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MARCH 1994

Irrigation scheduling is a consulting service offered to farmers in an effort to optimize application of irrigation water. Optimization is achieved through the use of a predictive simulation tool that is driver by measurements of soil condition, crop type and development, and weather data. Scheduling serves to control the moisture content levels of the soil to target levels. The result is generally the elimination of excessive watering and conservation of pumping energy.

Grant County Public Utility District (PUD) has proposed a conservation agreement to the Bonneville Power Administration (BPA) for obtaining energy saved through irrigation scheduling. Based on the 1991 season, Grant County PUD estimated that scheduling could conserve 206 kWh/acre at the irrigated site and water savings of 0.3 to 0.5 acre-ft per irrigated acre. Distribution energy (primary and secondary pumping) savings were estimated at 420 kWh/acre-ft of saved water. Energy gained by running the saved water through generation turbines at both Grand Coulee and Chief Joseph dams was estimated to be 450 kWh/acre ft.

The Pacific Northwest Laboratory (PNL)(a) is conducting an impact analysis of the Grant County PUD irrigation scheduling project. The specific goal of the evaluation is to estimate the on-site energy savings, off-site energy savings, timing of savings, and demand reduction, as well as amount of water saved, amount of saved water available for generation, and the corresponding power generation available to BPA.

This report begins with a summary of the work completed for the off-site verification plan and also a summary of the analysis of the 1992 and 1993 on-site data. Following the summaries is a detailed description of the second year's data collection and analysis of on-site water/energy consumption measurements. Estimates of energy and water savings and corresponding confidence intervals are included. The final season presents a discussion of the potential for future impact assessments with a reduction in field verification data.

Summary of Off-Site Work
There are several fundamental obstacles to making a quantitative assessment (based on measurement) of the off-site (primary and secondary) pumping savings in a complex dynamic system such as the Columbia Basin Irrigation Project. Possibly the largest obstacle is the small size of the effect being observed. Although the Grant County irrigation scheduling project is large (more than 33,000 acres), it encompasses only 6% of the total land area served by the Columbia Basin Irrigation Project. Current estimates of scheduling-related savings for this area are approximately 1% when expressed in terms of primary pumping levels in 1991. The uncertainties in measurements and simulations make it very difficult to verify this small change in primary pumping through direct measurements or a combination of measurements and simulation. The prospects of making a sufficiently accurate quantitative verification of off-site scheduling savings, through the comparison of Bureau of Reclamation (Bureau) records and simulation data, are very poor.

Modeling uncertainties originate in the
  • human element in farm and irrigation system control
  • size and complexity of the distribution system
  • variability in soil, materials, weather, and end use
  • unknown transport mechanisms

However, it is felt that useful qualitative information could be obtained from a model. It could serve for running sensitivity studies to estimate the impact of small load reductions on primary and secondary pumping. A general system model may also be useful to the Bureau for system management and efficiency optimization.

The potential value of qualitative modeling of the system is illustrated in the findings of Mr. Eldon Johns at the Bureau's E&R Center in Denver. His report, Columbia Basin Project Irrigation Efficiency and Return Flow Analysis (Johns 1992), describes the importance of considering water recovery and re-use characteristics of an area when assessing the impact potential of a conservation measure. For example, irrigation return flows from the upper portion of the West Canal and East Low Canal service areas find their way into Potholes Reservoir through wasteways and groundwater movement. This recovery into Potholes serves as a source primarily to the South District. Unfortunately, the needs of the areas served by Potholes Reservoir are generally in excess of the groundwater and return flows from the East Low and upper West Canal service areas. Columbia River "feedwater" must be fed directly to Potholes Reservoir to make up for the shortfall (approximately 20% of the total supply to Potholes). This is almost always diverted from the East Low Canal through one of its wasteways.

