Hydroclimatic Index for drought monitoring in the Colorado River Basin USA Decision Center for a Desert City Arizona State University Tempe, Arizona Institute for the Study of Planet Earth University of Arizona Tucson, Arizona Bureau of Reclamation National Oceanic and Atmospheric Administration

Hydroclimatic Index for Drought Monitoring in the Colorado River Basin, USA Project Objectives The broad goal of the Hydroclimatic Index project is to advance scientific understanding and mitigation of drought occurrence across the Colorado River Basin (CRB) within the southwestern United States using a new tool for representing hydroclimatic variability. The Hydroclimatic Index is aimed at improving upon the design of current drought monitoring tools. Application and testing of the HI within the CRB are on a finer spatial resolution than what is commonly used in drought monitoring and research. Rationale Unlike with all other hazards, the temporal bounds of a drought event are difficult to define due to the gradual nature of the onset and demise. This makes drought monitoring difficult; however, close monitoring can be an effective tool for mitigating drought impacts due to the time afforded by the generally slow onset and limitations of drought forecasting. Historically, efforts to portray drought have often been hampered by reliance on indices that are confusing, contain regional biases, and demonstrate limited relationships with the multiple dimensions of drought. Hydroclimatic Index Soil moisture is the focus of many drought indices, but the Hydroclimatic Index (HI) stops short of representing soil moisture in its characterization of the hydroclimatic condition. Only a few operational soil moisture sensors exist in the CRB, and the alternative of simulating soil moisture through time is difficult, as climate conditions, land surface characteristics (soil type, vegetation type, topography) and the amount of moisture relative to the soil’s capacity combine to control soil moisture. The HI simply represents the difference between precipitation (P) and potential evapotranspiration (PE) (P-PE) through time at a given location. PE is the climatic demand for water, or that amount of evapotranspiration that would occur from a grass-covered soil for which the water content is maintained at capacity. Negative P-PE values indicate the amount by which the climatic demand for water can not be met by precipitation and actual soil moisture would decline if not irrigated. Positive values represent the amount of excess water from precipitation that would recharge soil moisture, percolate to ground water, or run to streams and reservoirs through overland flow or interflow. Aggregates of P-PE are constructed for periods that represent short-term conditions (1-, 3-month), intermediate conditions (6-, 12-month), and long-term conditions (24-, 36-, 48-month), and the aggregate values are converted to percentiles to form the HI. The HI can then be stratified into drought categories that represent levels of drought intensity. NOAA Research The United States National Oceanic and Atmospheric Administration (NOAA) is funding the transition of the HI to the Arizona Drought Monitor and to the National Integrated Drought Information System (NIDIS) under their Transition of Research Applications to Climate Services (TRACS) program. Click here to learn more about this research. BOR Research The United States Bureau of Reclamation (BOR) is funding use of the HI to monitor drought across the Colorado River Basin and to research both the historical variability of drought and the implications of possible future climate change. Click here to learn more about this research.