Build Potentiometric Surfaces and Hydraulic-Gradient Arrows // Prepares groundwater-level observations from measured hydraulic head or from depth-to-water and land-surface elevation, interpolates potentiometric surfaces using thin-plate splines, inverse-distance weighting, ordinary Kriging, universal Kriging, or user-supplied methods, and creates raster, contour, diagnostic, support, and hydraulic-gradient products for review and export. Functions retain method conditions and fit diagnostics, validate explicit prediction tasks, inspect model- conditional uncertainty and monitoring-network sensitivity, identify limited prediction support, and check whether scaled hydraulic-gradient arrows remain within finite raster support and end at lower modeled head. Raster processing uses methods from 'terra' (Hijmans 2025) doi:10.32614/CRAN.package.terra, thin-plate splines use methods from 'fields' (Nychka et al. 2021) doi:10.5065/D6W957CT, and geostatistical interpolation uses methods from 'gstat' (Pebesma 2004) doi:10.1016/j.cageo.2004.03.012.