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Modeling the Ogallala Aquifer on the Texas High Plains
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The following document was extracted from the following paper: http://www-ne.cr.usgs.gov/highplains/bckgrnd.html Historical Mathematical
and Modeling Facts
About 20
percent of the irrigated land in the United States is in the High Plains, and
about 30 percent of the ground water used for irrigation in the United States is
pumped from the High Plains aquifer (Weeks and others, 1988). Many
studies of parts of the aquifer have been completed by irrigation districts,
local agencies, State agencies, the USGS, and other Federal agencies. A major
study that examined the physical features of the entire aquifer was completed by
the USGS. The High Plains Regional Aquifer-System Analysis (High Plains RASA)
described the geology and hydrology of the aquifer in detail (Gutentag and
others, 1984; Weeks and others, 1988). Computer models for each of three
regional subdivisions of the High Plains were developed during that study to
simulate the effects on the aquifer of several proposed water-management
practices. The analyses made as part of the High Plains RASA were based on data
collected before 1981. Beginning in 1988, water-level data again were
systematically collected and compiled in an aquifer-wide data base (Kastner and
others, 1989). The USGS and State
and local agencies have compiled water-level data collected since 1980 at more
than 12,000 locations. EXTENT
AND DESCRIPTION OF THE HIGH PLAINS AQUIFER The
High Plains aquifer is an extensive volume of saturated, generally
unconsolidated, deposits underlying about 174,050 square miles in the High
Plains region of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota,
Texas, and Wyoming. This aquifer formerly was known as the Ogallala aquifer, but
the different geologic units and ages of the deposits constituting the aquifer
necessitated a more inclusive designation. The High Plains aquifer consists
mainly of one or more hydraulically connected geologic units of late Tertiary or
Quaternary age; the Ogallala Formation is generally the principal unit (Gutentag
and others, 1984). FACTORS
AFFECTING WATER-LEVEL CHANGE If
the High Plains aquifer were unaffected by human activities, it would be in a
state of equilibrium in which natural discharge from the aquifer would be
approximately equal to natural recharge to the aquifer. Water levels would be
generally stable under these conditions. However, activities such as pumpage
from wells, surface-water diversions for irrigation and hydroelectric-power
generation, and cultivation and grazing practices result in nonequilibrium in
the aquifer; discharge does not equal recharge in many areas. This
nonequilibrium results in substantial changes in ground-water levels. Recharge Precipitation
is the principal source of natural ground-water recharge in the High Plains. In
a few areas, however, natural recharge can result from seepage losses from
streams and lakes. This recharge is particularly important along parts of the
Platte River system in Colorado, Nebraska, and Wyoming, where substantial
seepage losses of stream flow originating outside the High Plains result in
recharge to the High Plains aquifer. This recharge process has contributed to a
degree of stability of water levels in parts of central Nebraska. Recharge from
infiltration of water lost by seepage from surface-water diversions for
irrigation and hydroelectric-power generation has caused water levels to rise in
a few areas of the High Plains, particularly in south-central Nebraska. Recharge
from precipitation is quite variable in the High Plains, both in time and in
space. Factors that affect this variability include the precipitation regime,
evapotranspiration, soils, vegetation, land-use practices, and the
characteristics of the unsaturated zone between the soil zone and water table.
These factors determine that proportion of the precipitation reaching the
aquifer. The relation among these factors also makes the process of recharge
from precipitation complex and dynamic. The basic concept of ground-water
recharge from precipitation is expressed by an equation (Dugan and Peckenpaugh,
1985) describing the soil-water balance:
R = P + S - O - E - C,
(1) where
R = recharge, in inches;
P = precipitation, in inches;
S = antecedent soil water (stored), in inches;
O = surface runoff, in inches;
E = actual evapotranspiration, in inches; and
C = water-storage capacity of the soil zone, in inches. The
nonuniform distribution of precipitation, both in time and space, probably
causes most of the variability in recharge to the High Plains aquifer. Average
annual precipitation ranges from about 14 inches in the extreme western High
Plains to about 30 inches in parts of eastern Nebraska and central Kansas (fig.
