Water conservation practices for a river valley irrigated with groundwater1

Agricultural Water Management 38 (1999) 235±256

Water conservation practices for a river valley irrigated with groundwater1 Alan L. Boldt, Dean E. Eisenhauer*, Derrel L. Martin, Gary J. Wilmes Department of Biological Systems Engineering, University of Nebraska, Lincoln, NE 68583-0726, USA Accepted 17 July 1998

Abstract Water 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 (gross irrigation minus return flow) 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 (irrigation 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/year 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 wellmanaged pivots but high for some furrow irrigation systems based on the assumption that all deep percolation returns to the aquifer in the Central Platte Valley. Net depletion (gross irrigation minus return flow) of the groundwater for a center pivot with LEPA was 50 mm (17%) less than a center pivot with impact sprinklers. Ridge till had a net depletion 50 mm (25%) less than conventional tillage (double disk, plant) for furrow systems. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Water conservation; Computer simulation; Sprinkler irrigation; Furrow irrigation; Conservation tillage; Limited irrigation; Cropping systems

* Corresponding author. Tel.: +1-402-472-1637; fax: +1-402-472-6338; e-mail: [email protected] 1 Published as Paper No.11638, Journal Series Nebraska Agricultural Research Division. 0378-3774/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 9 8 ) 0 0 0 6 5 - 1

236

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

1. Introduction Changing economic and environmental concerns in irrigated agriculture have 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 the Central Platte Natural Resources District (Ron Bishop, Central Platte Natural Resources District, personal communication, 1992). Corn is the dominant crop and is irrigated for maximum yield. One concern in the area is the lowering of groundwater levels in a region north of 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 area of interest is shown in Fig. 1. The term `water conservation' has many meanings. Physical or hydrological definitions focus on potential water savings while behavioral definitions focus on using water more economically (CAST, 1988). In this paper, we concentrate on the hydrologic definition of water conservation and focus on the water balance of either a field or larger region such as a watershed or river basin. For the hydrologic system to be in balance, water inputs to a region must equal the water outputs (Fig. 2). Inputs include precipitation, available soil moisture at the beginning of a season, 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. When applied water is reduced one or more of the outputs will be concomitantly reduced, e.g., reduced ET. ET can be reduced by reducing one of its components, evaporation or transpiration. Crop yields usually are not affected when soil water evaporation is reduced. When

Fig. 1. Location of Central Platte Natural Resources District, Nebraska, USA.

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

237

Fig. 2. Hydrologic components in a river valley irrigated with groundwater.

evaporation is reduced, the potential for groundwater recharge and/or 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 viewed as a water conserving practice. Transpiration can be reduced by purposely limiting the available soil water so that plant stress occurs. With this practice, termed deficit irrigation, crop yield is reduced. An alternative method for reducing transpiration is to produce crops that require less water. In central Nebraska, alternative crops that have a lower ET than corn include grain sorghum and soybeans. 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 (Ron Bishop, Central Platte Natural Resources District, personal communication). To avoid runoff losses from a field, 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 during irrigation is assumed to be negligible with furrow irrigation. With sprinkler irrigation, evaporation losses can be a 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,

238

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

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. The groundwater in the Central Platte Valley is relatively low in salts and the area has a sub-humid climate, thus, 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 (Steele and Wigley, 1994), the Central Platte Natural Resources District (CPNRD), in cooperation with US Bureau of Reclamation, has proposed a surface water development/groundwater recharge project to augment the water supply within their district. 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 objectives of the evaluation were to: 1. Compare applied water, net depletion, and return flow for sprinkler and furrow irrigation systems producing maximum attainable grain yield for field corn in the Central Platte valley of Nebraska. 2. Determine the amount of water that is conserved by ridge till vs. conventional tillage (double disking, plant) when using a furrow irrigation system. 3. Determine the water conserved through deficit irrigation and the associated corn grain yield loss. 4. Evaluate applied water, net depletion, return flow, and grain yield for alternative cropping systems for both maximum production and deficit irrigation. 2. Methods Computer simulation models were used with a 29-year weather data base to evaluate several combinations of irrigation and tillage practices. The performances of a center pivot system with impact sprinklers and a center pivot with low energy precision application (LEPA) nozzles were simulated. With LEPA irrigation, water is placed below the crop canopy, near the soil surface, and 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 furrow simulations were performed for both conventional and ridge till practices. Conventional tillage in the Central Platte valley of Nebraska usually includes

