Potentials for expanding dry-land agriculture under global warming in water-stressed regions: a quantitative assessment based on drought indices

Agricultural sustainability under the warming climate highly depends on limiting the extreme use of the blue water resources within the irrigated lands, and instead, optimally applying green water resources within the expanded rainfed lands. In this research, potentials for expanding dry farming under climate change were analyzed in five climatic zones of Iran, based on the future projections of basic weather data and drought indices under 20 general circulation models (GCMs) and four emission scenarios, including RCP2.6, RCP4.5, RCP6.5, and RCP8.5. Drought indices included aridity index (AI), expressed as precipitation divided by evapotranspiration, and green water deficit (GWD), expressed as precipitation minus mines evapotranspiration. While annual predictions show a general increase in both precipitation and air temperature over the 2041-2070 period, given results at the monthly scale well revealed the spatial and temporal non-uniformity in the climate change projections. Such heterogeneities, together with those presented for different GCMs and RCPs, well indicated the importance of undertaking a wide range of uncertainties when implementing long-term planning. Based on GWD and AI, the arid and semi-arid regions, the backbone of the rainfed agriculture in Iran, are the most vulnerable to global warming. In these regions, green water availability will reduce by 0.8-167.5% especially within the wet seasons, and drought severity will increase by 0.1-33.2% through the rainfed cropping cycles. On the other hand, future climate may provide new opportunities for expanding dry farming within the hyper-arid region, which is now the most water-stressed region of the country. In this region, global warming projections show a positive increase of 0.3-146.2% in green water availability, and a reduction of 5.1-266.4% in drought severity. However, the humid region remained as the most suitable region for producing rainfed crops, although being widely allocated to irrigated-rice production under current condition. Based on the results, achieving sustainable dryland agriculture highly depends on regionally prioritizing susceptible area for dry farming, since the criteria indices showed an extreme spatial and temporal variability over the mid twenty-first century.