Space-time dynamics of soil water and process interactions in semi-arid terrain, Colorado, USA

Soil water is a dominant control of plant growth and hydrologic response in dryland (rainfed) agriculture. In agricultural fields, soil water is typically assumed to move vertically with no differential subsurface lateral flow in semi-arid regions. However, soil water dynamics in the profile can vary by landscape position in relation to terrain attributes and space-time soil and plant characteristics. Previous analyses have shown nested (fractal) characteristics of steady infiltration, terrain, and crop grain yield in these landscapes of eastern Colorado, USA. In this study, we measured soil water content across a landscape with varying topography to better understand soil/plant factors controlling the space-time dynamics. Rates of soil-water change at different depths and over multiple time scales were used to illustrate the space-time dynamics related to infiltration events, soil-water redistribution, and evapotranspiration. Dielectric capacitance sensors were used to measure (infer from frequency domain readings) hourly soil water content over five years (~2003 to 2008) at 18 landscape positions and four depths (30, 60, 90, and 120 or 150 cm) in a field with alternating strips of winter wheat-fallow rotation. Probes were fully buried to allow representative surface conditions and shallow tillage. Thus, after hand-augered installation of the probes and reconsolidation of soils, in-situ measurements represent field conditions at minimally disturbed sites. At summit and even some side-slope positions, profile soil-water dynamics may be explained primarily by vertical infiltration, evapotranspiration and redistribution processes. At downslope positions, however, complexities of overland flow and subsurface unsaturated lateral flow appear to influence soil water dynamics with depth. Rates of soil-water change at different depths and over multiple time scales were used to illustrate the space-time dynamics related to infiltration events, soil-water redistribution, and evapotranspiration. Process interactions in space and time further complicated the analyses of soil-water dynamics, as crop water use affected profile soil water during the growing season. Crop water use accounted for most of the inter- strip variability, while soil hydraulic properties and near-surface hydrology affected the variability across landscape positions within each strip. Both short-term hydrology and long-term soil development influenced the observed space-time patterns. The potential for surface flow accumulation may help explain the accumulation of subsurface lateral flow that also might affect the dynamics of soil water content at a given landscape position and depth. The topic of deconvolving surface infiltration from subsurface flow convergence is an ongoing research challenge. Feedbacks such as down-slope nutrient transport, differential soil development, and plant water uptake variability along the soil catena must be considered to fully explain space-time interactions. We propose application of a soil-terrain hydrology model that simulates variably saturated subsurface lateral flow in tandem with overland flow in semi-arid landscapes. Better understanding of such interactions should aid variable-rate management to enhance both production and sustainability.