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Data collected during this study included water use measurements for mountain meadows along Horse Creek, water use measurements for alta fescue and alfalfa at three sites in the Green River Basin, and climatological data at seven sites in the Basin. A major reason for obtaining water use measurements was to permit calibration of evapotranspiration models which could then be used with climatological data from locations where water use measurements were not taken to estimate evapotranspiration rates throughout the Basin. In addition, calibrated models can be used with historical climatic data to estimate long-term water use rates.
Transferability of Estimates
The transferability of the various models can be considered from the standpoint of the comparisons presented in the previous chapter in the section entitled "uncalibrated equation estimates vs measurements". The results of that section indicate that local calibration of the equations are desirable. However, the transferability of calibrated models within and between basins for similar vegetation and climatic conditions is still of concern. The question of the transferability of models can be considered only for those models for which required climatic and ET data are available at other locations. In most cases, this limits the analyses to models based on temperature data, unless special climatological measurements have been taken as was done in the Green River Basin during the period of this study.
Analyses of variations within the Green River Basin can best be done by considering alfalfa and alta fescue, since ET rates of alfalfa and alta fescue were measured at three widely spaced sites while ET rates of mountain meadows were measured at sites which were rather closely spaced. Analyses of variations between basins is much more difficult. In Wyoming, for example, the only recent measurements of water use consist of mountain meadow water use along the Little Laramie River (Borrelli, et al., 1982) and Kentucky bluegrass water use on lawns in Laramie and Wheatland (Borrelli, et al., 1981).
As already mentioned in the previous chapter, a calibrated version of the SCS Blaney- Criddle better accounted for variations in water use between locations within the Green River Basin than did the original Blaney-Criddle formula (Table 24). The difficulty with the calibrated SCS Blaney-Criddle is that a different kt is required for each crop, which makes the application of the formula somewhat cumbersome but still less complicated than methods such as the combination formulas. The larger spatial variation of the coefficients in the original formula indicates that the climatic differences between locations are not as well accounted for as in the SCS version of the Blaney-Criddle, except through inclusion in the coefficients themselves. This was, in fact, the reason for introduction of the kt value in the SCS Blaney-Criddle modification.
Considering some of the other equations and their ability to account for spatial variations within the Green River Basin, there was not a great amount of difference between the ASCE Penman, the FAO Blaney-Criddle, the FAO Penman, and the modified Jensen-Haise (Table 25). In all cases, a consistent pattern exists, with largest values of the crop coefficients at Seedskadee and smallest values at Daniel. This pattern can probably be attributed to the amount of advection that occurs at the various sites. Seedskadee has by far the greater oasis condition with Daniel the least.
Mountain meadow water use measurements for the Green River Basin and Little Laramie River Valley (Borrelli, et al., 1981) are shown in Table 26. Comparison of the use rates indicate seasonal values that are very close for the two Basins. Average temperatures, however, in the Little Laramie River Valley are about 7.5°F higher. Application of the SCS Blaney-Criddle formula, calibrated using Green River data (Table 21), to estimate mountain meadow water use in the Little Laramie River Valley reflect these comparisons. The estimated values are about 20% higher than measured values in the Little Laramie River Valley.
Comparisons for clipped grass in the Green River Basin and the eastern portion of Wyoming (Borrelli, et al., 1981) are given in Table 27. Again, application of the calibrated SCS Blaney-Criddle formula to estimate the Laramie and Wheatland seasonal values gave overestimates of 10% and 24%, for Laramie and Wheatland, respectively. The same trends, between water use rates and temperatures at the various sites, which were present for mountain meadows are evident for the clipped grass. The SCS Blaney-Criddle formula was used for the comparisons in both cases since the only available climatic data was temperature. Actually, some differences might be expected due to the different grasses involved, which were Kentucky bluegrass in Laramie and Wheatland and alta fescue in the Green River Basin. The bluegrass did show an apparent high temperature stress during the mid-portion of the summer at Wheatland.
Water Use Rates
Analyses of water use data and models for estimating ET indicate that water use rates vary considerably across the Green River Basin. Thus, for most accurate estimates of the water use rates, local calculations are desired. Local calculations require local climatic data as well as use of a model that accounts for the differences in conditions across the basin. Results from the analyses of the previous section indicate that some formulas are better suited than others for calculation ET at several locations within the Green River Basin. Because long-term climatic data consist of only temperature and precipitation, the temperature based equations must be used to determine estimates of long-term water use rates. In general, analyses showed that estimates using the calibrated SCS Blaney-Criddle were as accurate as estimates from the more data intensive formulas. This is probably because all estimates were for monthly periods. If shorter term estimates were required, then the data intensive equations might perform better.
