Mr. Gera ld R. Z imerman Execu t i ve D i r e c t o r Upper Colorado R i v e r Conmission 355 South 400 East S a l t Lake C i t y , Utah 84111

Dear Mr. Z imerman:

as,.

The f i n a l r e p o r t on the methods of comput ing evapo ra t i on f rom Lake Powel l i s enclosed. A d r a f t of t h i s r e p o r t was sen t t o you on October 30, 1984. The r e s u l t s o f t h i s r e p o r t may be used i n f u r t h e r s t u d i e s t o improve our d e t e r m i n a t i o n of i n f l o w t o Lake Powel l and bank s torage. The r e s u l t s o f these s t u d i e s may h e l p t o f u r t h e r r e f i n e t h e methods o f computing evapora- t i o n . Also, we hope sometime i n t he f u t u r e t o develop a procedure t o determine evapo ra t i on f o r use i n near r e a l - t i m e ope ra t i ons . Th is w i l l p robab l y i n v o l v e pan evapo ra t i on measurements and weather data .

I f you have any comnents o r quest ions , c a l l Lee Mor r i son a t 524-5573.

S i n c e r e l y yours ,

Reg iona l D i r e c t o r

Enc losure

cc: Reg iona l D i r e c t o r , Bou lder C i t y , Nevada

be: Chie f , D i v i s i o n o f P lann ing Techn ica l Serv ices , E&R Center A t t e n t i o n : 0-752 Power Opera t i ons Manager, Page, A r i zona

UNITED STATES OF AMERICA DEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATION

LAKE POWELL EVAPORATION

AUGUST 1986

UPPER COLORADO REGIONAL O F F I C E SALT LAKE CITY. UTAH

TABLE O F CONTENTS

INTRODUCTION ........................................................ ............................................ METHODS OF DETERMINATION

PROCEDURE ........................................................... MASS-TRANSFER METHOD ................................................ LAKE POWELL MEASUREMENTS ............................................ ADJUSTMENTS TO DATA ................................................. C A L I B R A T I O N C O E F F I C I E N T ............................................. COMPUTATIONS ........................................................

..................................... COMPARISON OF EVAPORATION RATES

................................................... GROSS EVAPORATION

N E T EVAPORATION ..................................................... ........................ EVAPORATION FROM PRE-RESERVOIR WATER SURFACE

STREAMSIDE VEGETATION ............................................... .......................................... TERRACE EVAPOTRANSPIRATION

............................................... BLANEY-CRIDDLE METHOD

REMAINING AREA ...................................................... RESULTS ............................................................. RECOMMENDATIONS ...................................................... REFERENCES ..........................................................

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27

INTRODUCTION

I n the e a r l y 1950's i t was expected t h a t a h i g h l y c r e d i b l e determinat ion of evaporat ion from reservo i rs i n the Colorado Basin was requ i red because of the need t o d i v i d e the water between the Upper and Lower Basins according t o the Colorado River Compact of 1922. flu. a.*.n JqaÑiMfUfiAW from Colorado River Storage Pro ject Reservoirs- (Lake Powell, Flami Navajo, Blue Mesa, Morrow Point and C r y s t a l ) acre- feet per year. l^msMffWfW o f t h i s i s evaporated Ç

-Therefore, a p lan was developed t o measure and c o l l e c t data. Since t h a t t ime i t has been determined t h a t the Upper Basin w i l l probably n o t be able t o develop the 7,500,000 a f pe r year a l l o t t e d by the Colorado R iver Compact*. Therefore, a t the present time, the accuracy o f the evap- o r a t i o n determinat ion i s not so c r i t i c a l . But the determinat ion i s s t i l l needed f o r severa l purposes inc lud ing water use and a v a i l a b i l i t y studies, streamflow fo recas t ing , and water budget s tud ies.

This r e p o r t describes and comparies several methods of determining evapora- t i o n from Lake Powell.

METHODS OF DETERMINATION

Although evaporat ion from a r e s e r v o i r cannot be measured d i r e c t l y , the re are several ways o f determining i t i n d i r e c t l y . . One way i s termed

od, the

b e c a u s e n o t a11 of the in f low can be measured and h-- - 0

n e a s u r e m e n t s can be used. This method i s being used as a check b u t i t has disadvantages i n t h a t Pans do not account f o r the change i n energy s torage nor for advected energy.

A t h i r d way i s t h e - ! - If a11 of the energy en te r ing and leav ing the Lake can be measured then the energy requ i red t o balance the energy-budget i s t h a t needed f o r evaporat ion. This method i s used as a check a t Lake Mead f o r annual values. This method requ i res data which i s expensive t o ob ta in f o r periods o f t ime shor te r than 1 year. Since sur face area i s changing dur ing the year i t was des i rab le t o use a method f o r which i t was p r a c t i c a l t o ob ta in data t o complete evaporat ion fo r periods o f t ime shor te r than a year.

