Soil water content monitoring for irrigation management: A geostatistical analysis

With the increasing attention to site-specific or variable rate irrigation management, it is helpful to reconsider the quantity and placement of soil water monitoring locations in this context. Volumetric soil water content (θ_v) was monitored using a neutron probe (NP) at 72 locations in a center pivot irrigated field in eastern Nebraska. Variance reduction and temporal stability analyses were performed on θ_v from shallow (∼top 46 cm) and full profile (∼122 cm) readings for four monitoring cycles in the 2015 growing season and 2016 preseason. Eleven additional cycles were included for a subset of the data for the temporal stability analysis. The spatial correlation scale for θ_v was found to be less than the closest spacing of monitoring locations in the study (i.e. <37 m). For this field site, approximately three neutron probe monitoring locations were required to determine mean soil water depletion (±2 cm) for the field or for a management zone. Little economy would be gained in variance reduction for areal mean θ_v from using a stratified network for management areas of reasonable size in a center pivot irrigated field. Temporally stable monitoring locations were identified. However, relatively low-cost spatial predictor variables, including elevation, deviation from mean elevation, apparent electrical conductivity, and mean relative difference of interpolated cosmic ray neutron probe surveys, were not consistent predictors of NP mean relative difference. The small range of variability of θ_v within the study field is thought to be a contributing factor. It is possible that for fields with similar variability, or for site-specific irrigation where zones have been selected to reduce within-zone variance, that sensor quantity is more important than sensor placement in quantifying the areal mean θ_v for irrigation management.