minimal Gordon (1985), in his study of effects of El abstraction above the station. Flow in the Nino Southern Oscillation on New Zealand lower catchment is, however, subject to the weather, established a relationship between influences of irrigation abstraction and seasonal rainfall and contemporary SOT. The groundwater-surface water interaction. Mean study indicated that there is a significant annual flow in the Kakanui River (Clifton Falls seasonal variation of SOI and rainfall Bridge) is 3.34 m's-'. correlation, with weak negative correlation The Shag River has a catchment area of 544 observed in Otago during spring and summer, km' and an elevation range of 1528 m. stronger negative correlation during autumn Comparatively low elevation hill country and positive correlation during winter. The forms the south and western boundary of the study also indicated that there is lag catchment. Flow at The Grange is influenced correlation between the SOT and subsequent by irrigation abstraction, particularly during weather elements in New Zealand. September to April. Flow in the lower For North Otago, the relationship between catchment is also affected by irrigation SOT and droughts was examined by Bee abstraction and groundwater-surface water (1984). This study showed that drought in interaction and the river sometimes dries up North Otago does not compare well with the naturally. Mean annual flow in the Shag River Southern Oscillation Index although most (The Grange) is 1.85 m3s-1. drought events occur at least partially during Most of the Kakanui river catchment a negative index phase. comprises greywackes, subschists and schists, This paper applies Herbst's method to while the downland topography is the most re-evaluate drought events and compares the distinctive of the region and is associated Historic Droughts in North Otago 31 • River level/flow station A Flow gauging station • Rainfall station 0 1 2 3 4 5 Kilometres WAIKOURA U A 1 U AIR 11 I AT CLIFTON FALLS BR LDE HORPE DO WINDSOR PARK NEWHAVEN AMARU ,11/ FUSHIA CREEK • Oamaru KAURU HILL DVIEW AT MILL DAM efr-THE DASHER NS Walanakarua DEEPDELLOK-Ai GOLDEN POINT • • HAG AT THE GRANGE GLENDALE 1iITrERS CREEK HAG AT CRAIG • STONEMAN ENTHORN OODWOOD PUMP PALMERSTON Palmersto SHEY PARK CENTREWOOD Figure 1. Study area, with location of rainfall and current surface water monitoring stations. 32 Historic Droughts in North Otago Station Station Name Station Type Record Catchment Area No Start End (km') 510001 Hilderthorpe Rainfall 1983 1998 140981 Waikoura Rainfall 1964 141901 Oamaru Airport Rainfall 1941 1985 141902 Oamaru AWS Rainfall 1982 15005A Dome Hills Rainfall 1975 150062 Fuchsia Ck Rainfall 1953 150072 Newhaven Rainfall 1952 150082 Elderslea Rainfall 1893 1908 150083 Windsor Park Rainfall 1892 1908 150085 Enfield Rainfall 1977 150091 Oamaru Rainfall 1867 1965 150161 The Dasher Rainfall 1936 150172 Kauru Hill Rainfall 1890 1930 150241 Islay Downs Rainfall 1969 150341 Glendale Rainfall 1961 150382 Trotters Ck Rainfall 1908 150431 Nenthorn Rainfall 1969 150471 Palmerston Rainfall 1969 150472 Bushey Park Rainfall 1908 1979 150571 Centrewood Rainfall 1912 1997 71703 Kakanui R. at Clifton Falls Br. River level/flow 1981 286 71713 Kakanui R. at Mill Dam River level/flow 1989 539 72603 Shag R. at The Grange River level/flow 1989 319 72604 Shag R. at Goodwood Pump Flow gauging 72627 Deepdell Ck at Golden Point Weir River level/flow 1985 72690 Shag R. at Craig Rd River level/flow 1993 428 Table 1. Rainfall and current surface water monitoring stations mainly with tertiary limestone deposit. In the significance of the frequency of drought Shag River, much of the catchment is occurrence in the North Otago region has often underlain by schist, while in the valley floor been referred to in earlier reports (Bee, 1984, quartz conglomerates, sandstones, mudstones Borlase, 1976 and Waugh et al., 1997). These and limestone occur and are interspersed with droughts have resulted in irrigation bans as volcanic outcrops.