2). About 75 percent of the annual precipitation normally occurs during the warm
season, April through September. Much of the precipitation during the warm
season comes from local thunderstorms, which can cause variable patterns of
precipitation, irrigation requirements, and recharge in a small area during a
given time period. Recharge, however, is dependent mostly on cool-season
precipitation (October through March) when evapotranspiration is less. Although
areal variability in precipitation generally is less localized in the cool
season than in the warm season, cool-season precipitation amounts can vary
substantially from year to year. The
amount of water in the soil that becomes available for recharge is dependent
largely on evapotranspiration. Potential evapotranspiration (fig. 2) is the
maximum water loss that would occur by evaporation and transpiration from an
area with complete vegetative cover, provided that an adequate supply of soil
water is available at all times to meet vegetation demands. Potential
evapotranspiration is determined by the amount of atmospheric energy available
for the removal of water from the soil either directly as evaporation or as
transpiration from vegetation. Potential
evapotranspiration is dependent on various climatic and atmospheric elements
that include solar radiation, temperature, humidity, and wind velocity. The
increase in potential evapotranspiration from northeast to southwest across the
High Plains is a consequence of (1) an increase in solar radiation, (2) an
increase in temperature, (3) a decrease in humidity, and (4) an increase in wind
velocity. Actual evapotranspiration is further dependent on the type of
vegetation, length of the growing season, and the The
seasonal relation of precipitation to evapotranspiration strongly affects the
recharge process. In the High Plains, recharge occurs mostly during the
nongrowing season, when evapotranspiration is minimal and soil water can
accumulate in the root zone and percolate downward. This accumulative process
may extend throughout the nongrowing season, with late-autumn precipitation
remaining in the soil column through the winter and percolating downward out of
the root zone only after spring precipitation and winter snowmelt cause the
soil's available water capacity to be exceeded. Thus, antecedent soil-water
conditions, winter snow, early-spring precipitation prior to the onset of the
growing season, and evapotranspiration are critical to recharge during the
nongrowing season. The
hydrologic characteristics of the soils can have a substantial effect on
recharge and resultant water-level change. Sandy soils permit greater recharge
than finer textured soils. Under equivalent vegetative and climatic conditions,
recharge can be several times larger for sandy soils than for silty clay-loam
soils as a result of the sandy soil's smaller available water capacity and
greater permeability (Dugan and Zelt, in press). Slope also affects the rate at
which precipitation infiltrates the soil and becomes soil water. Steeply sloping
soils with silty-clay loam texture, common in parts of the High Plains, are
characterized by large overland runoff and small infiltration during moderate-
to high-intensity rainfall. A more complete discussion and generalized map of
the hydrologic characteristics of soils and their effects on recharge in the
High Plains are found in Dugan and others (1990). Recharge
also is affected by vegetation type. Each vegetation type has a characteristic
consumptive water requirement (the amount of soil water that vegetation would
use, if available), which ultimately affects the soil water available for
recharge and irrigation needs. Therefore, variability of vegetation types across
the High Plains affects patterns of water-level change. Various
government programs to remove cropland from production, including the U.S.
Department of Agriculture's Conservation Reserve Program, which was implemented
in 1986 as part of the Food Security Act, have resulted in changes in vegetation
patterns in the High Plains. Under the Conservation Reserve Program, cropland is
returned to grassland or forestland for a minimum of 10 years. Although the
conversion of cropland to grassland in the High Plains could result in a
decrease in recharge because of the larger consumptive water requirement for
grasses than for most cultivated crops (Dugan and Zelt, in press), the resulting
effects on water levels likely will be more than offset by the opposing effects
of decreases in ground-water withdrawals for irrigation as a result of these
conversions from irrigated agriculture to nonirrigated uses. Intensity
of cultivation, cropping practices, and methods of tillage and land management
can have a substantial effect on recharge. Because consumptive water
requirements of most cultivated crops are less than that of native grasses,
recharge in those areas of intensive cultivation is enhanced considerably. These
areas include eastern and central Nebraska, central and southwestern Kansas, and
parts of the Texas Panhandle. In parts of the western High Plains where winter
wheat-fallow rotation is practiced, more than 25 percent of the land may be
fallow in any given year. Fallow conditions increase recharge by decreasing
transpiration of soil water from cropland. Certain methods of tillage and land
management can increase recharge by limiting water losses from runoff and
evaporation. Infiltration of soil water is increased by minimum tillage, land
leveling for gravity and flood irrigation, and terracing (Dugan and Zelt, in
press). Estimated
potential recharge to the High Plains aquifer during 1951-80 (fig. 3) is
calculated by the soil water balance method expressed in equation 1 and
described more fully in Dugan and Zelt (in press). The values shown in figure 3
are not the observed amounts of water entering the aquifer, but are values for
the surplus soil water available as recharge as determined by the climatic,
soil, and vegetation characteristics discussed previously. Furthermore,
potential recharge does not consider the process of soil water moving beyond the
soil zone through the unsaturated zone to the aquifer. This latter process may
occur within a few hours or through many months, depending on the depth to the
water table and the characteristics of the geologic materials in the unsaturated
zone between the soil zone and the water table. The
relation between precipitation (fig. 2) and estimated potential recharge (fig.
