Perspectives on Sustainable Development of Water Resources in KansasMarios Sophocleous, editorBulletin 2391998 239 pages, papers from 10 contributors, glossary, and an index |
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More than 98% of the water available for use enters the state of Kansas as precipitation. The statewide average annual precipitation is about 27 inches (68.5 cm). Evapotranspiration returns about 23.2 inches (59 cm) back to the atmosphere. Aquifer recharge uses approximately 0.9 inches (2.3 cm). Runoff to rivers that originate within the state represents approximately 2.6 inches (6.5 cm), and when combined with stream flow into the state from Nebraska and Colorado (equivalent to less than 0.4 inches or 1 cm), surface-water outflows to Missouri and Oklahoma account for almost 3 inches (7.5 cm). Annually, Kansans use about 1.6 inches (4 cm) of water. Ground-water use represents approximately 1.2 inches (3 cm) of that total (92% is used for irrigation), and surface-water use equals approximately 0.4 inches or 1 cm (75% is used by power plants).
Although the precipitation, evapotranspiration, and other factors in the water budget vary widely from year to year, the averages over several decades remain nearly constant. The evidence to date shows that the main water supply for Kansas--precipitation that falls on the state--has changed little in the last 150 years. What has changed is how water is used.
In western Kansas, ground water provides almost all the water, but at the present level of water use, ground-water resources are being depleted. Although the demand for water is continuously increasing, many areas in western and central Kansas are closed to further appropriations. In eastern Kansas. surface water is the principal water source; however, large withdrawals of ground water, which is closely coupled to the surface-water system, are obtained primarily from the alluvium of the Kansas River valley.
Kansas faces a number of water-availability and water-quality problems. Ground-water-level declines and stream flow reductions are especially significant in western Kansas. Saline-water intrusion, both natural and human-induced, affects surface-and ground-water systems. Pollution from non-point sources (mainly pesticides, fertilizers, and livestock), from point sources (waste disposal sites), and from mining activities are other water-quality problems that affect water resources in the state.
Ground-water management and management of surface waters must e conceived as a combined system. Because of the interdependence of surface and ground water, changes to any part of the system have consequences for the other parts.
Under natural conditions (prior to development by wells), aquifers are in a state of approximate dynamic equilibrium: over hundreds of years, wet times ( in which recharge exceeds discharge) offset dry times (when discharge exceeds recharge). Discharge from wells disturbs this equilibrium by producing a loss of water from aquifer storage. The decline of ground-water levels around pumping wells located near streams captures some of the ground-water flow that would have, without pumping, discharged to the streams. In fact, at sufficiently large pumping rates, these ground-water-level declines can induce flow out of a body of surface water into the aquifer ( a process known as induced recharge). The sum of these two effects leads to stream flow depletion. A new state of dynamic equilibrium can be reached with continued pumping only by an increase in recharge (induced recharge), a decrease in natural discharge, a loss of storage in the aquifer, or a combination of these.
Ground water pumped from the aquifer comes from two sources: aquifer storage and induced recharge of surface water. Storage refers to water that is naturally retained in the aquifer. Induced recharge is surface water that is added to the ground-water-level declines below the surface-water level. Initially, ground water pumped from the aquifer comes from storage, but ultimately it comes from induced recharge. The timing of the change from storage depletion to induced recharge is a key factor in developing water-use policies. Distinguishing between natural recharge and induced recharge to ascertain possible sustained yield is exceedingly difficult and is an area that needs further research.
Permanent stream flow or base flow is usually a result of ground-water discharge. Thus, if ground-water pumping lowers the elevation of the water table below that of the stream bed, stream flow will be reduced or interrupted. On the other hand, stream flow may be the major source of recharge to some alluvial aquifers, so that stream flow regulation or diversion may alter the recharge characteristics, and therefore the sustainable yield, of the ground water in the area. Stream-aquifer interactions also are important in situations of ground-water contamination by polluted surface water and of degradation of surface water by discharge of low-quality ground water. Coordinated, combined use of surface-and ground-water resource (conjunctive use) and management are required to improve the reliability and value of both resources.
Confined Aquifers
A confined aquifer is a porous and permeable geologic unit that is sandwiched between two relatively low permeability layers (unconfined aquifers are only bounded by a low-permeability layer below). Because the confining layers above and below these aquifer systems are usually regionally extensive, the recharge and discharge areas for these systems may be hundreds of miles apart. Confined aquifer systems are more sensitive to development than unconfined systems because of their hydrogeologic properties, such as their much lower storativity.
