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Introduction This memorandum describes the approach and results obtained under Task 2A, Agricultural Use. The determination of potential crop evapotranspiration (ET) requires climate, acreage, and crop type information. The determination of actual crop consumptive use requires knowledge of actual water supply available to meet the crop ET demand. The following subtasks were performed to determine monthly crop consumptive use in the Bear River basin for the study period 1971 through 1998:
Several terms used in this memorandum have been broadly used in other studies. The definitions used in this study are consistent with the American Society of Civil Engineers Manuals and Reports on Engineering Practice No. 70. They are simply stated as follows:
Evapotranspiration The total amount of water that would be used for crop growth if provided with an ample water supply. Also called potential consumptive use. In general, agricultural consumptive use is presented based on the basin division outlined in the Bear River Compact. The upper division consists of the Bear River Basin and its tributaries in Wyoming upstream of Pixley Dam. This includes the area above Woodruff Narrows Reservoir plus the areas in Wyoming just downstream of Woodruff Narrows Reservoir and to Pixley Dam. The central division consists of the Bear River Basin and its tributaries in Wyoming from Pixley Dam to the Idaho border. The lower division boundary does not include portions of Wyoming. Approach and Results - Climate Data Collection The Bear River basin climate stations in the upper and central division were reviewed to determine the appropriate stations for use in the consumptive use analysis. There are three stations with long term records in Wyoming near the towns of Evanston, Sage, and Border. There are also two climate stations located just outside the basin in Afton and Kemmerer. Table 1 summarizes the climate station elevations in feet above mean sea level, periods of record, and missing average monthly temperature and total monthly precipitation values for the study period 1971 through 1999. Table 1
The Border gage was not used in any further evaluation because the station does not cover the entire study period. As shown in Figure 1, average monthly temperature does not vary significantly across the southwestern area of Wyoming. Figure 2 shows that, with the exception of the Afton climate station in the Snake River basin, total monthly precipitation also does not vary significantly across the southwestern area of Wyoming. The Evanston climate station was chosen to use in the consumptive use analysis for irrigated acreage in the upper division and the Sage climate station was chosen for the central division and lands in the upper division below Woodruff Narrows Reservoir for the following reasons:
Figure 1
Figure 2 To determine historic consumptive use estimates for the entire study period, it was necessary to fill in missing temperature and precipitation data. The closest climate stations to the Evanston station are Sage and Woodruff, Utah. Both of these stations are missing the same monthly measurements that the Evanston gage is missing. There are relatively few missing temperature and precipitation values for the Evanston climate station; therefore, both missing temperature and precipitation were filled with long-term monthly averages for the station. The Sage climate station is close in elevation and proximity to the Border station. The Border station and the Sage station have 472 overlapping average temperature values, and 43 of the 47 missing measurements at Sage are available at Border. As shown in Figure 3, there is an excellent correlation between the two stations, therefore, the Border station was used to estimate the missing monthly temperature values at Sage. Four of the missing temperature values at the Sage station did not overlap with the Border station. For these months, the average monthly value for the entire period of record was used to fill the gaps.
