Groundwater pumping is expected to critically impact the flow of water to aquatic ecosystems in around half of the world’s watersheds with groundwater extraction by 2050, reports a modelling study published this week in Nature. Understanding the limits for groundwater resources is important because the flow of groundwater into rivers and streams has an important role in sustaining healthy ecosystems.

Groundwater extraction is becoming increasingly unsustainable; in some areas extraction already exceeds the rate that sources are recharged by rainfall and rivers. Depletion of groundwater can reduce streamflow - that is, the amount water that is discharged into rivers, lakes, wetlands and other ecosystems.

Inge de Graaf and colleagues investigate where and when the level of streamflow reaches the environmental flow limit for the first time, that is, the first time the streamflow reaches its critical point for at least three months over two consecutive years. The streamflow critical point occurs when there is no longer enough groundwater flow to support aquatic ecosystems. They create a global model that links groundwater pumping with the groundwater flow to rivers, producing a model that covers the period 1960 - 2100. The model indicates that the environmental flow limit has already been exceeded in around 20% of watersheds, mainly in drier regions of the world that rely on groundwater for irrigation (for example, parts of Mexico and the Upper Ganges and Indus basins). The research shows that by 2050, an estimated 42 - 79% of watersheds with groundwater pumping may have also reached this point at which streamflow can no longer sustain aquatic ecosystems.

The authors comment that these estimates are likely optimistic, as they do not consider any potential increase in demands for groundwater as a result of population growth or the development of emerging economies. They suggest that their findings could inform future efforts to encourage sustainable and efficient groundwater use.