The suitability of a small volume of soil for supplying the water needs of a plant depends not only on the properties within that particular volume, but also on its relation to neighboring soil properties. Impeding layers, preferential flow paths, and other features commonly affect soil moisture over considerable distances. Consequently the spatial heterogeneity of basic soil hydraulic properties needs to be treated as a fundamental soil characteristic, critical to the flourishing of individual plants and species, and to the interrelationships of different species. Important questions remain concerning the ecosystem functional role of intrinsic variation of soil hydraulic properties. We conducted infiltration/redistribution experiments in the Mojave National Preserve to evaluate soil water behavior in alluvial fan deposits. In each of three experimental locations we ponded water in a 1-m-diameter infiltration ring for 2.3 h, monitoring soil moisture during and after infiltration using subsurface probes, as well as electrical resistivity imaging (ERI) in cross-fan and down-fan directions. ERI is particularly useful for evaluating the intensity of lateral heterogeneity because it gives spatially continuous information over a wide domain, in our experiments about 6 m long and 1 m deep, unlike individual probes that monitor conditions only in their immediate vicinity.
Correlating with the lower plant cover of older soils, our results show that infiltration capacity declines modestly with increasing deposit age. Depths reached by infiltrated water are much reduced in older soils despite the possible augmentation of preferential flow paths by soil structural development. Our ERI results show that small-scale variation in water content is minimal in recently deposited sediments in an active wash, and is highly pronounced in a Pleistocene soil, especially in a cross-fan orientation traversing original sedimentologic depositional structure. Certain fractional-m3-sized parcels of the Pleistocene soil have especially great ability to retain water. The juxtaposition of these with parcels of soil that strongly transmit but weakly retain water creates a net enhancing effect on the root-accessible soil’s ability to hold water over extended time, as is vital where infiltration is infrequent. This sort of lateral heterogeneity therefore may allow some portions of the soil to concentrate and retain water that would be unavailable to plants if the soil were more homogeneous. The observed differences along cross-fan and down-fan directions suggest that alluvial fan architecture may influence water availability in ways that are vital for plant and ecosystem health.