How the MBDI Compares
Funder & Affiliates
The Moisture Balance Drought Index (MBDI) is being compared to greenness values for 17 sites located throughout Arizona.
Greenness values for four seasons were calculated from satellite imagery using the Normalized Difference Vegetation Index (NDVI). NDVI time series from 1989-2007 were assessed for 17 sites across Arizona, with bimonthly values averaged for a 2-kilometer radius – roughly 4,000 acres – surrounding each site. NDVI seasons were divided so that April, May and June are spring; July, August and September are summer; October and November are fall; and December through March represents winter.
The seasonal values for NDVI are being compared to MBDI values and to Standardized Precipitation Index (SPI) values. The 3-month scale most closely corresponds to contemporaneous NDVI seasons, while comparisons at longer scales can help relate the influence of previous seasons on greenness values.
The seasonal divisions were selected to reflect the dominant precipitation regimes in Arizona, distinguishing the two dominant wet seasons – the summer monsoon with its intense rainfall and high evaporation rates, and cool-season “winter,” when the precipitation that falls is more likely to soak into the landscape. The spring season, often known as “foresummer” in Arizona, tends to be dry. Fall precipitation is variable and subject to influence from tropical storms from the East Pacific.
Results will be reported as the analysis is completed.