The impact of El Nino and La Nina weather patterns on Canterbury water resources

Abstract

Water is an extremely important and increasingly contentious resource in the Canterbury region. An accurate assessment of the size and behaviour of the resource is fundamental to effective water management. This study attempts to calculate rainfall, runoff and evapotranspiration (ET) for Canterbury in order to ascertain a regional water balance as a means of quantifying a net excess or deficit of water in the hydrological budget. The effect of the El Nino Southern Oscillation (ENSO) on this water balance is investigated. Water balances are calculated for two ‘mega-catchments’; western or ‘Alpine’ Canterbury, from the Southern Alps to the foothills, and eastern or ‘Plains’ Canterbury. Long term averages (LTA) are compared with the two strongest years of positive and negative ENSO in the last thirty years, as measured by the Southern Oscillation Index (SOI). The water balance of the Alpine catchment proved problematic, with a significant deficit result. This is thought to be due to major underestimation of rainfall in the alpine region resulting from poor distribution of rainfall gauges. The rainfall figures were recalculated by addition of runoff and ET. The resulting rainfall figures show an increase in rain from LTA for El Nino years and an even greater increase for La Nina, although the high variability in rain means these differences are not statistically significant. This research indicates that there is an impact of strong ENSO events on the water budget components of Canterbury, New Zealand. La Nina conditions tend to produce increased rainfall and decreased evapotranspiration compared to El Nino conditions. The Plains catchment is where the pressure on the water resources is greatest. The LTA’s produce an annual excess of 94mm, while El Nino years with lower rainfall and higher ET, produced a deficit of 65mm. La Nina years have rainfall between the LTA and El Nino years, but a lower ET than either, and produces a deficit of 10mm. Due to data and modeling inaccuracies the La Nina deficit is not large enough to be considered certain. Availability of accurate measured data across the catchments proved to be a major issue for this study. As a result a mixture of measured and modeled data is used and results should be treated with caution. It is recommended that significant investment be made in increasing the capacity of the region to accurately quantify its water resources.