Another method i s based on M M à ‘ f ^ d à ‘ à ‘ w h says t h a t the r a t e o f evapor- a t i o n i s a f u n c t i o n o f the d i f fe rences i n the vapor pressure a t the water surface. IMl* * *Nothing i n t h i s repor t i s intended t o i n t e r p r e t the p rov is ions o f the Colorado River Compact (45 Stat. 10571, the Upper Colorado River Basin Compact (63 Stat . 31), the Water Treaty o f 1944 w i t h the United Mexican States (Treaty Series 994, 59 Stat . 12191, the Decree entered by the Supreme Court of the Uni ted States i n Arizona v. C a l i f o r n i a e t a]. (379 U.S. 3401, the Boulder Canyon Pro jec t Act (45 Stat. 10571, the Boulder Canyon Pro jec t Adjustment Act (54 Stat . 774; 43 U.S.C. 618a), the Colorado River Storage Pro jec t Act (70 Stat . 105.; 43 U.S.C. 6201, o r the Colorado River Basin P r o j e c t Act (82 Stat . 885; 43 U.S.C. 1501). Pursuant t o the Colorado River Basin Pro jec t Act (Pub l i c Law 90-537) of 1968.

I

PROCEDURE

I n the l a t e 1950's several evaporat ion pans were i n s t a l l e d near Lake Powell, Flaming Gorge, and Navajo Reservoirs. Data from pans a t the f o l l o w i n g loca t ions were co! lec ted f o r the May-October per iod f o r 1958 through and converted i n t o an average annual evaporat ion ra te .

Locat ion of Pan

H i t e Mexican Hat Moab Page

To ta l Average

~ v e r a & Annual Gross Evaporat ion Rate i n Inches

A pan was es tab l i shed a t Green River, Utah, bu t the data was no t used. A pan was i n s t a l l e d a t Wahweap i n 1962, b u t t h e data was n o t ava i lab le when the average annual evaporat ion r a t e was determined.

! A method was establ ished t o determine the evapo- t ransp i ra t ion from the pre-

I r e s e r v o i r Lake Powell area and data was c o l l e c t e d be fo re the Lake s t a r t e d i t o fill. The amount o f evaporat ion caused by Lake Powell was estimated by I

-. .

This evaporat ion has been used t o the present t ime (Table 8). The method

i and t h e computations f o r determining the evaporat ion from t h e equiva lent I pre - reservo i r i s i n the f i l e s o f the Water Operations Branch of the Upper 1 Colorado Region.

Studies o f severa l methods o f determining evaporat ion had been made a t Lake Hefner i n Oklahoma and Lake Mead. I t was decided t h a t c o l l e c t i n g data f o r the energy-budget method would be very expensive f o r per iods of t ime less than 1 year. Since i t was des i rab le t o determine evaporat ion f o r shor ter per iods o f t i m e , i t was decided not t o use the energy-budget method. I t was a lso des i rab le t o make a determinat ion independent o f the pan method. Therefore, the mass-transfer method was selected.

Three r a f t s were constructed, instrumented, and placed a t Wahweap Bay, Padre Bay, ( a l s o c a l l e d Crossing-of-the-Fathers), and B u l l f r o g Bay as shown i n F igure 1. The data was processed i n the e a r l y 1970's b u t the re was some concern as t o t h e c o e f f i c i e n t t o use i n the equation. A t t h a t t ime scien- t i s t s from several u n i v e r s i t i e s were i n v e s t i g a t i n g several aspects o f Lake Powel 1. The studies, sponsored by the Nat ional Science Foundation, were named the Lake Powell Research Pro ject . A cooperat ive agreement was made t o make an evaporat ion study as p a r t o f t h e i r water budget analys is . The Bureau fu rn ished inst ruments and another r a f t which was i n s t a l l e d a t Hi te . The Bureau p a r t i c i p a t e d i n t h i s t o o b t a i n data t o c a l i b r a t e a mass-transfer c o e f f i c i e n t f o r t h e da ta we had co l lected. New casset te recorders and sensors were i n s t a l l e d on t h e r a f t s along w i t h the o l d equipment which was s t i 11 being used. Data was co l l ec ted f o r about 2 years and analyzed by the energy-budget and mass-transfer methods and compared w i t h pan measurements.

Colorodo River

Figure i : Locations of Data C o l l e c t i o n System Rafts for Evapo- r a t i on Study o f L a k e P o w e l l

MASS TRANSFER METHOD

The following is a brief description of the mass-transfer method. A simple example of the same type as the Lake Hefner quasi-empirical equation is

in which E=evaporation, in inches per day; N=a coefficient of proportionality, hereafter called the mass-

transfer coefficient; u=wind speed, in miles per hour, at some height above the water

surface; a numerical subscript, if used, indicates the height in meters;

e0=saturation vapor pressure in millibars, corresponding to the temperature of the water surface;

eaavapor pressure of the air, in mi T I ibars; a numerical subscript, If used, indicates the height in meters.

Nearly all the mass-transfer equations to be found i n the literature have one thing in common: evaporation is considered to be proportional to the product of the wind speed, u, and the vapor-pressure difference, eo-ea. In a few equations, the wind speed, u. has an exponent, usually less than unity.

