Environmental Engineering Reference
In-Depth Information
5.1.1 Precipitation and Potential
Evapotranspiration
Arid areas that have sparse precipitation and high evapo-
transpiration rates when combined lead to an interesting
recharge pattern. In some parts of Arizona in the southwestern
United States, for example, the depth to water table can be in
excess of 300 ft (91.2 m). Recharge from recent precipitation
is almost nonexistent. It has been shown, instead, that ground-
water moves upward toward the land surface in response to
high evapotranspiration (Andraski et al. 2003). Not only is
no current recharge occurring, the evapotranspiration demand
causes ancient recharge that occurred thousands of years ago
to be brought to near the land surface. Deep water-table
conditions also occur in humid areas, such as the Sand Hills
regions of the Atlantic Coastal Plain, but the occurrence of
a similar vertical movement of groundwater in response to
evapotranspiration has not received much attention.
Jordan and Fisher (1977) examined the relation
between precipitation and evapotranspiration and recharge
on the island of St. Thomas in the Virgin Islands south and
east of the tip of Florida. The island is about 14 mi (22.5 km)
long and 2 mi (3.2 km) wide and has only two perennial
streams. Average precipitation is about 40 in./year (101 cm/
year) but high evapotranspiration rates permit only about
1-2 in. (2.5-5 cm) of recharge each year. Another effect of
high evapotranspiration is the enrichment of salts in the
remaining groundwater, up to 20 times more concentrated
than precipitation. Conversely, Long Island, New York, an
island of similar size but located in a more temperate cli-
mate, has recharge of more than 50% of the 40 in. (101 cm)
of annual precipitation. This is due to the highly porous
sediments of the underlying aquifers, high rates of ground-
water flow, lower levels of solar radiation for shorter annual
periods, and less potential evapotranspiration.
The effect of plant processes on recharge is no more evident
than during comparison of the occurrence of precipitation
relative to maximum evapotranspiration. Recharge can
occur only where and when precipitation exceeds evapo-
transpiration. Some areas of the United States, for example,
receive more precipitation during the summer and fall
months, such as the southeastern United States which is in
the path of hurricanes from across the Atlantic and the Gulf
of Mexico. However, recharge during this time is low,
because evapotranspiration is high. In the northeastern
United States, more precipitation occurs during winter and
spring (Fig.
5.1
).
Plants affect the timing and volume of recharge through
different processes. The presence of plants leads to a thick
layer of organic leaf litter that increases infiltration rates.
Extensive root systems increase the hydraulic conductivity
of the soil and unsaturated zone around the roots, which can
lead to increased infiltration amounts and rates. Johnston
(1987) observed that infiltration rates increased from 0.20
to 3.97 in./year (7.20-100 mm/year) in a planted field and
the time for infiltration to reach the water table decreased
over time. Soil porosity and infiltration rates were 9 times
faster for planted soils relative to bare soils (O'Conner
1985). However, increased infiltration rates support
increased transpiration rates which also may decrease
recharge. For some deep-rooted plants, infiltration rates
may not increase because of higher vertical root hydraulic
conductivities relative to lateral roots and fewer roots with
depth (Pate et al. 1995).
Fig. 5.1
The relation between
precipitation and potential
evapotranspiration,
ET
p
, actual
evapotranspiration,
ET
a
, and
recharge for a typical area in the
humid northeastern United States,
such as Pittsburgh, Pennsylvania
(Modified from Fetter 1988)
.
One
inch is equivalent to 2.54 cm.
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