Modelling surface water-groundwater interactions at the Palas Basin (Turkey) using FREEWAT

Palas Basin is a semi-arid closed basin located in the Central Anatolia region of Turkey. The major economic activity in the basin is agriculture; therefore, both surface water and groundwater are used for irrigation. However, intensive use of water resources threatens the hydrologic sustainability of a lake ecosystem (Tuzla Lake) located in the basin. In this study, we analyzed the relationships between agricultural water uses in the Palas Basin and water flows to the Tuzla Lake using groundwater flow model developed with the FREEWAT platform. The model grid with 250 m x 250 m resolution was created based on the entire watershed. Two hydrostratigraphic units were identified. The source terms defined in the model were rainfall recharge and the sink terms were evapotranspiration and wells. The model was run for one year at steady state conditions. Three scenarios were simulated to understand the effect of groundwater use on the lake hydrology. The first scenario assumed that there was no groundwater abstraction. As the second and third water management scenario, the model was run with 50% less and %50 more groundwater abstraction than that of the reference conditions. The model successfully simulated the direction of groundwater flow and groundwater levels in the basin. Annual groundwater recharge was simulated as 5.27 million m³. Groundwater losses were due to pumping (1.49 million m³/yr), leakance to DeÄŸirmen River (2.25 million m³/yr) and seepage to Tuzla Lake (1.53 million m³/yr). Water flows to Tuzla Lake were significantly related to pumping rates. Increasing groundwater pumping rates reduces groundwater flows to Tuzla Lake and lowers lake water levels. No groundwater abstraction and reduction in groundwater pumping rates increase water flows to Tuzla Lake and cause higher water levels. This analysis showed that protection of hydrologic characteristics of Tuzla Lake is only possible with more control on groundwater abstraction.