SDI and the declining Ogallala
Freddie R. Lamm and Todd P. Trooien
Northwest Research-Extension Center and Southwest Research-Extension Center
K-State Research and Extension
In 1995 there were nearly 2.24 million irrigated acres in 31 western-most Kansas counties comprising Region 1. The irrigation water source is the Ogallala aquifer along with some small alluvial aquifers and a small amount of highly contested surface water. In western Kansas, irrigation accounts for nearly 95% of the total water use. Though occupying a small percentage of the land area, irrigation has a significant effect on total crop production and economic stabilization of the region. The pressures facing irrigated agriculture in the western Kansas are generally similar to those experienced by other regions. These pressures include limited water resources, competition between urban, industrial, wildlife, recreation, and irrigation users, water quality degradation, and the economics. Western Kansas can be characterized as agriculturally based and producers have developed economically efficient crop and livestock systems that are well suited to the region. Few people question that adjustments will be required in the future (NRC, 1996, CAST, 1996). However, the scope of dependence on irrigated agriculture in the Great Plains demands that economical and technologically sound solutions be developed that can both maintain a higher level of sustainability and also lengthen the transition period to a less irrigation intensive society.
As irrigation sustainability becomes an increasing concern, irrigation improvements such as high efficiency subsurface drip irrigation (SDI) are likely to play an increasing role. Indeed, the irrigation surveys show an increase of 1000 acres/year from 1994-1996 (Irrigation Journal, 1995, 1996, 1997). The acreage is likely to increase at an increasing rate as producers become more familiar with SDI technology and as aging alternative irrigation systems are replaced. Many producers can increase irrigation efficiency by moving from surface irrigation to center pivot sprinkler irrigation. However, irregular field shape and sizes are in effect locking many producers out of this option. SDI on the other hand is easily sized to the field and water resource constraints. It is interesting to note that both of the recent irrigation reviews (NRC, 1996; CAST, 1996) specifically point out that the Ogallala region is especially good at adaptation and in embracing technological developments. These adaptations are partly forced by necessity. Examination of the water saving techniques in the Southern Plains in Texas (an area that is hit hard by water declines) show that producers are willing to make significant commitments to sustain the irrigated economy. SDI is one of the tools that producers in Texas are using to sustain the irrigated economy. Microirrigation in Texas increased 55% in two years from 65,000 to 101,000 acres according to the 1994 and 1996 irrigation surveys (Irrigation Journal, 1995; 1997).
A number of studies have indicated that microirrigation has the potential to save irrigation water or to increase water use efficiency (Dawood and Hamod, 1985; Tollefson,1985; Sammis, 1980; Clark, 1979; Safontas and di Paola, 1985, and Camp et al. 1989). Kansas State University has been actively involved in evaluating SDI for corn production since 1989. Lamm et al. (1995) concluded that SDI could reduce net irrigation requirements by 25% while still maintaining corn yields greater than 200 bushels/acre. Since the application efficiencies of alternative irrigation systems are typically 15 to 35% lower than SDI, the 25% reduction in net irrigation can actually be a 35-55% reduction in the gross irrigation amount.
Determining the effect of SDI adoption on the declining Ogallala aquifer can be approached from a number of different methods. Most of these methods require a number of arguable assumptions. The approaches to be discussed here are not exceptions. The results should not be considered facts, but rather as estimates of the effect of SDI adoption on water savings and extension of the useful life of the aquifer.
RETAINING IRRIGATED AREA AND EXTENDING AQUIFER LONGEVITY
Four counties (Grant, Haskell, Thomas and Sherman) were selected in western Kansas for a modeling effort to determine the effect of SDI adoption on aquifer life and irrigated area. The counties were not selected randomly and should not be considered representative of the whole region. Rather they were selected to demonstrate some of the different results that can be obtained depending on the characteristics of the counties (Table 1).
