Plant available water predicted by a flux-based approach

Plant available water (PAW) is an important indicator of soil suitability for crop growth and biomass production. Total available water (TAW) is defined as the difference between the water content at field capacity (FC) and at wilting point (WP). The readily available water (RAW) is a fraction of TAW defined as the water content between FC and the limiting point (LP). The challenge in determining TAW and RAW lies in the correct determination of FC, WP, and LP. We propose a process-based approach to address the issue, referenced as flux-based method (FBM). Five scenarios were used to assess the FBM: (1) generic soil-plant-atmosphere conditions, from which sensitivity analyses were performed; (2) a maize crop on several soils to compare the predictions of the FBM and the traditional FAO method; (3) mean plant-atmosphere conditions to map PAW from soil texture using the FBM and the FAO method; (4) a field experiment with a fully irrigated soybean crop; and (5) a field experiment with a common bean crop under water deficit. Resulting flux-based TAW and RAW showed high sensitivity to root length density and soil hydraulic parameters. The FBM tended to predict higher water contents at FC than the FAO method for maize crop scenarios. Texture triangles to predict TAW and RAW showed that the differences between the predictions of FBM and FAO are mostly due to the distinct values for FC, and for the LP, respectively. For both observed scenarios of soybean and common bean crops, the predictions of the FBM were plausible with time series of observed data. The FBM allows predicting PAW in Van Genuchten - Mualem type soils for different FC flux criteria, soil depths, root densities, and dynamic potential transpiration rates.