Kansas Geological Survey Publications Sample

Ground-water Recharge and Water Budgets of the Kansas High Plains and Related Aquifers Marios Sophocleous Bulletin 249 2004 102 pages, 25 figures, and references
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Introduction to Ground-water Recharge, Discharge, and Sustainability

As Kansas water resources become fully allocated and demand for ground water increases, ground-water managers are faced with the difficult task of ensuring the future viability of the resource. With the rise in public environmental awareness, ground-water managers also are concerned with protecting natural environments that are dependent upon the ground water, such as stream baseflows, riparian vegetation, aquatic ecosystems, and wetlands. Sustainable use of ground water must ensure not only that the future resource is not threatened by overuse and depletion, but also that natural environments that depend on the resource are protected. Trade-offs between ground-water use and potential environmental impacts always will exist, and therefore a balanced approach to water use between development and environmental requirements needs to be advocated. However, to properly manage ground-water resources, managers need accurate information about the inputs (i.e., recharge) and outputs (i.e., pumpage and natural discharge) within each ground-water basin, so that the long-term behavior of the aquifer and its sustainable yield can be estimated or reassessed.

Estimating recharge is critical in any analysis of ground-water systems and the impacts of withdrawing native water from them. In water-resource investigations, ground-water models are often used to simulate the flow of water in aquifers, and, when calibrated, may be used to predict long-term behavior of an aquifer under various management schemes. Without a good estimate of recharge and its spatio-temporal distribution, these models become unreliable. Accurate estimates of recharge and recharge mechanisms also are necessary to assess the risk of ground-water contamination, particularly diffuse agricultural contamination (such as from nitrates and pesticides). Clearly, understanding recharge is critical to managing most ground-water systems.

Under natural or virgin conditions and over long periods of time (before any development), ground-water recharge is balanced by ground-water discharge, i.e., Recharge = Discharge. Because ground water is nearly always moving, it will naturally flow from the recharge areas to the discharge areas. The discharge from the aquifers may occur in a variety of ways such as flow to streams, lakes, and springs; water use (transpiration) by phreatophytic vegetation that draws its water from the water table or its capillary fringe; evaporation from playas and areas of very shallow water table; leakage to adjacent aquifers; or flow to the sea.

Pumping ground water constitutes an additional withdrawal from the system that was in a natural state of balance under virgin conditions. In order for the system to reach a new equilibrium (a state of sustainability), the pumping must either cause the recharge to increase, and/or it must cause the discharge to decrease. Ground-water pumping usually has little impact on the recharge, especially under arid and semiarid conditions with deep water tables, because recharge is determined mostly by climatic conditions, although in areas of intense irrigation, return flows to the underlying aquifer could be significant. Pumping, however, can decrease ground-water discharge by lowering shallow water tables, thus reducing ground-water evapotranspiration and seepage to streams, springs, lakes, or wetlands. In hydrogeologic terms, pumping can capture ground-water discharge. The position of the water table, which normally reflects the distribution of the recharge and discharge areas, as well as the geometry of the aquifer and its hydrogeologic properties, will change as the system adjusts to the change in discharge. Thus, declines in ground-water levels are not necessarily an indication that the sustainable yield of an aquifer is being exceeded, but simply that the water balance has been altered (Cook et al., 2001), and may reflect a temporary decrease in aquifer storage that occurs before a new equilibrium is established.

In order for a ground-water system to be sustainable, pumping must be balanced by an equal capture of discharge and/or recharge. If pumping exceeds the total amount of natural recharge or discharge from the system, ground-water mining occurs, and the system is no longer sustainable. However, even without regard to the environment, it is not always possible to extract all of the natural aquifer recharge or discharge. In some cases, wells will run dry before natural ground-water discharges are reduced to zero. The fraction of recharge that can theoretically be extracted from an aquifer under steady-state conditions will depend on the geometry of the aquifer system, and, in particular, on the location of the pumping wells relative to the natural recharge and discharge zones (Bredehoeft et al., 1982; Sophocleous, 1998a, 2000a; Bredehoeft, 2002). Therefore, the sustainable yield of aquifers, and thus the environmental impact of ground-water extraction, depends not only upon the volume extracted, but also on the location of pumping wells relative to recharge and discharge areas, and sometimes also on the timing of the extraction. Prediction of environmental impacts of ground-water extraction always requires detailed investigation of natural ground-water recharge and discharge processes.

It is important to note that all levels of ground-water extraction will, in the long run, result in declines of natural discharges, with consequent environmental impacts. Sometimes such impacts will be small and not readily identifiable, while in other cases, they may be much more dramatic, such as in the drying up of springs and streams in western Kansas. However, there will always be a time lag between ground-water extraction and reduction in natural discharge, and therefore the current apparent health of an exploited aquifer and the ecosystems that depend upon it does not necessarily indicate that the situation will be sustainable in the longer term (Cook et al., 2001). The task of ground-water managers is to determine what limits of environmental impact are acceptable to the community and to manage extraction to maintain impacts within those limits.

Once the ground-water system is sufficiently perturbed, even cessation of pumping will not stop the adverse impacts. The impact of pumping after it is stopped persists for a variable time. The time lag between ground-water extraction and reduction in natural ground-water discharge will depend on the extraction rate of ground water relative to the natural recharge and discharge rates. For an aquifer discharging to a stream, this time lag is proportional to the square of the distance of ground water pumping from the stream and inversely proportional to the hydraulic diffusivity of the aquifer (usually expressed as the ratio of aquifer transmissivity to storativity). For relatively large ground-water basins with low recharge fluxes, this time lag can be many hundreds of years (Sophocleous, 1998a, 2000a). In some cases, this allows ground-water extraction at rates well in excess of recharge rates to continue for a number of years before the impact of this policy can be recognized.

Changes in land use, such as intensive irrigation, often result in increased deep drainage, which creates a pressure front that moves down through the soil towards the water table (Jolly et al., 1989). Until the pressure front reaches the water table, aquifer recharge continues at the same rate as it did before irrigation development. When the pressure front reaches the water table, aquifer recharge increases, causing the water table to rise. The time lag between the increase in deep drainage and the increase in aquifer recharge is related to the deep drainage rate, the initial water-table depth, and the soil-water content within the unsaturated zone. This time lag, and thus the manifestation of impacts of land-use changes vis-`a-vis ground-water recharge and discharge, can take many years to manifest.

In the following pages, the hydrogeologic framework for understanding natural recharge processes is set out in Part I, together with an outline of recharge estimation methodologies and related uncertainties and challenges facing the field of recharge assessment. A recharge-related glossary is presented as Appendix C of Part I. Part II summarizes most major recharge studies in the Kansas High Plains and associated aquifers as well as their water budgets, with emphasis on assumptions and limitations as well as environmental factors affecting recharge processes. Part III presents a conceptualization of the High Plains aquifer and its recharge characteristics. It also outlines appropriate techniques for quantifying recharge in the High Plains aquifer. Finally, in Part IV, EXCEL spreadsheets with county-by-county and districtwide recharge estimates for the Kansas groundwater management district regions and related statistics are compiled based on Kansas Geological Survey Bulletins and other publications.