How to avoid or postpone reaching the economic limit of groundwater-fed irrigation? Aggregated analysis for adaptation strategies from the farmers perspective.

Groundwater is a reliable and important source for irrigated agriculture but its use has consequences. In wetter regions, overuse of groundwater can threaten the health of streams that depend on discharge from the groundwater system. In drier regions or when groundwater withdrawals exceed the available groundwater recharge for a long time, groundwater resources will be depleted, and groundwater levels drop. The result is that farmers must extract water from increasingly deeper groundwater wells and incur greater costs for well construction and for the energy required to lift the water to the surface. Ultimately, a farmer can reach the economic limit for groundwater use when the cost of pumping water is larger than the revenue that can be generated with the crop. Farmers should consider this economic limit and adapt their cropping and production methods to safeguard economically sustainable production in the future. In order to evaluate possible adaptation strategies to avoid or postpone reaching the economic limit we developed a cost-benefit model at the local -farmers- level called HELGA (Hydro-Economic Limits as a Global Analysis) balancing the investment costs to deepen the well in the short term against the net present value of added profits from groundwater extraction in the long term. In HELGA, crop water requirements are calculated and satisfied with the available soil water and with irrigation from groundwater to meet the with consideration of the application losses. We include aquifer recharge and other sources of water use (surface water supply to dynamically account for the groundwater requirements of crops. Hence, we place groundwater irrigation within the context of other water resources and consider the groundwater exploitation costs in conjunction with the other costs to produce a crop. To include the impact of groundwater pumping on groundwater depth we couple HELGA to the water resource model PCR-GLOBWB, thus introducing farmer-scale hydro-economic analysis in a global-scale hydrological model with a resolution of 5 arc minutes (~10 x 10 km globally). In this manner, groundwater dynamics and surface hydrology are linked and the competition for groundwater with other sectors included. This coupling allows us to understand globally the implications of groundwater (over) use in the long term and how this defines the solution space from the aggregate farmers perspective. Our results show that farmers eventually reach the economic limit. Energy cost of groundwater pumping is one of the important drivers limiting groundwater use. Additionally, the increasing costs of the water infrastructure (i.e. deeper wells) is an important factor that explains the economic limit. Also, our analysis shows that variations in the irrigation water demand and the groundwater recharge as a result of climate variability strongly influences the profitability of groundwater-fed irrigated agriculture To counteract this, adaptation strategies such as changing the crop mix and increasing irrigation efficiency are effective in increasing the time to reach the economic limit and to extend the lifespan of aquifers. Farmers agency towards the management of a depleting resource make a difference in keeping this resource for future generations.