3) is apparent. Estimated average annual potential recharge amounts range from
about 25 percent of the average annual precipitation in eastern Nebraska to less
than 0.5 percent in the western parts of the High Plains. Generally, where
average annual precipitation exceeds 24 inches in the High Plains, the estimated
average annual potential recharge exceeds 4 inches. Potential
recharge exceeds 6 inches in parts of eastern Nebraska and central Kansas, where
the soils are sandy and precipitation exceeds 24 inches. In parts of the far
western High Plains, however, where precipitation is less than 16 inches,
evapotranspiration generally exceeds 60 inches, and soils are fine textured,
estimated average annual potential recharge is less than 0.25 inch (Dugan and
Zelt, in press). Estimates of annual recharge of 6 inches and 0.25 inch in these
areas, respectively, by Weeks and others (1988) compare well with those of Dugan
and Zelt (in press). Although
the effect of precipitation on estimated potential recharge is evident, other
factors also are important in defining local recharge patterns. Estimated
recharge that is large for the existing climatic conditions in parts of
north-central and southwestern Nebraska, northeastern Colorado, south-central
Kansas, and western Texas is largely attributable to sandy soils (Dugan and
others, 1990; Dugan and Zelt, in press). Agricultural
practices also affect local patterns of potential recharge in the High Plains.
Rates of potential recharge are increased in the southern Nebraska Panhandle,
most of the Kansas High Plains, northeastern Colorado, and parts of the Oklahoma
and Texas Panhandles as a result of large areas of fallow land associated with
winter wheat. Potential recharge also is increased substantially in eastern and
central Nebraska, central and southwestern Kansas, and parts of the Texas
Panhandle where other crops largely have replaced native grasslands (Dugan and
Zelt, in press). Average
annual potential recharge shown in figure 3 does not indicate the large
variability in recharge through time. Recharge throughout the High Plains may
vary considerably from year to year, principally because of variations in
precipitation. In many years, recharge may not occur in areas where annual
precipitation generally is small. Most of the long-term average recharge can
result from a few short, wet periods. The recharge process commonly is cyclical
in the High Plains--two or more consecutive years in which conditions are
favorable for recharge, followed by several years when these conditions are not
present and recharge is negligible. Discharge Water is
discharged from the High Plains aquifer by both natural and artificial
processes. Natural discharge from the aquifer occurs as evapotranspiration from
plants and soil where the water table is near the land surface and as seepage
from the aquifer through springs and to streams where the water table intersects
the land surface. Long-term natural discharge would tend to balance long-term
natural recharge. Water is discharged artificially from the aquifer
predominantly by pumping of wells. Artificial discharge can cause an imbalance
in the recharge-discharge relation in the aquifer; when discharge exceeds
recharge, some water is removed from storage. Part of the imbalance can be
offset by a decrease in natural discharge or an increase in induced recharge
from streams caused by the lowering of the water table. Comprehensive
data on the withdrawal and use of water from the High Plains aquifer are
collected and published at 5-year intervals by the USGS in cooperation with
State and local agencies (Solley and others, 1993). Some of these data are
derived from records of metered wells; however, most water withdrawn from the
High Plains aquifer is from wells that are not metered, particularly those used
for withdrawal of water for irrigation, rural domestic consumption, and
livestock. Estimated irrigation pumpage is extrapolated from (1) available
metered pumpage and (2) computations of pumpage based on consumptive irrigation
requirements, acreages of irrigated crops, and irrigation efficiency data. Rural
domestic water use and livestock consumption are extrapolated from average
consumption per capita and per head of livestock for a known population and
number of livestock, respectively. Withdrawals
from the High Plains aquifer declined between both 1980 and 1985 and 1985 and
