APPENDIX C

PAN EVAPORATION DATA

Pan evaporation data were used in this project to determine the monthly percentage distribution of FWS evaporation and to check on the usefulness of the ET data. Because there are so few pan evaporation stations in Idaho, it was necessary to try to use all of the available data.

Evaporation pans are typically read from May through September throughout most of Idaho, although in some years, the season is extended through the end of October. The stations that were used for this project are shown in Table C-1.

Table C-1. Pan evaporation stations
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Number Latitude Longitude Elevation Name
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100010 4257 11250 4410 Aberdeen Exp Stn
100448 4336 11555 3275 Arrowrock Dam
105275 4207 11118 5930 Lifton Pumping Stn
105466 4357 11340 6000 Mackay 4NW
106152 4644 11658 2660 Moscow - Univ of Idaho
106764 4322 11114 5385 Palisades Dam
106844 4348 11657 2220 Parma Exp Stn
107644 4349 11147 4920 Rexburg Ricks College
248783 4620 11405 3600 Western Ag Research Center at Corvallis
244328 4821 11400 3160 Hungry Horse Dam
242409 4512 11238 5230 Dillon WMCE
355160 4359 11701 2260 Malheur Branch Exp Stn
457938 4738 11732 2360 Spokane WSO Airport
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All available data for the April or May-October period were assembled. In many cases, there were a few days missing at the start or end of the measurement season. This would usually be attributed to a late spring or an early frost in the fall.

In some cases, the National Climatic Data Center would estimate in the few missing days to produce a total for the month. Most of these were flagged when published. The estimating method used was to determine the evaporation per day for the measured period and assume that the missing days had the same mean evaporation. For months with longer periods of missing records, usually 10 or more days, NCDC did not estimate a total for the month. For those months, the same type of estimating was done by the Idaho State Climate Office. That estimated total was then compared against the long-term average for those days, taking into account the fact that the month under consideration may or may not have been above or below normal in evaporation for the measured days.

For the period 1986-1990, a second estimating method was used. In this case, the mean evaporation for the measured days was determined and adjusted by the ratio of the mean temperature for the missing period to the measured period. When checked against known measurements of evaporation, this method yielded monthly evaporation values within five percent of the measured values.

Table C-2. Mean pan evaporation (in inches) data for stations in Idaho and surrounding states.

Station

Apr

May

June

July

Aug

Sept

Oct

Aberdeen Exp Stn

 

7.58

9.01

10.36

9.42

6.46

3.37

Arrowrock Dam

 

5.99

7.45

9.88

8.84

5.70

2.15

Lifton Pumping Stn

 

5.73

7.40

8.41

7.80

5.31

2.67

Mackay 4NW

 

6.96

8.75

10.49

9.07

6.68

2.64

Moscow - Univ Of Idaho

 

5.56

6.56

8.79

8.28

5.30

1.94

Palisades Dam

 

6.11

6.93

9.75

8.62

5.25

3.88

Parma Exp Stn

1.09

8.24

8.90

10.07

9.42

6.19

1.20

Rexburg Ricks College

 

5.15

7.60

7.98

8.45

5.81

 

Twin Fall WSO

1.00

8.35

9.30

10.32

9.13

6.66

4.42

Western Ag Research Cnt, Mt

 

4.59

5.21

6.30

5.58

3.34

1.86

Hungry Horse Dam, MT

 

4.43

5.77

7.59

6.45

3.17

1.26

Dillon WMCE, MT

 

4.64

4.98

5.93

5.10

3.21

2.57

Malheur Branch Exp Stn, OR

5.04

7.16

8.48

10.92

9.13

5.75

2.64

Spokane WSO Airport, WA

1.09

7.25

8.92

11.44

10.29

6.31

 

The actual monthly data, together with quality control flags, are available in the State Climate Office at the University of Idaho