Effect of Soil Management and Topography On the Spatial and Temporal Organization of Soil Moisture.

Soil water content (SWC) is a key variable for numerous physical, chemical and biological processes that take place at or near the soil surface. This work aims at increasing our understanding of the dynamics and organization of SWC at the field scale, in order to optimize SWC measurement strategies to evaluate the hydrological consequences of contrasting soil management systems.

An experimental field in SW Spain, where conventional and no-till management are being compared since 1983 on a clay soil, was periodically sampled at 54 locations for gravimetric SWC. Soil water retention curves were determined in the laboratory on minimally disturbed soil samples from each location.

A time-stable SWC pattern could be inferred and the wettest and driest locations were identified, as well as the points that best represented the field-average SWC throughout the monitoring period. Spatial and temporal SWC variations were characterized and related with the soil management systems, topography and slope orientation. SWC was found to be significantly higher in the no-till plots. Relationships between standard deviation, coefficient of variation, and spatially averaged SWC were determined for both soil management system, with maximum variability at SWCs of 100-120 g/kg. Using these equations, the required number of measurement points to estimate the average SWC with a certain reliability was estimated and showed that almost two times more points were required in no-till, with respect to conventional management. Using the fitted Brooks and Corey parameters and a stochastic analysis of unsaturated flow considering heterogeneous soil, enabled us to infer a theoretical relationship between standard deviation and SWC. Although the shape of both, theoretical and data-inferred relationships, were similar, the magnitude of the standard deviation was different, which could be attributed, at least in part, to the effect of topography.