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Introduction The spreadsheet model determines the mass balance at locations along the river. Therefore, historic or estimated river headgate diversions are used as the demands that drive the model. However, only a portion of the headgate diversion is actually available to satisfy crop consumptive use requirements, municipal demands, or industrial demands. The unused portion of the headgate diversions is either lost in route to the farm or treatment plant (conveyance loss) or lost during crop application (application or on-farm loss). Efficiencies and Return Flow Patterns This unused portion of headgate diversion either returns to the river as surface runoff during the month it is diverted, or "deep percolates" into the alluvial aquifer. The deep percolation portion returns to the river through the aquifer but generally lags the time of diversion by several months, or even years. It is important for the model to simulate both the percent of headgate diversions that return to the river, and the timing of which this unused portion returns. In the Bear River Basin, water from the river is reused many times from the headwaters to the Great Salt Lake. Diversion efficiency for agricultural use is the common measure of the portion of headgate diversion that is consumed, and therefore not returned to the river. Diversion efficiency for municipal and industrial use is the percent of headgate diversion that makes it to the treatment plant or industrial site. The remaining percent is lost during conveyance, and returns to the river as surface runoff or deep percolation. Diversions for agricultural use experience both conveyance losses and application losses, and both these loss percents return to the river as surface runoff or deep percolation. Conveyance Efficiency Factors that affect conveyance efficiencies include:
The report "Irrigation Conveyance Systems - Working Paper for the Bear River Basin Type IV Study", USDA and SCS, April 1976, estimated conveyance efficiencies throughout the Bear River Basin. The conveyance efficiencies for the Unita County irrigation systems ranged from 30 percent to 75 percent. The conveyance efficiencies for Lincoln County ranged from 50 percent to 85 percent. Tables are provided in that report showing conveyance efficiencies by major ditch system, however, there is no documentation provided on how these estimates were developed. Therefore, we estimated conveyance efficiencies based on main canal length and the proximity of irrigated acreage to the main canal. The maps provided in the Type IV Study report were used to measure canal lengths. In most cases, our estimated conveyance efficiencies were consistent with the efficiencies summarized in the Type IV Study report. Note that the ditch systems in the Bear River basin in Wyoming have changed very little since 1976. Table 1 shows the conveyance efficiencies estimated for the key ditch systems represented in the modeling effort for the upper division. The upper division aggregate ditch systems were assigned a conveyance efficiency of 60 percent, because they represent smaller ditches generally irrigating lands close to the river. Table 2 shows the conveyance efficiencies estimated for the key ditch systems represented in the modeling effort for the central division. The central division aggregate ditch systems were assigned a conveyance efficiency of 65 percent, again because they represent smaller ditches generally irrigating lands close to the river. The exception is the aggregate diversion representing the irrigated lands in Utah between Woodruff Narrows and Pixley Dam. These lands are dependent on each other's return flows for diversion supply, but because of the aggregation, all diversion will be modeled at one location. A conveyance efficiency of 45 percent was used to represent these diversions. Application (on-farm) Efficiencies Factors that affect application efficiencies include:
Ranges of irrigation application efficiencies are widely published. For this study, we used efficiency percents presented by Duane D. Klamm and John S. Brenner in the 1995 Evapotranspiration and Irrigation Efficiency Seminar sponsored by the American Consulting Engineers Council of Colorado and the Colorado Division of Water Resources. Flood irrigation application efficiencies were estimated to be 55 percent basin wide for the modeling effort. Center pivot and side-roll sprinkler application efficiencies were estimated at 85 percent, and hand-line sprinklers were estimated at 80 percent. Most crop acreage in the upper division of the Bear River basin is irrigated using flood application techniques. There is only one sprinkler irrigated quarter-section shown in the 1993 irrigated acreage assessment. The local water administrator further verified that all other farms use flood irrigation in the upper division. The central division is also mostly flood irrigated, however, there has been a greater trend towards sprinkler irrigation. Through interviews with water administrators and users, we have estimated the percent of acreage under the key ditch systems that apply water through sprinklers. Center pivot and side-roll sprinklers are most common; however there are a few hand-line sprinklers. In addition to conveyance efficiencies, Table 1 shows the suggested application efficiency for the key ditch systems in the upper division. Because lands in the upper division are flood irrigated, the application efficiencies are all 55 percent. An application efficiency of 55 percent is also used for aggregated ditch systems. Table 2 shows the suggested application efficiency for the key ditch systems in the central division. An application efficiency of 65 percent was used for aggregated ditch systems in the central division, which represents an average of key ditch system efficiencies. The model uses a diversion efficiency that represents the actual amount of headgate diversion used to satisfy crop consumptive use demands. It is calculated as the product of conveyance efficiency and application efficiency. The diversion efficiency is also provided in Tables 1 and 2. Table 1
Table 2
Return Flow Patterns As discussed above, the unused, or inefficient, portion of diversions are returned to the river either by direct surface runoff, or through the alluvial aquifer. For modeling purposes, an estimate must be made of both:
The location where unused water is returned to the river is based on the location of irrigated lands. Unless a "drain" has been constructed to remove excess water from lands with high water tables, return flows do not generally return to a specific point on the river. However, for modeling purposes, it needs to be returned to a specific node in the network. The return flows can be re-diverted at the return flow location node and any nodes downstream. For the larger ditch systems, unused water may return at more than one location. The irrigated acreage GIS theme was used to estimate these locations, shown in Table 3 for the upper division and Table 4 for the central division. Return flow patterns represent the timing of return flows back to the river. Generally, the surface runoff portion of unused diversion returns to the river the same month as diversions, whereas the deep percolation portion may take much longer. The timing of return flows can be determined using standard Glover equations, basin water budgets, or more detailed ground water modeling techniques. Return flow patterns were estimated in the publication "Water Budget Studies - Utah, Bear River Study Area", State of Utah Natural Resources, September 1994. Return flow patterns were developed for several river reaches, including both the upper and central portion of the Bear River Basin in Wyoming. The following return flow patterns were adopted from this study for the modeling effort, and represent the timing of the return of unused diversions. Note that month 0 is the month of diversion, month 1 is the 1st month after diversion, etc.
Return Pattern 1 = 50 % month 0, 25 % month 1, 15 % month 2, 10 % month 3 Return flow pattern 1 is used to represent larger ditch systems that serve lands further from the river. Return flow pattern 2 is used to represent smaller ditch systems that serve lands adjacent to the river. Return flow pattern 3 is used to represent municipal and industrial nodes, where water is returned to the river from treatment plants or industrial processes directly. In addition to the return flow node location in the river, Tables 3 and 4 show the return flow pattern used to represent each ditch system. Table 3
Table 4
Sources 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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