Page 296 - Hydrogeology Principles and Practice
P. 296

HYDC08  12/5/05  5:31 PM  Page 279






                                                      Groundwater resources and environmental management  279


                   Table 8.1 Groundwater budget information for the High Plains Aquifer based on the United States Geological Survey’s Regional Aquifer
                   System Analysis groundwater model.* After Luckey et al. (1986).
                   Budget parameter                         Northern High Plains  Central High Plains  Southern High Plains
                   Primary inflows (in cubic metres per year)
                                                                   8
                                                                                   8
                     Recharge from precipitation on rangeland and streams  5.98 × 10 †  4.66 × 10 †  1.97 × 10 8
                     Recharge from precipitation on agricultural land‡  2.89 × 10 9  –     1.43 × 10 9
                     Groundwater irrigation return (pumpage minus crop demand)  2.31 × 10 9  2.07 × 10 9  3.61 × 10 9
                     Recharge from other human activities (e.g. seepage from  2.31 × 10 9  –  –
                      reservoirs and canals)
                                                                                   7
                     Recharge from other aquifers across subunit boundary  –  1.88 × 10 §   –
                     Totals                                  8.11 × 10 9     2.55 × 10 9   5.24 × 10 9
                   Primary outflows (in cubic metres per year)
                                                                                   9
                     Total pumpage                           6.48 × 10 9     6.89 × 10 ||   8.59 × 10 9
                     Discharge to streams and shallow water-table areas  2.87 × 10 9  4.15 × 10 8  –
                     Discharge along eastern boundary        –               6.97 × 10 7   1.05 × 10 8
                     Totals                                  9.35 × 10 9     7.37 × 10 9   8.70 × 10 9
                   Net residual                             −1.24 × 10 9    −4.82 × 10 9   −3.46 × 10 9

                   * Assumptions: Inflow/Outflow values determined using 1960–1980 estimates; base of aquifer modelled as no-flow boundary; vertical flow
                   in aquifer considered negligible on regional scale.
                   † Recharge distributed unevenly based on soil type.
                   ‡ Additional recharge from precipitation on agricultural land because of changes in soil character due to tillage.
                   § Flow only from northern and southern subunits to central subunit.
                   || Municipal and industrial pumpage is 3.2% of this amount.


                   is taken from aquifer storage with potential long-term  a source of irrigation water has transformed the
                   impacts on groundwater levels.              mid-section of the United States into one of the major
                                                               agricultural regions of the world. Principal crops are
                                                               cotton, alfalfa and grains, especially wheat, sorghum
                   8.2.2 Large-scale groundwater development   and maize. Grains provide feed for the 15 million cat-
                   schemes                                     tle and the 4.25 million swine (1997) that are raised
                                                               over the aquifer. In addition, the aquifer provides
                   The understanding of aquifer conditions and the  drinking water to 82% of the people who live within
                   compilation of a water balance are central to water  the aquifer boundaries (Dennehy et al. 2002).
                   resources management. An example of a groundwa-  According to Dennehy et al. (2002), groundwater
                   ter budget for a large-scale aquifer system, in which  flow in the High Plains Aquifer is generally from west
                   the primary inflows and outflows to the aquifer are  to east, discharging naturally to springs and streams
                   tabulated, is shown in Table 8.1 for the United States  and is subject to evapotranspiration in areas where
                   High Plains Aquifer. The High Plains Aquifer con-  the water table is close to the land surface. Pumping
                   sists mainly of near-surface deposits of late Tertiary  from numerous irrigation wells is, however, the prin-
                   or Quaternary age forming one unconfined aquifer   cipal mechanism of groundwater discharge. Abstrac-
                                       2
                   and underlies 450,660 km in parts of eight States  tions greatly exceed recharge in many areas, causing
                   within the Great Plains physiographic province. The  large declines in water levels; for example, declines
                   Ogallala Formation of Miocene age, which underlies  of 30–43 m since the 1940s to 1980 in parts of Kansas,
                            2
                   347,060 km is the principal hydrogeological unit and  New Mexico, Oklahoma and Texas.
                   consists of a heterogeneous sequence of clay, silt,  From the groundwater budget shown in Table 8.1,
                   sand and gravel. Use of the High Plains Aquifer as   it is apparent that each region of the High Plains
   291   292   293   294   295   296   297   298   299   300   301