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Many methods exist for measuring evapotranspiration rates. These include inflow-outflow procedures, soil-moisture and rainfall balance methods, eddy correlation techniques, lysimeters, and various biological approaches. Lysimeters are the most direct and are usually considered to be the most accuratemethod. Lysimeters, in general, can be defined as weighing and non-weighing.Weighing lysimeters are the most precise, but are expensive to install and operate. Non-weighing lysimeters, in which water use rates are determined through maintenance of a water budget, are less expensive and are usually considered accurate for periods of one month or more. For shorter periods, the accuracy becomes more questionable as the periods become shorter. Nonweighing lysimeters were selected for this project because of the remote locations involved, the numerous lysimeters required due to the variable conditions, and the lower cost compared to weighing lysimeters.
Previous measurements of water use rates in the Green River Basin are limited to measurements by Burman and Louden (1967) for alfalfa in the Farson area. The soil-moisture water balance procedure was used for those measurements. Other measurements of agricultural water use rates in Wyoming consist of lysimeter measurements of mountain meadow water use along the Little Laramie River (Borrelli and Burman, 1982; Burman and Borrelli, 1984.
Methods and Locations
Fourteen non-weighing water balance lysimeters (Figure 3) were installed in the Green River Basin and operated for three years to obtain water use measurements for mountain meadows, alfalfa, and alta fescue. The lysimeters were installed during the fall of 1982 and spring of 1983. Ten of the lysimeters were located along a 20 mile stretch of Horse Creek between the BridgerTeton National Forest boundary and Daniel. These lysimeters consisted of eight with mountain meadow vegetation and one each of alfalfa and alta fescue. The other four lysimeters were one each of alfalfa and alta fescue located at both Farson and Seedskadee. Mountain meadow vegetation and alfalfa are the major agricultural crops in the Basin while alta fescue was used as a reference crop.
The lysimeters were constructed using one-eighth inch thick steel plate and measured 39.4 inches square by 60 inches deep (Figure 3). An aluminum tube, 1.5 inches in diameter by 60 inches long with the lower end sealed was installed in the center to provide access for the measurement of soil moisture by use of a neutron probe. A 4 inch diameter by 60 inches long perforated PVC pipe with a removable lid was installed for use as an access to measure water table depths and to remove water from the lysimeters when needed. The pipe was wrapped in fine mesh screen to prevent soil from entering the perforations. A small, 3/4 inch diameter by 60 inches long perforated PVC pipe was installed outside the lysimeter to monitor surrounding water table depths.
Figure 3. Details of Lysimeter Installation
Sites for the mountain meadow lysimeters were selected to represent as much as possible the variable conditions of vegetation, location, and soils. The differences in soil are indicated by specific yields and field capacities (Table D7). The alfalfa sites were selected to represent average to extreme conditions within the Basin. The Seedskadee site represented a very dry oasis type location. The high elevation at the Daniel site probably created near marginal conditions for alfalfa growth since little alfalfa is grown in the immediately surrounding area. The alta fescue was installed at each alfalfa site.
Installation was performed to insure minimal disruption of soil stratification with each layer replaced in reverse order of removal. To develop a vegetative cover, the sod was replaced in the mountain meadow lysimeters while the alfalfa and alta fescue were planted from seed. Thus, 1983 is considered to be an establishment year for all lysimeters because of the time required to develop the crops.
Data Collection and Reduction
Weekly water use measurements were made throughout the growing season. Actual dates of the start-up and close-out of the lysimeters were dependent upon climatic conditions, especially soil temperatures and snowfall. Frost had to be completely out of the soil before start-up. It was also desirable to delay starting the lysimeters until after the last snowfall because it was difficult to measure the true amount of precipitation that was received by the lysimeters, especially in the presence of wind. The same conditions were considered in the fall, except snowfall was the primary concern at that time. Therefore, the period of lysimeter operation was somewhat shorter than the actual growing season. Start-up and close-out were generally scheduled so that water use measurements were made for at least the entire months of June, July, August, and September.
Start-up and close-out of the lysimeters followed somewhat the same procedures and were conducted to provide a check on seasonal water use values. This procedure consisted of water being added to the lysimeters until they were completely saturated to the soil surface. During the start-up procedure a measured amount of water was then removed and the resulting water table depth 24 hours later was measured. These measurements provided an estimate of the specific yield of each lysimeter. During the close-out procedure, water was added and the amount required to completely saturate each lysimeter was measured. The total seasonal water use was then calculated by considering the amount removed during start-up, the amount added during close-out and the amounts added and/or removed each week during the operational period.
