Agriculture Increases River Flows

Clearing forests to grow crops has added to the amount of water that runs off of land into rivers by 2.5 percent in North America and 6 percent in Asia over the last three centuries.

That's because crops pump less moisture into the atmosphere through evaporation and plant transpiration than do forests.

The hydrological impact of converting land to agriculture has been countered somewhat by irrigation. Watering of crops contributes to evapotranspiration while decreasing runoff. Irrigation's influence on lowering river flows is most evident in the western US and Mexico during summer.

Still, even with irrigation factored in, some regions have considerably altered hydrological cycling since 1700. The greatest extent is in Southeast Asia where water draining into the Pacific Ocean has increased by 11 percent. In North America, 3.5 percent more freshwater flows into the Gulf of Mexico.

The largest changes to river flow, though, are the seasonal ones created by dams. In general, reservoirs produce larger runoff volumes in winter and less during spring snowmelt compared with natural regimes. Modelling of hydrological dynamics shows that reservoirs draining into the Arctic Ocean have caused influxes of water to be 20 to 40 percent higher in March.