Groundwater recharge, vegetation and climate change

Groundwater recharge and evapotranspiration are often strongly related to precipitation in temperate regions. However, climate change is expected to change precipitation patterns and alter evapotranspiration, which will inevitably have consequences for the fraction of precipitation that ultimately turns into recharge. Since evapotranspiration is highly sensitive to climate, predictions of future groundwater recharge depend on accurate representation of this parameter. Factors that shape evapotranspiration in vegetated regions are soil water availability, plant water use efficiency, global radiation and vapor pressure deficit, among others, all of which directly or indirectly affect plant water use. How plants adapt their water use to a changing climate is thus highly informative for predictions of future groundwater recharge.

Current climate models indicate an increase in potential evapotranspiration, especially under scenarios with strong CO2 emissions. But it is still far from clear whether actual evapotranspiration will also increase, especially in regions where summer-time ET is usually water-limited rather than energy-limited and how this affects recharge.

We simulated the consequences of increases in atmospheric CO2 and temperatures for both, evapotranspiration and groundwater recharge, and found that the results are very different for specific vegetations types. For example, some plants will experience an elongation of the growing period, thereby theoretically increasing annual soil water demand. But the growing season for crops may shorten because of faster growing and ripening, so that harvest may occur earlier in the year, thereby decreasing plant soil water demand. Increasing atmospheric CO2 will increase plant-water use efficiency, so that crops may need less water to grow. For trees the picture is even more complicated. Warming spring temperatures may lead to an earlier leafing, but soil water stress at the end the growing season may actually shorten the growing period of trees. Further an increase in leaf area will lead to more transpiration by trees through increased soil-water uptake by roots. But soil evaporation might decrease, as large canopy shading reduces sunlight reaching the ground under trees.

The net effect of plant water use and on groundwater recharge is worth studying, especially under the conditions of climate change, because groundwater will likely remain a valuable resource for future human water consumption.