In other words, conservation in the upper West and East Low Canal system service areas may not be effective (in reducing primary pumping) in itself because the waste and drainage from these areas is another area's supply. To illustrate this, assume that 60% of the excess water from pivot sprinklers in the Quincy District East Low Canal service area is collected at Potholes Reservoir via groundwater flow and, in turn, is delivered to the South District. A conservation measure in Quincy District yielding a savings of 10 in. would result in 4 in. of primary pumping savings (because 6 in. of feedwater to Potholes Reservoir would be needed to make up for the lost groundwater flow). The same measure implemented in the South district would yield a savings of 10 in. of primary pumping (because this directly reduces the demand for feedwater to Potholes). It can be argued that even if parallel conservation programs were run in the two districts, any primary pumping savings would rightly be credited at a 100% rate for areas served by Potholes and at a 40% rate (in this example) for areas that supply the groundwater system and Potholes Reservoir.(a)

Similarly, for second pumping, consideration must be given to farm location in determining the impact of the conservation measure. Savings on farms that are supplied from secondary pumping will cause a nearly one-to-one savings in secondary pumping. Conversely, savings on farms not supplied by secondary pumping will have no impact on secondary pumping demand. A rough estimate of secondary pumping savings for Grant County can be made from on-site savings data, the information on the fraction of Grant County farms served by secondary pumping, and the crop types on those farms. Detailed information on the number of farms served through secondary pumping is not readily available from the Bureau at this time but could potentially be tabulated.

The main objective of a modeling effort would be to determine primary pumping savings through an estimate of the re-use fraction for an area. In other words, the objective would be to estimate the portion of sprinkler-applied water that is available for groundwater movement and also the fraction of this that reaches Potholes Reservoir for distribution to other farms. As was described in the October 1992 work plan report, three different model development efforts can be undertaken:
  • Model A - steady-state farm model
  • Model B - dynamic farm model
  • Model C - general dynamic model (farm and distribution system)

Model A represents the smallest effort and is essentially a refinement of existing modeling technology being used by the Bureau and farm consultants. Basically, this model calculates the amount of water needed to maintain a constant moisture content level in a field. Tasks here are mainly the development of a database of typical seasonal crop coefficient profiles, adaptation of existing evapotranspiration code from the Bureau, and development of techniques for accessing Bureau weather data. A rough estimate for development time is 3 to 4 person-weeks.

Model B contains the functionality of Model A along with the capability of simulating the dynamic capacity of the soil/crop system and also the time-dependent nature of the sprinkler schedule. The difficulty here is the soil/crop model. Literature research and possibly some laboratory testing on various soil types are needed. A rough estimate for development time (Model A included) is 2 to 3 person-months.

Model C contains the functionality of Models A and B along with simulations for gravity-fed and nonscheduled farms as well as detailed simulation of the Columbia Basin's distribution system. Developing Model C involves an extensive characterization of the irrigation system and the development of various models of water transport mechanics (e.g., seepage from distribution vessels, groundwater movement, evaporation from lakes), along with a complex computer simulation of the control hierarchy. The model represents a complex network of interacting elements, all with dynamic water storage capacities. A rough estimate for development time (Models A and B included) is 12 person-months.

Models A and B are tools for predicting on-site levels of energy and water consumption for scheduled farms with central-pivot irrigation. They could potentially be used to estimate on-site savings relative to baseline (measured) water and energy consumption levels on unscheduled farms. Model C goes much farther by attempting to simulate the various types of unscheduled farms and also the distribution system and groundwater flows. Of the three models, C is the best suited for estimating re-use behavior of a specific location and, in turn, screening candidate conservation measures for conservation potential (primary pumping as well as on-site and associated secondary pumping savings).

In conclusion, verification of primary pumping savings is difficult to base directly on physical measurements. Possibly the only practical approach to the verification of primary pumping savings will come from a modeling effort and a qualitative assessment of the re-use characteristics of Grant County farms and the Potholes Reservoir system. Further analysis of the Bureau's water distribution records may provide some additional insight, but existing records to not give a complete answer (little is known about groundwater flow, waste flow, and losses). However, secondary pumping savings are closely related to the on-site savings from farms that are supplied by secondary pumping. This could be estimated by a tabulation of the number of acres, and corresponding crop type, served by secondary pumping in Grant County.

Note: sections of report not included in this summary are: Summary of On-Site Work 1992 Season & 1993 Season; 1993 Data Collection; Analysis; Results; Future Impact Assessments; Conclusions and Recommendations; References; Appendix A. editor

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