SPDSS_Task53-3_Assign Key Climate Information to Irrigated Acreage and Reservoirs

• Title: SPDSS Task 53.3 - Assign Key Climate Information to Irrifated Acreage and Reservoirs
• Description: This memorandum presents the general approach for the following Task 53 subtasks: 1. Assign key climate stations to geographic areas for the SPDSS modeling efforts. 2. Estimate average monthly reservoir evaporation rates for geographic areas.
• Decision Support - Doc Type: Task Memorandum
• Date: 2/1/2006
• DSS Category: Consumptive Use
• DSS: South Platte
• Basin: South Platte
• Contract/PO #: C153954
• Grant Type: Non-Reimbursable
• Bill Number: SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
• Prepared By: Leonard Rice Engineering

replace evaporation of water that was stored out of priority, for example water stored in an un-
lined gravel pit, then 70 percent of the total precipitation is considered effective and applied as a
credit. According to the State Engineer's Office, this assumes that 70 percent of precipitation on
the reservoir site was previously consumed by native vegetation, and 30 percent contributed to
stream flow. (Note that this 70 percent factor should not be confused with the 0.70 pan
coefficient generally applied to pan evaporation estimates to get gross evaporation estimates.)
Denver Water and the Bureau of Reclamation, however, are calculating amass-balance of
reservoir inflows and outflows; therefore they consider the full precipitation in their net reservoir
evaporation calculation. Similarly for the SPDSS Water Budget and Surface Water modeling
efforts, the full precipitation should be considered in the net evaporation calculation. Due to
inconsistencies noted, the reservoir evaporation rates from the Denver Water and NCWCD
models were not used.
In the absence of site-specific data, the following consistent method was adopted for the entire
basin, as explained in detail below:
1. Determine average annual gross evaporation based on NOAA publications
2. Determine average annual precipitation based on Colorado Climate Center publications
3. Distribute annual gross evaporation to monthly using State Engineer's Office procedure
4. Distribute annual precipitation to monthly using local climate station data
5. Estimate average net monthly evaporation rates by subtracting precipitation from gross
evaporation
Steps 1 and 2: Determine average annual gross evaporation and average annual precipitation
The CDSS GIS coverage includes the following gross evaporation and precipitation shape files
based on average annual estimates:
• NOAA Free Water Surface Evaporation published in June 1982, based on a 1956
through 1970 study period.
• Precipitation Isohyetal Map published by the Colorado Climate Center, based on a
report titled "Analysis of Colorado Average Annual Precipitation for the 1951-1980
Period".
These files were used to develop average annual gross reservoir evaporation and total
precipitation estimates for each water district or sub-district shown in Figure 1 above.
Step 3: Distribute annual gross evaporation to monthly
Average annual gross reservoir evaporation estimates developed in Step 1 above were distributed
monthly with the percentages used by the State Engineer's Office (presented by Wolfe and
Stenzel at a 1995 ET and Irrigation Efficiency Seminar and summarized in a paper titled
"Evaporation"). There are two average monthly distributions; one for below 6,500 feet and one
for above 6,500 feet above mean sea level (Table 2). The below 6,500 feet distribution was used
for Water Districts 2, 64, and the lower portions of Water Districts 1, 3, 4, 5, 6, 7, 8, and 9. The
above 6,500 feet distribution was used for the upper portions of Water Districts 1, 3, 4, 5, 6, 7, 8,
9 and Water Districts 23, 47, 48, 76, and 80. The resulting average monthly gross reservoir
evaporation estimates are provided in Table 3.
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