How the MBDI Compares
Funder & Affiliates
The Moisture Balance Drought Index (MBDI) fared reasonably well at predicting reservoir storage, based on a case study of two reservoir systems that together provide nearly half of the water supply for metropolitan Phoenix.
In general, the MBDI and Standardized Precipitation Index (SPI) both performed well when it came to explaining fluctuations in reservoir storage in both the Salt and Verde systems for the 30 years of record considered, from 1978 through 2007.
The SPI did slightly better at predicting storage in the Salt River system, especially in the cool months of October through February. The MBDI, meanwhile, generally performed better at predicting fluctuations in the Verde system during the warm months of April through September.
The better MBDI performance for the Verde may in part reflect that it is more susceptible to demand – including the demands of Phoenix-area residents who may water their plants more during hot, dry periods. Records indicate that water is preferentially released from the Verde in response to user demand.
In addition, the Verde system, constructed in 1946, is smaller than the Salt system, constructed in 1914. The Salt Reservoir system can store about seven times more water than the Verde Reservoir system, which holds about 93 billion gallons of water.
The size difference influences the time frames that best reflect reservoir storage fluctuations. For most months, the Verde storage was best predicted using a time scale of 12 months, while the Salt was best predicted using a time scale of 24 to 36 months.
The bar graphs show the results for two Phoenix-area reservoirs that were used in the analysis, with dams based on the Salt and Verde rivers. The bars indicate the amount of year-to-year variability (variance) in reservoir levels explained by the MBDI (orange) and the SPI (blue). The variability was estimated using linear regression.
Credit: Graphic designed by Jorge Arteaga based on data from Andrew Ellis.
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