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

239

two diskings before planting. Pivot simulations were only performed for conventional tillage practices. Simulations were performed for two soil series, Hord silt loam (fine-silty, mixed, mesic, Pachic Haplustolls) and Wood River silt loam (fine, montmorillonitic, mesic, Typic Argiustolls). These soils were chosen because of their relatively frequent occurrence in the Platte Valley plus they represent a large range of soil infiltration rates. The Hord series consists of deep well-drained, medium-textured soils with moderate permeability and high available water capacity (SCS, 1974). The Wood River series consists of deep, moderately well-drained soils that have a claypan. Permeability is slow and available water capacity is high (SCS, 1974). 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 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, discussed in the next section, was defined by the accumulation of growing degree days (GDD). Also in the crop growth model, four crops were simulated: a full season corn (1530 GDD for maturity), a short season corn (1440 GDD for maturity), soybeans, and grain sorghum. Growing degree days are expressed in 8C and calculated with a base temperature of 108C. The discussion of inputs and procedures refers to the corn with a 1530 GDD requirement. A short discussion of the other three crops is given in the results section. The crop model is one-dimensional ± it simulates a point in space. To account for the non-uniform infiltration 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. 2.1. Crop growth model The crop model estimates crop water use throughout the growing season. Evapotranspiration (ET) was calculated based on the reference crop evapotranspiration (ETr). ET was separated into potential evaporation and potential transpiration. Potential evaporation represents the maximum evaporation rate when the soil is wet. Potential transpiration represents the maximum transpiration rate when water is non-limiting to the crop. Basal crop coefficients were used to separate ET into the evaporation and transpiration components. The yield effect of plant stress is described through a linear yield±transpiration relationship (Martin et al., 1984). The relationship used in the model is defined by    T (1) Y ˆ Ym …1 ÿ b† ‡ b Tm where: Y is the Yield, Ym is the maximum attainable yield, T represents transpiration, Tm represents transpiration required for maximum attainable yield, and b is the empirical yield coefficient determined from water stress experiments conducted in Nebraska and surrounding states.

240

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

Initially, a season with optimum irrigation amounts for maximum yield was simulated for a center pivot system. This provided an estimate of the maximum ET (ETm) required for the maximum attainable yield (Ym) for the region. The maximum yield was estimated using University of Nebraska crop variety trials in central Nebraska and information from Pioneer Hybrids International. To impose deficit irrigation as a management practice, two limited irrigation seasons were placed on the crop. The yields for the limited irrigation simulations were calculated based on the simulated ET and T, the yield coefficient, b, and Eq. (1). The estimated maximum attainable grain yield for 1530 GDD corn was 11 900 kg/ha (at 15.5% moisture content) and the yield coefficient b used for corn was 1.30. Climatological data used in the simulation model were compiled from a National Climatic Data Center, US Department of Commerce (NCDC, 1963±1991). The climatological databases contain daily pan evaporation, temperature, and rainfall data collected at the Grand Island Weather Service Office from 1963 through 1991. Reference crop evapotranspiration was predicted using a pan coefficient calibrated using regression of locally collected data. The calibration equation is ETr ˆ 0:75 ETpan

(2)

where: ETr represents alfalfa reference crop ET and ETpan is the pan evaporation. The period simulated started on 1 March and ended each year on 30 November. Thus, the simulations included non-growing season precipitation and evaporation in the spring and fall but did not include water balance calculations when the soils are typically frozen. Predicting crop yield and return flow requires an accurate accounting of the amount of water that enters and what is retained in the crop root zone. The SCS curve number method was used to estimate precipitation runoff (SCS, 1972). Wood River silt loam is classified as being in Hydrologic Soil Group (HSG) C and Hord silt loam is classified as being in HSG B. The runoff curve number was reduced by 6% for ridge till planting as compared to conventional tillage and planting practices (Onstad and Otterby, 1979). The effect of basin tillage, used with LEPA nozzles, was accounted for by increasing the amount of surface detention by 15 mm. Drainage from the crop root zone was determined by the water holding characteristics of the soil, the root zone depth, the amount and distribution of infiltrated water, and the amount of water stored in the soil at the time of irrigation or rainfall. Information describing the Wood River and Hord silt loam soils were obtained from SCS (SCS, 1983). Table 1 contains the field capacities and permanent wilting points that we used for the Hord and Wood River soils. The beginning soil water depletions used in the model for 1 March of each year were equal to the ending depletions on 30 November of the previous year. A full profile was assumed for 1 March of the first simulation year, 1963. The maximum root zone depth was assumed to be 120 cm for maximum grain yield, 150 cm for the limited irrigation seasons, and 180 cm for dryland. Tillage effects on evaporation were accounted for by reducing evaporation in proportion to the fraction of the soil surface covered by crop residues. The approximate residue mass per unit area for corn after harvest for each irrigation season was: dryland, 5050 kg/ha; limited irrigation season, 9400 kg/ha; and irrigating for maximum yield, 11950 kg/ha. Each tillage operation reduces the crop residue. The fraction of after-harvest