Operation of the lysimeters included regular weekly irrigations and maintenance of a water table depth approximately the same as that of the surrounding fields. Detailed descriptions of the procedures are given in Appendix H. Weekly irrigation generally prevented soil moisture depletions in the layer above the water table from exceeding 50% (Appendix D3). The amount of water added was slightly in excess of the anticipated weekly ET. All water was added to the surface to simulate flood irrigation. Maintenance of the high water tables and surface irrigating the lysimeters served to give measurements which should simulate maximum water use rates.
Determination of the ET from each site was accomplished by use of a water balance considering precipitation, water added and/or removed, and the total change in soil moisture. The total change in soil moisture included the weekly fluctuation in water table depth and the change in the soil moisture in the layer above the water table, which was measured by use of a neutron probe. Occasionally, following a heavy rain, it was necessary to remove water from the lysimeters. This was done using a hand operated diaphragm pump.
Water use efficiencies measured in terms of yield/ET ratios in units of tons per acre per inch of water were determined for the mountain meadow and alfalfa lysimeters. The crops were harvested at approximately the same time that the surrounding fields were harvested. The cuttings were dried and weighed and used to determine crop yields as well as water use efficiencies. Harvesting was also done during close-out even though the surrounding fields were not harvested at that time. This harvesting permitted determination of seasonal water use efficiencies. The alta fescue was clipped once every week to maintain a height of 3 to 6 inches. Because of the difficulty in collecting the alta fescue clippings, no record of yields are available for the alta fescue lysimeters. Also, during 1983 the alta fescue and alfalfa sites were allowed to grow without cutting during the entire season. This permitted the newly seeded alta fescue and alfalfa to better establish themselves.
Operation of the lysimeters during 1983 differed slightly from that of 1984 and 1985. In 1983, all lysimeters were well-watered in order to minimize soil moisture depletions and obtain measurements simulating maximum water use rates. This procedure was also used during 1984 and 1985 for all lysimeters except four of the mountain meadow lysimeters. During 1984 and 1985, four of the mountain meadow lysimeters were operated in a manner similar to conditions outside the lysimeter. In this mode of operation, irrigation of the lysimeters was discontinued for the season at the same time irrigation of the surrounding fields was discontinued. This usually occurred at the time of harvest of the hay meadows. This operation permitted measurement of what will be termed actual water use as compared to maximum water use which was determined when continuing irrigations throughout the entire growing season.
Monthly summaries of the lysimeter and evaporation pan data taken at the various sites are given in Tables 6 through 8. Weekly summaries are given in Appendix C. Table 6 includes a classification of improved meadows, (grasses which have been sown). Water use rates (Table 8) and species composition (Table 11) for the improved meadows did not differ greatly from those for the mountain meadow vegetation. Thus, the analyses of data have been done without a distinction being made between these two types of vegetation. However, the raw data is listed by vegetation type for anyone wishing to treat the "improved meadows" and the "mountain meadows" separately. The first measurements indicate the approximate beginning of the growing season, although, there was obviously some water loss during the period of the year when measurements were not taken. The dates listed for the weekly periods in Appendix C are given so that 7 day weeks are used. Measurements of ET were not always on a 7 day schedule, and were not on the same date for all lysimeters for the same week. In order to have data that can more easily be compared, the measurements have been interpolated to the dates shown in Appendix C, thus actual measurement dates may differ from listed dates by as much as 3 or 4 days. Since most analyses which follow are based on monthly and/or seasonal periods, this interpolation of weekly data should not be of practical concern.
ANALYSIS OF EVAPOTRANSPIRATION DATA
In conjunction with the calibration of ET models and the estimation of water use for the Green River Basin, it is useful to evaluate the magnitudes of the ET measurements with respect to expected values. Various approaches might be considered for this verification. Techniques used in this report are comparisons of water use rates between crops within the study area, from other studies, and to values of evaporation from evaporation pans. In addition, an analysis of the consistency of measurements between years and sites should be of interest.
The variation of water use with location can best be depicted by comparing water use measurements at Daniel, Farson, and Seedskadee for alta fescue, alfalfa, and pan evaporation (Table 9). Results of accumulated ET and pan evaporation rates show the highest values at Seedskadee and the lowest at Daniel. Comparisons of measurements along Horse Creek for mountain meadows and evaporation pans show, at best, only a slight difference based on location (Table 10). The rates for both pan evaporation and mountain meadow ET are slightly greater at Daniel than at Merna.
