The development of a regional groundwater flow model for the Tahtali watershed

In this study, a regional groundwater flow model was developed for the Tahtalı watershed and surrounding area. The aim of the study was to determine the groundwater elevations of the region using a mathematical model, and to obtain the spatial distribution of the seasonal groundwater decline. Model was run as two-dimensional and steady-state conditions by application in unconfined aquifer.Within the scope of this study, field work was conducted for the area between the Nif Mountain and the Tahtalı dam reservoir in May and October 2007. Water table depths were measured at certain monitoring wells and these measurements were put into a geographical information system along with other information relevant to the modeling study. Model inputs were obtained from field work results and from literature review. Hydraulic conductivity and groundwater recharge, which constitute the key model parameters, were considered as calibration parameters. The model domain was divided into four recharge, and seven hydraulic conductivity zones. A water budget-based precipitation-runoff model was used to calculate the recharge rate for the zone representing the Tahtalı watershed. Measured water table elevations of May were used as targets in the calibration of the model, whereas October measurements were used in the verification. Model boundary conditions were altered during the verification process. Also, recharge rates of the model were reduced based on precipitation records, considering the summer drought. Thereby, the model was run both for winter and summer conditions to calculate seasonal decline of the groundwater levels.Groundwater elevation and flow direction maps were produced based on modeling results. Water budget results of the model revealed that groundwater recharge comprised 20% of the total water input for the entire study area. Furthermore, it can be noted that there is a hydraulic connection between the Nif Mountain and the Tahtalı reservoir. The spatial distribution of the seasonal groundwater decline was limited. Decline values were lowest in the Cumaovası plain and were highest in areas with relatively lower hydraulic conductivities.