When a field plot or a small watershed is repeatedly surveyed for soil water contents, locations can often be identified where soil is consistently wetter or dryer than the average across the surveyed area. Existence of such sites is important for soil management. It is also important for selection of sites to infer the area-average soil water content to use for coarse scale characterization and simulation, e.g., comparing remote sensing data and ground truth, or establishing field- or catchment-wide antecedent moisture conditions for runoff simulations. The phenomenon has been called the time stability, the temporal stability, or the temporal persistence in spatial patterns of soil water contents. The objectives of this work were: (a) to demonstrate the temporal persistence in soil water contents measured at field scale; (b) to upscale water contents from point measurements to the field scale; (c) to identify a representative location for monitoring total soil water content; (d) to estimate the length of time for soil moisture monitoring sufficient to characterize temporal persistence in water content.

Twenty four soil moisture multi-sensor capacitance probes were installed to monitor water content across a 6 ha area at the USDA-ARS OPE3 site in Beltsville, MD. These probes were located at depths of 10, 30, 50, 80, 120, 150, and 180 cm and were monitored every 10 min for 610 days. To quantify the temporal persistence, hourly average water contents were computed for all probe readings at one depth. Then the relative water contents were computed as ratios of the individual-probe water content measurement to the average water contents at that same depth (e.g. 10 cm depth). Based on these calculations, it appeared that the temporal persistence of soil-water contents was well pronounced at certain probe locations, and distributions of relative water content covered narrow ranges. Mean relative water contents were used to estimate missing data or correct erroneous sensor readings, thus substantially reducing uncertainty in average water content across the study area. Relative water contents also enabled identifying a representative location and reducing the number of sensors needed to obtain a specified accuracy of the average water content estimates. One month of soil moisture monitoring was found sufficient to evaluate distributions of the relative water contents and thus to characterize temporal persistence. Utilizing temporal persistence was a useful means to upscale water content from point measurements to the field scale and reduce uncertainty in soil water content monitoring.

Keywords: soil water content monitoring, temporal persistence, water content upscaling, field scale