Table 1. Summary of information for four western Kansas counties in 1995.
|Total Area, acres||368640||368640||691200||672000|
|Total Irrigated Area, acres||137600||213000||95000||103800|
|Grain Sorghum, acres||14400||9900||3000||3300|
|Other Crops, acres||3300||4400||11100||26700|
|Alfalfa NIR 50, Acre-Ft/acre||2.0||1.94||1.83||1.90|
|Corn NIR 50, Acre-Ft/acre||1.24||1.21||1.13||1.18|
|Grain Sorghum NIR 50, Acre-Ft/acre||1.09||1.05||0.98||1.03|
|Soybeans NIR 50, Acre-Ft/acre||1.03||0.98||0.91||0.97|
|Wheat NIR 50, Acre-Ft/acre||0.94||0.90||0.80||0.87|
|Other Crops NIR 50, Acre-Ft/acre||1.26||1.22||1.13||1.19|
|Surface Irrigation, %||44.4||62.9||7.4||21.4|
|Sprinkler Irrigation, %||55.6||37.1||92.6||78.6|
A spreadsheet template was constructed to simulate the effect of alternative irrigation scenarios on the number of irrigated acres over the course of several decades. The 1995 saturated thicknesses for each section in the county were obtained from maps prepared by the Kansas Geological Survey. It was assumed for the purposes of this modeling effort that irrigation would cease when the saturated thickness fell below 40 ft. An average available saturated thickness above this minimum value was calculated for the county based on these saturated thicknesses for the sections. Data from 1995 on irrigated area, crop mix , system type, and the net irrigation requirement were obtained from the Kansas Water Office, the Division of Water Resources, and the USDA-NRCS (Table 1). This data was used to calculate the initial gross irrigation requirement for the county. The crop mix was assumed constant for the entire duration of the modeling. Aquifer recharge from precipitation was assumed to be 0.1 inch annually for all the acres in the county. Multiplying the saturated thickness by an assumed storage coefficient (0.18) and the number of acres in each county gave an estimate of the amount of water in storage. The amount of storage was reduced by the irrigation requirement and a new saturated thickness was back-calculated. This was done in 10 year intervals. At the end of the 10 year interval, a check was performed to see if the section saturated thickness had fell below 40 ft. If so that area was excluded and the county gross irrigation requirement was adjusted downward accordingly. The modeling was then continued for another 10 year period until there were no longer any irrigated acres in the county. For the purposes of this model, each section was provided with the county percentage of irrigated acres and irrigation system type. Irrigated acres and system type are not typically spatially distributed in this manner, but was it was necessary in this model due to the lack of knowing the true spatial distribution. Application efficiencies of 65%, 85% and 100% were assigned to surface irrigation, sprinkler irrigation and SDI. Additionally SDI was allowed to reduce the net irrigation requirement by 25% as indicated by studies in Kansas (Lamm et al. 1995). The baseline scenario was that the present mix of irrigation system types (surface and sprinkler irrigation) continued for the life of the aquifer. In the second scenario, the entire surface-irrigated area was converted to SDI in 1995. No sprinkler-irrigated area was converted to SDI in this modeling effort, even though some sprinkler-irrigated areas are appropriate for using SDI.
The adoption of SDI on all the surface-irrigated acres had varied results across the four counties (Figure 1). In the southwest Kansas counties (Grant and Haskell) that still had a sizable percentage of surface irrigation in 1995, the adoption of SDI extended the life of the aquifer by 30-40 years and maintained a higher irrigated area for a longer period of time. In contrast, a very small amount of remaining surface irrigation existed in northwest Kansas in 1995 and the adoption of SDI had a relatively small effect. Comparing the areas under the curves of the different scenarios shows the overall effect of adoption of SDI. The ratio of Baseline with SDI to the Baseline was 1.33, 1.49, 1.04 and 1.15 for Grant, Haskell, Thomas and Sherman county, respectively. Increasing the usefulness of the aquifer by 133 and 149% for the two southwest Kansas counties is very significant since there is a higher dependence on irrigation for crop production because precipitation is low and evapotranspiration is higher. The difference between the two curves represents the number of acre-years of improvement by adoption of SDI. The SDI scenario resulted in an additional 2.3, 5.1, 0.2 and 1.1 million acre-years of irrigation for Grant, Haskell, Thomas and Sherman counties respectively. For an example, this in essence means adoption of SDI on the surface-irrigated acres in Haskell county would allow the retaining of 51,000 irrigated acres for 100 years beyond the baseline projection. Any increase in the aquifer life allows for other water technologies to be developed and implemented and also increases the time for regional economies to transition to a less irrigated agriculture.
Figure 1. Irrigated area for 4 western Kansas counties for 2 irrigation scenarios for the years, 1995-2145.
