Objective Drought Classification Using Multiple Land Surface Models

Abstract

Abstract The current generation of drought monitors uses physically based indices, such as the standardized precipitation index (SPI), total soil moisture (SM) percentiles, and the standardized runoff index (SRI) to monitor precipitation, soil moisture, and runoff deficits, respectively. Because long-term observations of soil moisture and, to a lesser extent, spatially distributed runoff are not generally available, SRI and SMP are more commonly derived from land surface model–derived variables, where the models are forced with observed quantities such as precipitation, surface air temperature, and winds. One example of such a system is the North American Land Data Assimilation System (NLDAS). While monitoring systems based on sources like NLDAS are able to detect droughts, they are challenged by classification of drought into, for instance, the D0–D4 categories used by the U.S. Drought Monitor (USDM), in part because of uncertainties among multiple drought indicators, models, and assimilation systems. An objective scheme for drawing boundaries between the D0–D4 classes used by the USDM is explored here. The approach is based on multiple SPI, SM, and SRI indices, from which an ensemble mean index is formed. The mean index is then remapped to a uniform distribution by using the climatology of the ensemble (percentile) averages. To assess uncertainties in the classification, a concurrence measure is used to show the extent to which the different indices agree. An approach to drought classification that uses both the mean of the ensembles and its concurrence measure is described. The classification scheme gives an idea of drought severity, as well as the representativeness of the ensemble mean index.