Brian Piette modeled evaporation from Lake Whatcom in Bellingham, Washington for the 2014 water year using the Penman method. He compared the modeled monthly evaporation totals from Lake Whatcom to pan evaporation data from the Post Point weather station. Total evaporation from Lake Whatcom was higher at 15.5 inches compared to 12.1 inches from the weather station pan. Evaporation was highest in July for both Lake Whatcom and the weather station. Brian also examined energy and mass transfer variables like radiation and wind speed that influence evaporation rates. He found evaporation was higher than monthly water withdrawals from the city of Bellingham in May, June and July and that physical water loss from evaporation is

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Memo
From: Brian Piette
To: Robert Mitchell
Date: November 6th, 2015
Re: Project 4: Evaporation at Lake Whatcom and at Post Point weather station
Introduction
It is often necessary to determine the amount of water that is evaporated off a free water surface, such as a lake,
in order to gain a better understanding of what factors drive evaporation. Various data collected from the Post
Point weather station in Bellingham, Washington (Figure 1) give results that determine the effectiveness of the
Penman Method model during the 2014 water year (WY). To observe patterns and frequencies of evaporation
that’s typical of a Pacific Northwest lake, I will compare monthly cumulative evaporation totals from Lake
Whatcom (LW) to pan evaporation totals at Post Point weather station. I will also consider other parameters that
drive evaporation, such as energy and mass transfer variables. In addition, data representing the amount of
water withdrawal from LW shall be examined and compared with total lake evaporation. It is important to
recognize the magnitude of lake evaporation in order to gain a better understanding of the local water budget.
Methods
Modeling lake evaporation involves the calculation of an energy-balanced equation that is considered in the
Penman Method. Calculated values are based on physical and empirical relations, which are computed in
Microsoft Excel. Raw data during the 2014 WY comes from the City of Bellingham (COB) Post Point weather
station. Figures generated in this report were conducted through ARC GIS and Microsoft Excel. Detailed
procedures can be found in appendix A.
Results
The Penman method result displays higher evaporation values in the warm, summer months and low
evaporation values during cold months (Figure 2). July 14th, 2014 appears to have the highest evaporation at
almost 0.20 inches while many days in November, December and January have zero evaporation (Figure 2).
Calculated monthly total evaporation values at LW are larger than monthly pan evaporation at Post Point, with
the exception of October (Figure 3). July has the most evaporation in WY 2014, both at LW (3.44 in) and at
Post Point (3.14 in). Total evaporation values at LW for WY 2014 is 15.5 inches, total pan evaporation is 12.1
inches (Figure 3). Observing the flux of shortwave radiation throughout WY 2014, highest values occur in
summer months while lowest values are observed in winter months (Figure 4). Sensible heat transfer shows a
very similar trend, high values in summer, low values in winter. Longwave radiation appears to have no trend
throughout WY 2014 (Figure 4). Monthly withdrawal totals from the COB are much higher during the summer

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months, July having 421.53 million gallons taken out of LW (Figure 5). February is the lowest at 242.47 MG.
Total evaporation over the entire lake is highest in July (462.7 MG) and lowest in January (0.40 MG). Only in
May, June and July evaporation rates are higher than water withdrawal from the COB(Figure 5).
Discussion
Evaporation takes place when there is no precipitation occurring. Therefore the change of evaporation rate from
low values in the winter months to high values in the summer months agree with the typical maritime climate
the Pacific Northwest offers (Figure 2). Less precipitation during the summer results in a gradient of high
concentrations of water vapor molecules at the boundary layer to low concentrations above the lake, which
drives evaporation. The amount of shortwave radiation received at LW (Figure 4) is correlated: due to the 23.5
degree tilt of Earth, changes in the angle of incidence of incoming radiation varies depending on the season. The
shorter the path of radiation, the less atmospheric depletion (Mitchell, in lecture), as seen in evaporation values
during the summer months. No significant changes in longwave Radiation (Figure 4) throughout WY 2014 were
observed. This may be due to the fact that lake surface temperatures are not accounted for in the Penman
Method.
There exists a difference between total monthly evaporation values at LW compared with pan evaporation rate
at Post Point (Figure 3). Because of the larger surface area, factors such as wind speed may affect evaporation
rate at the lake more compared with a small area of water, or the pan. Mass transfer mechanisms, such as the
wind, collects more water vapor downwind, decreasing evaporation rate. Depending on the average wind
direction at LW, data may have been collected where there is a higher deficit of water vapor molecules,
resulting in a higher calculated evaporation rate. The 1.2 meter wide pan is too small to have mass transfer
mechanisms occur (i.e. have a higher concentration of water vapor molecules on one side of the pan). Also,
August 5th and August 9th are very similar days (similar temperatures and cloudiness factors) but average wind
speed is nearly four times faster on August 5th (210 km/day). The total difference in evaporation at LW between
those two days in .06 inches (August 5th had more evaporation).
The Penman Method does not take into account precipitation. Therefore rainfall is going to affect the pan data
more than LW because the pan involves a smaller volume of water. On days where rainfall and evaporation
occur, such as a typical spring day, measurable water loss due to evaporation may be less than the actual value
of evaporation due to water recharge, resulting in incorrect values.
Total evaporation at LW and the parameters that most affect this will ultimately have an impact on the local
water budget. Evaporation is higher than the amount of water the COBwithdrawals during May, June and July
(Figure 5). Water resources managers must take this into consideration and understand that physical water

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withdrawal is not the only process that results in a net loss of water at LW during the dry summer months.
Considering evaporation off a free water surface is essential when assessing the cities’ local flux in available
water to the surrounding community in order to sustain a readily available resource.

