How the MBDI Compares
Funder & Affiliates
The MBDI subtracts Potential Evapotranspiration (PE) from Precipitation (P) values, using the Hamon formula to estimate PE as shown below:
Under the Hamon method, Potential Evapotranspiration is derived from the number of daylight hours (in units of 12 hours), days in the month and saturated water vapor density, which changes with temperature.
The Hamon method estimates not only evaporation from the soil but transpiration from plants, basing its estimates on empirical measurements of how much water evaporates from well-watered turf when moisture is 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.
For more details on calculating Potential Evapotranspiration, see: Hamon, W.R. 1961. Estimating the potential evapotranspiration. Journal of the Hydraulics Division, Proceedings of the American Society of Civil Engineers 87:107-120.