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

• Observation of soil variation in 100 locations in a small Rocky Mountains catchment.
• Translate observations into hydraulic properties to use in a hydrological model.
• Explore effects of spatial variation of model inputs on model output variation.
• Variation in soil properties is main contributor to spatial variation of model output.

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 fluxes (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 field and laboratory work on 100 soil profiles 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 fluxes. Hydrological model results indicated that the effect of soil variation on the variation of hydrological model outputs was larger than the effects of variation in topographic influenced 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 include spatial variation in the prediction of dry season streamflow in semi-arid catchments.