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

241

Table 1 Field capacity and permanent wilting point volumetric water contents for the two soils used for simulationa Depth

Wood River silt loam Field capacity

(cm)

(cm3/cm3)

0±30 30±105 105±180

0.38 0.38 0.38

a

Hord silt loam Permanent wilting point

Field capacity

Permanent wilting point

0.17 0.24 0.21

0.38 0.35 0.35

0.17 0.21 0.21

(SCS, 1983).

Table 2 Fraction of after-harvest residue remaining on the surface following tillage for two tillage systemsa Tillage type

Fraction of after-harvest residue After harvestb (1 October)

Conventional Ridge till a b

0.5 1.0

Before spring tillage (1 April)

After spring tillage (1 April)

After planting

After furrowing

(1 May)

(25 June)

0.4 0.8

0.15 0.8

0.05 0.4

0.05 0.25

Based on data presented in Rawls et al. (1980) and MidWest Plan Service (1992). Assumes medium disking soon after harvest.

residue remaining on the surface following tillage for conventional and ridge till is shown in Table 2. The approximate dates of each tillage operation are also listed in Table 2. Timing of irrigations was controlled by the capacity of the irrigation system and the length of the allowable irrigation season. For maximum yield, irrigations were scheduled when the soil water depletion reached 50% of available water in the root zone. For the center pivot systems, the gross irrigation depth applied with each irrigation was 33 mm. The depth applied with the furrow systems varied depending on irrigation system and infiltration rate with the latter being influenced by tillage practice. The length of each limited irrigation season was defined by the accumulation of growing degree days (GDD). The first limited season started when 560 GDD had accumulated and ended when 1220 GDD had accumulated. For corn, this is approximately from vegetative stage 15 to beginning dent (Ritchie et al., 1986). On average, this is a 5-week irrigation season. The second limited season started at 720 GDD and ended at 1110 GDD growth stage (R1±R4) and is approximately a 3.5-week irrigation season. Discussion of results from the two limited seasons are referenced by 5week season and 3.5-week season. The fully irrigated season (maximum yield) ranged from 8 to 10 weeks in length. For the limited irrigation seasons the 50% allowable depletion criteria was still used during the irrigation period. However, in dry years, soils for the limited irrigation season treatment could be drier than a 50% depletion before the first irrigation occurred or after the last irrigation ended. Grain sorghum and soybeans were irrigated when 50% of the available soil water had been depleted.

242

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

The application efficiency for center pivot irrigation was assumed to be 85% with impact sprinklers (Schneider and Howell, 1995) and 92% with LEPA (Lyle and Bordovsky, 1981). We assumed that 8% of the applied water was lost to drift and evaporation with impact sprinklers and 1% with LEPA. Runoff losses were assumed to be 7% of the applied water for both impact sprinklers and LEPA. Because of the mildly sloping topography and the medium or high infiltration rate soils in the region, it was assumed that the runoff eventually infiltrates the soil and percolates relatively rapidly to the groundwater either within the field or near the field boundary. 2.2. Furrow irrigation model In a furrow irrigated field, infiltration varies from the water inlet end to the downstream end of the field. This causes variation in the grain yield and the amount of return flow. A representative yield and deep percolation response to the varied irrigation and tillage practices was determined using 10 points along the furrow. SIRMOD was used to estimate infiltration at these 10 points along the furrow. Mean yield and deep percolation were determined by integrating results from the 10 points. A field length of 400 m and a slope of 0.2% was used in the simulations. The crop row spacing was 91 cm and every furrow was irrigated. 2.2.1. Irrigation description The blocked end system prevents water from running off of the field. The dike causes a pond at the downstream end of the field which often increases the potential for return flow caused by deep percolation beneath the pond (Cahoon et al., 1995). Furrow inflows must be managed so that the volume of ponded water and deep percolation is not excessive. An alternative is to use a runoff recovery system. With runoff recovery, a larger furrow inflow rate is acceptable because a high proportion of the runoff water will be recycled. The higher inflow rate leads to faster water advance and more uniform distribution of infiltration. Both a blocked end system and a system with runoff recovery were simulated for a continuous flow irrigation (conventional furrow irrigation). For surge flow irrigation, where water is applied intermittently, only a runoff recovery system was simulated. SIRMOD output includes the volumes of water that was applied (Va) and that ran off (Vr). Given an efficiency for a recovery system (return ratio, Rt), and a runoff ratio, Rr (Vr/Va), the amount of water that must be supplied to the field (Vm) is Vm ˆ Va …1 ÿ Rr Rt †

