A simple model of karst spring flow using modified NRCS procedures

A simple model of spring flow in a karst watershed with numerous sinkholes is presented. The watershed is divided into subwatersheds and runoff volume calculated using the NRCS curve number procedure with corrections for actual antecedent moisture conditions using the 5-day antecedent rainfall volume as a parameter. The peak discharge for each subwatershed is calculated with the TR-55 unit discharge equations with time of concentration corrected for the flow through the epikarst and routed exponentially to the spring, using a calibration coefficient. Total discharge at the spring is calculated by summing attenuated peaks from each subwatershed, using a weighting factor based on the predicted arrival time for each peak flow. The model was calibrated on long-term flow data collected at the spring. The calibrated model was then evaluated on four storms measured subsequent to the calibration. The results were acceptable for all but one storm, but indicate the need for improved runoff volume calculation methods in karst watersheds.