Evaluating the benefit-cost  ratio of groundwater abstraction for additional irrigation water on global scale.

Abstract: Projections show that to feed a growing population which is expected to reach 9.1 billion in 2050 would require raising overall food production by some 70 percent by 2050. One of the possible ways to increase agricultural production is through increasing yields by expanding irrigation. This study assesses the potential costs and benefits associated with sustainable groundwater abstraction to provide for irrigation.The feasibility of groundwater abstraction is determined using a combination of three indicators:groundwater recharge, groundwater quality (salinity) and sustainability (no depletion). Global groundwater recharge estimates used, are simulated with the Lund-Potsdam-Jena dynamic global vegetation model with managed lands (LPJmL). The cost of groundwater abstraction is determinedon a spatially explicit scale on global level at a grid resolution of 0.5°. Groundwater abstraction cost is divided into two parts: capital costs and operational costs. The potential benefit of increased water supply for irrigation is given by the water shadow price which is determined by using a Model of Agricultural Production and its Impact on the Environment (MAgPIE). The water shadow price for water is calculated in areas where irrigation water is scarce based on the potential increase in agricultural production through additional water and it reflects the production value of an additional unit of water. The water shadow price is given on a 0.5° grid resolution in US $/m3. Combining the cost of abstraction and the water shadow price, the benefit cost ratio is calculated globally on a spatially explicit scale to determine where investment in groundwater irrigation wouldbe beneficial. Finally, the results are analysed in global, regional and country perspectives. The results show that groundwater abstraction is beneficial for an area of 135 million hectares which is around 8.8% of the total crop area in the year 2005. Europe show the highest potential with an area of ~ 50 million hectares with a majority of the area located in France, Italy, Germany and Poland. Second is North America with an area of ~ 43.5 million hectares located in the Eastern states where the irrigation infrastructure is less developed as compared to the Western states. Sub-Saharan Africa shows a potential of ~ 15.4 million hectares in the Southern and Eastern countries of Zimbabwe, Kenya, Malawi, Tanzania, Ethiopia and some parts of South Africa. South Asia despite extensive groundwater extraction shows only a moderate potential of ~ 9 million hectares, mostly located in India whereas China shows almost no potential. This is due to extensive groundwater depleted areas which were removed from the analysis and low water shadow prices which made abstraction not beneficial. Well installation costs play an important role in developing countries in regions of Sub-Saharan Africa and South Asia, where a reduction in costs would lead to an increase in area by more than 30%. Subsidy analyses shows that substantial increase in crop land areas where a benefit cost ratio >1 takes place in India with subsidised energy prices but this effect is found to be negligible in Mexico. This study is, to the author’s knowledge, the first to assess the benefit cost ratio of groundwater abstraction on a global scale by determining spatially explicit abstraction costs. The results show that a great potential for groundwater abstraction exists in all regions despite problems of groundwater depletion due to disparity in distribution and development of groundwater resources. Energy subsidies and cheap well installation techniques are the two factors that could bring down the abstraction costs which are quite important in developing regions where farm incomes are low. Also, groundwater irrigation potential not only exists in arid areas of Africa and South Asia where irrigation is needed but also in humid areas of Europe and North America where groundwater irrigation can play an important role in building resilience to events of drought. However, it is essential to not to follow the path that has led to groundwater depletion in many parts of the world and develop this potential in a sustainable way through groundwater use regulations, policies and efficient technologies.