A Physical Agricultural Drought Index Based on Root Zone Water Availability: Model Development and Application
Meixian Liu, Alexander Y. Sun
Abstract
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.