Figure 3 Total monthly precipitation at Sage station was compared to precipitation at the other four stations reviewed in the analysis. Clear relationships were not found. Therefore, missing precipitation values for the Sage station were filled with the long-term monthly average of the measured data. Approach and Results - Crop Consumptive Use Methodology Blaney-Criddle approaches to determining crop evapotranspiration (ET) are widely used due to limited climate data requirements. Blaney-Criddle methods require average monthly temperature and total monthly precipitation, whereas other methods may require daily parameters including temperature, precipitation, wind speed, vapor pressure, and solar radiation. One of the most widely used Blaney-Criddle approach is the Soil Conservation Service (SCS) method, published in Irrigation Water Requirements Technical Release No. 21 (TR-21) by the Soil Conservation Service, April 1967 revised September 1970. This methodology can be adapted to better represent known local conditions by calibrating the climatic coefficient (kt) and the crop coefficients (kc) so empirically calculated ET estimates represent measured ET. This procedure was followed in a cooperative project sponsored by the Bear River Commission in the published Duty of Water Under the Bear River Compact: Field Verification of Empirical Methods for Estimating Depletion - Research Report 125, January 31, 1989, Robert W. Hill, et al (Research Report 125). The SCS Modified Blaney-Criddle equation is as follows:
ET = kc x kt (t x p/100)where ET = monthly consumptive use, inches A local climatic coefficient (kt) was developed for Research Report 125 to represent the entire Bear River Basin region. Monthly crop coefficients (kc) were calibrated for Research Report 125 for several locations in the basin so that ET estimated using the SCS Modified Blaney-Criddle approach represented measured ET values in those areas. Calibrated crop coefficients developed near the climate stations of Evanston, Woodruff, and Border are believed to best represent our study area. Therefore, the crop coefficients for these three sites were averaged to represent an appropriate set of crop coefficients for the analysis. Research Report 125 provides an approach that is, presumably, acceptable to all three compact States, therefore appropriate to use in the Bear River Plan. Therefore, the SCS Blaney-Criddle method was used to estimate potential evapotranspiration with the following parameters put forth in Research Report 125:
In addition, irrigation water requirements were determined by estimating effective precipitation to be 80% of growing season rainfall, instead of using the SCS effective rainfall method. This was done to be consistent with the approach used in Research Report 125. Blaney-Criddle estimated ET, using the averaged calibrated crop coefficients from Research Report 125 and the crop coefficients recommended in TR-21, were determined for both alfalfa and irrigated meadow (pasture) using the Evanston climate station and the Sage climate station precipitation and temperature data. Many Blaney-Criddle models are available to easily calculate estimates using the SCS Blaney-Criddel aproach. For this study, a public domain model, developed by Leonard Rice Engineers staff for the State of Colorado, was used. The StateCU model is available of the Colorado CDSS Web Site at http://cdss.state.co.us. Figure 4 shows the average monthly ET for irrigated meadow for the period 1971 through 1990 at the Evanston climate station. Figure 5 shows the average monthly ET for alfalfa for the period 1971 through 1990 at the Sage climate station. For both crop types, the total annual ET estimates using local parameters are around 25 percent higher, with the increase seen in the spring and early summer months.
Figure 4
Figure 5 Table 2 shows Blaney-Criddle TR-21 crop coefficients and the averaged crop coefficients recommended for use in determining the historic crop consumptive use in the Bear River Basin, Wyoming. Crop coefficients are shown for both pasture and alfalfa. The growing season in the Bear River Basin in Wyoming generally extends from mid April through mid October. The differences shown in Table 2, TR-21 coefficients higher early and late in the growing season and calibrated coefficients higher during the summer months, are similar to calibrated coefficients in other higher elevation basins in the Western U.S. Table 2
Approach and Results - Irrigated Acreage Irrigated acreage estimates for the Bear River basin were developed for the WWDC during the 1998 feasibility study by States West Engineering. Table 3 shows the breakdown of irrigated acreage in the basin. Figure 6 shows the location of irrigated acreage in the basin. Table 3
Wyoming Agricultural Statistics, reported each year by county, were perused to determine how acreage has changed over the study period. County agricultural statistics generally report cash crops only, therefore are not a good indicator of actual acreage in the Bear River basin. A large majority of the acreage is irrigated meadow that is either used for grazing or hayed for on-farm use. However, information from local water users indicates that irrigated lands have been fairly consistent through the period 1971 through 1998. Since 1971, water rights for approximately 2,900 additional acres have been permitted in the basin, representing less than 5 percent of the total acreage. Based on this information, a constant acreage was used in the consumptive use analysis.
