Determination of irrigation scheduling thresholds based on HYDRUS-1D simulations of field capacity for multilayered agronomic soils in Alabama, USA

The use of soil matric potential (ψ) sensors to support irrigation decisions has become common practice among many producers. However, ψ values at which irrigation should be initiated (ψlim) based on a pre-defined irrigation depth is still lacking. The main objectives of this study were: (i) evaluate the impact of different negligible drainage flux on estimated ψ at field capacity (ψfc) using HYDRUS 1D simulations; (ii) identify ψlim values and its corresponded irrigation depth under different soil profile depth at representative soil types in Northwest and Southeast Alabama. The ψ-θ relation at field capacity (ψfc and θfc, respectively) were estimated by a numerical internal drainage flux experiment for multilayered soils using HYDRUS-1D software simulations. Among the different negligible drainage fluxes (qfc) tested, a qfc value of 0.01 and 0.025 cm d−1 yielded the best results for the soil located at Northwest and Southeast Alabama, respectively. For a soil water depletion of 35% and a soil profile depth of 0.6 m, the ψlim ranged from − 103 to − 133 kPa for the soils located at Northwest Alabama and − 38 to − 51 kPa for soils located in Southeast Alabama. It returned an irrigation depth varying from 20 to 24 mm for Northwest Alabama soils and 15–33 mm for Southeast Alabama soils. For a same irrigation depth, it was observed that the ψlim increased (became less negative) as soil profile depth considered for irrigation calculations increased. Additionally, if the same pre-defined irrigation depth is used during the entire growing season, there is a high change that plants could be under stress due to the high level of water deficit. Using the same irrigation depth during the entire crop growth season could be a flawed irrigation management strategy; therefore, irrigation depth should dynamically change over the growth season as the plant roots reach deeper soil layers.