(3)

The water that is reintroduced into the system from the runoff recovery system is in lieu of pumped groundwater. An 85% efficiency (Rtˆ0.85) for the recovery system was assumed. It was assumed that the remaining 15% of the runoff returned to the groundwater through seepage from the return reservoir and runoff ditches. The total time water was applied to a furrow (the irrigation inflow time) for continuous irrigation was 720 min. During a surge flow irrigation, water is still applied for 12 h but is applied onto two irrigation sets (left and right side of the surge valve), thus a furrow only has water applied for 6 h total.

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

243

Table 3 Furrow inflow rates for furrow simulations Soil type

Tillage

Season

Irrigation event

Continuous irrigation

Surge irrigation

Blocked end

Runoff recovery

Runoff recovery

Furrow inflow, (l/s) Hord

Wood River

Conventional

Maximum yield

Ridge

Limited Maximum yield

Conventional

Limit Maximum yield

Ridge

Limit Full Limit

First Later First First Later First First Later First First Later First

1.5 1.1 1.8 1.9 1.4 2.3 0.8 0.6 0.9 1.0 0.7 1.2

1.9 1.3 2.3 2.3 1.6 2.7 1.1 0.7 1.3 1.3 0.9 1.6

2.3 1.5 2.7 2.7 1.8 3.2 1.5 0.9 1.8 1.8 1.1 2.2

A fixed cutoff ratio, defined below, was used for each irrigation system Cutoff ratio ˆ

Advance inflow time to end of furrow Total inflow time of irrigation

(4)

Advance inflow time is the inflow time required for water to advance across the field. Iterative computer simulations were needed to find the inflow rate that resulted in the desired cutoff ratio for the fixed total inflow times. Based on the work by Cahoon et al. (1995), the cutoff ratio was set at 0.75 for the blocked end case minimizing the amount of return flow. Systems with a runoff recovery system used a cutoff ratio of 0.5. Inflow rates were limited to the maximum allowable non-erosive stream which is 3.15 l/s for a furrow with a slope of 0.2% (SCS, 1984). Inflow rates and advance on-times used in the furrow simulations are listed in Table 3. 2.2.2. Infiltration assumptions Cumulative infiltration is calculated in SIRMOD by using the empirical Kostiakov equation. The Kostiakov equation is given by Z ˆ ktoa

(5)

where: Z is the cumulative infiltration per unit of furrow length (m3/m), to represents infiltration opportunity time (min), k and a are empirical parameters. The infiltration parameters were based on data collected at three field sites in Central Platte Valley. The authors collected data at two of the sites and data for the third site was from Ostermeier (1992). The volume balance method (ASAE, 1995) was used to determine the parameters. The resulting infiltration functions compared well with the Soil Conservation Service Infiltration Family (SCS, 1983) for each soil.

