The Challenges of Estimating Plant Available Water

The availability of moisture in the soil to plants is critical to detecting and monitoring agricultural drought and its adverse outcomes, including lowering crop yields, the drying of wildfire fuels, and intensification of heatwaves. One way to monitor water availability to plants is through estimating their fractional available water (FAW). Using measures of wilting point and field capacity, FAW nominally ranges from 0 to 1, representing limited to abundant moisture access conditions respectively. One of the main challenges of deriving FAW conditions is estimating the wilting point (WP) and field capacity (FC) of the soil in which the sensors are placed. Estimates of these thresholds can be based on analysis of local pedon soil samples, extracted from U.S. Department of Agriculture’s gridded database (gNATSGO), or through time series analyses of soil moisture observations. In this study, several measures of WP and FC were analyzed at each U.S. Climate Reference Network (USCRN) station depth (i.e., 5, 10, 20, 50, and 100 cm) to derive and evaluate their respective estimates of FAW. Distributions of FAW over each station’s period of record were evaluated to determine their reasonableness, and ability to respond to evolving drought conditions across the United States.