Alan L. Boldt, Dean E. Eisenhauer, Derrel L. Martin, Gary J. Wilmes, Biological Systems Engineering, University of Nebraska-Lincoln, Lincoln, NE.
ABSTRACTWater conservation strategies for center pivot and furrow irrigation in the Central Platte Valley of Nebraska were evaluated using computer simulation. Irrigation requirements, grain yield, return flow and net depletion of groundwater were simulated for a period of 29 years for Hord and Wood River silt loam soils. Grain yields were simulated for a typical corn variety for non-limiting water supplies (maximum attainable yield), for two levels of deficit irrigation where irrigation was limited to certain growing periods, and for dryland conditions. Additional simulations were performed for a short-season corn, grain sorghum, and soybeans. The impacts of tillage practices on water conservation were also investigated.
Center pivot irrigation on the Hord silt loam required 75-125 mm/yr less water application than furrow irrigation. For the Wood River silt loam, water applications were the same for both irrigation systems. Applied water depths were reduced by an additional 75-125 mm using deficit irrigation with only a small reduction in yield. Return flow to the groundwater was small for well managed pivots but high for some furrow irrigation systems. Net depletion (gross irrigation minus return flow) of the groundwater for a center pivot with LEPA sprinkler was 50 mm less (17%) than a center pivot with impact sprinklers. Ridge till had a net depletion 50 mm less (25%) than conventional tillage (double disk, plant) for the furrow systems.
INTRODUCTIONChanging economic and environmental concerns in irrigated agriculture has caused reevaluation of irrigation water usage in central Nebraska. The Central Platte Valley of Nebraska is heavily irrigated using groundwater. There are over 320 000 ha irrigated in a four-county region (Ron Bishop, Central Platte Natural Resources District, personal communication). Corn is the dominant crop and is irrigated for maximum production. Two major concerns in the area are the lowering of groundwater levels and the reduced stream flows in the Platte River. There is interest in water conservation to reduce net depletion of groundwater. Net depletion is defined as water withdrawn minus water that returns to the groundwater reservoir (CAST, 1988).
The term "water conservation" has many meanings. Hydrologically, the focus is on the water balance of either a field or larger area such as a watershed or river basin. Water inputs to a region must equal the water outputs. Inputs include precipitation, available soil moisture, and applied irrigation water. Outputs include crop evapotranspiration (ET), evaporation, deep percolation, leaching requirement, and runoff. One way to reduce overall water usage (conservation) is to reduce the applied water. To reduce applied water one or more of the outputs must be concomitantly reduced, e.g., reducing ET.
ET can be reduced by reducing one of its components, evaporation or transpiration. Crop yields are usually not affected when soil water evaporation is reduced. When evaporation is reduced, the potential for groundwater recharge increases and/or for crop transpiration increases. Conservation tillage systems have lower soil water evaporation than clean tillage systems because of the partial surface cover by crop residues. Thus, conservation tillage is often thought of as a water conserving practice.
Transpiration can be reduced by purposely limiting the available soil water so that plant stress occurs. This practice is often refereed to as deficit irrigation. Crop yield is reduced with deficit irrigation. An alternative method for reducing transpiration is to produce less water intensive crops. In central Nebraska, alternative crops that have a lower ET than corn include grain sorghum and soybeans. When deficit irrigation or different cropping systems are used to conserve water there are economic impacts that must be considered.
Improving irrigation technology to increase irrigation system efficiency is another approach for conserving water. The goal is to reduce the "losses" associated with irrigation: evaporation, runoff, and deep percolation. Furrow irrigation, using gated pipe, is practiced on about 90% of the irrigated area in the Central Platte Valley. To avoid runoff losses from a field, which is a legal requirement in Nebraska, it is common to block or dike the downstream end of the furrows so that all runoff is retained on the field. Improved systems that are considered for water conservation include furrow irrigation with runoff recovery, surge flow furrow irrigation, center pivot sprinkler irrigation, and center pivot LEPA (low energy precision application) irrigation (Lyle and Bordovsky, 1981).
Evaporation from irrigation is assumed to be negligible with furrow irrigation. With sprinkler irrigation, evaporation losses can be the major loss of water during application.
Runoff during irrigation with groundwater can be lost from the aquifer but still be available for use within the watershed or basin (CAST, 1988). As mentioned earlier, irrigators in Nebraska are required by law to prevent runoff from leaving the field boundary. In effect, the runoff must either be reused or must return to the aquifer. With furrow irrigation, efficiency might be higher with runoff recovery when compared to blocked-end systems because water can be applied more uniformly. With well designed and properly managed sprinkler irrigation, runoff should not occur.
