OPTIMIZING PUMPING RATES TO CONTROL PIEZOMETRIC LEVELS - A CASE-STUDY

Recharge from irrigated areas and inadequate drainage have resulted in rising piezometric levels, and is increasing the potential for waterl ogging and land salinisation in the Wakool Irrigation District (WID) o f south-eastern Australia. Since 1981, a sub-surface drainage scheme w ith 48 pumps, 110 tube wells and 2000 ha of evaporation basins has bee n developed in two stages to protect 47 000 ha of irrigated land. The existing pumping scheme is operating at non-optimal levels resulting i n the removal of more saline groundwater than necessary, and incurring operating costs which are unnecessarily high. A method has been devel oped to determine the optimum pumping rates required to control the pi ezometric levels in the WID. It comprises four steps. Firstly, a linea r programming model (PUMP-MAN-1), was used to estimate net recharge (e xcluding pumping) to the unconfined aquifer in a typical year. Secondl y, PUMPMAN-1 was verified using a groundwater simulation model (MODFLO W). Thirdly, the groundwater simulation model was used to predict futu re piezometric levels using net recharge estimated earlier (PUMPMAN-1) and corrected for pumping. Finally, another linear programming model (PUMPMAN-2), using net recharge estimated in step 1 and piezometric le vels estimated in step 3, was used to estimate the optimal pumping rat es to maintain piezometric levels at or below critical depths from the surface. The results show that the piezometric levels can be lowered to provide adequate protection from waterlogging and salinisation even when the pumping rate is reduced from 41 M1.d-1 to 31 M.d-1. This rep resents approximately a 25% reduction in saline groundwater to be disp osed and a savings of AUD 75 000 in annual operating costs.