Using the Van Genuchten Model to Establish Irrigation Scheduling Thresholds

An efficient irrigation system requires information on soil water status to determine irrigation application amounts and timing. Knowledge of the boundary of the lower and upper limits of soil water content is necessary to optimize irrigation applications by avoiding crop water stress and leaching. However, in situ measurements of soil water characteristics needed to establish these boundaries are labor and time intensive. The objectives of this study were thus 1) to propose a new method of calculating field capacity (FC) from the van Genuchten Model (vG Model), 2) to utilize the parameter as the upper boundary in irrigation scheduling, and 3) to evaluate the performance of vG Model in irrigation scheduling. Physical property data of nine typical soil series in Tift County, Georgia were input into the RETC (RETention Curve) program for parameters of the vG Model. Soil volumetric water content (VWC) and matric potential (Ψ_m) at FC calculated in the current study were consistent with those from previous research in soils with similar textures. Irrigation needed to bring the soil profile to FC was calculated. In order to evaluate the performance of the vG Model in irrigation scheduling, measurements of Ψ_m in six cotton fields in the southern Georgia were input into the vG Model to calculate the water inputs associated with increases of Ψ_m. The water balance was also calculated using the water balance equation with records of irrigation and rainfall and adjusted daily evapotranspiration in each cotton field. The water balance calculated from the vG Model was equal to or higher than that calculated from the water balance equation, but they were linearly correlated (R² = 0.67 ~ 0.83). Therefore, the vG Model can be calibrated for utilization in irrigation management in the aspects of the upper limit accuracy and irrigation scheduling.