Modeling Soil Water Availability to Crops Requires Accurate Knowledge of Near-Wilting Soil Hydraulic Properties

Water availability to crops depends on dynamic processes in the soil, however, predictions of characteristic water contents like field capacity or permanent wilting point are usually based on a fixed pressure head value alone. We hypothesize that crop water availability and the characterizing conditions depend on soil hydraulic properties, especially those for the dry soil close to wilting point, making an evaluation only by pressure head insufficient. On its turn, agricultural management affects the hydraulic properties, hence the evaluation of changes in crop water availability should include their determination. We verified the changes in water availability limits in some Brazilian soils with different management histories and, based on obtained values, made predictions of the available water fraction and how it is affected by soil management. We performed evaporation experiments measuring soil water content by gamma-ray attenuation readings. Undisturbed soil samples (7 cm diameter and 7 cm height) were taken from several soils. Samples were saturated by capillarity, closed at the bottom and then submitted to natural evaporation from their upper surface. Evaporation rate was determined by weighing. Measurements of attenuation of a collimated gamma ray, produced by a ¹³⁷Cs source were performed at several times and depths below the sample surface, during some weeks after the onset of evaporation, until samples reached a very low (quasi air-dry) water content. Samples were finally oven dried and one last reading was performed to determine the attenuated reading of the dry sample. Water contents corresponding to attenuated readings were calculated using Beer´s law. From these measurements, hydraulic properties (retention and conductivity) were obtained as a function of pressure head using Richards equation-based inverse modeling. The obtained hydraulic properties allowed to determine field capacity by a flux density criterion at the lower soil boundary. Critical and wilting point conditions could be estimated by a matric flux potential approach. Determined values for field capacity and critical and wilting point conditions were evaluated by comparison to values obtained by the common practice using fixed pressure heads. The set of information will allow to discuss the validity of the common (fixed pressure head) method, suggest improvements for its interpretation and limitations to its use.