SUSTAINABILITY OF CONJUNCTIVE WATER-USE FOR SALINITY CONTROL IN IRRIGATION AREAS - THEORY AND APPLICATION TO THE SHEPPARTON REGION, AUSTRALIA

The long term sustainability of conjunctive water use for controlling irrigation salinity is affected by increase in groundwater salinity ov er time. This paper uses mass conservation of salt and water to assess groundwater degradation over long time scales. Management options whi ch affect this rate of degradation are also examined. The groundwater model developed is illustrated using data from the Shepparton Irrigati on Region in the Murray Basin, Australia. The model predicts rapid gro undwater deterioration when conjunctive use is conducted over only a f raction of the area of influence of a groundwater pump. Where the pump ed aquifer is underlain by deeper groundwaters, the rate of groundwate r degradation is also affected by leakage into or out of the conjuncti ve use system. Surface redistribution of groundwater from pumps instal led in zones of regional groundwater discharge to areas recharging the regional groundwaters, reduces excessive degradation in the zones of discharge. With optimal surface distribution of groundwater, the rate of degradation is low. The rate of groundwater degradation also depend s on salt inputs from irrigation water and rainfall, and the average d epth from the soil surface to the base of the aquifer. The rate of deg radation resulting from applied salts in surface water and rainfall is typically about 0.01 dSm(-1) per year for shallow aquifers in the She pparton region, but the rate is lower where deeper aquifers are pumped . Partial irrigation also reduces the rate of degradation because of t he reduced rate of salt inputs. Where poorer quality groundwater lies within the area of influence of the groundwater pump, a greater rate o f deterioration in the quality of pumped groundwater can be expected f rom groundwater mixing. In some irrigation regions limited export of g roundwater through surface water conveyance structures to a river is p ossible, so that a regional surface salt balance could be maintained. However, salt exports made equal to the rate of surface imports into t he irrigated area will only significantly impact groundwater salinity in the very long term, or where only shallow aquifers can be pumped. I n addition, this export can be costly for downstream water users, or i f construction of additional conveyance infrastructure is extensive; e xport can have a detrimental impact on riverine ecosystems. Other mana gement options such as the depth of pump installation and the spatial distribution of irrigation water and pumped groundwater, which affect the redistribution of salts within the groundwater system, have the po tential to have a much greater impact on local groundwater salinity.