The mass-transfer coefficient, N, represents a combination of many variables in the published mass-transfer equations. Among these are the manner of the variation of wind with height, the size of the lake, the roughness of the water surface, atmospheric stability, barometric Pressure, and density and kinematic viscosity of the air.

The following mass-transfer equation was developed for Lake Mead and used to make monthly determinations of evaporation rates. These determinations have been checked on an annual basis by the energy-budget and pan- evaporation methods.

E = evaporation in inches per day.

U z = is wind speed in knots.

en= is saturation vapor pressure at the temperature of the water surf ace.

e2= is the vapor pressure of the air computed in millibars at 2 meters above the water surface.

FIGURE 2

5

1. Air Temperature 2. Water Temperature 3. Relative Humid i ty

LAKE POWELL MEASUREMENTS

The f o l l o w i n g data was c o l l ec ted from each of the three r a f t s ; water sur- f ace temperature, a i r temperature, and r e l a t i v e humidity. The wind was measured by an anemometer placed two meters above the water surface. The anemometers were read once each week. A t each of the th ree r a f t s , water surface temperature, a i r temperature, and r e l a t i v e humid i ty were recorded on 7 day c i r c u l a r char ts as shown i n F i g u r e 2 . Wind was measured and recorded by anemometers placed 2 meters above the water sur face which were read once each week. The data was manual ly read and recorded on t h e form shown i n F igure 3.

ADJUSTMENTS TO DATA

The wind r u n was requ i red on a d a i l y bas is b u t t h e r a f t anemometers were o n l y read weekly on t h e r a f t s . Therefore, the d a i l y wind r u n from anomeme- t e r s a t t h e Uahweap Marina and Page were used t o pro r a t e the weekly r a f t wind r u n readings f o r each day.

Some o f t h e data was n o t recorded o r was obvious ly wrong. Therefore, methods were developed t o est imate miss ing data e i t h e r by in te rpo la t ion , observing trends be fo re and a f t e r the miss ing data, o r by c o r r e l a t i o n w i t h data from other r a f t s . Wind has a l a r g e e f f e c t on evaporat ion and i s h i g h l y va r iab le . Therefore, a procedure was developed t o est imate miss ing wind data us ing a h ie ra rchy o f opt ions s t a r t i n g w i t h the most p re fe r red method. This procedure i s expla ined i n the f i l e s o f t h e Water Operat ions Branch. Other d e t a i l s and computer p r i n t o u t s are a lso a v a i l a b l e i n these f i l e s .

CALIBRATION OF COEFFICIENT

The evaporat ion r a t e as determined by t h e Lake Powell Research P r o j e c t ( L P R P ) was used t o c a l i b r a t e a mass-transfer c o e f f i c i e n t . For those days i n 1974 when t h e r e was concurrent data as c o l l e c t e d by the LPRP instruments and Bureau instruments a regress ion analys is was made f o r each of the three data s t a t i o n s . Thus there were 295 data sets a t Wahweap Bay, 143 da ta sets a t B u l l f r o g Bay and 133 data se ts a t Padre Bay. The regress ion coef- f i c i e n t s were; .53 a t Wahweap; .72 a t Padre Bay, and .55 a t B u l l f r o g Bay. These c o e f f i c i e n t s are no t very h igh ma in ly because o f the determined d i s t r i b u t i o n o f wind data. I f the regress ion analys is were done f o r weekly o r longer per iods, t h e regress ion c o e f f i c i e n t s would be much higher. A compari--- monthly r a t e s i s shown i n Table 1. From the regress ion ana- l y s i s fl -

DATA FOR CO: A

BY MAS

! COMPUTATION OF EVAPORATION AT LAKE POWELL

M r "-TRANSFER METHOD WATER A N D POWER RE:

D I V I S I O N OF WATER A N D R E S E R V O I R REGULA

FIGURE 3

3F EVAPORATION 'ELL WATER AND P O W E R RESOURCES SERVICE

3 '"THOD D I V I S I O N OF WATER AND LAND O P E R A T I O N S R E S E R V O I R R E G U L A T I O N B R A N C H

1 I J I I I

I 4 I

! I ' 1- I

I

i I

I I ! \

I 1 1 I

I I . , I 1 ,

I \ I I . , --

I I ' ! -- , i ! 1 1 '

!

I , I , I ! I I : 1

' I I

- I I I I

i I s I 1 ! ; 1 I ! : I I -

i ! I

I 1 I 1 , . I

----------------- 1 1 ' I i

I l l I

! \ I I

I

COMPUTATIONS

Equation (3 ) was used t o compute a d a i l y evaporat ion oped from equat ion ( 2 ) which was developed specifics

r a t e . It was deve l l y f o r Lake Mead.

( 3 )

Where

E = evaporat ion i n inches per day

u = average wind speed i n m i les per hour

eo = s a t u r a t i o n vapor pressure a t water sur face temperature i n mi 11 i bars

Hmax = maximum r e l a t i v e humid i t y as percentage

Hmin = minimum r e l a t i v e humid i ty as a percentage

eas = s a t u r a t i o n vapor pressure i n m i l l i b a r s a t average d a i l y a i r temperature.