TABLE 24. AREAL COMPARISON OF BLANEY-CRIDDLE CROP GROWTH STAGE COEFFICIENTS*
=========================================================================
LOCATION
-------------------------------- AVERAGE OF
CROP METHOD DANIEL FARSON SEEDSKADEE LOCATIONS
-------------------------------------------------------------------------
ORIGINAL 1.34 1.50 1.83 1.55
B-C
ALFALFA
SCS 0.94 0.98 1.06 0.99
B-C
========================================================================
THE VALUES SHOWN ARE FOR THE SEASON MAY THROUGH SEPTEMBER. THE VALUES
ARE FOR COMPARISON PURPOSES ONLY. SEASONAL VALUES ARE NOT TO BE USED
TO ESTIMATE CROP WATER USE.
TABLE 25. AREAL COMPARISON OF PENMAN, FAO, AND MODIFIED JENSEN-HAISE METHODS*
==============================================================================
LOCATION
-------------------------------- AVERAGE OF
CROP METHOD DANIEL FARSON SEEDSKADEE LOCATIONS
----------------------------------------------------------------------------
ALFALFA PENMAN 0.88 0.90 1.01 0.93
FAO B-C 1.08 1.11 1.16 1.12
FAO PENMAN 0.88 0.93 1.05 0.96
MODIFIED JEN-HAISE 0.95 1.02 1.05 1.02
============================================================================
* THE VALUES SHOWN ARE SEASONAL CROP COEFFICIENTS FOR MAY THROUGH SEPT.
TABLE 26. MOUNTAIN MEADOW WATER USES FOR GREEN RIVER VS LARAMIE RIVER
===========================================================================
AVERAGE WATER USE* AVERAGE TEMPERATURE
--------------------------- ---------------------------
LARAMIE RIVER GREEN RIVER LARAMIE RIVER GREEN RIVER
MONTH (INCHES) (INCHES) (F) (F)
--------------------------------------------------------------------------
MAY 2.45A 1.76C 47.0 43.5
JUN 6.67 5.95 58.0 49.9
JUL 6.46 6.73 65.0 59.3
AUG 3.96 5.27 61.5 55.0
SEP 3.14B 2.89 56.9 43.4
--------------------------------------------------------------------------
SEASON 22.66 22.60 57.7 50.2
==========================================================================
YEARS ARE 1979 & 1980 FOR LARAMIE RIVER AND 1984 & 1985 FOR GREEN RIVER
A-MAY 14 THROUGH 31
B-SEP 1 THROUGH 18
C-AVERAGE FROM MAY 19 THROUGH 31
TABLE 27. CLIPPED GRASS WATER USE FOR GREEN RIVER VS LARAMIE AND WHEATLAND
===========================================================================
AVERAGE WATER USE* AVERAGE TEMPERATURE
-------------------------------- ------------------------------
GREEN RIVER LARARMIE WHEATLAND GREEN RIVER LARAMIE WHEATLAND
MONTH (INCHES) (INCHES) (INCHES) (F) (F) (F)
--------------------------------------------------------------------------
MAY 2.41A 4.49 4.88 47.7 47.1 55.9
JUN 5.88 6.50 6.14 54.3 58.8 66.4
JUL 6.42 7.20 6.61 63.2 65.5 73.0
AUG 6.24 5.28 4.88 59.9 61.2 67.8
SEP 4.38 3.66 4.45 47.3 54.5 49.5
--------------------------------------------------------------------------
SEASON 25.33 27.13 26.96 54.5 57.4 64.9
==========================================================================
* YEARS ARE 1984 & 1985 FOR THE GREEN RIVER BASIN AND 1976-1978 FOR LARAMIE
AND WHEATLAND. STATIONS IN THE GREEN RIVER BASIN INCLUDE DANIEL, FARSON,
AND SEEDSKADEE.
A-AVERAGE DATE FROM MAY 12 THROUGH 31
Comparisons of estimates using the calibrated versions of the SCS Blaney-Criddle, ASCE Penman, modified Jensen-Haise, and FAO Blaney-Criddle and Penman formulas for the seven sites at which intensive weather data were measured during 1984 and 1985 are given in the Appendix E. The results show that the estimates using the calibrated SCS Blaney-Criddle formula compare favorably with those from the other equations. However, as shown in Table 15 of Chapter 4, water use estimates using the SCS Blaney-Criddle without calibration for mountain meadow vegetation may be considerably lower than the estimates obtained herein using a calibrated version of the formula.
Measurements of water use taken during 1984 and 1985 in the Green River Basin were taken to permit calibration of ET formulas for estimating maximum consumptive use rates. Actual consumptive use rates under irrigation are expected to be less than the estimated maximum rates. Some of the factors that need to be defined to estimate actual irrigation water use include the total irrigated acreages, lengths of irrigation season, agronomic and irrigation practices, depth of ground water tables, and soil fertility and salinity levels. In addition, irrigation water use is also affected by the amount of precipitation received.
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