244

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

The crop growth model required data for each irrigation event of the season. Typically, infiltration decreases significantly from the first irrigation to the second irrigation. Generally after the second irrigation there is less change (Eisenhauer et al., 1984). Two groups of infiltration parameters were used to define an irrigation season, one for the first irrigation and one for subsequent irrigations. Cumulative infiltration after 6 h of wetting decreased approximately 33% for later irrigations as compared with a first irrigation for both soils. The infiltration parameters were for an irrigation season where maximum yield was desired. When the irrigation is delayed, as is the case for deficit irrigation, infiltration is greater because the increased soil moisture deficit results in a larger hydraulic gradient and perhaps soil cracking. Infiltration for a silt loam soil was increased by 20% when the irrigation was delayed by approximately 2 weeks (Enciso-Medina, 1991). To account for this, the `k' in the Kostiakov equation was increased by 20%. The original infiltration parameters were based on conventional tillage (two diskings, plant). Ridge till increases the amount of residue on the furrow surface and increases the infiltration rate. The combination of increased hydraulic roughness and increased infiltration causes a slower water advance. On average, ridge till will increase the infiltration by 22% for both first and later irrigation events (Eisenhauer et al., 1984). The maximum adjustment to infiltration was for a delayed first irrigation using a ridge till system. The combined adjustment increased the infiltration by 46%. Again the Kostiakov coefficient `k' was adjusted to account for the change in infiltration. Parameters for surge irrigation were found by adjusting the continuous flow parameters using the cycle ratio±time model developed by Blair and Smerdon (1988). The empirical parameters for the Kostiakov equation for each simulation are listed in Table 4. Fig. 3 illustrates the infiltration depths for the various irrigation and tillage combinations. 3. Results 3.1. Irrigating for maximum grain yield Simulation results for grain yield, applied water, net depletion, and return flow for center pivot and furrow irrigation on Hord and Wood River silt loam soils are shown in Fig. 4. All simulations, with the exception of continuous flow irrigation with runoff recovery, showed yields at the maximum yield of 11 900 kg/ha for 1530 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% soil water 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

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

245

Table 4 Kostiakov parameters for continuous and surge irrigation Soil

Hord

Wood river

Tillage

Season

Conventional

Full

Ridge till

Limit Full

Conventional

Limit Full

Ridge till

Limit Full Limit

Irrigation event

First Later First First Later First First Later First First Later First

Continuous irrigation

Surge irrigation

Kostiakov parameters

Kostiakov parameters

ac

kc (m3/m mina)

as

ks (m3/m mina)

0.57 0.71 0.57 0.57 0.71 0.57 0.35 0.50 0.35 0.35 0.50 0.35

0.0043 0.0012 0.0052 0.0053 0.0015 0.0063 0.0083 0.0023 0.0100 0.0102 0.0028 0.0123

0.49 0.65 0.49 0.49 0.65 0.49 0.27 0.42 0.27 0.27 0.42 0.27

0.0055 0.0015 0.0066 0.0067 0.0018 0.0081 0.0108 0.0030 0.0130 0.0132 0.0037 0.0160

ac and as ± Kostiakov `a's' for continuous and surge, respectively. kc and ks ± Kostiakov `k's' for continuous and surge, respectively.

effects of the basin tillage. The basins increased the effective rainfall by 26 mm for the season because of less runoff. Applied water depths on the Hord silt loam soil were 28% less for center pivot compared to furrow irrigation. Return flow was considerably lower (64% lower) for center pivots than for furrow irrigation. Net depletion was similar between center pivot and furrow irrigation when conventional tillage was used. The results were somewhat different with the Wood River soil where the applied water depths and return flows were nearly equal for center pivot irrigation and furrow irrigation. The different response with the Wood River soil was due to its lower infiltration rate (Fig. 3). Infiltration from irrigation rarely exceeded the soil moisture deficit in the low quarter of the field with the Wood River soil. This was not always the case for the Hord soil. 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. Net depletion was not reduced with surge irrigation even though water applications were reduced ± a result of the assumption that deep percolation returns to the aquifer. 3.2. 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 (Fig. 4). Net depletion for the surge flow irrigation system with conventional tillage and Hord soil was 275 mm while for the same

246

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

Fig. 3. Furrow irrigation infiltration for various irrigation events for Hord and Wood River silt loam soils and conventional tillage and ridge till systems.

system with ridge till it was only 215 mm. The water savings (lower net depletion) with ridge till is caused by a combination of less rainfall runoff and less soil water evaporation due to increased residue cover. The net affect of the reduced evaporation and runoff was an increased proportion of rainfall that went to return flow, thus decreasing the net depletion. Table 5 summarizes the water balance data for two tillage types and continuous flow irrigation with blocked ends. 3.3. Deficit irrigation Grain yield, applied water, net depletion, and return flow for a limited irrigation season of 5 weeks are illustrated in Fig. 5. The results for the 3.5-week limited season are shown in Fig. 6. 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 but had a more noticeable impact on grain yields for center pivot irrigation. On a Hord silt

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

247

Fig. 4. Simulation results for grain yield, applied water, net depletion, and return flow when irrigating for maximum yield with 1530 GDD corn for center pivot and furrow irrigation on Hord and Wood River silt loam soils.

