Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 41

a water-table depth of less than 8 ft (2.4 m) was about 109 ft 3 /
s (cubic feet per second), or 3 (ft 3 /s)/mi 2 (Lee 1912). Similar
tank experiments also were conducted by Parshall (1937),
but the results were inconclusive, and the ET rates reported
were larger than what would be expected for field conditions.
Lee's results were used by Robinson (1958) to determine
the relation between ET and temperature and depth to
groundwater (Fig. 2.9 ). It can be seen that as the depth to
groundwater increases, ET decreases, and as the temperature
increases, ET increases. Moreover, the rates of ET were
less than pan evaporation, in most cases from 68% to 75%
of pan evaporation (Robinson 1958). A conclusion from

this result is that the pan evaporation rate can provide an
upper bound on potential ET at sites being evaluated for
phytoremediation.
Tank experiments to investigate ET from riparian trees also
were conducted and the results were reported by Robinson
(1970). Twelve 30 ft 2 (2.78 m 2 ) tanks between 7 and 10 ft (2.1
and 3 m) deep were planted with willows, greasewood, and
rabbitbrush, and one tank was left unplanted. Robinson (1970)
reported ET as a volume per foliage or quantity of water per
foliage, which was affected by depth to groundwater, length of
growing season, and nutrient toxicity.

Fig. 2.9 Relation of saltgrass
transpiration to ( a ) depth to water
table at six sites in the
southwestern United States where
the shallow water table resulted in
high transpiration, and ( b )
average air temperature, with
high temperatures resulting in
high ET rates, especially if the
water table was closer to land
surface (Modified from Robinson
1958). One foot is equivalent to
0.304 m.