The issues surrounding the sustainability of water resources in confined systems are complex and involve both the quantity and the quality of ground water. These systems will not respond to development in the same manner as unconfined systems because of differences in the sources and amounts of recharge, and in the mechanisms that release ground water to pumping wells. Because of their greater average depth below the surface, confined systems are more likely to contain ( or be connected to other aquifers that contain) unusable ground-water. As a result, water quality can be a very important factor in determining the usability of the aquifer.
Sustainability of a confined aquifer system may be possible only in regions that are close to either the regional recharge/discharge areas or to areas of hydraulic connection with other aquifer systems. Management on the basis of sustainable yield may be more realistic in this part of the confined system for planning horizons on the order of a few decades. Ultimately, however, sustainability may not be a viable management concept for confined aquifers.
The primary management tools to control declines in confined aquifers are well spacing, restrictions on the rates of withdrawal from the aquifer, and artificial recharge. Depletion of the aquifer will occur if production wells are too close together, or the rates of withdrawal from the aquifer are unregulated.
Water Chemistry
An essential consideration in development of an aquifer is the chemical quality of water produced because the quality of water limits its use. In an undeveloped aquifer, ground water is, for the most part, at a chemical equilibrium with its surroundings because ground-water is, for the most part, at a chemical equilibrium with its surroundings because ground-water flow is generally very slow. Ground-water movement induced by pumping may change the ground-water chemistry. One of the ways that ground-water chemistry may be affected is by recharge of contaminated surface water into unconfined aquifers: surface water may contain incompletely processed sewage effluent, residual agricultural chemicals, or other undesirable chemicals. Another way is through mixing of poor-quality (for example, saline) water into unconfined or confined aquifers: an aquifer containing good-quality water in hydraulic continuity with an aquifer containing poor-quality water may be affected by the poor-quality water.
All aquifers contain water that is chemically stratified. Pumping a well causes mixing of the stratified ground water, which may result in dissolution (corrosion) or precipitation (encrustation) of solids, sorption or desorption of metals or organics, ion exchange of cations or anions, and oxidation or reduction of redox elements that affect their mobility in ground water. In addition, fluctuation of the water table that results from pumping or pumping in concert with irrigation can change conditions in the unsaturated (soil) zone, the gateway to an aquifer. The unsaturated zone is an important chemical-buffer zone to aquifers, the importance of which geochemists are only beginning to understand.
Ground-water use that results in deterioration of water quality may cause irreversible damage to an aquifer. Consideration of water-quality effects in sustainable yield assessments represents not just one more test of the "sustainability" of aquifer development, but an essential part of the evaluation.
To protect ground-water supplies from over exploitation, some state and local agencies have enacted regulations and laws based on the concept of "safe yield." Safe yield is defined as the attainment and maintenance of a long-term balance between the amount of ground water withdrawn annual and the annual amount of recharge. Safe yield is a management concept that allows water users to pump only the amount of ground water that is replenished naturally through precipitation and surface-water seepage. As defined, safe yield ignores discharge from the system.
Under long-term equilibrium conditions, the amount of recharge to an aquifer equals the amount of water discharged into a stream, spring, or seep. Consequently, if pumping equals recharge, the streams, marshes, and springs eventually dry up. Continued pumping in excess of recharge eventually depletes the aquifer (e.g., the Ogallala aquifer in parts of Kansas, Texas, and New Mexico). Thus, natural recharge should not be part of the well-field water budget unless natural discharge is also factored into the formula.
Policy makers are primarily concerned about aquifer draw down and surface-water depletion; both are related to the rate and duration of pumping, location of the well, and aquifer properties. Natural recharge is unrelated to these parameters and is irrelevant to ground-water and surface water depletion. Despite its irrelevance, natural recharge is often used in ground-water policy to balance ground water use under the banner of safe yield. It is a misconception that the natural rate of recharge represents a safe rate of yield.
Use of the traditional concept of "safe yield" of ground water persists today despite being repeatedly discredited in the scientific literature. Misconceptions about safe yield and its use in ground-water management lead to continued ground-water depletion, stream dewatering, and loss of wetland and riparian ecosystems.
Sustainable Systems
Safe yield has often been used as a guide for the sustainable use of a single product--the number of trees that can be cut, the number of fish that can be caught, the volume of water that can be pumped from the ground or river, year after year, without destroying the resource base. However, experience has repeatedly shown that a single product goal is too narrow a definition of the resource, because other resources inevitably depend on, or interact with, or flow from the exploited product. We can maximize our safe yield of water by drying up our streams, but when we do, we learn that the streams were more than just containers of usable water.