503a (114)b 516 (122) 13

(mm)

Effective rainfall

368 (94) 350 (109) ÿ18

490 (112) 523 (132) 33

Gross irrigation or applied water

ETr: Reference crop evapotranspiration. ET: Crop evapotranspiration. a Values are the average of 29 years. b Standard deviation of 29 years. Total rainfallˆ544 mm. Grain yield ± both tillage typesˆ11 926 kg/ha.

Wood river silt loam soil Conventional 485 (109) Ridge till 505 (117) Difference 20

Hord silt loam soil Conventional Ridge tillage Difference

Tillage system

330 (81) 297 (94) ÿ33

305 (81) 274 (81) ÿ31

Net irrigation

5 (1) 4 (1) ÿ1

3 (1) 3 (1) 0

Number of irrigation

371 (94) 353 (109) ÿ18

493 (114) 526 (132) 33

Depth of infiltration

81 (43) 130 (61) 49

216 (56) 318 (86) 102

Return flow

287 (91) 220 (105) ÿ66

274 (98) 205 (103) ÿ69

Net depletion

1372 (132) 1372 (132) 0

1372 (132) 1372 (132) 0

ETr

231 (38) 191 (28) ÿ40

234 (38) 191 (30) ÿ43

Evaporation from soil

780 (58) 732 (53) ÿ48

780 (56) 729 (48) ÿ51

ET

Table 5 Water balance data for conventional tillage and ridge till systems for a Hord and Wood river soil series, continuous flow irrigation with blocked ends, and irrigating for maximum yielda

248 A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

249

Fig. 5. Simulation results for grain yield, applied water, net depletion, and return flow when the irrigation season was limited to 5 weeks with 1530 GDD corn for center pivot and furrow irrigation on Hord and Wood River silt loam soils.

250

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

Fig. 6. Simulation results for grain yield, applied water, net depletion, and return flow when the irrigation season was limited to 3.5 weeks with 1530 GDD corn for center pivot and furrow irrigation on Hord and Wood River silt loam soils.

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

251

loam, the yield was reduced by 14% while on the Wood River soils the yields were reduced 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 irrigation 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 simulated. Net depletion was reduced on average of 5%. Return flow was reduced on average 36% for the Hord soil for both tillage systems. On Wood River soils, return flow was reduced by 65% and by 55% for conventional tillage and ridge till, respectively. 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, 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 (1530 GDD) of 4625 kg/ha for the Hord silt loam soil and 4200 kg/ha for the Wood River silt loam. These yields are 39% and 35% of the maximum attainable yield. 3.4. Cropping systems The results for the four crops: corn (1530 GDD), corn (1440 GDD), grain sorghum, and soybeans, are shown in Figs. 7±9. These results are for center pivot with impact sprinklers and furrow irrigation with surge flow using conventional tillage and the different irrigation seasons. When irrigating the Hord soil for maximum production with a center pivot with impact sprinklers and conventional tillage, the yield, water applied, and net depletion was reduced 5%, 6%, and 9%, respectively, when comparing 1440 GDD corn to the 1530 GDD corn. Compared to 1530 GDD corn, the net depletions were 36% and 16% less for maximum yield grain sorghum and soybeans, respectively. When the irrigation season was limited to 5 weeks, the yield and net depletions were reduced by 1% and 6%, respectively, for grain sorghum, compared to irrigating for maximum yields. 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 depletions 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.

252

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

Fig. 7. Simulation results for grain yield, applied water, net depletion, and return flow when irrigating for maximum yield with four cropping systems for center pivot with impact sprinklers and surge flow irrigation on Hord and Wood River silt loam soils.

