Quantifying the Importance of Selected Drought Indicators for the United States

Using information theory, our study quantifies the importance of selected indicators for the U.S. Drought Monitor (USDM) maps. We use the technique of mutual information (MI) to measure the importance of any indicator to the USDM, and because MI is derived solely from the data, our findings are independent of any model structure (concep-tual, physically based, or empirical). We also compare these MIs against the drought representation effectiveness ratings in the North America Drought Indices and Indicators Assessment (NADIIA) survey for Ko center dot ppen climate zones. This reveals 1) agreement between some ratings and our MI values [high for example indicators like standardized precipitation evapo-transpiration index (SPEI)]; 2) some divergences (e.g., soil moisture has high ratings but near-zero MIs for ESA Climate Change Initiative (CCI) soil moisture in the Western United States, indicating the need of another remotely sensed soil moisture source); and 3) new insights into the importance of variables such as snow water equivalent (SWE) that are not included in sources like NADIIA. Further analysis of the MI results yields findings related to 1) hydrological mechanisms (summertime SWE domination during individual drought events through snowmelt into the water-scarce soil); 2) hydrocli-matic types (the top pair of inputs in the Western and non-Western regions are SPEIs and soil moistures, respectively); and 3) predictability (high for the California 2012-17 event, with longer-time scale indicators dominating). Finally, the high MIs between multiple indicators jointly and the USDM indicate potentially high drought forecasting accuracies achievable using only model-based inputs, and the potential for global drought monitoring using only remotely sensed inputs, espe-cially for locations having insufficient in situ observations.SIGNIFICANCE STATEMENT: Drought maps from the U.S. Drought Monitor and the Objective Short-and Long-Term Drought Indicator Blends and Blend Equivalents are integrated information sources of the different types of drought. Multiple indicators go into creation of these maps, yet it is usually not clear to both public and private stake-holders like local agencies and insurance companies about the importance of any indicator in any region and season to the drought maps. Our study provides such objective information to enable understanding the mechanism and type of drought occurring at a location, season, and possibly event of interest, as well as to potentially aid in better drought monitoring and forecasting using smaller custom sets of indicators.