A better approach would address the sustainability of the "system" and its water yield-not just the trees, but the whole forest; not just the fish, but the marine food chain; not just the ground water, but the running streams, wetlands, and all the plants and animals that depend on them. Such a holistic approach, however, is fraught with difficulty. We cannot use a natural system without altering it, and the more intensive and efficient the use, the greater the alteration.
Sustainable development of water resources refers to a holistic approach to development, conservation, and management of water resources, an approach that considers all components of the hydrologic system. It is inherently intergenerational because it implies that we must use the water resources in ways that are compatible with maintaining them for future generations. This intergenerational perspective constrains our management of water. The mechanisms to bring about these changes are not clear-cut and are still a matter of debate. The concept is a dynamic one and will be continually refined.
Although the ideas of sustainable yield have been around for many years, a quantitative methodology for the estimation of such yield has not yet been perfected. Since the 1980's, three-dimensional numerical models of the complete stream-aquifer hydrogeologic system have been employed to provide a predictive tool explaining the connection between well-field withdrawal and surface water depletion.
Water-resource Management
In view of persistent declines in ground-water levels, especially in western Kansas, the Kansas Legislature in 1972 passed the Kansas Ground water Act authorizing the formation of local ground water management districts (GMD's) to help control and direct the development and use of ground-water resources. As a result of these ground-water level declines, especially since the mid-ground-water level declines, especially since the mid 1970's, stream flows of western and central Kansas streams have been decreasing. In response to these stream flow declines, the Kansas Legislature passed the minimum instream flow law in 1982, which requires that minimum desirable stream flows (MDS) be maintained in different streams in Kansas.
The three western GMD's (1,3, and 4), which have the least precipitation and the highest rate of ground-water level declines, adopted a planned ground-water depletion policy (although GMD 4 switched to a zero depletion policy for new wells in 1990); the two GMD's in central Kansas (2 and 5), which have more precipitation and smaller rates of ground-water-level declines, adopted modified forms of "safe yield" or "sustainable yield" policies, thus attempting to maintain a balance between water-resource inputs and outputs.
Wise management of water resources needs to be approached not only from the standpoint of quantity and quality, but should also take into account the impact of ground-water exploitation on the natural environment, including human, ground water, surface water, and riparian ecosystems. This kind of integrated management approach is now taking hold in Kansas. The progressive evolution of Kansas water management from the establishment of local GMD's, and the progression of their policies, to the adoption of integrated resource planning and management by the Kansas Water Office and Division of Water Resources (Kansas Department of Agriculture) bodes well for the sustainable development of water resources in Kansas.
Water resource management should be based on the best information we have today, but should be flexible enough for change and complexity because natural systems are inherently variable, "patchy," and complex. This also implies managing in a probabilistic and risk assessment framework that recognizes the inherent unpredictability of nature. Instead of determining a fixed sustainable yield, managers should recognize that yield varies over time as environmental conditions vary.
Our understanding of the basic principles of soil and water systems and processes is fairly good, but our ability to apply this knowledge to solve problems in complex local and cultural settings is relatively weak. Additional lines of communication from the research fields to the water users in the field are required. Communication breakdowns probably account for the persistence of simplistic but misguided concepts such as conventional safe-yield management strategies. A strong public education program is needed to improve understanding of the nature, complexity, and diversity of ground-water resources, and to emphasize how this understanding must form the basis for operating conditions and constraints. This is the only way to positively influence, for the long term, the attitudes of the various stakeholders with water-resource interests.
Reservoir Yields
Yield is used to characterize the capacity of a water resource to serve as a long-term water supply. Yield determinations affect many facets of water policy. In Kansas, a yield estimate places an upper limit on the amount of water supply that can be marketed fro a reservoir. This de facto rationing of water resources can have significant implications for entities in search of water supplies. Therefore, it can affect the larger regional water supply. Yield determination also have corollary implications for policies pertaining to reservoir recreation, reservoir fisheries, and downstream stream flows and riverine habitat.
Reservoir yields depend primarily on inflows and reservoir storage. As such, reservoir yields decrease with time due to reservoir sedimentation. The loss of storage results in a loss of reservoir yield unless compensatory actions are taken. These measures can include augmentation of inflow, increasing the conservation storage capacity via structural or institutional means, or physically removing the accumulated sediments. Except for increasing the conservation storage via institutional means, these approaches (augmentation of inflow, structural increases in storage capacity, removal of bottom sediments) are generally not feasible due to economic, environmental, and political concerns.
