Effects of initial soil water content and saturated hydraulic conductivity variability on small watershed runoff simulation using LISEM

Abstract

Soil water content (θ) and saturated hydraulic conductivity (Ks) vary in space. The objective of this study was to examine the effects of initial soil water content (θi) and Ks variability on runoff simulations using the LImburg Soil Erosion Model (LISEM) in a small watershed in the Chinese Loess Plateau, based on model parameters derived from intensive measurements. The results showed that the total discharge (TD) and peak discharge (PD) were underestimated when the variability of θi and Ks was partially considered or completely ignored compared with those when the variability was fully considered. Time to peak (TP) was less affected by the spatial variability compared to TD and PD. Except for TP in some cases, significant differences were found in all hydrological variables (TD, PD and TP) between the cases in which spatial variability of θi or Ks was fully considered and those in which spatial variability was partially considered or completely ignored. Furthermore, runoff simulations were affected more strongly by Ks variability than by θi variability. The degree of spatial variability influences on runoff simulations was related to the rainfall pattern and θi. Greater rainfall depth and instantaneous rainfall intensity corresponded to a smaller influence of the spatial variability. Stronger effects of the θi variability on runoff simulation were found in wetter soils, while stronger effects of the Ks variability were found in drier soils. For accurate runoff simulation, the θi variability can be completely ignored in cases of a 1-h duration storm with a return period greater than 10 years, while Ks variability should be fully considered even in the case of a 1-h duration storm with a return period of 20 years.