Identifying soil moisture thresholds for agricultural drought sensitivity through coupling modeled and satellite data in a humid temperate catchment

Agricultural drought assessment requires understanding the conditions under which soil moisture (SM) deficits translate into measurable vegetation stress. This study presents a geospatial, threshold-driven framework to assess agricultural drought vulnerability by coupling modeled SM with the satellite-derived Vegetation Condition Index (VCI) in the Rur catchment, western Germany (2000–2024). Long-term SM observations and VCI data were aggregated to a common spatiotemporal scale and standardized using Z -scores to identify relative drought anomalies. An optimal SM threshold of ≤64% nFK (available field capacity) was then identified to discriminate water-limited from energy-limited conditions, resulting in a substantial improvement in SM–VCI coupling from r = 0.109 to 0.354 (+225%), corresponding to an increase in explained variance from 1.2% to 12.5%. Under these conditions, a dynamic seasonal pattern emerged, with water-limited areas expanding from the northern agricultural lowlands in May–June to the entire catchment by July, and peak coupling occurring in August (mean r = 0.501). Spatially, relatively strong correlations ( r > 0.50) were concentrated in agricultural and grassland areas, whereas forests, particularly in the early summer, showed persistent decoupling. This regime-specific sensitivity provides an ecologically informed basis for drought monitoring, with agricultural land and grasslands being most responsive. These findings demonstrate that effective drought monitoring in humid catchments requires first diagnosing ecologically meaningful water-limited regimes using locally defined SM thresholds and then focusing on moisture-sensitive land-use types. This regime-aware, spatially explicit framework provides an actionable foundation for targeted drought management in heterogeneous humid temperate landscapes. • A SM threshold ≤64% nFK isolates water-limited conditions, increasing SM–vegetation r² from 1.2% to 12.5%. • “Coupling curtain” maps reveal agricultural drought sensitivity expanding from northern lowlands across the catchment. • Agricultural land and grassland show strongest SM–vegetation coupling, while forests remain largely decoupled.