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References
Mitchell, Robert. “Evaporation”. Environmental Studies building, Western Washington University. October-
November 2015. Lecture.

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Figures
Figure 1. Location of Lake Whatcom and Post Point weather station. Bellingham, Washington.

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Figure 2. Total evaporation (inches) per day during the 2014 WY at Lake Whatcom, located in Bellingham, Washington.
Figure 3. Total monthly evaporation of WY 2014 for Lake Whatcom versus total monthly pan evaporation from Post Point. Both locations are in
Bellingham, Washington.
0.000
0.050
0.100
0.150
0.200
Evaporation(in/day)
Water Year (Oct-sept)
0
0.5
1
1.5
2
2.5
3
3.5
Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
Evaporation(in/month)
Month
Lake Pan
Total lake:15.5 in
Total pan:12.1 in

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Figure 4. Shortwave, longwave radiation and sensible heat transfer fluxes though the 2014 WY at Lake Whatcom, Bellingham, Washington.
Figure 5. A look at the water budget for Lake Whatcom during the 2014 WY. Shown are monthly withdrawal values aloing with monthly evaporated
water values.
-400.00
-200.00
0.00
200.00
400.00
600.00
800.00
CalcubicCm/Day
2014 WY (Oct-Spet)
Shortwave Radiation Longwave Radiation Sensible Heat Transfer
0
50
100
150
200
250
300
350
400
450
500
MillionsOfGallons
Evaporated Monthly Withdrawal

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Appendix A.
GEOL 472/572 Surface-Water Hydrology
F2015 –Project 4
Out: 27October
Due: 6 November
Your objective forthisprojectistouse the Penmanmethodtoestimate the free-waterevaporationfromLake Whatcom
for the 2014 wateryear (WY) and compare the modeledmagnitudestopanevaporationdatafromthe weatherstation
at Post PointinBellinghamandwaterwithdrawal volumesfromthe cityof Bellingham(COB).Youwill alsoexaminethe
energyandmass-transfervariablesthatcontrol evaporation.
Findthe Project4 folderin ...classesGeol472-572Projects.The 2014 data setand yourfuture model isanExcel file
calledPenman_Model_WY14.xls. Copythisfile intoyourworkspace folder.
1. Your firsttask isto type the equationslistedinthe Penmanmethodhandout(Penman_Method.pdf)intoExcel under
the appropriate parameter.Use the Oct 1 valuesat the top of the table (yellow)tovalidate yourresults.The area
(AL) of Lake Whatcomrequiredfor KE is 20.07 km2
.
2. Aftercompletingandvalidatingthe model,plotthe dailymodeledevaporationasafunctionof days(Figure 2 for
your report;Figure 1 isthe locationfigure withthe PostPointlocation).
3. Create a columnplot(Figure 3) that illustrates Ein/month of the modeledevaporationstartingwithOctoberalong
withthe pan evaporationdatafromthe PostPointtreatmentplant.The PostPointvaluesincludedinthe Excel
spreadsheethave beenmultipliedbyapan coefficient(0.8).
4. Calculate the total numberof inchesforthe year(Ein/year) of the evaporation. Putthe total yearlyevaporationina
textbox inFigure 3. You can report thisvalue inyourresultsordiscussionsection.
5. Refertothe energybalance equation (whenAw,G,and Q are ignored) inthe Penman_Method pdf. K,L andH in
thisequationare the heatinputs(oroutputs) of the lake.Manipulate thisequationtodeterminethe magnitude of H
for eachday. Create a figure (Figure 4) andplotthe magnitudesof K, L and H as a functionof dayson one figure.
Scale the plotfrom -400 to 800 cal cm-2
day-1
Examine theirrelativemagnitudes.Thinkaboutwhyduringcertain
timesof the year,some valuesare negative.
6. There isanotherExcel file (COB_WY2014.xlsx) inthe Project_4folderthatliststhe dailyvolume of water(millions of
gallons(MG)) withdrawnfromLake Whatcomby the COB duringthe 2014 wateryear.Giventhat the average lake
surface area is20068331m2
, determinedthe dailyvolume of water(MG) evaporatedfromLake Whatcom. Create a
columnplot(Figure 5) that illustratesMG/month of waterevaporatedstartingwithOctoberalongwiththe monthly
COB withdrawal volumesforthe 2014 WY.
7. Use the model toexplore the sensitivityof othervariablesonevaporation.It’squite obviousthatevaporationwill
increase withtemperatureandsolarradiation,butwhataboutthe cloudinessfactor, C?The cloudinessfactoristhe

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mostdifficultparametertoquantify,buthasa large impact onevaporation—checkoutthe Kand L and Ein/day
valuesforthe Aug10 and12 dates(similartemperaturesandwindspeeds).The windspeedalsomakesadifference.
For example,examine the windspeed(va) valuesforAugust5 and9 (similartemperaturesandcloudinessfactors)
and observe howitchangesthe total evaporation.
Deliverables
You will integrate yourworkforthisprojectintoatechnical reportguidedbythe reporttemplate.Youreportwill
include the following:
1. Figure 1 – the locationfigure showingLake Whatcomwiththe PostPointlocation
2. Figure 2 – plotof the modeledevaporationininchesasa functionof time (days)
3. Figure 3 – monthlycolumnplotof the modeleddatainincheswiththe pandatain inches
4. Figure 4 – the plotsof dailyK,L, andH
5. Figure 5 – the columnplotillustratingthe monthlyevaporatedandCOBwithdrawal volumes.
6. ThisproceduresdocumentasAppendixA.
7. Do not turn inthe data from the large Penmanmodel spreadsheet.
Submityourreportin a digital formattoyo