Dryland yields for the three additional crops were 4775 kg/ha for 1440 GDD corn, 4300 kg/ha for grain sorghum, and 2150 kg/ha for soybeans. 4. Discussion Usually the goal of implementing a water conservation practice is to `save' water. Does saving water mean reducing water application or reducing net depletion of the water resource? Our work shows that if the goal is to reduce net depletion, one approach is to reduce ET. Other approaches are to reduce nonrecoverable losses such as runoff to salt sinks, deep percolation to salt sinks or to perched water in silt and clay sediments, or when contamination makes the water unusable for other purposes. For the Central Platte Valley, we assumed that deep percolation losses are recoverable and that small amounts of runoff from irrigation are recoverable, such as from the center pivot. We assumed that rainfall runoff is not recoverable. Obviously, rainfall runoff is not necessarily a `loss' from a watershed or river basin because it is available to the water users in the lower part of the basin. The water conservation benefits of ridge tillage and of center pivot LEPA irrigation, i.e., less net depletion, came as a result of lower evaporation and lower rainfall

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

253

Fig. 8. Simulation results for grain yield, applied water, net depletion, and return flow when the irrigation season was limited to 5 weeks with four cropping systems for center pivot with impact sprinklers and surge flow irrigation on Hord and Wood River silt loam soils.

runoff. In the case of LEPA, evaporation was reduced during water application. For ridge tillage, soil water evaporation was reduced. Reducing evaporation saves `wet' water because it reduces the transfer of liquid water to vapor. The rainfall runoff `losses' that were reduced by LEPA (a result of the micro-basins being in place year round) and by ridge tillage could have been recovered downstream of the field or region. The water is not lost to the river basin per se. This points out the importance of specifying the system boundary when choosing water conservation practices. Practices that `save' water at the field level do not necessarily conserve or save water on a watershed or basin scale. The reduction in rainfall runoff with LEPA came as a result of having micro-basins in place year round. Most of the 26 mm/year that were saved were saved during May and June. Having micro-basins in place at that time may not be practical or feasible due to the other field operations, e.g., planting and cultivating, that takes place during that time frame. This research also shows that using irrigation systems that are more efficient, i.e., systems where less water application is required to produce the desired crop yield, does not necessarily reduce water supply depletion. For example, it is clear that center pivot irrigation reduces water applications because of higher application efficiencies as compared to furrow irrigation. Even so, net depletions were not reduced by implementing

254

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

Fig. 9. Simulation results for grain yield, applied water, net depletion, and return flow when the irrigation season is limited to 3.5 weeks with four cropping systems for center pivot with impact sprinklers and surge flow irrigation on Hord and Wood River silt loam soils.

this practice because of the assumption that deep percolation losses from less efficient systems, for example, furrow irrigation, were recoverable within the region. Another example of the irrigation efficiency dilemma is the ridge-till furrow irrigation combination. Here the irrigation efficiency actually decreased (more water application) when using the water conservation practice even though net groundwater depletion was reduced. The other benefits of increased irrigation efficiency are important to this discussion. In groundwater irrigated regions, energy is required to pump water. Reducing water applications obviously has the benefit of reducing energy requirements. An additional benefit is reduced irrigation labor requirement with lower water application. Further, an important benefit of implementing some water conservation practices, such as more efficient irrigation and deficit irrigation, is the reduction in return flow caused by deep percolation. Water quality degradation caused by nitrate±nitrogen is a problem in the Central Platte Valley of Nebraska (Exner and Spalding, 1990). The degradation is the result of the leaching of nitrate±nitrogen below the crop root zone. Practices that reduce deep percolation potentially reduce the risk of contamination. The use of center pivots, surge irrigation on the Hord soil, deficit irrigation, and changing crops all showed significant potential for reducing return flow due to deep percolation.

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

255

Finally, the potential impacts of implementing deficit irrigation as both a water conserving practice and a practice for reducing contamination from return flow is evident from this work. Clearly, shortened irrigation seasons, such as the 5-week irrigation season, that reduce water applications, return flow, and net depletion, have potential for significant environmental benefit with relatively small reductions in production. 5. Conclusions When comparing management alternatives with conventional practice, i.e., irrigating conventionally tilled 1530 GDD corn for maximum yield with continuous flow blocked end furrow irrigation, we draw the following conclusions: 5.1. Impacts of irrigation systems and ridge tillage  Center pivot LEPA irrigation reduced water application.  Center pivot sprinkler irrigation reduced water applications only with the Hord soil.  Water applications increased with the ridge till planting system on the higher infiltration Hord soil.  Water application decreased with ridge till on the lower infiltration Wood River soil.  Return flow was reduced by center pivot irrigation.  Surge flow irrigation reduced return flow.  Return flow was increased by using ridge tillage except where used in combination with surge flow irrigation on the Wood River soil.  Net depletion was reduced by center pivot LEPA irrigation and by ridge till.  Grain yields were not significantly influenced by irrigation system or tillage system. When comparing to irrigating a full season corn for maximum production, we conclude the following: 5.2. Impacts of deficit irrigation and cropping systems  Limiting the corn irrigation season to 5 weeks significantly reduced water application, return flow, and net depletion, while only slightly reducing grain yields  Limiting the corn irrigation season to 3.5 weeks significantly reduced water application, return flow, net depletion, and grain yield  Relatively small water savings resulted with shorter season corn or with soybeans  Net depletions were ordered from highest to lowest for the following cropping systems regardless of the irrigation strategy employed: 1530 GDD corn, 1440 GDD corn, soybeans, grain sorghum Acknowledgements Partial funding for this project was from Central Platte Natural Resource District, Grand Island, Nebraska and the Nebraska MSEA Project.