Figure 6 Approach and Results - Key Ditch Systems and Diversion Data Collection Although there are over 100 diversions for irrigation in the Bear River basin in Wyoming, approximately 75 percent of the irrigated acreage are served by 36 larger diversion structures. These 36 "key" ditch systems are modeled individually, both in the consumptive use analysis and in the surface water simulation model. The 36 ditch systems were chosen based on a cutoff minimum diversion capacity of 9 cfs, plus input from the local water administrators. In general, less than 9 cfs ditch systems were considered key if they are important to the compact or state administration, for instance if they have high priority water rights, or they receive supplemental water from one of the three larger reservoirs; Whitney, Sulphur Creek, or Woodruff Narrows. Table 4 shows the key ditch systems for the upper division in upstream to downstream order. All the key ditch systems in the upper division divert from the mainstem Bear River. In addition, the average annual diversions for the period 1971 through 1999 are shown. Table 4
Table 5 shows the key ditch systems for the central division in upstream to downstream order by source. In addition, the average annual diversions for the period 1971 through 1999 are shown. Table 5
Clarence Kemp walked the river and major tributaries with the local water administrators for both the upper and central division. The headgate location of the key ditch systems was determined, using a hand-held GPS unit, and photographs were taken documenting the condition of the structures. Detailed operating memoranda were developed for each of these systems, and are provided separate from this memorandum. Diversions are measured or observed throughout the irrigation season by the local water administrators, as required by the Bear River Compact. Complete daily diversion records in Wyoming were obtained for the key ditch systems from the Compact Administrator for 1971 through 1999; therefore no data filling was required. The most current years were provided in digital form. Paper copies were provided for the earlier years and digitized as part of this study. Because our study is being performed using a monthly consumptive use model, and a monthly river simulation model, only average monthly rates, in cfs, and total monthly volumes, in acre-feet, were digitized. Tables showing diversion records for the key ditch systems are provided with the operating memoranda described above. Approach and Results - Crop Type and Irrigation Practices Crop ET requirements and irrigation methods can vary by crop type. Interviews were held with the local water administrators and individual water users to determine crop types for the key ditch systems and to gain an understanding of local irrigation practices. As shown in Table 6, most of the acreage in the basin is irrigated pasture. Table 6
Most of the Upper Division applies surface water to satisfy crop ET requirements using flood irrigation techniques. The only exceptions in the Upper Division are the Cornelison Pump, which diverts water from Sulphur Creek to a center pivot sprinkler serving a quarter section (~160 acres), and the SP Ramsey ditch, which applies diversions through a sprinkler to approximately 80 acres. Sprinkler irrigation is more prevalent in the Central Division. Hand line, side-roll and center pivot sprinkler systems are used. Table 7 shows the percent of lands flood irrigated versus sprinkler irrigated for the major ditches in the Central Division. Table 7
Irrigation in both the Upper and Central Divisions generally begins in mid-May and continues into September. In dry years, irrigation in the central division is more likely to end earlier than irrigation in the upper division; the upper division has access to storage in Whitney Reservoir, Sulphur Creek Reservoir, and Woodruff Narrows Reservoir. There are no major storage reservoirs in the central division in Wyoming. Approach and Results - Irrigated Land/Diversion Association In order to determine the actual consumptive use in a basin, it is necessary to associate water supply with crop consumptive demand. As discussed above, irrigated acreage in the basin was developed under a previous contract. As part of the Bear River basin study, water rights have been mapped to irrigated parcels identified in the GIS irrigated acreage coverage. Each water right permit contains point of diversion information, including water source and name of diverting structure. In addition, the water rights define the amount of acreage allowed to be irrigated under the permit, up to 70 acres per 1 cfs. The irrigated parcel mapping was prepared in a general fashion, without an attempt to create a one-to-one relationship between water rights and irrigated acreage. Therefore, analyses were required to match water rights with their associated irrigated acreage so diversion records could be compared to the irrigation water requirements. In most cases, more than one water right serves an irrigated parcel. In addition, many water rights are tied to more than one parcel. In data terms, the irrigated acreage and water right GIS data tables display a many-to-many relationship. Water rights were tied to irrigated acreage parcels based on the following general procedure:
This process was iterative in nature, since key structure water rights were associated with the same irrigated parcels as other key water rights. Therefore, this procedure did not lend itself to automation and was performed interactively by using the GIS linking capabilities and engineering judgement. This procedure resulted in the following information, required to complete the supply-limited consumptive use analysis.
