To quantify the impact of the location of storage on evaporation in the Rio Grande Basin, Guardians conducted an analysis of the monthly reservoir storage and evaporation in 2010 and 2013. Guardians used the Rio Grande Compact Commission’s annual reports as the source of the actual monthly reservoir volumes (acre-feet), reservoir elevation (feet), and monthly reservoir evaporation (inches). Area-capacity tables for each reservoir were used to determine the estimated surface area. We routinely used elevation as the basis for determining the surface area.

A few exceptions to this data and method of estimating surface area were made due to inconsistencies in the data. First, instead of using the elevation listed for Cochiti Reservoir in 2010 (where the elevation appears to have been misreported), we used the reservoir volume in the area capacity table to estimate the elevation for each month that year. Second, in 2013, the annual reports did not provide any monthly evaporation data for Caballo Reservoir. We used the monthly evaporation data for January through November 2012 and the data for December 2011 to fill this void. No data existed for December 2012.

To calculate the estimated monthly evaporation for each reservoir in acre-feet, the following method was used:

- The monthly pan evaporation (measured in inches) found in the Rio Grande Compact Commission Reports was converted to feet by dividing the evaporation number by 12. This resulted in the pan evaporation in feet per month.
- The pan evaporation data (measured in feet) was then converted to open water evaporation. Open water evaporation is considered 70 percent of PAN evaporation. Thus, we multiplied the pan evaporation data (measured in feet) for each month by 0.7 to get open water evaporation. This resulted in the open water evaporation in feet per month.
- Evaporation in acre-feet was then calculated by multiplying the open water evaporation in feet by the surface area of the specific reservoir at the given elevation indicated by the volume. We consulted area-capacity curves for each of the six reservoirs to determine the elevation and surface area associated with the specific volume.

These calculations were made using the actual monthly data in 2010 and in 2013 and are located in Appendix A, Tables 1A to 1F (2010) and 2A to 2F (2013). A summary of the actual monthly storage data in 2010 and 2013 is located in Appendix A, Table 1G (2010) and Table 2G (2013). The monthly evaporation calculated for each reservoir and the annual totals are located in Appendix A, Table 1H (2010) and Table 2H (2013).

View 2010 Appendix Tables 1A to 1R [PDF] »

View 2013 Appendix Tables 2A to 2N [PDF] »

The same method was used to calculate the evaporation losses in the alternate scenarios. In these scenarios, the quantity of water stored in low-elevation reservoirs (Elephant Butte and Caballo) each month was redistributed upstream to the high-elevation reservoirs. Once the redistribution was determined, then the formula detailed above was used to determine the given evaporation based on the new storage volumes in each of the reservoirs.

The quantity of water determined for storage in upstream reservoirs was distributed based on one simple assumption—the highest-elevation reservoirs should be filled to capacity before any water is stored at a lower elevation—in order to conserve the most water due to evaporation loss. Thus, low-elevation storage was used to fill Heron first (the highest-elevation reservoir) to its capacity of 401,300 acre-feet. Thereafter, any remaining low-elevation storage was redistributed to El Vado to its capacity of 196,500 acre-feet, followed by Abiquiu. In 2010, we assumed that it could be possible to acquire additional storage in Abiquiu beyond the 200,000 acre-feet, up to the additional 520,000 acre-feet identified in the U.S. Army Corps of Engineers 1980 Study (a maximum storage of 720,000 acre-feet). For simplicity, none of the scenarios required additional storage in Cochiti Reservoir, but the same method could be used to allocate storage to Cochiti Reservoir and calculate the evaporation losses therefrom if desired.

Appendix A, Tables 1I to 1L (2010 Scenario #1), Tables 1M to 1R (2010 Scenario #2), Tables 2I to 2K (2013 Scenario #1), and Tables 2L to 2N (2013 Scenario #2) show the redistribution and calculation of evaporative losses for two alternative scenarios for 2010 and 2013. The data from the Rio Grande Compact Commission Reports and the area-capacity curves used to calculate these amounts are included as references to the report.

National Academy of Sciences Reservoir Reoperations Study

Draft Scope of Work

**Goal 1: Identify how much storage physically and legally exists in high-elevation (Heron, El Vado, Abiquiu, Cochiti, Jemez Canyon, Galisteo) and low-elevation reservoirs (Elephant Butte and Caballo).**

Tasks:

- Identify existing storage and break down each by purpose (flood control, sediment, storage for irrigation, municipal, or other uses).
- Identify possible additional storage in each reservoir (e.g., Corps 1987 review of Abiquiu Reservoir identifies an additional 467, 000 acre-feet of storage, above flood and sediment control and San Juan-Chama Project storage, that could be utilized given additional hurdles like easement acquisition and legal constraints).
- Identify legal constraints on use of each reservoir (e.g., congressional authorizations, Rio Grande Compact, channel capacities, etc.).

**Goal 2: Determine the surface area versus stage and volume for each reservoir.**

Task:

- Find and review the most recent sedimentation or other study that contains surface area-capacity curves for each reservoir.

**Goal 3: Determine the evaporation losses from each of the high-elevation (Heron, El Vado, Abiquiu, Cochiti, Jemez Canyon, Galisteo) and low-elevation reservoirs (Elephant Butte and Caballo).**

Tasks:

- Find the monthly PAN evaporation data for each reservoir.
- Determine the monthly evaporation rates at each storage site.
- Determine if models already exist to estimate reservoir evaporation losses in a given year or series of years.

**Goal 4: Determine the amount of water savings that could occur if more water is stored upstream. Develop examples of where, how, and when water can be stored and released, creating several alternative scenarios representing dry, wet, and average streamflow years.**

Tasks:

- Determine how refined our period of analysis needs to be to establish benefit of the proposed movement of storage. Is it one year, three years, five years, ten years, etc.?
- Can the post-1974 period of record be used without grossly overestimating the amount of water in storage looking forward, based on climate-change predictions?
- Identify three periods of record that represent the wet-, dry-, and average-year scenarios on the Rio Grande. For example, 2011–2013 might represent the dry-year scenario.

What is the shortest period that would give us the full picture?

- Identify or develop method/model/procedure for evaluating the savings by moving storage upstream.

**Goal 5: Determine the amount of existing carriage losses of status quo water management, based on typical water delivery/movement patterns.**

Tasks:

- Model or use another method to determine how much water is lost in transport.
- Determine modifications needed to existing flow patterns to facilitate additional upstream storage and calculate the additional carriage losses between reservoirs and delivery points.
- Determine net water savings of a new proposed water storage plan, factoring in carriage losses.

**Goal 6: Develop a reservoir release plan that allows for the transport and delivery of water downstream while prioritizing river health and securing flows for native species in the Rio Chama and Rio Grande.**

Tasks:

- Determine what the ideal flow pattern (timing/amount) is from U.S. Fish and Wildlife Service for native species, including but not limited to the Rio Grande silvery minnow, Southwestern willow flycatcher, and yellow-billed cuckoo.
- Determine the ideal flow pattern to facilitate natural river processes now lacking in the Middle Rio Grande ecosystem (e.g., sediment transport, overbank flooding, cottonwood/willow regeneration, etc.).
- Determine channel capacity limitations.