The Moisture Balance Drought Index (MBDI) was developed to monitor drought occurrence on a monthly basis across the Colorado River Basin (CRB) of the southwestern United States. The index is based on the balance of precipitation with an air temperature-driven estimate of the climatic demand for moisture, or potential evapotranspiration. This common approach to characterizing the hydroclimate is taken based on the premises that (1) representing only precipitation can be misleading in warm and/or dry climates or seasons in which evaporative loss dominates precipitation, (2) simulating soil moisture requires location-specific soil and vegetation characteristics that make accurate estimate difficult, and (3) soil moisture is characterized by a finite lower limit that restricts portrayal of overall hydroclimatic variability at the dry end of the distribution. The balance of precipitation and potential evapotranspiration is the basis for estimating soil moisture, and it can also represent the level of demand for water supply (e.g., agricultural, municipal) and water loss from open water bodies. Furthermore, the balance of precipitation and potential evapotranspiration can be used to directly compare conditions for different seasons and regions. The MBDI is based on the monthly difference of precipitation minus potential evapotranspiration and accumulated values of the difference over an array of multi-month timeframes that potentially relate to different drought impacts. Values are expressed in percentiles for ease of interpretation from a historical perspective.

In this paper we assess the utility of the MBDI and the value of including representation of climatic demand in drought monitoring within the CRB, and we present a cursory look at application of the index across the continental United States. First, the MBDI and indices of its individual components of precipitation and potential evapotranspiration were matched with basin runoff and groundwater level data from eight select sub-basins of the CRB. Second, the indices were matched with Normalized Difference Vegetation Index (NDVI) values at select sites across the region to assess relationships with vegetation greenness. Third, the indices were matched with historical records of wildfire occurrence and coverage on a regional basis. We present and discuss the results of statistical analyses of the relationships between the historical variability in these impact variables and the MBDI, precipitation, and potential evapotranspiration. Results indicate clear benefits in representing the balance between precipitation and the climatic demand for moisture in drought monitoring within the arid southwest United States. To expand the spatial coverage of the MBDI for additional regional testing, the method has been applied to a newly derived five-kilometer resolution data base for the continental United States created at the National Climatic Data Center. We present a cursory look at historical drought periods and recent conditions across the country using this new data set, and we discuss its potential use for future drought monitoring.