Seasonal agreement of GLDAS and FLDAS soil-moisture products with precipitation in the Lake Urmia Basin, northwestern Iran

Study Region The Lake Urmia Basin is a semi-arid endorheic watershed in northwestern Iran affected by lake shrinkage, intensive water use, drought and strong hydroclimatic gradients. Basin-wide long-term in-situ soil-moisture observations were unavailable. Study Focus We examined seasonal agreement between GLDAS-Noah and FLDAS-Noah near-surface soil moisture and GPCC precipitation during 2000–2020. Monthly fields were spatially harmonized on a common basin overlay, and January, April, July and October represented winter, spring, summer and autumn. After screening an initial 25-station precipitation pool, GPCC was compared with 24 retained meteorological stations; one missing station-month value was excluded, yielding 6047 valid paired observations. Correlations, error metrics and spatial maps were used as an indirect hydroclimatic consistency assessment, not direct validation of soil-moisture accuracy. New Hydrological Insights for the Region Agreement varied by month and location. The station comparison showed small mean GPCC bias in January and July, positive bias in April and October, and modest point-grid correlations, indicating that GPCC better represents basin-scale precipitation magnitude than local station variability. GLDAS showed strongest precipitation correspondence in January, FLDAS in April, and the two soil-moisture products agreed more closely in July and October after correction of a July/October label issue. The products can support regional drought screening, but use should remain conditional on season, physiography and independent soil-moisture validation needs.