Variogram Assessment of Sub-Field Scale Soil Water Variability for Irrigation Management

When comparing soil water sensors installed in close proximity, observed differences in readings may reflect spatial heterogeneity in soil hydraulic properties, intrinsic sensor variability, or both. Partitioning sources of variability is essential for informed sensor selection and placement. This study used variogram analysis to quantify small-scale (9 m) soil water variability at three irrigated corn sites in southeastern North Dakota during the 2022 and 2023 growing seasons. Volumetric water content (θv) was measured along transects parallel and perpendicular to the crop rows using an Acclima TDR-310H sensor and contrasted with previously deployed sets of different types of soil water sensors. Descriptive statistics of the collected θv data revealed variability within transects, with mean θv ranging from 0.19 to 0.24 cm3 cm−3. Directional variograms showed anisotropic spatial structures, where perpendicular transects exhibited a higher nugget effect than parallel transects (square roots of nugget 0.019–0.030 cm3 cm−3 vs. 0.012–0.022 cm3 cm−3, respectively), likely due to stem flow patterns along corn rows. Similarly, sill and range parameters were also higher for perpendicular than for parallel transects. Comparison of variogram-derived variances with sensor-to-sensor differences indicated that the observed variability arises from both spatial heterogeneity and sensor type. For sensor comparison studies, these results demonstrate that sensors installed parallel to crop rows are less affected by spatial variability compared with sensors installed perpendicular to crop rows and that such installation minimizes confounding factors of spatial heterogeneities, while demonstrating the broader use of variogram analysis for partitioning measurement variability.