Sentinel-2-based predictions of soil depth to inform water and nutrient retention strategies in dryland wheat

The thickness or depth of fine-textured soil (z(f)) dominates water storage capacity and exerts a control on nutrient leaching in semi-arid agroecosystems. At small pixel sizes (< 1 m; 'fine resolution'), the normalized difference vegetation index (NDVI) of cereal crops during senescence (Zadoks Growth Stages [ZGS] 90-93) offers a promising alternative to destructive sampling of z(f) using soil pits. However, it is unclear whether correlations between z(f) and NDVI exist (a) at larger pixel sizes (1-10 m; 'intermediate resolution') and (b) across field boundaries. The relationship of z(f) to NDVI o(f) wheat (Triticum aestivum L.) was tested using images from a combination of multispectral sensors and fields in central Montana. NDVI was derived for one field using sensors of fine and intermediate spatial resolution and for three fields using intermediate resolution sensors only. Among images acquired during crop senescence, z(f) was correlated with NDVI (p < 0.05) independent of sensor (p = 0.22) and field (p = 0.94). The z(f) relationship to NDVI was highly dependent on acquisition day (p < 0.05), but only when pre-senescence (ZGS <= 89) images were included in the analysis. Results indicate that cereal crop NDVI of intermediate resolution can be used to characterize z(f) across field boundaries if image acquisition occurs during crop senescence. Based on these findings, an empirical index was derived from multi-temporal Sentinel-2 imagery to estimate z(f) on fields in and beyond the study area.