On the Role of Hydrological Losses in Estimating Event Runoff Coefficients Using the NRCS Method

The NRCS-CN method is widely used to estimate runoff depth. It is based on a key parameter called the Curve Number (CN), which is used to estimate the runoff coefficients of rainfall-runoff events. Since both storm and catchment characteristics for each event are dynamically variable, treating the CN of a basin as constant seems to be an unrealistic assumption. To take this variability into account, we have developed an empirical equation that estimates the event-based runoff coefficients using a correction factor obtained from the 24-h and maximum daily intensity of rainfall as well as from a minimum CN. The proposed CN method was tested in both calibration and validation stages using an event-based rainfall-runoff dataset from the Kuhsookhteh Watershed, Iran. The results showed good performance for both the calibration and validation datasets, with NS = 0.75 and R-2 = 0.77 for the calibration set, and acceptable with NS = 0.6 and R-2 = 0.61 for validation set. This study also highlights a variable threshold in form of the slope intercept equation which its initial abstraction could be seen as a descriptor of the loss component. The results for storm events with multiple continuous rainfalls show that the basin infiltration capacity reaches its minimal value and excess rainfall is expected to be controlled by the interaction between potential retention and & lambda;. The obtained runoff coefficient from this methodology was encouraging however such relations for long rainy events should be tested in more catchments.

Automated summary

The NRCS-CN method is widely used for estimating runoff depth, but assuming a constant CN for a basin is unrealistic. This study proposes an empirical equation that estimates event-based runoff coefficients by considering the variability of storm and catchment characteristics. The proposed method was tested and showed good performance in a watershed in Iran. The study also highlights a variable threshold in the form of a slope intercept equation, which describes the loss component. The obtained runoff coefficient from this methodology is encouraging but needs to be tested in more catchments for long rainy events.