Monitoring drought in ungauged areas using satellite altimetry: The Standardized River Stage Index

The lack of in-situ hydrological observations, especially in remote and developing areas, is one of the main challenges of drought monitoring. This paper proposes a novel drought index, the Standardized River Stage Index (SRSI), based on data retrieved from satellite altimetry missions. The novelties of the SRSI are that it can quantitatively monitor drought of river water deficits at multiple timescales and that it does not require in-situ observations. The Mekong River is used as the case study to demonstrate and validate the SRSI calculated from the multi-mission satellite altimetry data (Envisat and Jason-2/3). The log-logistic distribution is best fitted for SRSI calculation for the highest goodness-of-fit for river stages for the Mekong River. The comparison with the Standardized Streamflow Index (SSI) from the in-situ discharge observations shows that the SRSI can reasonably capture the drought of river water deficit. It is effective in identifying the timing and intensity of severe drought events, such as the 2019 Thailand drought. The SRSI is also compared with the Standardized Precipitation Evapotranspiration Index and the GRACE Drought Index in revealing drought features such that the river water deficit of Mekong River has time lags of 2 to 3 months for precipitation deficits and water storage deficits. Hence, the SRSI is more suitable for monitoring drought of river water deficit, because it directly links to the streamflow via physical stage-discharge relationship. Overall, the SRSI provides an effective indicator of drought of river water deficit when in-situ observations are unavailable. It can serve in the real-time drought early warning in remote and developing areas.