Comparison and Determining of best recession curve formula in different karstic regions of IRAN

Karstic water resources are of the most important fresh water sources. Analyzing recession curve of Karstic rivers and springs is one of the common methods for determining Karstic aquifer characterizations. In this survey for achieving the best formula of Karstic recession curve، in three selected watersheds (Khersan، Gamasiab and Zymkan)، nine hydrometric gauges were selected and six index years in each station were chosen by using Standard Index of Annual Precipitation (SIAP). After separating recession curves، different formulae (Boussinesq (1903)، Maillet (1905)، double exponential function (Horton 1933) and Hyperbolic function (Drouge 1973))،were fitted to them. N coefficients were calibrated in Drouge and Horton equations by fitting estimated and observed discharges. Also we use RMSE and correlation coefficient. Results show that in 72. 22% of the cases hyperbolic function is the best formula and its N coefficient is between «1. 5 - 2. 1».