The vapor pressure of the a i r i s approximated by m u l t i p l y i n g the average d a i l y humid i ty by t h e sa tu ra t ion vapor pressure a t the average d a i l y a i r temperature. Thus the .005 i n the equation i s from d i v i d i n g by 100 t o con- v e r t from percentage and d i v i d i n g by 2 t o get an average. The computations were done by a computer program termed EVAP. The d a i l y evaporat ion ra tes were t o t a l e d f o r each month. They are shown i n Table 2 .

COMPARISON OF EVAPORATION RATES

Table 3 i s pan evaporat ion as measured a t Wahweap. Table 4 i s the Lake Powell evaporat ion r a t e i n inches from the adjusted c lass A pan a t Wahweap Arizona. This adjustment i s described i n Lake Powel 1 Research 8ul l e t i n Number 48. The average annual evaporat ion r a t e f o r t h e evaporat ion pan method was 69.44 inches f o r the 1962-1975 per iod w h i l e the r a t e f o r the mass-transfer method was 68.32 inches f o r the same per iod.

GROSS EVAPORATION

NET EVAPORATION

kcc-^Yop "Â¥ which i s the amount o f increased evaporat ion caused by t h e r e s e r v o i r i s needed f o r some purposes. I t

w q e n m m sMtRR , a l s o

'

TABLE 1

'̂ "a 3.65

\ a re LPRP I/ Bay Bureau 2/ 4 .17

I Bullfrog bay Bureau 21 Adjusted Pan 3/ - -- -~ --

I / As computed by Lake Powell Research Project - 2/ AS computed using equation ( 3 ) using mass-transfer c o e f f i c i e n t derived by r e g r e s s i o n a n a l y s i s . - 3/ Based on adjusted class a pan data from the c lass a pan at Wahweap. -

TABLE 2 LAKE POWELL EVAPORATION RATE

(IN INCHES)

MONTHLY (CAN 2.W

- . .

T & l e 3 T o t a l Pan Evaporation i n Inches at Wahweap, Arizona (1962-197~)~

Year Jan Feb Mar Apr May Jun Jul Auq Sept Oct Nov D e c Total

Mean 2 . 0 8 3 . 0 3 6.51 10.23 14.68 16.44 17-08 15.05 11-60 7.51 3 . 7 6 2 . 6 9

Number of Years i n the Mean

1 5 9 1 4 13 1 4 14 14 14 13 9 1

a = The pan w a s u s u a l l y not operated a l l 12 months of each year. When there was no measurement available the mean of the r e s t of the years for that month was used in calculating the esti- mated values- Original pan data from U.S. Weather Bureau Climatological Data for Arizona.

b = Estimated d a t a .

Year

1 9 6 2

1 9 63

1964

1 9 6 5

1 9 6 6

1967

1 9 6 8

1 9 6 9

1 9 7 0

1 9 7 1

1 9 7 2

1 9 7 3

1 9 7 4

1 9 7 5

T a b l e 4 E v a p o r a t i o n a t Lake Powell Based o n Adjusted Pan Data f r o m t h e Class A Pan a t Wahweap, A r i z o n a

Jan Feb Mar Apr May J u n

2 . 0 8 2 .05 3 .04 5.28 5 . 7 0 7 .99 b 2 . 0 8 2 .20 3 . 5 0 4.57 5 .82 8.00 b b b

2 . 0 8 2.12 3.12 4 . 3 5 5 . 2 8 7.54

2 . 0 8 ~ 2 . 5 6 3.10 3.79 5 . 2 3 6.61

2 . 0 8 ~ 1 . 4 8 3 .74 5 . 2 6 5 .72 8.84

2 . 0 8 ~ 2 . 1 2 ~ 3.2bb 4 - 8 0 4.69 7 .15 b b

2 .08 2 . 1 2 3 . 4 0 3 .96 5.27 8 . 0 1 b 2 . 0 8 2.12 2 . 9 8 4.29 5 . 6 7 7 .16 b b

2 .08 2.12 3 . 8 5 4 .79 5 . 9 6 7 . 8 9 b b b 2.08 2.12 3 . 2 5 4 . 7 6 4 .96 8.01 b b b 2 - 0 8 2.12 3 . 2 5 5 . 3 5 5 . 7 4 7 . 1 5 b b

