Irrigation freshwater withdrawal stress in future climate and socio-economic scenarios

Although perceived as an abundant resource, freshwater is disproportionally distributed across the globe determining 8% of the current population to live in severely water scarce regions. Irrigated agriculture, currently representing 70% of all freshwater withdrawals and thus a major source of water stress, could continue to expand its water requirements as a consequence of expected economic and population growth. In this research, future pressure over freshwater resources coming from irrigated crop production is captured by an Irrigation Withdrawal to Availability (IWA) indicator derived through a global Computable General Equilibrium framework. The metric is calculated for several socio-economic development pathways and considers technological evolution through differentiated irrigated and rainfed crop yield changes. The RESCU model employed explicitly uses freshwater as a factor in crop production, whilst clearly distinguishing between irrigated and rainfed production functions. Two scenarios are applied to three alternative SSPs (SSP1, SSP2, SSP5) - inherent yield improvements under a 'no climate change' assumption and yield changes due to climate change in the A1B carbon emissions pathway. Results show that freshwater withdrawals continue to expand in most of the regions that are currently water-stressed with the IWA increasing in some cases by more than 50% from 2004 levels. Other regions, such as China, benefit from yield improvements and thus shift from irrigated to rainfed crop production. Climate change leads to a further increase in the IWA for India and a decrease for Northern Africa, the rest of South Asia and the Middle East. The research uses a global dynamic recursive CGE model, RESCU (RESources CGE UCL). The opportunity to use a CGE framework is justified by the need to incorporate the effects of expected economic growth, evolution of factor supply and technological change over agricultural production. From a baseline output for each SSP scenario, the CGE model enables us to distinctly determine the impacts of productivity changes of inputs in crop production over freshwater demand. The model uses the GTAP 8.1 database for economic data. The GTAP regions are aggregated to 20 wider RESCU regions with a grouping done to reflect differences in terms of agro-ecological conditions and economic development. India and China are represented distinctly for a better distinction of their future freshwater challenges. The crop producing sectors are well represented in RESCU with each of the 8 crop types initially present in GTAP having distinct rainfed and irrigated production functions. Thus the GTAP database crop sectors are being split using rainfed and irrigated production information from the biophysical Global Crop Water Model GCWM (Siebert & Döll 2010). The value of irrigation is also disaggregated from the value of land used in irrigated crop production by using the value of lost production in a no irrigation world derived from the GCWM 'no irrigation' model run. Changes in freshwater demand in irrigation are thus captured at different levels: a) Changes in output of irrigated crops induced by economic growth b) Relative changes in output of different crop classes each with different irrigation water intensities c) Technological changes leading to input intensification or input efficiency gains. The research uses a global dynamic recursive CGE model, RESCU (RESources CGE UCL). The opportunity to use a CGE framework is justified by the need to incorporate the effects of expected economic growth, evolution of factor supply and technological change over agricultural production. From a baseline output for each SSP scenario, the CGE model enables us to distinctly determine the impacts of productivity changes of inputs in crop production over freshwater demand. The model uses the GTAP 8.1 database for economic data. The GTAP regions are aggregated to 20 wider RESCU regions with a grouping done to reflect differences in terms of agro-ecological conditions and economic development. India and China are represented distinctly for a better distinction of their future freshwater challenges. The crop producing sectors are well represented in RESCU with each of the 8 crop types initially present in GTAP having distinct rainfed and irrigated production functions. Thus the GTAP database crop sectors are being split using rainfed and irrigated production information from the biophysical Global Crop Water Model GCWM (Siebert & Döll 2010). The value of irrigation is also disaggregated from the value of land used in irrigated crop production by using the value of lost production in a no irrigation world derived from the GCWM ‘no irrigation’ model run. Changes in freshwater demand in irrigation are thus captured at different levels: - Changes in output of irrigated crops induced by economic growth - Relative changes in output of different crop classes each with different irrigation water intensities - Technological changes leading to input intensification or input efficiency gains The future freshwater withdrawal resulting from the model scenarios are then intersected with future freshwater availability (mean annual-run off) data coming from the ISI-MIP hydrological models. This allows for the computation of the indicators of irrigation water withdrawals relative to resource availability. The research produces a wide range of values for indicators of water stress induced by irrigated agriculture. 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