Climate Moisture Indices for the World Water Development Report II

Global field of long-term average annual climate moisture index (CMI, Willmott and Feddema, 1992) computed using the ratio of annual precipitation (P) to annual potential evapotranspiration, (PET), specifically CMI = (P/PET) when P < PET and CMI = 1 - (PET/P) when P >= PET. The indicator ranges from to +1, with wet climates showing positive CMI, and dry climates negative CMI. PET was estimated using the physically-based function of Shuttleworth and Wallace (1985), which is a modification of the standard Penman-Monteith PET equation. Long-term average precipitation was computed from Mitchell et al. (2003). Coefficient of variability (CV) is computed as the annual standard deviation divided by long-term annual mean.