2.08 2.12 2 . 5 8 4.22 4 . 7 0 6 . 9 2 b b b 2 . 0 8 2.12 3 . 2 5 4.66 5.55 7 . 8 5

b b 2 . 0 8 ~ 2.12 3 . 2 5 3.66 3 .69 6 . 7 7

J u l

1 0 . 7 8

1 1 . 1 9

9 . 7 0

8 . 2 8

1 0 . 0 0

8 . 8 6

8.60

8.58

8 . 5 2

1 0 . 8 3

8.99

8 . 3 3

8 . 2 4

8 . 3 2

Auq S e p t O c t Nov D e c

1 1 . 1 5 9 . 0 4 5 .79 4.82 5.38

8 .48 7 . 6 3 6 .32 5 . 7 3 5.38 b

1 0 . 5 0 9 . 7 2 8.12 6 .12 5.38 b

1 0 . 0 0 9.66 6 . 3 7 5 .84 5.38 b

1 1 . 0 6 8 . 6 4 6 . 7 8 6 . 4 5 5 . 3 0 b

8 . 6 5 7 . 7 7 6 .94 6 . 8 6 5 . 3 8 b

7 . 7 5 9 . 4 4 5 .94 6 . 5 8 5.38 b

9 . 0 8 8 . 2 3 6.03 5 . 9 0 ~ 5 . 3 ~ ~ b 8 . 9 2 9 . 4 6 6 . 1 5 5 . 9 0 5 . 3 0 ~ b 9 . 9 1 8 . 1 2 6.04 5.90 5 . 3 8

b

b b 7 .73 8.24 6.38 5 . 9 0 5 - 3 0 b

9 .37 8 . 3 0 6.84 6 . 0 3 S.3ab

8 .89 8 .36 5 . 6 3 4 .74 5 . 3 ~ ~ b

7.80 7 .59 6 .09 5 . 9 0 5 . 3 8 b

Annual T o t a l

M o n t h l y A n n u a l Mean Mean 2 . 0 8 2 . 1 2 3 . 2 6 4 . 5 0 5 .28 7 - 6 6 9.22 9 . 1 8 8 . 5 8 6 . 3 8 5 - 9 0 5.38 69.44

a = The p a n w a s usually n o t o p e r a t e d a l l 12 m o n t h s o f each y e a r . When there was no mea- surement a v a i l a b l e t h e mean o f the rest of the years for t h a t month was used in c a l - c u l a t i n g the ad ju s t ed values. Original pan d a t a from U.S. W e a t h e r B u r e a u Cl imatolo- gical D a t a for Arizona."

b = Based on e s t i m a t e d v a l u e s as per n o t e a a b o v e ,

TABLE 5 LAKE POWELL GROSS EVAPORATION

( I N ACRE-FEET)

wan JAN r ? ~ nnu wà n w JuN JUL BUC SCP OCT NW ntc TOTBL

1965 9753

1966 18316

1967 79bS

1968 11097

1969 la792

1979 16457

1971 17389

1972 27913

1973 3HZ4

074 41985

1975 37039

1976 33714

1977 28892

1978 28939

1979 43369

m l v REAM 24331

For t h i s s tudy the evapotranspi rat ion t h a t must be subtracted was determined i n f o u r par ts .

EVAPORATION FROM THE PRE-RESERVOIR WATER SURFACE

The f i r s t p a r t was the evaporat ion f rom t h e p re - reservo i r r i v e r water sur- face. I t i s computed by m u l t i p l y i n g t h e area of t h e r i v e r by the gross evaporat ion ra te . The r i v e r water su r face area was determined us ing the f o l l o w i n g c r i t e r i a .

( 1 ) The aÑWyAÑlfMpf from which the Lake Powell topography came were f lown i n September and October o f 0 --

(2) The average f low of the r i v e r over the per iod o f record was about 18,000 c.f .s. a t Lees Ferry.

( 3 ) T a i l water curves taken i n t h e v i c i n i t y o f Glen Canyon Dam i n d i c a t e a r i s e of o n l y about 5 f e e t f o r an increase i n f l o w o f 15,000 c.f .s.

( 4 ) Since the water l i n e shown on the topography represents a f l ow o f about 6.000 c.f.s.. i t was assumed t h a t the contour c loses t t o t h i s water l i n e would represent average f l o w cond i t i ons .

(5) Where a contour crossed t h e r i v e r on a given topography sheet, a l i n e represen t ing the average f low l i n e was drawn i n and the area planime- t e r e d accordingly.

( 6 ) Sandbars t h a t d i d no t show ve a t i o n were assumed t o be inc luded i n I -...

b .

The r i v e r - e l e v a t i o n r e l a t i o n s h i p i s shown i n F igure 4. The gross evapora- t i o n r&e f rom Ta l e 4 was used. The r e s u l t i n g r i v e r evaporat ion i s shown d.

STREAMSIDE EVAPOTRANSPIRATION

The second p a r t was the evapo t ransp i ra t ion t h a t would have occurred from t h e vegetat ion, which i s mos t l y phreatophyte, and evaporat ion from the wetted s o i l on the streambank i f the r e s e r v o i r d i d not e x i s t . I t was com- puted by m u l t i p l y i n g the evapo t ransp i ra t ion r a t e by the streamside area. The evapotranspi rat ion r a t e was computed by the Blaney - Gr idd le method 7/ as shown i n Table 7 and described below. The streamside areas were deter- mined under a c o n t r a c t between the U n i v e r s i t y o f Utah and the Bureau of Reclamation i n the l a t e 1950's be fo re Lake Powell s t a r t e d t o f i l l 9/. The vegetat ion dens i t y was measured along transects. These were used as an index t o make ocu la r est imates o f the remaining areas. The areas were measured by p lan imeter from maps which were prepared f rom over lay maps and t a b u l a r data from the f i e l d . The area-e levat ion r e l a t i o n s h i p i s shown i n F igure 5. The main source o f water f o r t h i s p a r t i s f rom p e r c o l a t i o n w i t h a small c o n t r i b u t i o n from p r e c i p i t a t i o n . The streamside evapotranspi rat ion i s shown i n w m b

