Irrigation using groundwater for watertable control: a model of water and salt balance limitations

Abstract

One of the land management practices for alleviating irrigation waterlogging and salinity in S.E. Australia is groundwater pumping from aquifers to lower shallow watertables. Pumped groundwater may be mixed with channel supply waters to obtain an acceptable water quality then used for irrigation. A model was developed to determine the effect of irrigation using saline groundwater, diluted with channel waters, on nett recharge i.e. the quantity of deep drainage minus the quantity of pumped groundwater. The model was used to assess nett recharge in rice and pasture based landuse by reviewing field studies on groundwater use. Combined with crop salt tolerance data and a leaching requirement model, the nett recharge model enabled both water and salt balance limitations to groundwater pumping to be assessed. Nett groundwater recharge was expressed as a function of the salinity of the groundwater, surface water, and the applied water using two empirical relationships which relate the leaching fraction and the infiltration amount to the salinity of the applied water for given soils and crops. In its simplest form the model equates nett recharge to the difference between the leaching fraction and the relative salinity of the applied water. For low salinity channel water (0.1 dS/m), the analysis for rice indicated that zero or negative nett recharge would only occur if at least half the applied water was pumped from low salinity aquifers (<2dS/m). For white clover based summer pastures, on permeable fine sandy loam soils, more than half of the irrigation water must be pumped from relatively low salinity aquifers (2-4 dS/m). For white clover summer pasture on a relatively impermeable clay loam, an aquifer salinity up to 5 dS/m could be used at high dilution and zero nett recharge achieved.