Only many-to-many water rights and irrigated acreage associations were found in the Upper Division. However, there are unique lands served by both Myers #1 and Myers Irrigation diversions that are not served by other diversions. Two unique associations between water rights and irrigated acreage were found in the Central Division - Quinn Bourne, and VH Canal. Table 8 shows the estimated acreage in the basin assigned to individual diversion structures or aggregate groups of diversion structures. Table 8
During the analysis to associate acreage with diversion structures, wells permitted for irrigation were also tied to irrigated lands and to ditch systems. The following diversions or aggregate diversions were determined to have supplemental ground water:
Approach and Results - Irrigation Water Requirements As discussed in the Crop Consumptive Use Methodology section, monthly crop consumptive use was determined using the SCS Blaney-Criddle approach outlined in TR- 21, with locally calibrated crop coefficients. The monthly effective precipitation was subtracted from the crop consumptive use to estimate irrigation water requirement. A Blaney-Criddle computer model, developed by staff at Leonard Rice Engineers, was used to automate the calculation process. Inputs to the model include annual irrigated acreage and corresponding crop types (estimated to be constant from 1971 through 1998), calibrated crop coefficients, and monthly temperature and precipitation records (monthly from 1971 through 1998). Table 9 provides the average irrigation water requirements for the Bear River basin in Wyoming for the Upper and Central Divisions. Figure 7 shows the annual irrigation water requirements for 1971 through 1998. Table 9
Figure 7 Approach - Water-Supply Limited Consumptive Use Irrigation water requirement is an estimate of the amount of water crops would use if they had a full supply. However, in the arid west most farming operations by necessity grow crops with less than a full supply. Water-supply limited consumptive use is the amount of water actually used by the crop. In the Bear River Basin in Wyoming, there are five measurable sources of agricultural supply:
Another indication of well-watered versus limited water conditions is crop yield. However, because much of the irrigated meadow in the Bear River Basin is used for grazing or hayed for on-farm use, consistent yield information is not available. A farm budget/soil moisture mass-balance approach can be used to estimate supply- limited consumptive use. The StateCU Blaney-Criddle program used in the irrigation water requirement analysis has a farm budget/soil moisture routine that operates based on the procedure outlined in Figure 8.
Figure 8 The following general procedure was followed to estimate the supply-limited consumptive use and agricultural ground water use in the Bear River basin in Wyoming:
As shown in Figure 7, the determination of water-supply limited consumptive use requires records or estimates of the following:
Diversions for Index Structures. The structures identified as having a unique relationship between diversions and acreage were used as index structures to estimate the overall basin water supply-limited consumptive use. Monthly shortages for the study period for the Myers #1 and Myers Irrigation Ditch combined diversions were estimated to represent shortages seen in the Upper Division. Their combined water rights represent the water right priorities seen in the Upper Division. Almost the entire Upper Division irrigates meadow, so the crop types are consistent between the index structures and structures to be estimated. In addition, nearly all the Upper Division acreage above Woodruff Narrows has access to supplemental water from a combination of Whitney and Sulphur Creek Reservoirs. The Upper Division acreage downstream of Woodruff Narrows shares in supplemental water from Woodruff Narrows Reservoir. The monthly shortages for Quinn Bourne and VH Canal were averaged and estimated to be representative of shortages seen in the Central Division. Again, the water rights for these two ditches represent the water right priorities seen in the Central Division, and the crop types are consistent with crop types seen throughout the Central Division. There are no major reservoirs in the Central Division. Neither Quinn Bourne nor VH Canal has permits for supplemental ground water. Maximum Conveyance and Application Efficiencies. Diversion records provide the amount of water diverted at the river headgate. However, only a portion of the headgate diversion is actually available to satisfy crop consumptive use requirements. The unused portion of the headgate diversion is either lost en route to the farm (conveyance loss) or lost during crop application (application or on-farm loss). Conveyance and application efficiencies were estimated in the Spreadsheet Modeling Support - Efficiencies and Return Flow Patterns memorandum developed for this study. The efficiencies estimated for the index structures are in the middle range of efficiencies estimated for structures in the basin. Soil Moisture Holding Capability. The Bear River Basin hydrograph, like most basins in the west, show the peak flow in late spring or early summer coinciding with snowmelt runoff. However, the peak crop demand generally occurs in mid summer, corresponding with more hours of sunlight and warmer temperatures. The practice of applying more water to the land in early spring, when water is available, than the crops actually need is extremely common. Much of this excess supply is stored in the