The irrigation-groundwater nexus at the global scale
Insitute of Physical Geography, Goethe University Frankfurt
In many regions of the globe, irrigation affects groundwater recharge and storage. According to simulations with the global water resources and use model WaterGAP, irrigation accounts for about 90% of global consumptive water use, while about 40% of the consumptive water use in irrigation stems from groundwater. Irrigation with surface water leads to an increase of groundwater recharge in e.g. Southeast and Central Asia, while in some semi-arid and arid regions with groundwater irrigation, net groundwater abstractions are so high that even a decline of baseflow to zero cannot prevent groundwater depletion, i.e. a steady decline of groundwater storage. Comparing WaterGAP groundwater depletion to independent depletion estimates from local modeling, well observations or GRACE data, we found that depletion is simulated best if we assume that farmers in depletion areas irrigate at 70% of optimal water requirement (Döll et al. 2014). During 2000-2009 about 15% of the globally abstracted groundwater was taken from nonrenewable groundwater. Considering the significant environmental impacts of irrigation, it is noteworthy that according to the Global Crop Water Model, global cereal production would decrease by only 20% if currently irrigated crops were not irrigated (Siebert and Döll 2010). Of course, regional impacts would be much stronger, with losses of 66% in Northern Africa and 45% in Southern Asia.