WSIBal: a coupled water, conservative solute and environmental isotope mass balance model for lakes and other surface water bodies.

The calculation of a water balance for a surface water body is often subject to large errors. These errors arise because of the uncertainty associated with individual components of the water balance, particularly the volumes of groundwater inflow and outflow. Coupling the water balance with balances for conservative solutes and environmental isotopes can assist in reducing the size of the error in the mass balance. WSIBal is a computer model that simultaneously solves the coupled mass balances of water, a conservative solute tracer (e.g. chloride) and environmental isotope tracers (e.g. delta H-2, delta O-18). The model can be used to solve for unknown components of the lake water balance, with the coupled environmental isotope tracer balance in WSIBal providing additional model constraints that extend the conventional water and solute balance approach. Features of the model are illustrated by its application to two case studies. The first investigates the effect of pumping on Blue Lake, a deep maar lake located near Mount Gambler in South Australia, which has no surface inflows or outflows, apart from pumping for water supply. The impact on the groundwater components of the lake water balance caused by the pumping is investigated using WSIBal. The second case study examines the seasonal cyclical variations in the composition of Lake Jasper in the D'Entrecasteaux National Park, in the southwest of Western Australia. These simulations are used to constrain the magnitude of the water balance components of the lake, particularly groundwater inflow and outflow.