Pareora – Waihao River: Water Resource Summary Report No. R06/20 ISBN 1-86937-602-1 note CHAPTER 5 DRAFT - not ready Prepared by Philippa Aitchison-Earl Marc Ettema Graeme Horrell Alistair McKerchar Esther Smith August 2005 1 Report R06/20, ISBN 1-86937-602-1 58 Kilmore Street PO Box 345 Christchurch Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru Phone (03) 688 9069 Fax (03) 688 9067 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 2 Executive summary The water resources of the rivers and associated groundwater resources draining The Hunters Hills in South Canterbury are described and quantified. These rivers include the Pareora River in the north and the Waihao River in the south, and those between. None of the rivers is large and while they sometimes flood, their usual state is to convey very low flows. Given the population in the region, including Timaru which takes much of its water supply from the upper Pareora River, the region is arguably the most water deficient in the country. More than 100 years of record are available for several raingauges in the region. Mean annual rainfalls are less than 600 mm on the coast, increasing to about 1200 mm at higher points in The Hunters Hills. Monthly rainfalls show a slight tendency toward lower values in winter months. Typical (median) monthly totals range from about 25 mm to about 80 mm. Particularly dry periods occurred in 1914-1916, 1984-1985, 1989-1999 and 2001 to 2003. Since 1996, annual rainfalls have exceeded the mean annual values in only two or three years. Overall, however, no trends or shifts are evident in the annual rainfalls. Both temperature and wind records show substantial variation between day and night and between seasons. On the coast, prevailing summer wind direction is northerly to southeasterly: prevailing winter wind is southwesterly. Northwesterly wind, commonly associated with Canterbury climate, is very infrequent. Gaugings of stream flows undertaken at sites across the catchments since the 1950s are assembled, adjusted for the effects of abstractions and correlated with corresponding flows at recorders. Fitted regression equations are used to estimate flow statistics at the gauging site, given normalised statistics for the recorder sites. Typically along the main channels, flow is lost to unconfined gravels and even in the absence of abstractions, middle reaches of most of the streams cease flowing in low flow conditions. Some recovery of flows occur in the lower reaches and the lost water also sustains flows in a number of lowland spring-fed streams. A map of the seven-day mean annual low flows per unit area of catchment includes estimates of losses and gains along main channels. A map of mean annual discharge per unit area of catchment shows that annual runoff ranges from zero near the coast to more than 20 l/s/km2 (631 mm/yr runoff) in catchment headwaters. A very tentative water balance for the Wainono lagoon is presented. The largest uncertainty appears to be the barrier through flow and the groundwater contribution. More investigations are needed to improve the water balance model but valuable insight is gained from the measurement campaign around the lake. In recent years, groundwater has become the main source of water for irrigation. Groundwater resources include: shallow aquifers comprised of Quaternary age alluvium located within the river valleys and adjacent to the coast which the are recharged by and discharge to the main rivers and streams; aquifers in the Pliocene-Pleistocene Cannington gravels (particularly in the marine sequence); deeper aquifers within the Tertiary age sediments of the Southburn Sands and Taratu Formation. A preliminary water balance is presented which accounts for inputs from land-surface recharge (rainfall and irrigation return water) and river recharge, and outputs to spring-fed streams, the Wainono Lagoon, abstractions and offshore leakage. Groundwater, particularly from the Quaternary Alluvium and Cannington Gravel aquifers, accounts for 69% of total water allocation of 413,000 m3/day. Surface water use has plateaued for most 3 catchments since the mid 1980s. Actual water use is less than allocated, and in a dry year such as 1997-98 would be expected to be around 42.5% of the average daily rate over a 150 day season. Recommendations for resource use focus on setting minimum flow/groundwater levels, and seasonal groundwater allocation limits. For the lower Pareora River where continual monitoring of river flow is impracticable, a management regime for abstractions based on observed groundwater levels in a hydraulically connected well is suggested. Acceptable residual flows need to be determined through the processes set out in the Natural Resources Regional Plan. For groundwater, a management approach for each of the distinct aquifer types based on an allocatable volume of annual recharge is presented. 4 Water Resource Summary: Pareora-Waihao Executive summary..............................................................................................................................3 1 Introduction................................................................................................................................12 1.1 Planning Framework..........................................................................................................14 1.2 Scope and Objectives.........................................................................................................15 2 Climate.......................................................................................................................................15 2.1 Rainfall...............................................................................................................................15 2.2 Temperature .......................................................................................................................19 2.3 Wind...................................................................................................................................20 2.4 Sunshine.............................................................................................................................23 2.5 Evaporation........................................................................................................................24 2.6 Soil moisture deficit...........................................................................................................24 2.7 Changes over time..............................................................................................................26 3 Geology and Soils......................................................................................................................31 3.1 Geology..............................................................................................................................31 3.2 Soils....................................................................................................................................33 4 Surface Water Resources ...........................................................................................................36 4.1 Introduction........................................................................................................................36 4.2 Methodology......................................................................................................................36 4.2.1 Naturalising flows......................................................................................................38 4.2.2 Normalising flows......................................................................................................39 4.3 Pareora River .....................................................................................................................40 4.3.1 Introduction................................................................................................................40 4.3.2 Pareora River at Huts.................................................................................................41 4.3.3 Flows at other locations in the Pareora catchment.....................................................42 4.3.4 Gains and losses along the main channel...................................................................46 4.4 Waihao River .....................................................................................................................48 4.4.1 Introduction................................................................................................................48 4.4.2 Waihao River at McCulloughs Bridge.......................................................................49 4.4.3 Flows at other locations in the Waihao catchment ....................................................53 4.4.4 Gains and losses along the main channel...................................................................55 4.4.5 Buchanans Creek .......................................................................................................57 4.5 Otaio River.........................................................................................................................63 4.5.1 Introduction................................................................................................................63 4.5.2 Otaio River at Gorge..................................................................................................63 4.5.3 Flows at other locations in the Otaio catchment and gains and losses along the main channel