A Daily Drought Index Integrating GNSS Precipitable Water Vapor and Precipitation Observables

The efficiency of combining GNSS precipitable water vapor (PWV) with precipitation measurements in developing monthly meteorological drought indices has been successfully demonstrated in many regions globally. However, its ability to monitor daily or more frequent drought evolutions for practical drought monitoring and forecasting remains elusive. Here, we propose a new paradigm to enable more accurate and daily-sampled drought index, the weighted average of standardized precipitation efficiency (WASPE) at 30-day time scale, based on GNSS PWV and precipitation data, over the middle and lower reaches of the Yangtze River basin, China, during 2011-2022. The WASPE was then compared with the commonly used operational Meteorological Drought Composite Index (MCI), namely the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI). The results show that WASPE agrees well with MCI/SPI/SPEI at the 20 analyzed GNSS stations in the study region, with correlation coefficients ranging from 0.59 to 0.85. Deviations between WASPE and MCI/SPI/SPEI decrease from ~12% to ~6% with the increase of drought intensities. In particular, we demonstrated that WASPE is more consistent with SPI and SPEI for drought intensities for the abnormal, moderate and severe droughts dominated by precipitation, while coincides with MCI in the intensity of extreme affected by multiple variables. Additionally, WASPE can simultaneously maintain its drought detection capability and eliminate the occurrence of discontinuous enhancement of droughts due to the new statistical weighting method. The proposed daily WASPE could be an effective supplement for timely drought monitoring towards improvement of disaster response.