The effect of spatial soil variation on the hydrology of a semi-arid Rocky Mountains catchment

Soil properties can exhibit strong spatial variability, even at the small catchment scale. However, the hydrological implications of actual variability remain widely unknown since the required data are not easily collected. This is especially true for observations of covariation between local soil properties and local hydrological ¿uxes (e.g. evapotranspiration and drainage) and/or vegetation. We studied the impact of soil variation on the discharge of an incised catchment in the Colorado Rocky Mountains. Soil variation was determined by ¿eld and laboratory work on 100 soil pro¿les in the catchment. Soils were found to have substantially variable properties but had on average sandy texture, weak structure and limited depth to bedrock. Observed soil properties were translated into hydraulic properties using pedotransfer functions and then used in a 1D hydrological model based on Richards' equation to quantify the effect on hydrological ¿uxes. Hydrological model results indicated that the ef-fect of soil variation on the variation of hydrological model outputs was larger than the effects of variation in to-pographic in¿uenced parameters. Dependent on the hydrological model output, variation in soil hydraulic parameters is more important than the variation in soil depth and vice versa. Spatial variation of hydrological characteristics is underestimated when spatial variation of the soil information is unknown. As a consequence, knowledge on the spatial variation of input data is important for policy and water-management in order to in-clude spatial variation in the prediction of dry season stream¿ow in semi-arid catchments