ANNUAL WATER SAVINGS AND THE ECONOMIC EFFECTS
Another approach to examining the effect of the adoption of SDI on the declining Ogallala aquifer is to examine the annual water savings that could occur. It should be noted that this approach is only a "snapshot" in time and is harder to translate through time. Still, it does allow for a simple calculation of the amount of water that could be saved at that point in time, and then an economic value can be placed on the water.
In 1995, there were 735,141 surface irrigated acres (32.9% of total irrigated acres) in the 31 western Kansas counties in Region 1. Using the 1995 crop mix and the net irrigation requirements of those crops, the total net irrigation requirement for all the surface-irrigated acres was calculated to be 820,386 acre-ft. Using an application efficiency of 65% results in a gross irrigation requirement of 1,262,133 acre-ft. If SDI with a 100% application efficiency and the 25% reduction in net irrigation was applied on those same 735,141 acres, the gross irrigation requirement would be 615,290 acre-ft. The water savings accruable to the adoption of SDI would be 646,843 acre-ft annually. Each additional inch of irrigation water will produce 10-20 bushels of corn when applied in an efficient manner. If the corn is priced at $2.35/bushel, the annual water savings could translate into a value ranging from $182,409,798 to $364,819,596. Of course these values are not savings the irrigator can directly pocket, but are more a potential savings to the irrigator or to society in the long run. The in-pocket savings in pumping costs attributable to adoption of SDI might approximate $25,000,000 annually, although higher pressure requirements of SDI might erode some of those savings. In comparison to both of these calculations of the value of the water savings, it can be noted that the conversion of the 735,141 acres to SDI at $540/acre (O'Brien et al. 1997) would require $396,976,140. However, this value is a one-time cost to be spread out over the SDI system life, which is estimated to be 10-20 years.
A case has been presented that the adoption of SDI on the surface-irrigated acres in western Kansas could have a significant effect on the declining Ogallala aquifer. Most water planners and resource managers in the Ogallala region recognize that there will be no magic bullet that will remove all of the region's water problems. Instead there is a growing realization that it will take many tools working together to help avoid the significant disruption in the economies and societies grown accustomed to widespread irrigation use. These tools will include a wide variety of activities such as value-added products, precision agriculture, cloud seeding, enviro-tourism, wildlife and game preserves, alternative crops, and improvements in irrigation. SDI is just one of the many tools, but it is a tool that has a lot of pluses. It does not require a change in occupation. It can reduce waste of water to a negligible amount. It is environmentally friendly. The SDI system can be economically sized to the available water source. It has low operational energy costs and is well suited for off-peak electrical load management strategies. Irrigation events can be fine-tuned to spoon feed water just in time to avoid water stress. It can more efficiently augment the summer pattern precipitation of the region than any other irrigation method.
It has been said that the depletion of the Ogallala aquifer may eventually be considered one of the environmental catastrophes in the history of the world. Whether that is hyperbole or not is probably not within our powers to know. The question is whether we take proactive steps towards preventing its depletion or at least providing for a smoother endgame. The easiest course is to do nothing. Clearly SDI is not the current irrigation system of choice in the region. Many might say the economics are not right for adoption of SDI. Others might argue that if not now, then when, and will there be a resource still to save? Placing a value on this national resource introduces a set of economics not usually considered in traditional economic models.
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O'Brien, D., Rogers, D., Lamm, F. and G. Clark. 1997. Economic comparison of SDI and center pivots for various field sizes. Kansas State University AES and CES, Manhattan, Kansas. Irrigation Management Series, MF-2242. 6 pp.
Sammis, T. W., 1980. Comparison of sprinkler, trickle, subsurface, and furrow irrigation methods for row crops. Agron. J., 72:701-704.
Safontas, J. E., and J. C. di Paola, 1985. Drip irrigation of maize. Chapter in Volume 2 of Drip/Trickle Irrigation in Action, Proceedings of the Third International Drip/Trickle Irrigation Congress, Fresno California. pp. 575-578.
Tollefson, S., 1985. Subsurface drip irrigation of cotton and small grains. Chapter in Volume 2 of Drip/Trickle Irrigation in Action, Proceedings of the Third International Drip/Trickle Irrigation Congress, Fresno California. pp. 887-895.
This paper was first presented at the 15th Annual Water and the Future of Kansas Conference, KSU Union, Kansas State University, Manhattan, Kansas, March 4, 1998.
Comments or questions can be directed to
Research Agricultural Engineer
KSU Northwest Research-Extension Center
105 Experiment Farm Road
Colby, Kansas 67701