The Role of Science in Agricultural Water Management
Thomas L. Huntzinger
Water Appropriations Program
Kansas Department of Agriculture
Scientific methods provide credibility, consistency, and an impartial perspective on water management. Data and results of scientific analysis quantify the comparison of choices available to make complex decisions required to effectively use and protect an aquifer. The presentation is organized into the fundamental components of the water cycle and explains how this general scientific principle is applied to water management of a regional aquifer like the High Plains – Ogallala aquifer in western Kansas. Throughout the remainder of this paper the aquifer will be referred to simply as the Ogallala aquifer.
The Ogallala aquifer is a regional aquifer that supports agricultural production in western Kansas. The climate is semi-arid and requires irrigation to grow feed grain and forage for an established livestock feeding and meat processing industry. The area relies nearly exclusively on this industry for its economic future. Kansas has an established water management system in place to address the challenges of effectively using and protecting this vital ground water resource.
Geographic Setting of Kansas Water Resources
Kansas is in the center of the continental United States. It is a semi-arid short grass prairie with cropland in the west and semi-humid tall grass prairie savannah and cropland in the east. Winter wheat is a primary dryland crop in the west, and corn is a major irrigated crop in the west and a dryland crop in the east. Other row crops grown are alfalfa, grain sorghum and soybeans irrigated in the west and dryland in the east. Minor crops are sunflowers and recently cotton. Western Kansas has some of the largest beef processing facilities in the world. Tall grass prairies and cattle dominate central Kansas where shallow soils and steep topography prevent production of grain crops. The capital city is Topeka a city with a population of about 200,000. Kansas City and Wichita are the two metropolitan areas of the state and each exceed populations of about 600,000.
The annual precipitation varies from about 102 centimeters in the southeast to about 31 centimeters in the west. Water supplies for eastern Kansas come from the rivers and reservoirs that supply urban users. Most of the water use is for municipal and industrial use in the east. Some surface water is used for irrigation. The Arkansas River supplies some of the Wichita users but the river is nearly dry in the western half of the state.
Western Kansas obtains most of its water from groundwater. The Ogallala aquifer is nearly the sole source of water for the western third of the state. It supports a vast rural agricultural area.
Nearly all the water use is for irrigation of crops for livestock feed. The groundwater resources for the central and east are some alluviums in general of limited extent along major streams that serve primarily municipal users and some irrigation.
The quantity of water in storage in an aquifer defines the extent of the aquifer’s ability to sustain water use demands through variations in climatic conditions over time. Bedrock elevations throughout the Ogallala were determined and refined by the US Geological Survey and Kansas Geological Survey for the past more than 30 years. Estimates of the total water in storage in the Ogallala each year is obtained from annual water level measurements made at a network of wells distributed throughout the aquifer area (Laflen and Miller, 2004). The annual measurements are made in the winter season when wells have not pumped for several months. Records of the water levels are used to determine absolute changes in storage and saturated thickness and to define declining rates where the aquifer is depleting. Water level measurements are made by the Division of Water Resources, the State’s water management agency, and the Kansas Geological Survey which is the earth science research organization for the state. The data base is maintained by the Kansas Geological Survey.
Annual natural recharge to the aquifer is the quantity of water that is sustainable over the long term. Long term average annual withdrawals that exceed this amount will result in depletion. To ensure a stable long term ground water supply, development ultimately must be limited by natural recharge. Mean annual recharge estimates have been made for the Ogallala aquifer by the U.S. Geological Survey (Hansen, 1991). The Kansas Division of Water Resources uses the estimates of natural recharge to define a term called safe yield in the law of the state. All new development within the state now must not exceed the safe yield, or natural recharge, in the area. The Ogallala is depleting at this time because the safe yield limit did not become law until after much of the development had already occurred. If depletion is to occur it must be planned and managed with the expectation that sometime in the future the development must return to within the limits of natural recharge.
Crop demand must be satisfied by the application of irrigation water. Crop demand depends on the climatic conditions each season. Many energy balance equations have been developed that relate evapotranspiration of crops to measurable climatic variables such as temperature, wind, humidity, and solar radiation. Complex methods that apply equations that are assumed more accurate require the most sophisticated and costly climatic data (Natural Resource Conservation Service, 1993). Therefore some balance between the data collection requirements and accuracy must be reached for a practical approach to determining crop demand. There are long term climatic data sites in Kansas that are maintained by the National Weather Service and supplemented in some places by state operated stations.
