How the MBDI Compares
Funder & Affiliates
Depth to well water
The Moisture Balance Drought Index (MBDI) tends to perform best in situations where the evaporative demand has time to wield an influence on water resources. This holds true in an assessment of Arizona groundwater levels, as measured by the depth to water in wells.
Groundwater levels reflect not only the supply of precipitation but also the power of evaporative demand. It can take weeks for surface water to seep below the reach of plant roots. Until then, the water remains subject to evaporation – especially evaporation through plants, known as transpiration.
Because of this, it makes sense that the MBDI, which considers the effect of evaporative demand, generally performed better than the Standardized Precipitation Index (SPI) at explaining the amount of variability in groundwater levels.
Even if it makes it past plants, water can take years to travel all the way down to the groundwater table. Given that, it makes sense that groundwater levels in studied wells in Arizona correlated best with the drought indices at the scale of three or four years (36 to 48 months).
Twelve of the 16 groundwater wells analyzed showed statistically significant correlations between fluctuations in groundwater levels and the drought indices, an indication that the MBDI can predict groundwater fluctuations under some conditions.
The analysis pared down 16 wells from an initial 45 designated as drought monitoring sites by two agencies, the U.S. Geological Survey and the Arizona Department of Water Resources. The analysis excluded wells that exhibited a trend (which could indicate an influence from groundwater withdrawals) or did not have sufficient records for the period of study, 1956-2007.
Typically, depth of well water was measured once a year, with the timing of the measurement ranging from October to February. Results were compared to the climatic conditions at the scale of sub-basin, as depicted on the figure below.
The bar graphs show the results for the 16 wells from the seven sub-basins that were used in the analysis. The bars indicate the amount of year-to-year variability (variance) in groundwater 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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