soil root zone and can be used by the crops to continue growing after the surface water supply is exhausted. Depending on the soil characteristics, up to a month of additional growing capabilities is common. Water stored in the soil root zone is the cause for the "dry-up" period after irrigation ends and prior to harvesting. Research Report 125 estimated the usable capacity for soil moisture near Randolph UT to be 10 inches, with a corresponding root depth of 6.0 feet. The StateCU Farm Budget/Soil Moisture Accounting procedure requires soil moisture holding capacity input in inches per inch of root depth, allowing the program to account for varying root depth during the growing season and varying root depths by crop. Randolph UT is on the Bear River near Wyoming and was estimated to be representative of soils seen in both the Upper and Central Divisions. Therefore, 0.1389 inches per inch was used to estimate the soil ability to store excess water. If an average root depth of 3-feet is used, this provides a soil "reservoir" capable of holding over 25,000 acre-feet of moisture under the 63,900 acres of irrigated land in the basin. (63,900 acres) x (0.1389 inches/inch) x (3 feet) = 26,627 acre-feetAvailability of Supplemental Ground Water. Structures that are permitted for supplemental ground water are identified in the Irrigated Land/Diversion Association section. Results - Water-Supply Limited Consumptive Use The StateCU model was used to automate the Farm Budget/Soil Moisture Accounting procedure for the index structures. The shortages were applied on a monthly basis to other structures within the same Division, as discussed above. Only structures with supplemental ground water were estimated to have a full supply. Ground Water Use Estimates. Ground water consumptive use was estimated to be the difference between irrigation water requirement and surface water supply-limited consumptive use for those structures with ground water permits. The amount of ground water pumped to meet the deficit demand for these structures is dependent on the application method. More losses are incurred using flood irrigation techniques than sprinkler techniques; therefore more ground water must be pumped to meet the same demand. Table 10 shows the average ground water consumptive use and corresponding pumping estimated by structure, for the study period. Table 10
Table 11 shows the average annual irrigation water requirement, water supply-limited consumptive use, and percent shortage estimated for the Upper and Central Divisions. The supply-limited consumptive use includes consumptive use of both surface and ground water sources. Table 11
Figures 9 and 10 show the annual irrigation water requirement, consumptive use of surface water, and consumptive use of ground water in the Upper and Central Division for 1971 through 1998.
Figure 9
Figure 10 Summary and Conclusions The Bear River Basin in Wyoming generally has a reliable source of water for irrigation. The three larger reservoirs in the Upper Division allow demand to be satisfied except in the driest years. Supplement ground water, although not widely permitted, helps relieve some of the shortages seen in the Central Division. Figure 11 shows the average annual crop consumptive use for the Upper and Central Divisions from 1971 through 1998. Figure 12 shows the average annual crop consumptive use by supply for 1971 through 1998 for the entire Bear River basin in Wyoming.
Figure 11
Figure 12 Comments and Concerns The method of determining supply-limited consumptive use based on index structures can be improved upon if more detailed mapping of water rights to irrigated lands is undertaken in the future. In addition, more detailed soil analysis and conveyance loss studies may also provide better estimates. However, based on discussions with local water users and water administrators, the estimates prepared in the memorandum are considered an accurate representation of the crop water use. Estimating ground water consumptive use using the method described in this memorandum is common engineering practice in lieu of better information. However, often farmers sprinkler irrigating from wells will use their ground water as their primary source. This method of estimating ground water use likely underestimates the actual pumping for irrigation use. However, ground water use for irrigation continues to be a small percentage of the water used to meet crop demands. Sources American Society of Civil Engineers Manuals and Reports on Engineering Practice No. 70, Evapotranspiration and Irrigation Water Requirements, 1989. Soil Conservation Service, Irrigation Water Requirements Technical Release No. 21, April 1967, revised September 1970. Robert W. Hill, et al, Duty of Water Under the Bear River Compact: Field Verification of Empirical Methods for Estimating Depletion - Research Report 125, January 1989. Larry Pochop, et al, Consumptive Use and Consumptive Irrigation Requirements in Wyoming, 1992. Don Shoemaker, Water Superintendent, Personal Communication Kevin Wilde, Water Superintendent, Personal Communication USDA and SCS, Irrigation Conveyance Systems - Working Paper for the Bear River Basin Type IV Study, April 1976. State of Utah Natural Resources, Water Budget Studies - Utah, Bear River Study Area, September 1994. Duane D. Klamm and John S. Brenner, excerpts from the 1995 Evapotranspiration and Irrigation Efficiency Seminar sponsored by the American Consulting Engineers Council of Colorado and the Colorado Division of Water Resources. |
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