5.2 Capture of Evapotranspiration

Groundwater evapotranspiration includes both evaporation from the water table and transpiration through plant roots that tap the uppermost part of the saturated zone. Evaporative losses from the water table involves a flux through the unsaturated zone to the atmosphere. This flux will be largest if the distance from the water table to the soil surface is shortest (that is, the water table is at or immediately below the land surface). It is also expected that there exists a depth below which evaporation from the water table becomes negligible. Similarly, transpiration from the saturated zone by phreatophytes depends on root penetration, which is most extensive when the water table is shallowest. Phreatophytes are plants that depend for their water supply upon groundwater that lies within reach of their roots (Robinson, 1958). However, there is a maximum depth that plant roots can penetrate. The limiting depth of the water table ‑‑ below which no evapotranspiration can occur ‑‑ is called the extinction depth or cutoff depth (McDonald and Harbaugh, 1988).

Overall, the groundwater evapotranspiration flux is inversely proportional to the depth of the water table below the land surface. Thus, as the water table declines in response to pumping and storage depletion, the potential evapotranspiration will decrease (i.e., it is captured). An extreme example of this effect is illustrated by photos of a riparian zone of a stream in an arid climate, where a long‑term loss of vegetation due to large water‑table declines reflects a capture of evapotranspiration (Figure 13).

Photos showing loss of riparian vegetation where the water table declined
Figure 13 – Loss of riparian vegetation where the water table declined: a) 1942 photograph of a reach of the Santa Cruz River south of Tucson, Arizona, showing stands of mesquite and cottonwood trees growing in the riparian zone of the river (left photograph, Arizona Game and Fish Department); b) photograph of the same site in 1989 showing that the riparian vegetation has largely disappeared (right photograph, R.H. Webb, USGS). Data from two nearby wells indicate that the water table has declined more than 30 m because of pumping; this pumping (and its consequences) appears to be the principal reason for the vegetation loss (Healy et al., 2007).

The potential for salvaging groundwater evapotranspiration to help balance groundwater pumping was recognized by Theis (1940). With the goal of optimizing well locations and minimizing drawdown and streamflow depletion, Theis (1940) stated “pumps should be placed as close as economically possible to areas of … natural discharge where ground water is being lost by evaporation or transpiration by non-productive vegetation.” A small decrease in the rate of groundwater evapotranspiration over a large area can yield a large volume of water.


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