Plant available water capacity of soils at regional scale: Analysis of fixed and dynamic field capacity

Abstract

Estimation of the plant available water capacity (PAWC) of soils at a regional scale helps in adopting better land use planning, developing suitable irrigation schedules for crops, and optimising the use of scarce water resources. In the current study, 72 soil profiles were sampled from the Barossa Region of South Australia to estimate pedo-transfer functions deduced from easily estimated soil properties. These functions were then used to estimate the fixed (10, 33 kPa) and dynamic pressure head ( h fc ) water content at field capacity (FC) for minimum drainage flux ( q = 0.01 and 0.001 cm/day), which serves as the upper boundary for plant-available water in the soils. The estimated crop lower limit (CLL, θ wp ) was corrected for subsoil constraints, especially the exchangeable sodium percentage (ESP). The data showed that the mean values of h fc in sand-dominated light and medium textures (i.e. sand, loamy sand, sandy loam, and loam) varied in a narrow range (15.8–18.2 kPa), while in the clay-dominated heavy textured soils (i.e. clay loam) showed the widest range of h fc (11.3–49.3 kPa). The data showed wide variability in the PAWC for dynamic FC (PAWC fc ) and fixed FC at 10 kPa (PAWC 10 ), 33 kPa (PAWC 33 ), and a mix of 10 and 33 kPa (PAWC 10,33 ) pressure heads depending on soil texture. Normally, the difference between PAWC at h fc and 10 kPa (ΔPAWC 10 ) was positive, whereas the corresponding difference between PAWC at 33 kPa and h fc (ΔPAWC 33 ) was negative across all sites. Nevertheless, the estimation of PAWC assuming a fixed FC at 10 and 33 kPa pressures (PAWC 10,33 ) for sandy, clay, and silty soils reduced the deviation between dynamic and fixed pressure PAWC < 10% across the region. The estimation of PAWC was improved by incorporating the impact of subsoil constraints, such as high ESP in the soil, which was more pronounced for clay and silty texture soils occurring at lower depths. These findings demonstrate the inherent inconsistencies between static pressure and flux-based dynamic field-capacity estimations in soils. Soil heterogeneity, intra-texture variability, subsoil constraints, and swell–shrink clays can have great impacts on the water retention capacity in response to dynamic and fixed pressure FC values.