Temporal persistence in vertical distributions of soil moisture contents

When a field or a small watershed is repeatedly surveyed for soil water content, sites often can be spotted where soil is consistently wetter or consistently dryer than average across the study area. The phenomenon has been called time stability, temporal stability, or temporal persistence in spatial patterns of soil water contents. Relatively less is known about temporal persistence of water content at various depths. The objectives of this work are to demonstrate the temporal persistence in soil water contents measured on a vertical two-dimensional grid, and to propose a technique to use this persistence to remedy the effect of probe malfunctioning on the estimates of the average water content in the layer. Sixty time domain reflectometry (TDR) probes (two rods) were installed along the trench in loamy soil at 12 locations with 50-cm horizontal spacing at five depths (15, 35, 55, 75, and 95 cm). The water content data were incomplete due to malfunctioning of connections in the automated measurement system. When all probes worked, some probes at a given depth consistently showed water contents below average whereas others showed water contents above the average. To quantify the persistence, we computed relative water contents as ratios of individual-probe water contents to average water contents from the same depth. Average relative water contents were used in a technique we proposed to correct estimates of depth-average water contents by accounting for missing data. A numerical experiment showed the efficiency of the proposed technique. Corrections for temporal persistence can be useful in estimating layer-averaged water contents and their uncertainty.