256

A.L. Boldt et al. / Agricultural Water Management 38 (1999) 235±256

References ASAE, 1995. Evaluation of irrigation furrows. ASAE EP419.1. ASAE Standards 1995, American Society of Agricultural Engineers, St. Joseph, Michigan, pp. 737±742. Blair, A.W., Smerdon, E.T., 1988. Modeling surge irrigation infiltration. J. Irrig. Drainage Eng. ASCE 104(1), 35±41. Cahoon, J.E., Mandel, P., Eisenhauer, D.E., 1995. Management recommendations for sloping blocked-end furrow irrigation. Appl. Eng. Agric. 11(4), 527±533. CAST (Council for Agricultural Science and Technology), 1988. Effective Use of Water in Irrigated Agriculture. Report No. 113. Eisenhauer, D.E., Frank, K.D., Dickey, E.C., Fischbach, P.E., Wilhelm, W., 1984. Tillage practice effects on water conservation and the efficiency and management of surface irrigation systems ± Project completion report. University of Nebraska, Lincoln, IANR (September). Enciso-Medina, J., 1991. An infiltration model for managing furrow irrigation with limited water supplies. Ph.D. Thesis, University of Nebraska, Lincoln. Exner, M.E., Spalding, R.F., 1990. Occurrence of pesticides and nitrate in Nebraska's groundwater, 1990. WC-1, Water Center, University of Nebraska, Lincoln. Lyle, W.M., Bordovsky, J.P., 1981. Low energy precision application irrigation system. Trans. ASAE 24(5), 1241±1245. Martin, D.L., Watts, D.G., Gilley, J.R., 1984. Model and production function for irrigation management. J. Irrig. Drainage Eng. 110(2), 149±164. MidWest Plan Service, 1992. Conservation Tillage Systems and Management. MWPS-45. Iowa State University, Ames, IA. NCDC (National Climatic Data Center), 1963±1991. Climatological Data, Nebraska, US Department of Commerce. Onstad, C.A., Otterby, M.A., 1979. Crop residue effects on runoff. J. Soil Water Conserv. 34(2), 94±96. Ostermeier, K.A., 1992. Verification and application of fertigation model for surge irrigation. M.S. Thesis, University of Nebraska, Lincoln. Rawls, W.J., Onstad, C.A., Richardson, H.H., 1980. Residue and tillage effects on SCS runoff curve numbers. Trans. ASAE 23(2), 357±361. Ritchie, S.W., Hanway, J.J., Benson, G.O., 1986. How a corn plant develops. Special Report No. 48. Iowa State University, Ames, IA. Schneider, A.D., Howell, T.A., 1995. Reducing sprinkler water losses. Proc. Central Plains Irrigation Short Course. Garden City, KS, 7±8 February, pp. 60±63. SCS (Soil Conservation Service), 1972. Hydrology. National Engineering Handbook, Section 4, US Department of Agriculture. SCS (Soil Conservation Service), 1974. Soil Survey of Buffalo County. Nebraska, US Department of Agriculture. SCS (Soil Conservation Service), 1983. Nebraska Irrigation Guide. US Department of Agriculture. SCS (Soil Conservation Service), 1984. Furrow Irrigation, National Engineering Handbook. Ch. 5, Section 15, US Department of Agriculture. Steele, G.V., Wigley, P.B., 1994. Groundwater level changes in Nebraska, 1992. Nebraska Water Supply Paper Number 72. Conservation and Survey Division, University of Nebraska, Lincoln. Utah State University, 1989. SIRMOD ± Surface irrigation simulation software. Irrig. Software Eng. Div., Utah State University (USU), Logan, Utah.