Infiltration on alluvial fans in arid environments: Influences of fan morphology

Mountain-front recharge through highly permeable alluvial fans can be an important source of groundwater recharge in arid climates. To better understand the geomorphic factors (e.g., fan slope, fan area, active channel proportion of fan area, sediment permeability, and entrenchment of the active channel) that control flow and infiltration on alluvial fans, we developed a coupled numerical model of steady surface water flow and Green-Ampt-type infiltration. The model was applied to synthetic alluvial fans using random walkers to create connected distributary networks. The purpose of this approach is to predict where and how recharge occurs on fans as a function of fan morphology. Using the numerical model, we examined how the fan shape and the sequence of fan surfaces influenced where infiltration occurred on the fan. We also investigated how fan morphology influenced the partitioning of infiltration between the fan and the valley floor. Finally, we examined how infiltration influenced the spatial distribution of flooding. The greatest amount of infiltration occurred on low gradient fans where water spread laterally with shallower ponded water depths, although the large inundation area often included less permeable sediments outside of the active channel. The ratio of the incision depth to the input flow depth was an important predictor of the amount of infiltration. The greatest amount of infiltration occurred on fans with incision depths slightly smaller than the input flow depth. These results have implications for groundwater resource assessment and for development of monitoring networks on fans in arid environments.