1990. The estimated total volume of water withdrawn from the High Plains aquifer
was 20,519,000 acre-feet in 1980 (Wayne Solley, U.S. Geological Survey, oral
commun., 1988), 17,071,500 acre-feet in 1985, and 16,534,800 acre-feet in 1990
(table 3). Thus, withdrawals decreased about 17 percent between 1980 and 1985
and about 3 percent between 1985 and 1990. The
3-percent decrease in total withdrawals between 1985 and 1990 was due largely to
a nearly 4-percent decrease in water withdrawals for irrigation and livestock
use during that period. Although comparable livestock water-use data are not
available for 1985, available data indicate that livestock water use probably
increased slightly from 1985 to 1990 (Carr and others, 1990; Solley and others,
1993). Thus, the decrease in total agricultural water use was attributable
almost entirely to decreases in irrigation water use. Nonagricultural water uses
increased about 15 percent between 1985 and 1990 (table 3). Further comparison
of changes in nonagricultural water uses between 1985 and 1990 is not possible
because of differences in water-use classification. The
ground-water withdrawal statistics in table 3 apply only to individual years,
which are not necessarily representative of long-term water use. Withdrawals,
particularly for irrigation, are affected by climatic conditions, particularly
precipitation, which can cause large fluctuations in water requirements. None of
the water-use reporting years--1980, 1985, and 1990--appear to represent
extraordinary precipitation conditions in the High Plains. Some of
the decreases in withdrawals between 1980 and 1985, however, are attributable to
precipitation increases and, consequently, smaller irrigation requirements in
parts of the High Plains in 1985, particularly in Nebraska (Steele, 1988). The
apparent 10-year decline in total ground-water withdrawals from the High Plains
aquifer largely is a result of reduction in withdrawals for irrigation. This
reduction is the result of several factors to be discussed in greater detail in
subsequent sections of this report:
(1) Climatic conditions since 1980 have been conducive to decreased water
demands. Prolonged droughts generally were absent from 1980 to 1993 in most of
the major irrigated areas of the High Plains. Precipitation was above normal in
nearly all parts of the High Plains in 1981-92, averaging 1.81 inches above
normal (National Climatic Data Center,1981-92).
(2) There has been a long-term decrease in the amount of irrigated
cropland in parts of the Central and Southern High
Plains as a result of the changing economics of irrigation.
(3) Advances in irrigation technology and management practices have
reduced the volume of water needed to meet the consumptive irrigation
requirements of crops. Kansas,
Nebraska, and Texas, which have nearly 75 percent of the land area overlying the
High Plains aquifer (table 1), accounted for about 85 percent of both the total
withdrawals and withdrawals for irrigation from the High Plains aquifer in 1990
(table 3). Three major areas of large withdrawal rates occur in the High Plains:
(1) eastern and south-central Nebraska; (2) southwestern and south-central
Kansas; and (3) the northern part of the Southern High Plains in Texas and New
Mexico (fig.4). Areas with slightly smaller withdrawal rates include an area
encompassing northeastern Colorado, southwestern Nebraska, west-central Kansas,
and the northwestern Panhandle of Texas. In all of these areas, the saturated
thickness of the aquifer, well yields, soils, and topography are conducive to
irrigated agriculture. The
ground-water withdrawals rates by county shown in figure 4 principally represent
irrigation development because ground-water irrigation accounted for about 95
percent of all ground-water withdrawals in the High Plains in 1990 (table
3).Thus the withdrawal rates (fig. 4) are largely a function of both irrigated
acreages in the county and withdrawal rates per irrigated acre. The large
withdrawal rates per county in parts of southwestern Kansas and the Texas
Panhandle reflect, in part, the large withdrawal rates per irrigated acre in
these areas. Some counties in south-central Nebraska have among the largest
percentage of area irrigated in the High Plains, but withdrawal rates per
irrigated acre are relatively small in this area (Dugan and Zelt, in press).