3500

0 < > Ill Ñ UJ

FIGURE 4

RIVER WATER SURFACE AREA ACCUMULATED ABOVE GLEN CANYON DAM

10 12 13 i 4 I 5 16 17 18 19 SURFACE AREA IN 1000 ACRES

FIGURE 5

STREAMSIDE AREA ACCUMULATED ABOVE GLEN CANYON DAM

3300 -2 18 20 2 2 24 26 28 30

AREA IN 1000 ACRES

t Ill It.

z 2 0 I- < > w _1 UJ

Fl(3URE 6

TERRACE AREA ACCUMULATED

ABOVE GLEN CANYON DAM

AREA IN 1000 ACRES

TERRACE EVAPOTRANSPIRATION

The t h i r d p a r t i s the evapotranspi rat ion from the te r race areas under pre- r e s e r v o i r condi t ions. I t was computed by m u l t i p l y i n g the evapotranspira- t i o n r a t e by t h e te r race area. The evapotranspi rat ion r a t e was determined by the Blaney-Criddle method TJ described below. The areas were deter- mined by t h e same survey r e f e r r e d t o above. This water reaches the r o o t zone most ly by c a p i l l a r y a c t i o n w i t h some by p r e c i p i t a t i o n . The te r race area-e levat ion curve i s shown i n F i g u r e 6. The r e s u l t i n g evapotranspira- t i o n i s shown i n Table 8 Column 4.

BLANEY-CRIODLE METHOD

The Blaney-Criddle method of computing evapotranspi rat ion (consumptive-use) was chosen because of the minimal data requirements and accuracy acceptable f o r t h i s purpose. The basic equation i s :

u=f k Where u i s the evapotranspi rat ion r a t e i n inches. k i s an emperical coef- f e c i e n t dependent on the type of vegetat ion. Consumptive use c o e f f i c i e n t s have been determined mostly f o r i r r i g a t e d crops so t h e r e has been l i t t l e i n v e s t i g a t i o n of n a t i v e vegetation.

From values g iven f o r k by Blaney 10/ f o r n a t i v e vegetat ion f o r several dens i t i es o f vegetat ion 1.1 was s e K c t e d f o r the streamside area and 0,9 was used f o r t h e te r race area. The v a r i a b l e f i s ca lcu la ted as ( t ) ( p ) / 1 0 0 . Where t i s the mean monthly a i r temperature i n degrees f a r e n h e i t and p i s the monthly percent o f annual d a y l i g h t hours and i s dependent upon the l a t i t u d e . 39O n o r t h l a t i t u d e was used. Therefore u=( t )x(p)x(k) / lOO. Table 6 shows the computation o f (p)x(k)/100 f o r the streamside area and t h e te r race area. This was m u l t i p l i e d by the average temperature f o r the month t o o b t a i n t h e evapotranspi rat ion rates.

EVAPOTRANSPIRATION OF REMAINING AREAS

The f o u r t h p a r t i s the evapotranspi rat ion from the remaining area of the r e s e r v o i r under p re - reservo i r cond i t i on .

Most of the p r e c i p i t a t i o n f a l l i n g on the area o f Lake Powell would not have produced runof f because i t would have evaporated from t h e vegetat ion and s o i l o r i t would have t ransp i red from the vegetat ion. I n consumptive use s tudies t h i s i s termed e f f e c t i v e p r e c i p i t a t i o n . I t does not inc lude p r e c i - p i t a t i o n t h a t causes r u n o f f . The method o f determining the e f f e c t i v e pre- c i p i t a t i o n used i s described i n a r e p o r t by R. A. Schleusener 8/. The p r e c i p i t a t i o n was computed from seven s ta t ions : Page, Wahweap. Bu l l f rog , Canyonlands, Hi te , Mexican Hut, and Natura l bridges. Since each o f these s t a t i o n s were loca ted above the maximum e leva t ion o f Lake Powell, and s ince p r e c i p i t a t i o n increases w i t h e levat ion, i t was necessary t o reduce the pre- c i p i t a t i o n t o an equiva lent f o r Lake Powell a t a represen ta t i ve e leva t ion of 3,600 f e e t . Th is was done by p l o t t i n g the average annual p r e c i p i t a t i o n f o r 1963 through 1980 against t h e e l e v a t i o n f o r the seven s t a t i o n s and drawing a curve throught the po in ts . The r e s u l t i n g average annual p r e c i p i - t a t i o n from t h i s curve i s 6.33 inches. The r a t i o o f 6.33 t o t h e average annual p r e c i p i t a t i o n at each s t a t i o n was m u l t i p l i e d by the monthly p r e c i p i -

TABLE 6 BLANEY-CRIDDLE

COMPUTATION OF ^ Streamside Terrace

January

February

March

Apri l

May

June

July

August

September

October

November

December

Total

t a t i o n t o reduce i t t o the equiva lent p r e c i p i t a t i o n a t 3,600 f e e t . The equ iva len t p r e c i p i t a t i o n f o r a l l s t a t i o n s was then t o t a l e d and d iv ided by t h e number o f s ta t ions on a monthly basis. Table 6 was adapted from the r e p o r t by R. A. Schleusener 4/.

