A Physical Agricultural Drought Index Based on Root Zone Water Availability: Model Development and Application

Abstract

Physical agricultural drought indices generally use soil moisture to represent root-zone water availability (RZWA). The uncertainty in root-zone properties, especially in deep-rooting regions, may lead to significant uncertainty in the results. This study adopted a conceptual model that requires no specific root-zone properties to model the RZWA. A RZWA-based drought index (AgDI) was then developed by standardizing the root-zone water deficit. A comparison over 20 catchments with different terrain and vegetation demonstrated the effectiveness of AgDI in characterizing agricultural droughts, with the correlations between vegetation indices (normalized difference vegetation index [NDVI] and gross primary productivity [GPP]) and AgDI (mean ~0.54) being significantly higher than those between the same vegetation indices and the Palmer Drought Severity Index (PDSI, ~0.22), Standardized Precipitation-Evapotranspiration Index (SPEI, ~0.33), and Standardized Soil moisture Index (SSI, ~0.37). Compared to the SSI, the AgDI was not equally advantageous everywhere, but it generally showed better performance in deep-rooting regions.