Nonlinear analysis of flow recession curves

Flow recession curves of rivers are commonly simulated by the equation of a single linear reservoir which is determined by one parameter, the retention constant. In reality, however, retention-discharge characteristics are hardly linear and recession curves can be approximated only sectionwise by linear reservoirs with retention "constant" values which increase while discharge decreases. In the present study a nonlinear reservoir algorithm with an exponential relationship between storage and discharge of the type S = a-Çt is applied on given recession curves of gauging stations in different regions. The coefficients are determined by iterative curve fitting or a non linear regression procedure. Resulting computed recession curves show an excellent fit to the given data. In most cases the obtained power coefficient b is about 0.4, thus the storage-discharge relationship is far from being linear. The model parameters are related to catchment properties for regional analysis and physical interpretation.