Monitoring Meteorological and Hydrological Droughts at a Daily Scale: Simple Physical Models and Derived Indexes

The day-to-day monitoring of drought is required by decision-makers. Treating flood/drought as an instantaneous state, we have developed a physical model to describe the time change in the state, and proposed the derived WAP (Weighted Average of Precipitation) index, which uses precipitation only and monitors meteorological drought. Evaporation is implicitly included in the model as one of the dissipation components. In the present study, we modify the model to express evaporation explicitly, making the “flood extent” forced by both precipitation and evaporation. The derived WAPE index serves as a water-balance-based drought indicator that reflects the day-to-day variation in moisture conditions, with particular emphasis on soil drying processes. Compared with WAP, WAPE captures further changes in drought extent during dry periods, corresponding to soil moisture evolution. The WAPE reasonably describes two real physical processes: (1) during dry spells with strong evaporation, drought tends to be aggravated; and (2) when local drought is severe and evaporation weakens, drought may be locally mitigated due to the restoring force from horizontal and vertical soil moisture gradients. The daily-resolution and physically based nature of the WAPE index also suggests its potential applicability to the identification and dynamic monitoring of flash droughts under climate warming.