Environmental Engineering Reference
In-Depth Information
that were deposited by streams flowing east-
ward from the ancestral Rocky Mountains. The
land surface is characterized by gently sloping
plains that range in elevation from 2300 m in
the west to 335 m in the east. Agriculture is the
main industry, including dry-land and irrigated
farming and ranching. Climate varies from arid
to semiarid to subhumid with average annual
precipitation ranging from 350 mm in the
southwest to 800 mm in the northeast (McGuire
et al
.,
2003
).
The High Plains aquifer is one of the most
productive and most intensely developed aqui-
fers in the United States. The unconfined aqui-
fer consists of poorly sorted sand, silt, clay, and
gravel. The large areal extent and thickness
(up to 300 m) of the aquifer provide storage for
as much as 3.7 trillion cubic meters of water
(McGuire
et al
.,
2003
). Water in the aquifer has
accumulated over thousands of years. Recharge
rates prior to the onset of development for irri-
gation in the 1940s were estimated to range
from 0.8 to 26 mm/yr in the south and central
parts and 2 to 39 mm/yr in the northern part
of the aquifer (Luckey
et al
.,
1986
; McMahon
et al
.,
2003
). Depth to the water table is vari-
able, ranging from 0 to about 150 m. Decades
of pumping for irrigation have resulted in sub-
stantial water-table declines (in excess of 50 m)
in parts of the south and central High Plains
aquifer.
It is difficult to generalize about recharge
mechanisms in an area as large as the High
Plains. Undoubtedly, there are multiple sources
of recharge throughout the region. Under range-
land vegetation, most recharge in the arid to
semiarid southern High Plains occurs as focused
recharge from surface-water bodies. The south-
ern High Plains is extremely flat, and surface
water tends to drain to ephemeral lakes and pla-
yas, which effectively serve as focused, natural
recharge basins (Scanlon and Goldsmith,
1997
;
Gurdak and Roe,
2009
). Diffuse recharge can
occur under cropped land in the southern High
Plains. Scanlon
et al
. (
2007
) applied the unsatu-
rated-zone chloride mass-balance method at 19
locations in dry-land croplands and determined
a median rate of drainage through the unsat-
urated zone of 24 mm/yr. McMahon
et al
. (
2006
)
used a similar approach at two sites under irri-
gation and estimated drainage rates of 17 and
32 mm/yr. Wood and Sanford (
1995
) applied the
chloride mass-balance method by using ground-
water concentrations and estimated an average
recharge rate of 11 mm/yr for a large part of the
area. Historically, groundwater discharge in the
southern High Plains was to springs along the
eastern escarpment. Falling groundwater levels
have virtually eliminated discharge from most
of these springs over the past several decades.
Extraction of groundwater for irrigation is now
the dominant discharge mechanism. Gurdak
and Roe (
2009
) presented a review and synthe-
sis of 175 publications on recharge in playa and
interplaya settings, mostly within the southern
High Plains.
Diffuse and focused recharge are both
important in the central High Plains. Diffuse
recharge rates are highest in areas with sandy
soils, and rates tend to increase from west to
east in tandem with the trend in annual pre-
cipitation (Luckey and Becker,
1999
). Reported
estimates of diffuse recharge under native
rangeland vary from 5 mm/yr, as estimated
with the unsaturated-zone chloride mass-
balance method (McMahon
et al
.,
2006
), up to
177 mm/yr in sandy areas with a shallow water
table, as estimated with the water-table fluctu-
ation method (Sophocleous,
1992
). As with the
southern High Plains, enhanced recharge is
expected under agricultural crops. McMahon
et al
. (
2003
) used the unsaturated-zone tritium-
profile method and estimated unsaturated-zone
drainage rates of 21 and 116 mm/yr beneath two
irrigated fields. Natural discharge before devel-
opment was to gaining streams throughout the
central High Plains. Many of these streams have
dried up because of declining groundwater lev-
els (Sophocleous,
2000
) and are now ephemeral,
losing streams that focus recharge when runoff
from precipitation flows through stream chan-
nels. Results from a groundwater-flow model
of the southern part of the central High Plains
aquifer showed that recharge rates in 1997
were greater than those under predevelopment
conditions (
Figure 9.4
) but that water storage
within the aquifer had been reduced by about
18% (Luckey and Becker,
1999
).
