How the MBDI Compares
Funder & Affiliates
Supply and Demand
The first step in calculating the Moisture Balance Drought Index (MBDI) involves subtracting moisture demand from supply for pixels of 4,000 acres each, then summing the results across the area of interest. Supply means precipitation, which is estimated based on the PRISM approach. Demand relates to potential evaporation, which increases as temperatures rise and days lengthen.
The MBDI uses the Hamon method to approximate demand. The method uses temperature and day length along with constants to estimate not only evaporation from the soil but transpiration from plants. Estimates for Potential Evapotranspiration are based on how much water theoretically could evaporate from well-watered turf if moisture were unlimited.
Although moisture is rarely unlimited in arid environments such as those in the Colorado River Basin, plants and animals experience stress from the extra pull of evaporation as conditions dry out and heat up. The Hamon method accounts for this stress by considering potential evaporation rather than limiting its calculation to actual evaporation of only existing moisture.
Comparing similar precipitation levels during two different months at Payson, Arizona, can illustrate how the MBDI works. In 2001, about 2.77 inches of rain fell in January, compared to 2.60 inches in July. Yet the effect of that amount of precipitation on Payson differed because of the influence of temperature and day length. That year, January temperatures averaged 37.8° F, while July temperatures averaged 74.8° F. The evaporative toll on the landscape extended across longer days in July, when the sun shines on average about 14 hours a day, compared to 10 hours in January.
After taking into account how the hotter, longer days influence Potential Evaporation for these two months, the MBDI shows January benefited from a surplus of about 2 inches of moisture, while July ended up with a deficit of about 4 inches of moisture. That’s the equivalent of about 6 inches difference in the moisture balance from the same amount of precipitation. For MBDI calculations, a deficit or surplus does not carry over into the next month or other time frame.
Caption for first in-text graphic: Temperature and the number of hours of daylight drive evaporation from the surface and transpiration from plants under the Hamon method, which uses these two factors to derive values for Potential Evapotranspiration (shown here in millimeters of water).
Credit: Graphic design by Jorge Arteaga
Caption for second in-text graphic:Because of high rates of potential evapotranspiration, much of the Colorado River Basin has surplus water only during cool months when averaged by month.
Credit: Graphic design by Jorge Arteaga, based on a figure by Kirsten Ironside for a section of the Colorado Plateau. The values for Potential Evapotranspiration were provided by Andrew Ellis using PRISM climatic data.