Modelling of a groundwater system to assess links between reduced rainfall and groundwater levels

Amgad ELMAHDI, CSIRO LAND AND WATER
Annette Barton, CSIRO LAND AND WATER

Abstract

The Warrion region in south west Victoria, Australia, has been subjected to land cover changes, groundwater pumpage and irrigation. Declining groundwater levels over the past decade has focused attention on the impacts of irrigation practices and research to date has identified groundwater extraction as the primary cause of falling groundwater levels and the drying of a significant group of lakes. However, recent declines in groundwater levels have coincided with reduced rainfall, and it has been hypothesised that severe drought over the past ten years has affected groundwater levels in the Warrion more significantly than has been thought. To test this hypothesis a simple one-dimensional model was developed to determine how well groundwater behaviour could be estimated from rainfall and evapotranspiration. The region is understood to be a local groundwater system with recharge occurring along the elevated ridge of the central range and groundwater discharge into the lakes on either side. The system was conceptualised as three connected reservoirs, with an upper, recharge, reservoir (the groundwater store) draining to two lower, discharge, reservoirs (the lakes). Input to the upper reservoir is a function of rainfall and evaporation, while outflow to the lakes is a function of the height difference between the upper and lower reservoirs and hydraulic conductivity –- in accord with Darcy's Law. A system dynamics approach has been taken to modelling the system using the object oriented programming language, VENSIMTM. The model was calibrated to recorded monthly groundwater level measurements. The best fit was obtained where the model was calibrated using the last half of the record (R2 = 0.95) and validated using the first half. An R2 of 0.925 was obtained for the full historic record (total 83 monthly values). No lag was required for the rainfall record. Results show groundwater fluctuations can be simply modelled using rainfall and lake levels data. Recharge is estimated at around 30%. This has led to the conclusion that groundwater trends are strongly linked to reduced rainfall.