Ground-water withdrawals from the High Plains aquifer in 1990 ranged from less
than 0.02 acre-foot per acre (0.24 inch) in areas of minimal ground-water
development to more than 1.0 acre-foot per acre (12 inches) in Haskell County in
southwestern Kansas (figs. 1 and 4). Ground-water
withdrawals for irrigated crops do not represent the actual consumption or water
permanently removed from the aquifer. Only that water consumed by
evapotranspiration or that which runs off into drainage ways is actually lost.
Because runoff generally is minimal for most irrigation systems currently in use
and because the volume of water applied to crops often exceeds
evapotranspiration, a substantial volume of the applied water may infiltrate
through the soil and unsaturated zones and return to the aquifer as recharge.
Thus, ground-water withdrawals alone do not fully explain water-level changes. Consumptive
Irrigation Requirements in the High Plains The
consumptive irrigation requirement provides an estimate of the volume of water
consumed by irrigated crops through evapotranspiration and, therefore, actually
lost from the aquifer. It is an estimate of the minimum irrigation water
required to maintain adequate soil water for optimal plant growth. This
requirement, which is unique for each crop, is dependent largely on
(1) potential evapotranspiration,
(2) the growth characteristics of the crop,
(3) soil water available at the beginning of the irrigation season, and
(4) irrigation-season precipitation. The
hydrologic characteristics of the soil have only a limited effect on the
consumptive water requirement of a specific crop
(Dugan and Zelt, in press). The
consumptive irrigation requirement does not represent a minimum pumpage
requirement, which assumes some inefficiencies in irrigation-distribution
systems. The consumptive irrigation requirement as an estimate of water lost
from the aquifer is based on the assumption that irrigation efficiency is 100
percent and that any excess water applied either becomes runoff (usually
negligible) or infiltrates back to the aquifer. Average
annual consumptive water requirements during 1951-80 for corn, the principal
irrigated crop in the High Plains, ranged from about 8 inches in northeastern
Nebraska to about 22 inches in northeastern New Mexico (fig. 5). The combination
of larger average annual potential evapotranspiration and smaller average annual
precipitation caused the largest consumptive water requirements to occur in the
southwestern part of the Central High Plains. Average annual (1951-80)
consumptive irrigation requirements in areas of substantial irrigation ranged
from 8 to 16 inches in Nebraska, 15 to 20 inches in southwestern Kansas, and 15
to 21 inches in the Texas Panhandle. Net
Water Withdrawals from the High Plains Aquifer Water-level
declines in areas of the High Plains irrigated with ground water should largely
reflect the net withdrawals of water (for irrigation) from the aquifer since it
accounts for 95 percent of the total ground-water withdrawals. Actual net
withdrawals can be the expressed by the following equation:
Wn = Ip - (Si + R), (2) where
Wn= net ground-water withdrawals for irrigation, in inches;
Ip= irrigation pumpage, in inches;
Si= seepage of excess irrigation water back to aquifer, in inches; and
R= recharge from precipitation, in inches. Observed
data for irrigation pumpage (Ip), seepage (Si), and recharge (R), however, are
not available. An alternative method of estimating net withdrawals is expressed
by the following equation:
Wn = Ci - R, (3) where
Ci = consumptive irrigation requirement. Consumptive
irrigation requirement (Ci) is an approximation of Ip - Si because the
consumptive irrigation requirement is the minimum irrigation required to
maintain adequate soil water for optimum plant growth and because excess
irrigation pumpage would return to the aquifer as seepage (Si), assuming runoff
is negligible. Net
withdrawal rates in figure 6 are based on equation 3 and represent the combining
of recharge in figure 3 and the consumptive irrigation requirement (corn) in
figure 5. Although only the consumptive irrigation requirement for corn was
available for these calculations, it does provide a uniform comparison of
regional differences in net demand on available ground-water resources. Estimated
average annual net withdrawal rates during 1951-80 ranged from about 2 inches
(acre inches per acre) in parts of northeastern Nebraska to 20 inches in parts
of the Oklahoma and Texas Panhandles, southeastern Colorado, and northeastern
New Mexico. In the areas of substantial ground-water withdrawals (fig. 4), net
withdrawals for corn ranged from 4 to 14 inches in central Nebraska, 16 to 18
inches in southwestern Kansas, and from 12 to 20 inches in the intensively
irrigated parts of the Texas
High Plains. The
process of irrigation development and associated water-level changes in the