Deep percolation, another output, is often considered a loss with irrigation, especially with surface irrigation. However, as pointed out by CAST (1988), deep percolation is only lost when it flows to a salt sink or accumulates in fine textured sediments which makes the water unattainable by wells. In the Central Platte Valley, the groundwater is relatively shallow and deep percolation returns to the aquifer rather quickly without capture in saline sinks or in formations that make it unattainable.
Since the groundwater in the Central Platte Valley is relatively low in salts and the area has a sub-humid climate, leaching requirements with irrigation for salinity management are negligible. It is presumed that deep percolation of rainfall satisfies the needs for salt leaching.
Because of groundwater depletion in the area (Ellis and Wigley, 1988), the Central Platte Natural Resources District (CPNRD), in cooperation with the U.S. Bureau of Reclamation, has proposed a surface water development/groundwater recharge project to augment the water supply within their district. Since water conservation could be an alternative to a water development project, the CPNRD asked for our evaluation of the impact of potential water conservation practices on net depletion of groundwater. The specific objectives of our project were to:
METHODS
In this paper we provide you with only a brief description of the modeling procedures. A paper is currently in press by the authors that explains the details of the modeling. The modeling required linkage of a ET-crop yield model, a soil water balance model, and a surface irrigation model.
Computer simulation models were used to evaluate management systems. A data base containing 29 years of weather data was used to determine responses to irrigation water conservation practices. Several combinations of irrigation and tillage practices were simulated. The performance of a center pivot system with impact sprinklers and a center pivot with low energy precision application (LEPA) nozzles were simulated. LEPA nozzles place water below the canopy. With LEPA irrigation, water is applied much more rapidly than the soil can infiltrate. Thus, interrow tillage which creates micro-basins is required to store water on the soil surface until it has time to infiltrate. The performance of three furrow irrigation systems were simulated: (1) continuous flow irrigation with blocked (diked) ends, (2) continuous flow irrigation with runoff recovery, and (3) surge flow irrigation with runoff recovery. The surface simulations were performed for both conventional and ridge till practices. Conventional tillage in the Central Platte valley of Nebraska usually includes two diskings before planting. Pivot simulations were only performed for conventional tillage practices.
A crop growth soil water balance model (Martin et. al., 1984) was used to evaluate the effect of water stress on crop grain yield, return flow and net depletion. To determine this effect, four lengths of the irrigation seasons were simulated; an irrigation season required to produce maximum yield, two limited irrigation seasons (deficit irrigation), and no irrigation (dryland). The length of each limited irrigation season was defined by the accumulation of growing degree days (GDD). Also in the crop growth model, four crops were simulated: a full season corn (2750 GDD for maturity), a short season corn (2600 GDD for maturity), soybeans, and grain sorghum. The ET component required adjustment for evaporation reduction due to residues on the surface and ET reductions when the crops experienced moisture stress. The approximate tillage dates and crop residue remaining after tillage are given in Table 2. The residue reduction due to tillage was based on data presented by Rawls, et. al. (1980).
In the soil water balance model, runoff from precipitation was reduced by adjusting the SCS runoff curve number using the information presented by Onstad and Otterby (1979). The curve number was 6% less for ridge tillage than for conventional tillage. A final assumption in the soil water balance model is that surface storage of water increased 15 mm because of the presence of micro-basins when LEPA irrigation was used. This storage reduced the amount of runoff from rainfall and irrigation.
Limited irrigation was imposed by shortening the irrigation season. A 5 week season and a 3.5 week season were simulated. The 5 week season began at about vegetative stage 15 (Ritchie et. al., 1986) and ended at beginning kernel dent. The 3.5 week season started at stage R1 and ended at stage R4. These shortened seasons are in contrast to an irrigation season of 9 to 10 weeks that is required for maximum grain production.
Simulations were performed for two soils: Hord silt loam and Wood River silt loam. The water holding properties for these soils, given in Table 1, were obtained from SCS (1983).
The crop model is one dimensional--it simulates a point in space. To account for the non- uniform application depth of a furrow irrigation system, another model, SIRMOD (Utah State University, 1989), was used to create irrigation inputs for the crop model at multiple points in the field.