Comparisons between crops show a much greater ET rate for alfalfa than for alta fescue and a slightly greater rate for alfalfa than for mountain meadows when comparing rates at the same locations. In addition, the measured ET rates for alfalfa are consistently greater than evaporation rates from class A evaporation pans. It should be recalled that measured ET rates were maximum rates. Tovey (1963) has shown that the consumptive use of alfalfa grown with surface irrigation but without a water table is less than with a static water table. His results indicate a water use rate of approximately 22% less without the water table. A reduction of alfalfa ET rates to approximately 80% of measured values gives rates slightly less than measured rates from evaporation pans. Such conditions (well watered alfalfa without a water table), are usually considered when comparing alfalfa ET rates with evaporation from a Class A evaporation pan. Measured ET rates for the mountain meadows varied somewhat between lysimeters. Reasons for this difference are not apparent, although some differences did exist in plant composition in the lysimeters (Table 11).
Results of water use efficiency calculations show that alfalfa had higher yield/water use ratios than did the mountain meadows (Tables 12-14. Considering only the first harvest gives average yield/water use ratios of 0.142 tons per acre per inch of water used for alfalfa (lysimeters 2B, 4A, and 6A) and 0.126 tons per acre per of inch water used for mountain meadows. Water use efficiencies beyond the first harvest are erratic.
TABLE 6. LYSIMETER AND SITE DESCRIPTIONS FOR WATER USE MEASUREMENTS
==========================================================================
SITE OPERATION EVAP PAN
ID LOCATION CROP METHOD INCLUDED
---------------------------------------------------------------------
2A FARSON ALTA FESCUE MAXIMUM NO
2B FARSON ALFALFA MAXIMUM NO
3A MERNA MOUNTAIN MEADOWS MAXIMUM YES
3B MERNA MOUNTAIN MEADOWS ACTUAL YES
3C MERNA IMPROVED MEADOWS ACTUAL *
3D MERNA MOUNTAIN MEADOWS MAXIMUM *
3E MERNA IMPROVED MEADOWS ACTUAL *
3F MERNA IMPROVED MEADOWS MAXIMUM *
4A DANIEL ALFALFA MAXIMUM YES
4B DANIEL ALTA FESCUE MAXIMUM YES
4C DANIEL MOUNTAIN MEADOWS MAXIMUM YES
4D DANIEL MOUNTAIN MEADOWS ACTUAL YES
6A SEEDSKADEE ALFALFA MAXIMUM YES
6B SEEDSKADEE ALTA FESCUE MAXIMUM YES
=======================================================================
* THE PAN LOCATED AT THE MERNA SITE IS APPROXIMATELY 5 MILES FROM
THESE LYSIMETERS.
TABLE 7. MONTHLY MEASURED PAN EVAPORATION*
=========================================================================
MONTH
STATION YEAR MAY JUN JUL AUG SEP OCT
--------------------------------------------------------------------------
MERNA 1984 -- 7.06 7.37 5.45 5.04 1.99B
1985 1.29A 7.47 7.17 6.79 4.17 --
DANIEL 1984 -- 6.54 7.86 6.84 4.72 1.38C
1985 1.82A 7.78 6.58 7.50 5.14 --
SEEDSKADEE 1984 -- 10.13 11.09 8.70 6.76 2.16B
1985 3.01A 11.21 11.41 12.49 7.09 --
=========================================================================
* ALL MEASUREMENTS ARE IN INCHES
A = MAY 23 - MAY 31
B = OCT 1 - OCT 18
C = OCT 1 - OCT 11
TABLE 8. MONTHLY MEASURED EVAPOTRANSPIRATION*
=================================================================
LYS MONTH
# YEAR MAY JUN JUL AUG SEP
----------------------------------------------------------------
2A 1983 -- -- 3.98A 7.66 7.18
1984 3.39 4.68 6.38 5.28 4.48
