In this study, we adopt the Irrigation Water Supply Reliability
(IWSR) index as the metric of irrigation water availability. IWSR is
defined as the share of potential irrigation demand that is realized
through actual consumption, on an annual basis. Potential demand
is the demand for irrigation water in the absence of any water
supply constraints, whereas actual consumption of irrigation
water is the realized water demand, given the limitation of water
supply for irrigation. If this index equals one, then all demand is
met and there is no irrigation shortfall. In a global analysis
undertaken at the level of 126 hydrological basins and 281 ‘‘foodproducing
units’’ (FPUs), Rosegrant et al. (2012a) estimate IWSRs
for 2000 and 2030 using the IMPACT-WATER model (see Appendix
A.2). Since the analysis was focused on demand side changes, the
1951-2000 monthly climatology was used to derive irrigation
availability, which represents average climate condition over that
period. Therefore, extreme weather events such as droughts and
floods were not considered.