Measurement and modeling of the spatial variability of infiltration on rangeland watersheds

Infiltration processes on rangeland watersheds are highly variable in both space and time due to heterogeneities in soil properties and temporal variability of rainfall, as well as vegetation, cover and topographic characteristics. Infiltration processes at the plot scale are often described and modeled using a single hydraulic conductivity (Ke) parameter. In this study, the spatial variability of infiltration processes at the plot scale is examined using an integrated measurement and modeling approach. A newly developed variable intensity rainfall simulator is effectively used to measure changes in plot scale infiltration rate with changes in rainfall intensity. In addition, process-based hydrologic simulation models are used to determine the amount of complexity needed to accurately model the observed runoff response from the rainfall simulator experiments. First, a soil box lysimeter is used to measure variability of infiltration processes and runoff response due to soil texture and rainfall intensity. Second, five natural field plots in a rangeland watershed are used to measure the same processes with additional complexity in the form of cover and topographic characteristics. Steady-state infiltration rates increased with rainfall intensity on all of the plots, indicating partial area response and spatial variability of infiltration within each plot. Evaluation of the soil box lysimeter infiltration experiments showed that the location of areas with higher and lower infiltration capacities along a flow path does have an important effect on runoff response and that the response changes with changes in rainfall intensity. Using the detailed measurements of the field plot vegetative cover and surface characteristics of the plots to discretize the plots into separate overland flow planes improved the ability to model the variability of infiltration processes and the observed runoff response. Multi-plane configurations parameterized based on soil and plot characteristics resulted in a significant improvement over single plane configurations. The measurement and simulation model results show that the rainfall runoff relationship cannot be accurately described or modeled using a single Ke value at the plot scale