Assessing the Impact of Rainfall Variability on Watertables in Irrigation Areas

Abstract

This report describes three different methods i.e. Quartiles, Deciles and a Standard Precipitation Index (SPI) of quantifying rainfall variability. The quartile and decile methods divide the data sets into discrete intervals and therefore describe rainfall variability as step functions. However, SPI is based on continuous statistical functions and therefore can describe rainfall variability on a continuous basis. Due to the development of shallow watertable conditions in the irrigation areas smaller variations from the average rainfall conditions can be responsible for dramatic fluctuations in the watertables. Therefore the continuous SPI method can provide a better way of quantifying rainfall variability and correlating it with changes of shallow watertables. The rainfall variability in the three irrigation areas in the south eastern NSW i.e. Murrumbidgee, Coleambally and the Murray irrigation areas has been quantified using the quartile, decile and the SPI methods. The shallow piezometric level fluctuations in the Murrumbidgee Irrigation Area (MIA) show a very strong correlation with winter rainfall variation. The shallow piezometric levels in the Coleambally Irrigation Area (CIA) show a lesser degree of correlation with the SPI due to local and regional groundwater dynamics and changes in rice water use. The piezometric levels in the Murray Valley show least correlation with the SPI, which may be attributed to lower impacts of management practices and complex nature of the groundwater recharge and discharge zones in this area. The SPI method provides an excellent opportunity for year to year reporting of seasonal and yearly climatic variability. Its correlation with piezometric levels can be adopted for environmental reporting and used as a method of distinguishing between climatic and management impacts on watertables. Differences in piezometric response in years with similar winter and yearly SPI values may be caused by changes in management practices.