Characterizing the severity and risk of drought in the Poudre River, Colorado

The implementation of water use restrictions in a given district is very much related to the severity of an evolving drought; thus, water management agencies such as City Water Departments often face the question of how critical or severe a particular drought event is. While this question has typically been examined using drought indices (e.g., the Palmer Drought Severity Index), unfortunately they do not provide any information on the frequency or recurrence time of the drought or on the risk that specific drought events may occur in a given time horizon. In this paper, the severity of alternative drought events are determined following a similar concept as for the severity of floods, i.e., the return period of a flood event, e.g., the 500-year flood. We develop procedures for determining drought severity based on historical data, generated data, and mathematical algorithms. In particular, we focus on the drought that has been occurring in the Western United States and take the Poudre River, Colo., as an example. The analysis was based on 119 years of records of naturalized flows of the Poudre River. A stochastic model was used for generating long-term synthetic flows, from which the return period and risk of drought events were determined. In addition, we used a mathematical algorithm for determining the distribution of drought events and their return periods. Assuming the sample mean as the water demand threshold, drought severity, frequency, and risk statistics were obtained; e.g., the severity of the 3-year (2000-2002) drought (i.e., 3-year drought with D > 404,011 acre-ft) is of the order of a 1,000-year return period and the risk that such a 3-year drought will occur in a 25-year period is about 2.4%.