TABLE 7 E f f e c t i v e Precipi tat ion i n Inches

Month Jan Feb Mar A p r i l May June J u l y Aug Sept Oct .6 -6 .8 .8 .8 .8 1.0 1.0 1.0 -8

Nov Dec .8 .8

The p r e c i p i t a t i o n up t o the l i m i t shown i n Table 7 was m u l t i p l i e d by the remaining area. This i s shown as column 5 i n Table 8. The remaining area i s the t o t a l area o f the r e s e r v o i r f o r the month minus the t o t a l o f the water sur face area, the streamside area, and the te r race area. From these computations i t was determined t h a t about 95 percent o f the p r e c i p i t a t i o n on the r e s e r v o i r area would have evaporated and t ranspi red.

RESULTS

Table 8 shows t h e computed monthly evaporat ion f o r each of t h e fou r p a r t s and the r e s u l t i n g ne t evaporat ion f o r the per iod January 1965 through May 1979.

Table 9 shows t h e ne t annual evaporation, as described under PROCEDURE, which has p r e v i o u s l y been used on a p r e l i m i n a r y basis, compared t o the n e t annual evaporat ion computed from t h i s study (Table 8). The ne t eva- p o r a t i o n i s l ess under t h i s study when the r e s e r v o i r i s low b u t greater when t h e r e s e r v o i r i s high. The average evaporat ion under t h i s study i s 5 percent h igher than the p re l im inary f i gu res . This i s most ly due t o the 3 percent h igher gross evaporat ion r a t e o f 68.3 inches per year compared t o 66.2 inches from the former estimates.

RECOMMENDATIONS

I t i s recommended t h a t the r e s u l t s o f t h i s study be used t o make a water budget ana lys is f o r Lake Powell. Th is w i l l improve the est imate o f the amount o f water s tored i n the banks. I t might a lso help t o determine the amount o f bank s torage t h a t i s recoverable.

I t i s a lso recomeneded t h a t the r e s u l t s o f t h i s study be used t o determine a s impler method o f es t imat ing evaporat ion. A method t h a t cou ld be used i n operat ions would be b e n e f i c i a l . One method tha t should be considered i s t o use pan evaporat ion a t Wahweap.

LAKE I,....= 8

POUELL NET EVAPORATION (IN ACRE-FEET)

196C 1 31 1965 2 4!0 186s 1 ;; IMS lOCS 6 3 1 1965 6 38 1965 7 31 1965 8 3 1 1965 9 30 1965 1@ 31 1965 1 1 39 1965 I2 31

CALENDER YEAR TOTALS:

CALEHOO VEM TOTALS!

3753 13893 16384 19347 31686 W226 33322 47651 4PeW 15586 17B92 19755

392883

JB316 13374 19643 2917P 37W7 48336 45478 52336 37846 33W9 17985 1S674

367333

K373 TOOT

14ne 2-582 24140 3S9B7 m69 41182 36725 32È 181ZB 2SSIS

3@9123

ST31 366Ã 4565 S4IS 8857 8242 8191 lies? 11 733 3704 4U6b 4641

77060

4276 3135 4EQÃ 6717 8309

16827 10292 12148

BtWfJ 7966 4159 3057

85324

8 9 6 22-77 xse seas 639 1 wn 10135 >7sa ma 7723 4363 636B

7589s

1685 1794 £33 25 38 3608 4189 4% 4549 3495 2871 ^w^ Kg?

35987

1374 1452 2486 3186 4247 4646 5e4a 4706 37eG 2791 ewl 1492

37M 1

1373 1757 86 19 saw 3831 4357 5ÈS 4S74

2% SViZ mt

36È1

3636 Bill 6791 9567

17577 16801 17118 66568 30196 6318 9335

12982

162262

11830 sen leas6 16211 21 177 38329 27133 32457 22885 21C76 9974 91 44

2186%

5125 w e 5174 I0479 IBBCT 1VSZ7 22119 2294s sum WVZ3 B199

taw?