uplands of the High Plains generally proceeded from south-to-north. Numerous
factors, including ease of developing water resources, availability of readily
irrigable land, pre-existing cropping systems, and perceived economic benefits
affected the patterns of irrigation development. Documented
water level changes generally began to occur soon after irrigation began in the
various parts of the High Plains. Water-level
declines were apparent in the Southern High Plains by 1940, the Central High
Plains by 1950, and the Northern High Plains by 1960. There were notable
exceptions to this pattern of development, such as in Box Butte County, in the
Nebraska Panhandle, where ground-water development followed by water-level
declines began in the early 1950's. Water-level rises began in the uplands of south-central and
southwestern Nebraska in the early 1940's as a result of seepage from Platte
River surface-water diversions for irrigation and power generation. Some areas
of the High Plains, including north-central Nebraska, South Dakota, and parts of
Colorado and Wyoming are not suitable for extensive irrigation development
principally because of soil, topographic, or hydrologic limitations; therefore,
water levels have remained essentially stable in these areas. The
water level changes from initial development or predevelopment to 1980 are shown
in figure 7. Predevelopment water levels, as used here, are the estimated water
levels that existed prior to any effects imposed by human activity. A
predevelopment water level generally represents seasonally high water-table
conditions, usually occurring in early spring. By 1980,
the largest decline in water levels had occurred in the Southern High Plains of
Texas and New Mexico (fig. 7). Water levels declined more than 50 feet in a
large part of this area, with a maximum decline of nearly 200 feet occurring in
Texas. Declines exceeding 50 feet
occurred in several smaller areas of the Central High Plains of southwestern
Kansas, the north-central Panhandle of Texas, and the central Panhandle of
Oklahoma. Maximum declines in the Central High Plains exceeded 100 feet in two
small areas of southwestern Kansas. Only small areas of 10- to 50-foot declines
occurred in the Northern High Plains by 1980. In addition to the area of
water-level rise associated with surface-water diversions in south-central and
southwestern Nebraska, smaller areas of rise occurred in central Nebraska, along
the Kansas-Oklahoma border, and the extreme Southern High Plains of Texas. The
rise in central Nebraska also is associated with surface-water diversions (Luckey
and others, 1981). The
regional differences in magnitude and areal extent of water-level declines from
predevelopment to 1980 (fig. 7) are partly time dependent. The absence of areas
of decline exceeding 50 feet in the Northern High Plains is attributable partly
to the later development of irrigation in that area. The
saturated thickness of the High Plains aquifer in 1980 ranged from less than 100
feet in much of the High Plains to more than 1,000 feet in parts of
north-central Nebraska and southeastern Wyoming (fig. 8). The saturated
thickness averaged about 340 feet in Nebraska (table 1), but the saturated
thickness of the aquifer averaged only about 110 feet in the remainder of the
High Plains (Gutentag and others, 1984). The saturated thickness was less than
100 feet in 46 percent of the High Plains area and exceeded 600 feet in only 5
percent of the area (Gutentag and others, 1984, p. 23-24). Saturated thickness
in the major irrigated areas of Kansas, Nebraska, and the northern Panhandle of
Texas generally was 200 to 400 feet and 100 to 200 feet in the Southern High
Plains of Texas. The large water level declines from predevelopment to 1980 in
parts of Kansas and Texas (fig. 7) resulted in a substantial reduction in
saturated thicknesses in these areas by 1980. Large
but discontinuous parts of the High Plains aquifer in the Central High Plains
had little or no saturated thickness in 1980 (fig. 8). The largest of these
areas were in west-central Kansas, northeastern New Mexico, and southeastern
Colorado. In the Southern High Plains, a large part of the aquifer in
east-central New Mexico had little or no saturated thickness. In the Northern
High Plains, only two small areas in northwest and west-central Kansas had
little or no saturated thickness. Areas
that had little or no saturated thickness in 1980 generally are along the
boundaries of the High Plains or where deposits of late Tertiary and Quaternary
age largely have been removed by erosion. The extent of the areas of little or
no saturated thickness in 1980 probably is comparable to predevelopment
conditions because these areas could not have sustained any substantial
ground-water development. Adequate supplies of ground water even for livestock
and rural domestic purposes generally are not available in these areas.