Withdrawals from the aquifer for irrigation must be managed to protect the aquifer and at the same time allow the greatest beneficial use. To ensure that an equitable quantity of water is available for each user the maximum reasonable quantity needed for irrigation is determined. This is based on a concept called net irrigation requirement. Net irrigation requirement is defined by law in Kansas as the average potential evapotranspiration of a given crop less the average available precipitation that can be recovered from the soil moisture storage. Each user is not authorized to use more water for irrigation than the net irrigation requirement which allows more users a share in the opportunity to benefit from a finite water source.
Irrigation water use varies with climatic conditions as water users pump water to meet the crop water demand or the net irrigation requirements. During some years, water use reaches the maximum allowable limits and other years it does not. Sometimes an individual user may choose not to plant an irrigated crop for a season. In Kansas, the typical reported use is less than 70% of the maximum allowable amount. Therefore the amount actually pumped must be monitored to determine the actual withdrawal from the aquifer. Flow meters are installed in the distribution lines from the pump to the distribution system. Meters are read at the beginning and end of the irrigation season or the year. Reporting water use on an annual basis is a requirement by Kansas law for each irrigation user. A user is fined if a report is returned after the required annual
deadline for reporting. Water use information is used to project the demands and to determine the amount of pumping that will cause the aquifer to be depleted (Kansas Department of Agriculture, 2000). It also is used to determine if users have exceeded their maximum allowable quantity. The US Geological Survey includes the data from Kansas with other states to publish a national water use summary every five years (Hutson, et al. 2004).
Protecting the Water Resource for the Future
Aquifer depletion is a challenge that is common to many areas. This is a challenge in the Ogallala as irrigation development far exceeded natural recharge before Kansas law put limits on irrigation. The area’s economic success now relies on extensive withdrawals from the aquifer. It is essential that depletion be monitored so future use can be managed to use the remaining resource most effectively. Declines in groundwater levels have been determined from the data collected from the annual water level surveys described previously (Schloss, et al., 2000). One useful product from this data is a map of the percent change in saturated thickness that was prepared by the Kansas Geological Survey. It shows declining rates in areas where resources are consumed most rapidly.
Typically there is not an initial economic motivation to limit pumping to natural recharge in areas where the initial saturated thickness is ample, recharge is minimal and pumping is large. It may be more reasonable to manage the depletion by setting an allowable depletion rate that puts a limit on withdrawals that is an acceptable projection of the remaining saturated thickness at some time in the future. Through the past 25 years the allowable depletion in the Ogallala in southwest Kansas was to allow the saturated thickness to decrease by 40 % in 25 years but not to allow the aquifer thickness to become less than 12 meters. When the projected use of existing wells reached this limit, no new wells were allowed to be drilled in the Ogallala aquifer. In all of southwest Kansas, this limit has been reached or was exceeded before the limit was imposed. Now, no permits are being issued to drill new wells other than to replace existing wells. New water users must now acquire existing permits from willing sellers.
Development that allows depletion to occur means that in the future the aquifer will not produce adequate well yields to sustain the existing water uses. It is important to know when that time may occur to plan for alternate water management strategies. There are now areas within the Ogallala that have reached the point that well yield will no longer sustain the past water uses. A useful analysis to plan strategies for this challenge is the projected useable lifetime of the aquifer. This analysis combines a projection of past pumping rates, the saturated thickness, and aquifer
characteristics. It presents the results in terms of time that all users can understand and interpret. The results of this analysis, for the Ogallala, show the geographic variability of the Ogallala and the areas where the time remaining to develop an alternate water management strategy is very short. Evaluation of alternative strategies must rely on more sophisticated projections that may require numerical modeling to define the outcome of management decisions (Lucky, et al., 1986).
Organizational Structure for Water Management
Effective water management in Kansas relies on a structured approach defined by state water law and associated rules and regulations. This structure is established, refined, and implemented by users working together through a series of organizations of farmers, local interest groups and ground water management districts. The State laws are developed through elected legislative representatives and administered by rules and regulations established by state agencies that work with local users and their representative groups (Huntzinger, 2005).
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