In the surface irrigation modeling the key adjustments that had to be made were in the infiltration function. Infiltration is a process that has a large influence on the fate and distribution of the applied water. The infiltration function was adjusted for the following: tillage effect (Eisenhauer et. al., 1983), irrigation number, effect of irrigation delay (Encisco, 1993), and the effect of surging (Blair and Smerdon, 1985). The effect of these practices on the infiltration function are shown in Figure 1.
RESULTSIrrigating for Maximum Grain Yield. Simulation results for grain yield, applied water depth, net depletion, and return flow for center pivot and furrow irrigation on Hord and Wood River silt loam soils are shown in Figure 2. All simulations, with the exception of continuous flow irrigation with runoff recovery, showed yields at the maximum yield of 11900 kg/ha for 2750 GDD corn. Generally, yields for continuous flow irrigation with runoff recovery and a Hord silt loam soil were 1-2% below the maximum. Yield reductions in continuous flow simulations were the result of the irrigation scheduling method. Furrow irrigations were scheduled when the point that represented 90% of the furrow length reached 50% depletion. The remaining 10% of the field exceeded 50% depletion before an irrigation occurred. This often reduced yields in the least watered 10% area.
The center pivot with LEPA nozzles resulted in an applied water depth 8% less than the center pivot with impact sprinklers. Net depletion with LEPA was 17% less than for impact sprinklers. The decrease in net depletion is a result of lower evaporation and the effects of basin tillage. The basins increased the effective rainfall by 26 mm for the season (less runoff).
Applied water depths for the Hord silt loam soil were 28% less for center pivot compared to furrow simulations. Return flow was considerable lower (64% lower) for center pivots than for furrow irrigations. Net depletion was similar between center pivot and furrow irrigations when conventional tillage was used. Results with Wood River silt loam were similar when comparing irrigation systems.
Applied water depths for continuous flow irrigation with blocked ends and continuous flow with runoff recovery were similar. Surge flow irrigation reduced the amount of applied water compared to continuous flow irrigation for Hord soil by 18% and for Wood River soil by 8%. Surge flow irrigation reduced return flow by 40% compared to continuous flow irrigation for the Hord soil and 25% for the Wood River soil.
Tillage Comparison. Ridge till systems consistently showed less net depletion than conventional tillage while grain yield was not affected by tillage practice. The average net depletion for ridge till was 24% less than for conventional tillage (Figure 2). Net depletion for the surge flow irrigation system with conventional tillage and Hord soil was 275 mm while for the same system with ridge till it was only 215 mm.
The water savings (lower net depletion) with ridge till is due to a combination of less rainfall runoff and less soil water evaporation.
Table 3 summarizes the water balance data for two tillage types for a Wood River soil and continuous flow irrigation with blocked ends.
The reduced evaporation is the result of the larger quantity of crop residue on the soil surface. The net affect of the residue and reduced runoff was one less irrigation event for the season and an increased opportunity for rainfall to be included as return flow, thus decreasing the net depletion.
Deficit Irrigation. Results for a limited or deficit irrigation season are given in Figures 3 and 4. Grain yield, applied water, net depletion and return flow for a limited irrigation season of 5 weeks are illustrated in Figure 3. The 5 week season is an approximate length of season while the actual length is controlled by accumulation of growing degree days as explained previously. The results for the 3.5 week limited season are shown in Figure 4.
The 5 week irrigation season resulted in little yield reduction for center pivot compared to irrigating for maximum yield. On a Hord silt loam, limiting the irrigation season to 5 weeks reduced the grain yield compared to the maximum yield by only 2% and for a Wood River soil by only 3%. The hydrologic impacts of the shorter irrigation season were more significant. Applied water was reduced by 19% compared to irrigating for maximum yield for both soil types. Net depletion was reduced by 13% and return flow by 50% compared to maximum yield conditions.
Limiting the irrigation season to 3.5 weeks decreased applied water depths further and had a more noticeable impact on grain yields for center pivot irrigation. On a Hord silt loam, the yield was reduced by 14% while on the Wood River soils the yields were reduced by 16% compared to the maximum yields. Applied water decreased by 39% with both Hord soil and Wood River soils. Also for both soils, net depletion was reduced by 33% and return flow by 60% compared to maximum yield conditions.
Results from furrow simulations with a limited 5 week season showed less than 1% yield reduction for both Hord and Wood River soils. Reduction in applied water was similar between furrow irrigation types and soils. An average reduction of 20% was observed. Net depletion was reduced an average of 5%. Return flow was reduced on average 36% for Hord soil for both tillage systems. On Wood River soils with conventional tillage, return flow was reduced by 65% and for ridge till it was reduced by 55%.
