Multiscale Assessment of the Water Balance Components in Arizona Simulated by the National Water Model

ABSTRACT Arizona, located in the Desert Southwest of the U.S., faces chronic water scarcity and has been strongly affected by the multidecadal Millennium Drought. As the state increasingly turns to water augmentation strategies, accurate, high‐resolution estimates of water balance components are essenctial. To support these efforts, this study evaluates the skill of version 3.0 of the NOAA National Water Model (NWM) 1‐km retrospective hydrologic simulations across Arizona for the period 2003–2022. Model skill was assessed against streamflow at 124 daily and 98 hourly gauges, daily evapotranspiration ( ET ) at nine eddy covariance towers, and daily snow water equivalent ( SWE ) at 19 stations. Results show that the NWM performs better for high flows than low flows, particularly during winter in snow‐dominated basins in central and northern Arizona. Lower skill for summer high flows is linked to precipitation forcing deficiencies, while poor baseflow simulation likely reflects the model's inability to represent channel transmission losses. ET daily variability is generally captured, though modest seasonal biases remain. SWE seasonality is represented, but magnitudes are consistently underestimated, likely due to biases in solid precipitation forcings. This study provides guidance on the regions and seasons in Arizona where NWM‐derived water balance components can be used with confidence and identifies where bias correction may be needed to support water management and augmentation planning.