Comparison of synthetic unit hydrograph methods for flood assessment in a dryland, poorly gauged basin (Napostá Grande, Argentina)

Abstract

This paper examined and compared the reliability of two popular unit hydrograph methods for flood assessment in a dryland, poorly gauged basin: the Soil Conservation Service (NRCS-UH) method and the geomorphologic instantaneous unit hydrograph (GIUH) model. In addition, two different estimates of the basin’s time of concentration were compared, along with varying values of the runoff curve number, to compute the watershed lag. Simulations were performed for the upper Napostá Grande (SW Buenos Aires, Argentina), using eight historic rainfall-runoff events to validate the resulting hydrograph at the basin outlet. Validation used runoff volume, peak flow, and recession time as an alternative to time to peak, for which only mean daily data were available. Results revealed great discrepancies in unit hydrograph parameters for varying determination methods, time of concentration estimates, and basin lag factors, as well as lower-than-standard peak rate factors for GIUH hydrographs. The comparison of simulated with observed hydrographs suggested a better agreement of GIUH for the highest retardance factor, as it produced the smaller peaks with the longer recession. This study informs on the complex relationships involved in unit hydrograph (UH) determination for the studied basin and warns about the variability of obtained results depending on the applied methodology, the caution needed in the systematic use of standard parameters, and the importance of verifying the accuracy of results. This provides a valuable framework for flood assessment within regional, ungauged basins with similar characteristics, which may exhibit comparable total runoff volumes for the same rainfall event but not necessarily equivalent flood hydrographs.