On the determination of pixel-based groundwater withdrawals using SWEO water accounting procedures. Part 2: validation

ABSTRACT Figure shows a scatter plot of yearly total groundwater withdrawals per county (Mm³ year-1) from SWEO with USGS reports and average yearly total groundwater withdrawals per county over irrigated areas (mm year-1). The Scalable Water balance from Earth Observation (SWEO) framework proposes an approach to deriving key hydrological fluxes and state variables from remotely sensed data, as a basis for policy formulation and definition of sustainable water allocations. These issues are particularly important in those areas where water is scarce, and current usage depends on unsustainable groundwater use. Validation was carried out across three contrasting basins: the Aconcagua (Chile), Upper North Platte, and Upper San Joaquin (USA) over the period 2003–2019, using the full available dataset. Validation followed four steps combining in situ and independent remote sensing data. First, SWEO monthly total water storage compared well with gravity recovery and climate experiment (R2 = 0.62–0.73). Second, the different water storage components were validated: snow and surface water agreed with United States Department of Agriculture and United States Geological Survey (USGS) observations (R2 = 0.84 and 0.81; MAE = 35.3 mm month−1 and 9.59 Mm3 month−1), and groundwater levels over 17 years matched SWEO (R2 = 0.90; MAE = 10.75 m). Third, SWEO river discharge showed robust agreement with observed streamflow (R2 = 0.73; MAE = 12.48 Mm3 month−1). Fourth, USGS five-year groundwater reports validated total withdrawals per county, agreeing strongly at basin scale (R2 = 0.91; MAE = 13.79 Mm3 year−1) and over irrigated areas (R2 = 0.90; MAE = 36.15 mm year−1), with pixel-level patterns matching well density and literature ratios. Overall, SWEO effectively captures monthly water balance dynamics at 250 m resolution across diverse basins, providing a robust tool for long-term water management, allocation policy, and climate impact assessments.