I S M

TABLE 8 LAKE POWELL NET EVAPORATION

( I N ACRE-FEET)

1 1 ) (2) ( 3 ) ( 4 ) (5) ( 6 1 ( 7 ) <6*3È4È (1-a)

W M ' . ~ : HO. OF GROSS EUflPORftTION EWOIWTIOK E m - EUftPO- TOTAL NET M Y S EW#PORATIOM FROM RIVER FROft TOWSPIMHON TMKSPlRftTlW ADJUSTTtMT EUAPORATIOH

S T R E W S l M FRW ERRWE FROn ~ ~ c ~ I M I N O

CM.ENCCT YEAR TOTfllS* 3 7 9 5 9 WS19 1 4 4 9 3 35043 17927 157Q81 222478

683 767 986

1169 16Ç I M S z 1645 1233 m2 641

1 Si?? 1 B83 2418 6875 4144 4 6 4 5 5082 4793 3639 a m eem 1614

TABLE 8 LAKE POWELL NET EVAPORATION

( I N ACRE-FEET)

AREA . . . . . . . . . . . . . . . - RCRE-FEET ..........................................

CALENDER VEftR TOTALS*

CALENDER Vâ‚ TOTALS1

615 73<

ID38 1285 ices 1 g68 2193 1969 1694 1236 936 5 78

IS751

6-19 764 I209 1358 1726 1964 m 1934 1669 1232 Has 671

16072

514 692 957

1233 1607 1981 21% 1m \w 1344 Ã 3̂ 69Ã

15886

TAB, LAKE POWELL N E l EVAPORATION

( I N ACRE-FEET)

SSP 666

1174 1 x 4 1823 21Z6 a166 1999 1751 1907

718 GI6

15932

712 846

113s I461 1929 2133 ew9 18W I *is I M 7 76Ã 632

159C6

9S7 sea

1241 161Ã 2998 2172 5122 1767 Ill* 1 6 4 611 626

18394

CALENDER Y E W TOTflLSl

CflLEMDtR YEW TOTnLSI

TABLE 8 LAKE POHELL NET EVAPORATION

( I N ACRE-FEET)

C a l e n d a r Year

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

T o t a l

A v e r a g e

TABLE 9 LAKE POWELL NET EVAPORATION

1,000 ACRE-FEET

P r e l i m i n a r y * N e t E v a p o r a t i o n

Net E v a p o r a t i o n T h i s Study

* Computed from pan e v a p o r a t i o n r a t e a s descr ibed u n d e r PROCEDURE.

REFERENCES

1. U.S. Geo log i ca l Survey, 1954a, Water-Loss I n v e s t i g a t i o n s : Lake Hefner Stud ies , Techn i ca l Report : U.S. Geol . Survey Prof. Paper 269. Prev ious1 y p u b l i s h e d as U.S. Gee\. p u r v e y C i r c , 229 (1952) and as U.S. Navy E l e c t r o n i c s L a b o r a t o r y Repor t 327.

2. U.S. Geo log i ca l Survey, l954b. Water-Loss I n v e s t i g a t i o n s : Lake Hefner Stud ies , Base Data Reoort : U.S. GeoL Survey Pro f . Paper 270; a l s o pub1 i shed as U S . Navy E l e c t r o n i c s L a b o r a t o r y Repor t 328.

3. NOAA Techn ica l Report NWS33, 1959, "Evapora t ion A t l a s f o r t h e Cont iguous 48 S t a t e s M Dept. o f Comm. NWS, June 1982.

4. Harbeck e t a l . U.S. Geo log i ca l Survey, 1958, Water Loss I n v e s t i g a t i o n s : Lake Mead Stud ies , P ro fess iona l Paper Number 298.

5 . Jacoby, Gordon C. Jr., e t a1. 1977, Lake Powe1 Research P r o j e c t B u l l e t i n , Number 48, N a t i o n a l Sc ience Foundat ion 1 n s 1 Phys ics U n i v e r s i t y of C a l i f o r n i a , Los Angeles.

6. G. E a r l Harbeck, Jr., U.S. Geo log i ca l Survey, 1962, A P r a c t i c e F i e l d Technique f o r Measur inq Rese rvo i r Evapo ra t i on U t i l i z i n q Mass-Transfer Theory, U.S.G.S. P r o f e s s i o n a l Paper No. 2 7 2 4 .

7. Blaney, K.F., C r i dd le , W . O., 1962, Determin ing Consumptive Use and I r r i g a t i o n Water Requirements, U.S. Department of A g r i c u l t u r e Tech. B u l l e t i n 1275 .

8. Schleusener, R.A. , Crow, L.W., 1961, Ana l ys i s o f P r e c i p i t a t i o n Data i n t he Upper Co lorado R i v e r Basin, Colorado S t a t e U n i v e r s i t y .

9. Woodbury, A. M., Du r ran t S. D., F lowers, S., 1959, A Survey of Vegeta t ion i n t h e Glen Canyon Rese rvo i r Bas in An th ropo log i ca l Papers Number 36, U n i v e r s i t y o f Utah Press.

10. B laney H . F., C r i dd le , 1948, F i n a l Repor t Eng ineer ing Adv i so ry Committee t o Upper Co lorado R ive r Bas in Compact C o m i s s i o n .

11. Jensen, M.E., E d i t o r , 1973, Consumptive Use o f Water and i r r i g a t i o n Water Requirements, American S o c i e t y o f C i v i l Engineers.