Generalized Relationship Linking Water Balance and Vegetation Productivity across Site-to-Regional Scales

Abstract

Evapotranspiration (ET) is a pivotal component in catchment-scale water balance and is essential for informed watershed management. Nevertheless, uncertainties in ET observation or modeling have been hindering effective water resources management. This study addresses this gap by establishing a robust, generalized linear relationship between ET and gross primary productivity (GPP) at the catchment scale. We test the linearity of the relationships between monthly GPP and ET data at 380 near-natural catchments across various climatic and landscape conditions in the contiguous US, yielding Pearson’s r≥0.6 for 97% of the 380 catchments. We then develop a regionalization strategy to parameterize this GPP-ET relationship at the catchment scale by identifying and using the linkages between the parameter values and extensively available hydroclimatic and landscape data. We demonstrate the efficacy of the proposed GPP-ET relationship and parameter regionalization strategy by their combined predictive capacity, where the predicted monthly GPP matches well with remote-sensing–based GPP product, achieving Kling-Gupta efficiency (KGE) values ≥0.5 for 92% of the catchments. In addition, we verify the relationship and its parameter regionalization at 35 AmeriFlux sites with KGE ≥0.5 for 25 sites, suggesting that the new relationship is transferable across the site, catchment, and regional scales. Our findings are valuable for improving remote-sensing–based estimation of monthly ET and diagnosing coupled water-carbon simulations in land surface and Earth system models.