1985 1.26B 6.42 6.25 7.52 4.65
2B 1983 -- -- 5.47A 10.43 4.86
1984 5.16 9.55 8.81 11.09 5.27
1985 1.60B 8.50 9.96 10.24 5.23
3A 1983 -- 2.12C 3.32 5.01
1984 1.97D 6.78 7.32 8.38 --
1985 1.18B 6.27 5.97 5.67 2.05
3B 1983 -- 2.56C 5.21 3.87
1984 1.08D 6.03 7.48 6.40 1.56
1985 0.68B 6.11 6.22 2.43 1.71
3C 1983 -- 2.33C 4.26 3.77
1984 1.66E 4.14 4.94 2.22 1.51
1985 0.73B 4.38 4.96 1.49 1.36
3D 1983 -- 3.19C 4.44 3.99
1984 1.16E 4.04 5.34 4.18 4.04
1985 1.16B 5.41 6.50 3.47 2.73
3E 1983 -- 2.84 4.26 4.29
1984 1.44E 4.35 5.26 3.23 1.12
1985 1.14B 5.86 6.48 1.76 1.18
3F 1983 -- 4.30C 4.06 4.36
1984 1.81E 4.26 5.92 4.96 2.02
1985 1.31B 6.19 7.11 4.50 2.01
4A 1983 -- 2.43C 6.10 4.80
1984 1.59F 6.45 8.83 5.55 3.98
1985 1.79B 9.13 11.98 6.71 5.17
4B 1983 -- 2.04C 4.11 4.03
1984 0.98F 3.54 5.96 5.08 4.91
1985 1.61B 6.37 5.39 5.94 3.33
4C 1983 -- 4.15C 5.83 3.77
1984 2.53F 6.07 7.12 5.58 4.21
1985 2.95B 8.60 8.59 5.40 3.17
4D 1983 -- 3.43C 4.89 4.04
1984 2.44F 6.19 6.89 2.94 2.13
1985 1.72B 6.27 6.84 1.80 1.56
6A 1983 -- -- 5.65A 8.69 3.89
1984 6.96 11.51 12.70 14.00 4.71
1985 2.58B 12.46 14.14 16.26 5.40
6B 1983 -- -- 7.56A 7.96 7.06
1984 5.34 6.39 7.68 6.61 4.43
1985 1.87B 7.88 6.88 6.98 4.47
================================================================
* ALL MEASUREMENTS ARE LISTED IN INCHES.
A = JULY 8 - JULY 31
B = MAY 23 - MAY 31
C = JULY 15 - JULY 31
D = MAY 17 - MAY 31
E = MAY 15 - MAY 31
F = MAY 16 - MAY 31
TABLE 9. ACCUMULATED MEASURED ET AND EVAPORATION RATES*
====================================================================
EVAPOTRANSPIRATION
ALTA FESCUE ALFALFA EVAPORATION
MONTH 4B 2A 6B 4A 2B 6A DANIEL MERNA SEEDS
--------------------------------------------------------------------
JUN 5.0 5.6 7.1 7.8 9.0 12.0 7.2 7.3 10.7
JUL 10.6 11.9 14.4 18.2 18.4 25.4 14.4 14.5 21.9
AUG 16.2 18.3 21.2 24.3 29.1 40.5 21.6 20.7 32.5
SEP 20.3 22.9 25.7 28.9 34.3 45.6 26.5 25.3 39.5
====================================================================
* ALL VALUES ARE GIVEN IN INCHES.
TABLE 10. ACCUMULATED ET FOR MOUNTAIN MEADOWS*
=====================================================
LYSIMETER SITE
MONTH 3A 3D 3F 4C
-----------------------------------------------------
JUN 6.5 4.7 5.2 7.3
JUL 13.2 10.7 11.8 15.2
AUG 20.2 14.5 16.5 20.7
SEP 22.3 17.9 18.5 24.4
====================================================
* ALL VALUES ARE GIVEN IN INCHES.
TABLE 11. CLASSIFICATION OF VEGETATION ON LYSIMETERS*
====================================================================
LYSIMETER SCIENTIFIC NAME COMMON NAME PERCENT
--------------------------------------------------------------------
3A Phleum pratense TIMOTHY 30
Melilotus SWEET CLOVER 25
Taraxacum officinale DANDELION 25
Agrostis stolonifera RED TOP BENTGRASS 15
Poa pratensis KENTUCKY BLUEGRASS 5
3D Melilotus SWEET CLOVER 50
Phleum pratense TIMOTHY 40
Taraxacum officianle DANDYLION 10
3F Phleum pratense TIMOTHY 92
Taraxacum officinale DANDELION 5
Hordeum pusillum LITTLE BARLEY 3
4C Melilotus SWEET CLOVER 25
Poa pratensis KENTUCKY BLUEGRASS 25
Agropyron trachycaulum SLENDER WHEATGRASS 20
Hordeum pusillum LITTLE BARLEY 10
Phleum pratense TIMOTHY 5
Taraxacum officinale DANDYLION 5
Poa fendleriana MUTTON BLUEGRASS 5
===================================================================
* TRACES OF OTHER VEGETATION MAY ALSO BE PRESENT. DATE OF THE
SURVEY WAS AUG 8, 1985. VEGETATION CLASSIFICATIONS FOR THE OTHER
LYSIMETERS ARE GIVEN IN APPENDIX D.
