Public Release: 

A missing link in water modeling

American Association for the Advancement of Science

A process that is largely overlooked in earth system models may shape large-scale soil evaporation and plant transpiration more than scientists thought, a new study suggests, helping quantify the global water cycle. The results provide insights that current approaches into complex water cycle interactions are lacking. Evapotranspiration, or ET, is the sum of evaporation from the soil (E) and transpiration (T) from plants - and it represents a critical part of the water cycle on Earth. Because T depends on plant processes, however, whereas E depends on shallow soil moisture, the two respond differently to physical drivers. Recently, scientists have debated how T and E are uniquely affected by variables including one not captured in existing water models: the movement of water through the subsurface to a stream or other drainage body, also known as lateral groundwater flow. Here, Reed Maxwell and Laura Condon leveraged a model that couples groundwater and surface water flow to simulate water flux, both with and without lateral groundwater flow, over most of the major river basins in continental North America. They ran their model simulations hourly for one year. Lateral groundwater, their simulations revealed, provided a key additional water source for transpiration uniquely. In fact, in some cases, it caused transpiration to be more than 30 times greater than evaporation from soil. Plants' access to deeper water allows them to draw water after the shallow soil is dry, the researchers propose. Their results underscore the importance of including lateral groundwater flow in earth system models, particularly as changes in the water table become more common.

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