How the MBDI Compares
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The Moisture Balance Drought Index (MBDI) generally performed well at predicting the amount of water flowing in rivers draining from eight Colorado River Basin watersheds studied.
The results are based on 60 years of record, from 1948 to 2007. Measurements relate climatic conditions within the basins to the amount of river flow exiting the drainage area, using the lowermost stream gages available from the monitoring agency, the U.S. Geological Survey.
The time frames of the cool season versus the warm season vary by basin, as the intent was to include in the cool season any moisture from melting snow and related processes.
Three of the four more northerly basins – namely, the Animas, Tomichi and Yampa – have cool seasons that extend from October through June. Three of the four southerly basins – the Upper Gila, Little Colorado and Verde – have cool seasons ending two months earlier, thus extending from October through April. The remaining basins – the southerly Salt and the northerly Virgin – have intermediate cool seasons extending from October through May.
The northerly versus southerly basins also differ in their optimum time frames for predicting variability. In the southerly basins, time frames from 1 to 12 months work best at explaining variability. In the northerly basins, the optimum time frames for predicting river flow cluster around 6 to 12 months for spring and summer but 12 to 48 months for winter.
The results for the MBDI were generally comparable to results for the Standardized Precipitation Index (SPI), although the SPI did a better job of explaining variance during the cool season for two Colorado River Basin sub-basins, the Upper Gila and the Virgin. In other cases, the differences in the percentage of variability explained by the drought indices were too small to be considered statistically significant.
More details on these results can be found in an article submitted for publication to Water Resources Research or by clicking here.
The bar graphs show the results for eight Colorado River Basin rivers draining the sub-basins identified. The bars indicate the amount of year-to-year variability (variance) in river flow 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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