Observation wells, if present in these areas, probably do not represent
water-level conditions in the High Plains aquifer. Subsequent discussions of
water-level changes in the High Plains aquifer do not consider the areas that
had little or no saturated thickness in 1980. An
estimate of the volume of ground water in storage in the High Plains aquifer can
be calculated from average saturated thickness and aquifer specific yield. The
area-weighted average saturated thickness of the High Plains aquifer (table 1)
was about 190 feet in 1980 (Gutentag and others, 1984). The specific yield (the
volume of water that will drain by gravity from aquifer pore spaces) averages
about 15 percent (0.15) of the total volume of saturated material (Gutentag and
others, 1984). Therefore, the
estimated total volume of water that could drain from the aquifer in 1980 was
about 3,250 million acre-feet (table 1). Pumping costs in relation to well
yields, however, would limit the availability of much of this water. GROUND-WATER-LEVEL
OBSERVATION-WELL NETWORK Water-level
changes in the High Plains aquifer are monitored through an extensive network of
observation wells that are indicated in figures 9 and 10. Water-level changes
between 1980 and 1993 were based on measurement of water levels in 6,206 wells.
Observation wells added to the network after 1980 increased the number of
observations used to compute changes between 1992 and 1993 to 8,053 (table 4).
Although the difference in number of observations available from 1980 to1993 and
from 1992 to 1993 do not permit exact statistical comparisons of water-level
changes between the two time periods, general comparisons are possible because a
large majority of observation wells had water-level measurements in both time
periods. The additional observation wells measured in 1992 and 1993 tend to be
located in areas where additional ground-water development for irrigation
occurred after 1980. The
observation-well network used for monitoring water-level change both between
1980 and 1993 and between 1992 and 1993 reflect the addition of wells since the
initial report in this series (Kastner and others, 1989). In that report, 1980
to 1988 and 1987 to 1988 changes were based on 4,719 and 6,203 wells,
respectively. Most of the network expansion resulted from acquisition of
additional observation-well data from local sources in Texas. The
number of observation wells in the network varies from year to year. Some
observation wells have been permanently lost from the network, particularly
those providing long-term observations (1980 or earlier). These are typically
older wells that often are no longer measurable because of structural collapse
and subsequent plugging, or insufficient depth to penetrate a declining water
table. Short-term loss of some observations occurs annually from the network as
a result of the measurement period coinciding with recent pumping of the
observation well or nearby wells and water levels not fully recovering. These
yearly Variations
in the database are usually small and randomly distributed spatially and do not
significantly affect analysis of water-level change. The
variable density and location of observation wells within the network (figs. 9
and 10) are governed principally by the intensity of ground-water irrigation
development. Areas with larger percentages of irrigated land tend to contain a
greater density of observation wells. In contrast, those areas with little or no
irrigation development tend to have lesser densities of observation wells. In
many areas, particularly where the aquifer has little or no saturated thickness,
few measurable wells are available. Most of
the observation wells in the network are privately owned irrigation wells with
access ports and sufficient diameters to permit ready access with a measuring
device. Irrigation wells are especially well suited for monitoring water-level
changes because their large diameter and large pumping capacity make them less
prone to plugging, which is common in small-diameter, small-capacity wells. A
small percentage of the observation wells are designed specifically for water
level monitoring, and some are equipped with recording devices for continuous
monitoring of water levels. Few wells designed for municipal, domestic, or
livestock uses are suitable for water level monitoring. Water
levels are measured by personnel of numerous Federal, State, and local agencies
in the High Plains. Local water and natural resource conservation districts are
responsible for the largest number of these water-level measurements. The USGS
is responsible for compiling the water-level data and maintaining the
water-level databases in most States. Measurements
usually are made in the winter and early spring when water levels generally have
recovered fully from pumping during the previous irrigation season and represent
the highest water levels during the year. Irrigation of winter wheat in the
western High Plains in the late winter and early spring occasionally requires
selection of alternative measurement periods in those areas. In some areas,
particularly in the Northern High Plains, measurements are made only in the fall
following the irrigation season because winter and spring weather typically is
not conducive to field work. Consistency from year to year-in date of Measurement
is perhaps more critical than the measurement season because this consistency
provides a more logical comparison among annual water levels.
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