TABLE 12. HARVEST DATES OF LYSIMETERS FOR 1984 AND 1985
=============================================================================================
1984 1985
------------------------- --------------------------
LYSIMETER SITE FIRST SECOND CLOSE OUT FIRST SECOND CLOSE OUT
LOCATION I.D. CROP HARVEST HARVEST HARVEST HARVEST HARVEST HARVEST
---------------------------------------------------------------------------------------------
FARSON 2A ALT.FES.
FARSON 2B ALFALFA JUL 12 SEP 06 OCT 17 JUL 17 AUG 28 OCT 07
MERNA 3A MTN.MED. AUG 29 AUG 29 OCT 09
MERNA 3B MTN.MED. AUG 29 AUG 29 OCT 07
HORSE CR. 3C IMP.MED. AUG 09 AUG 08 OCT 09
HORSE CR. 3D MTN.MED. AUG 09 AUG 08 OCT 09
HORSE CR. 3E IMP.MED. AUG 30 AUG 08 OCT 09
HORSE CR. 3F IMP.MED. SEP 06 AUG 08 OCT 08
DANIEL 4A ALFALFA JUL 18 OCT 19 AUG 07 OCT 08
DANIEL 4B ALT.FES.
DANIEL 4C MTN.MED. JUL 19 OCT 19 AUG 07 OCT 08
DANIEL 4D MTN.MED. JUL 19 OCT 19 AUG 07 OCT 08
SEEDSKADEE 6A ALFALFA JUL 11 AUG 28 OCT 19 JUL 10 SEP 06 OCT 08
SEEDSKADEE 6B ALT.FES.
===========================================================================================
TABLE 13. HARVEST YIELDS AND YIELD/ET RATIOS OF LYSIMETERS FOR 1984*
=============================================================================================
FIRST HARVEST SECOND HARVEST CLOSE OUT HARVEST
-------------------- -------------------- ----------------------
LYSIMETER SITE SITE CROP YIELD/ET SITE CROP YIELD/ET SITE CROP YIELD/ET
LOCATION ID ET YIELD RATIO ET YIELD RATIO ET YIELD RATIO
--------------------------------------------------------------------------------------------
FARSON 2B 16.7 1.97 .118 17.1 1.92 .112 4.20 .54 .128
MERNA 3A 24.3 2.17 .089
MERNA 3B 21.0 2.00 .095
HORSE CR. 3C 11.5 1.64 .143
HORSE CR. 3D 11.7 1.84 .157
HORSE CR. 3E 14.0 2.22 .159
HORSE CR. 3F 17.3 2.15 .124
DANIEL 4A 13.8 2.40 .174 14.00 .53 .038
DANIEL 4C 12.9 2.29 .178 14.01 .30 .093
DANIEL 4D 13.2 2.27 .172 7.10 .45 .063
SEEDSKADEE 6A 19.0 2.23 .117 21.8 1.69 .078 7.3 1.22 .167
===========================================================================================
* CROP YIELD IS ABOUT 12% MOISTURE CONTENT.
YIELD/ET IS IN UNITS OF TONS PER ACRE PER INCH.
TABLE 14. HARVEST YIELDS AND YIELD/ET RATIOS OF LYSIMETERS FOR 1985*
===========================================================================================
FIRST HARVEST SECOND HARVEST CLOSE TO HARVEST
----------------- ------------------ -------------------
LYSIMETER SITE SITE CROP YIELD/ET SITE CROP YIELD/ET SITE CROP YIELD/ET
LOCATION ID ET YIELD RATIO ET YIELD RATIO ET YIELD RATIO
--------------------------------------------------------------------------------------------
FARSON 2B 17.0 3.00 .176 12.5 0.67 .054 6.7 0.24 .036
MERNA 3A 18.2 1.01 .055 3.1 0.10 .032
MERNA 3A 15.3 1.42 .093 2.1 0.06 .029
HORSE CR. 3C 10.3 0.83 .081 2.3 0.03 .013
HORSE CR. 3D 14.2 1.57 .111 5.4 0.02 .004
HORSE CR. 3E 14.4 1.05 .073 2.3 0.06 .026
HORSE CR. 3F 16.1 2.32 .144 4.9 0.11 .022
DANIEL 4A 24.5 1.86 .076 10.6 1.25 .118
DANIEL 4C 21.8 4.92 .226 7.6 0.64 .084
DANIEL 4D 15.5 2.16 .139 2.9 0.07 .024
SEEDSKADEE 6A 20.8 4.45 .214 27.2 2.23 .082 3.6 0.55 .153
==========================================================================================
* CROP YIELD IS ABOUT 12% MOISTURE CONTENT.
YIELD/ET IS IN UNITS OF TONS PER ACRE PER INCH.
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