Uncertainty and Risk
Water-resources planners realize that knowledge of the hydrologic system is fraught with uncertainties, but decisions still must be made. Water-yield estimates are the result of a hydrologic-balance calculation, based on the initial stock of water, and all inflows and outflows to the system. These quantities vary with time and location and can only be estimated, and thus may carry significant uncertainty. All sources of uncertainty need to be recognized, and their impact on the variables, such as water yield, need to be evaluated. Because uncertainty and risk are companions, evaluating uncertainty allow the assessment of risks and the management of its consequences. Uncertainty needs to be incorporated into the analysis in a quantitative fashion, by means of probabilities. By doing this, decision makers can set policies that meet acceptable levels of risk.
Natural systems can never be perfectly known. Fortunately, uncertainty can be evaluated and policies adopted that minimize the risk of undesirable consequences, such as depleting an aquifer at a rate faster than desired. By recognizing uncertainty we can make the most of the available information, thus leading to better decisions.
Over the last 150 years, most of the land area of Kansas (over 90%) has been changed by agricultural development. Sustainable crop production without irrigation has been a matter of developing management practices that increase the effectiveness of a limited water supply and protect the soil from excessive erosion. Adoption of conservation practices that decrease runoff and reduce evaporation losses have been important. In much of the state, the effectiveness of these practices has resulted in more efficient use of water for grain and forage production.
Since water use by agriculture is a consumptive use that results in evaporation of water from the land surface, more efficient irrigation practices mean that less water becomes runoff or ground-water recharge. As a result, with the development and adoption of ways to use water for agriculture more efficiently in Kansas, less water is available for non-agricultural uses, especially in the drier regions of the state. In the future, these effects will probably result in further decreases in the amount of water available for appropriation by other users. In the western half of the state in particular, stream flows have been reduced by up to 50% since 1950 by a combination of agricultural practices, including withdrawal of ground water for irrigation along streams. In the eastern half of the state, the effect has been limited because of the difference in climatic conditions.
Climate Change
A rapidly growing body of evidence suggests that we are entering a somewhat warmer and definitely more variable world. Sustainable water yields may or may not be reduced in the long-term average, but they will almost certainly by less reliable in the short term. Climate warming may increase demand for water at a rate even greater than that predicted on the basis of economic development. Present-day reservoirs, well fields, and water laws and rights are consistent with the climate as it was. These rights and structures or at least their present mode of operation can not be expected to yield the same results in the now and future climate.
Sustainable yield depends on the assumption of basic stability of the hydrologic cycle: long-term consistency in precipitation and the flow of surface water and ground water, and factors such as temperature, wind, and sunlight that control evaporation and transpiration. Climate is subject to both natural and human-induced changes on scales ranging from local to global.
The local and regional hydrologic effects of global changes cannot be reversed or stabilized at a local level. Adaptation is the only near-term management option. Such adaptation will almost certainly require reconsideration of the concepts, as well as the present water quantities, associated with "safe yield" and ideas of sustainability.
Climate change will have uncomfortable results, but it need not be disastrous if action is taken now to prepare the state and its citizens to mitigate the worst effects of climate change and take advantage of possible benefits. Some of the possible approaches include:
Chapter 2: On the Elusive Concept of Safe Yield and the Response of Interconnected Stream-aquifer Systems to Development, by Marios Sophocleous
Chapter 3: Evolving Sustainability Concepts--Modern Developments and the Kansas Experience, by Marios Sophocleous, R. W. Buddemeier, and R. C. Buchanan
Chapter 4: Is Sustainability a Viable Concept in the Management of Confined Aquifers in Kansas?, by P. Allen Macfarlane
Chapter 5: Water Chemistry and Sustainable Yield, by G. L. Macpherson and M. A. Townsend
Chapter 6: Yield Estimates for Surface-water Sources, by David I. Leib and Thomas C. Stiles
Chapter 7: Effects of Agricultural on Water Yield in Kansas, by James K. Koelliker
Chapter 8: Climate Change and Sustainable Water Yield, by Robert W. Buddemeier
Chapter 9: Managing Uncertainty in Yield Estimates, by Hernan A. M. Quinodoz
Chapter 10: Concluding Comments on Managing Water-resources Systems--Why "Safe Yield" is Not Sustainable, by Marios Sophocleous
Chapter 11: Selective Glossary of Hydrology and Environmental Sustainability-related Terms, compiled by Marios Sophocleous