Can limits of plant available water be inferred from soil moisture distributions?

Abstract

Robust assessment of crop water availability requires effective integration of soil moisture data within the range of field capacity (θ_FC) to permanent wilting point (θ_PWP). Emerging needs for spatiotemporally dynamic θ_FC and θ_PWP are difficult to achieve with lab determinations. Therefore, we used long-term data from 182 sites across the United States to evaluate whether soil moisture extremes defined by 95th and 5th percentiles represent θ_FC and θ_PWP, respectively. Soil moisture extremes and lab-measured θ_FC and θ_PWP were well correlated (R² = 0.71−0.92), however, both 95th and 5th percentiles overestimated θ_FC and θ_PWP at most depths (RMSE = 6%–16% vwc). Percentiles of soil moisture distribution that corresponded to lab-determined θ_FC and θ_PWP varied widely and were a function of precipitation received at the site and site- and soil-depth specific clay content. These findings imply that while θ_FC and θ_PWP may not be broadly represented by soil moisture extremes (95th and 5th percentiles), there may be potential to statistically infer the positioning of θ_FC and θ_PWP within long-term soil moisture distributions using biophysical determinants such as aridity and soil characteristics.