When furrow irrigations were limited to a 3.5 week season, large yield reductions occurred. For Wood River soils with conventional tillage and continuous flow irrigation, yields were reduced by 10%, and for surge flow they were reduced by 19%. With ridge tillage, yields were reduced by 8% for all irrigation types. The yield reductions were similar with Hord soils being 7% for the two types of tillage. The exception was continuous flow with runoff recovery using ridge till. Here the yield reduction was 14%. Net depletion for continuous flow with runoff recovery and ridge till on a Hord soil was reduced 37%. Net depletion for surge flow using conventional tillage on a Wood River was reduced 34%. Net depletion results for all other furrow simulations showed an average reduction of 20%. The higher reduction in net depletion for the two unique cases explain the greater yield reduction for these cases. Return flow was reduced an average of 60% for the Hord soils and 75% for the Wood River soils.
Dryland management resulted in a grain yield for corn (2750 GDD) of 4625 kg/ha for the Hord silt loam soil and 4200 kg/ha for the Wood River silt loam.
Cropping systems. The results for the four crops: corn (2750 GDD), corn (2600 GDD), grain sorghum, and soybeans, are shown in Figures 5-7. These results are for center pivot with impact sprinklers and furrow irrigation with surge flow using conventional tillage and the different irrigation seasons.
For the Hord soil, center pivot with impact sprinklers, and conventional tillage, the yield, water applied, and net depletion was reduced 5%, 6%, and 9%, respectively, for the 2600 GDD corn compared to the 2750 GDD corn when irrigating for maximum production. The maximum production grain sorghum and soybeans had net depletions of 36% and 16%, respectively, less than 2750 GDD corn.
Compared to irrigating for maximum yields, when the irrigation season was limited to 5 weeks, the yield and net depletion were reduced by 1% and 6% respectively, for grain sorghum. The 3.5 week irrigation season resulted in grain sorghum yield and net depletion reductions of 5% and 21%, respectively.
For soybeans, yield and net depletion were reduced by 1% and 3%, respectively, for the 5 week growing season and 8% and 25%, respectively, for the 3.5 week growing season compared to irrigating for maximum yield.
Dryland yields for the three additional crops was 4775 kg/ha for corn (2600 GDD), 4300 kg/ha for grain sorghum, and 2150 kg/ha for soybeans.
CONCLUSIONS(Irrigating for maximum yield)
When irrigating for maximum grain yield for 2750 GDD corn, center pivot irrigation requires 28% less applied water than furrow irrigation for a Hord silt loam soil, but for a Wood River silt loam soil, applied water depths were similar between systems.
Net depletion is reduced when LEPA irrigation is used compared to impact sprinklers.
Return flow was 64% less when using a center pivot verses a furrow irrigation system with Hord silt loam soils while little difference was observed between irrigation systems with Wood River soils.
Surge flow when compared to continuous flow irrigation, reduced return flow by 40% for Hord soils and 25% with Wood River soils.
(Tillage comparison)
Ridge till reduced net depletion by 25% when compared to conventional tillage using furrow irrigation. The reduction was a result of increased effective rain and reduced soil water evaporation.
(Limited irrigation season - 5 week)
Limiting an irrigation season to 5 weeks resulted in only a 1-3% corn yield reduction while applied water was reduced by 20% for all soil, tillage, and irrigation combinations.
Net depletion for center pivots was reduced by 13% and for furrow irrigation, by 5% when the season was limited to 5 weeks.
Return flow was reduced 36 - 65% as a result of the limited irrigation season.
(Limited irrigation season - 3.5 weeks)
Larger yield reductions occurred when the irrigation season was limited to 3.5 weeks. Center pivot corn yields were reduced by 15% while furrow irrigation showed a reduction ranging from 7-19%.
Net depletion was reduced by 33% for center pivots and 25% for furrow irrigation.
Return flow was reduced by 60% with center pivots and 60% - 75% for furrow.
(Comparison of different crops)
Relatively small water savings were observed by switching to a shorter season corn (2600 GDD) or soybeans from a 2750 GDD corn.
Grain sorghum required 26% less applied water and had 30% less net depletion compared to other crops.
REFERENCES
Return to 1996 Platte River Basin Ecosystem Symposium
Last updated by Darren A. Jack on 3/17/97