Leaf Temp and Evapotranspiration by Jeff Hunter

Agricultural and Forest Meteorology, 32 ( 1 9 8 4 ) 4 1 - - 5 3

Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

E V A L U A T I O N OF CANOPY T E M P E R A T U R E - - E V A P O T R A N S P I R A T I O N MODELS OVER V A R I O U S CROPS* J.L. H A T F I E L D

USDA-ARS, Plant Stress and Water Conservation Research Unit, Route 3, Lubbock, TX (U.S.A.) R.J. R E G I N A T O a n d S.B. IDSO

U.S. Water Conservation Laboratory, Phoenix, A Z (U.S.A.) ( R e c e i v e d May 2, 1 9 8 3 ; r e v i s i o n a c c e p t e d F e b r u a r y 27, 1 9 8 4 )

ABSTRACT Hatfield, J. L., R e g i n a t o , R. J. a n d Idso, S. B., 1 9 8 4 . E v a l u a t i o n o f c a n o p y t e m p e r a t u r e - e v a p o t r a n s p i r a t i o n m o d e l s over various crops. Agric. For. Meteorol., 32: 4 1 - - 5 3 . C a n o p y t e m p e r a t u r e s , w h e n m e a s u r e d r e m o t e l y , o f f e r a m e t h o d o f e s t i m a t i n g evapo t r a n s p i r a t i o n w i t h surface e n e r g y b a l a n c e m o d e l s . E q u a t i o n s w h i c h have b e e n dev e l o p e d by o t h e r s have b e e n e v a l u a t e d o n l y at a l i m i t e d n u m b e r o f l o c a t i o n s a n d w i t h a few crops. O u r s t u d y was c o n d u c t e d at several l o c a t i o n s w i t h weighing l y s i m e t e r s w i t h a variety o f c r o p s a r o u n d t h e U n i t e d S t a t e s : Brawley, CA; T e m p l e , T X ; L i n c o l n , NE; St. Paul, MN; F a r g o , ND; K i m b e r l y , ID; a n d Davis, CA, to e v a l u a t e e v a p o t r a n s p i r a t i o n utilizing c a n o p y t e m p e r a t u r e as a n i n p u t i n t o t h e surface e n e r g y balance. T h e results s h o w t h a t e v a p o t r a n s p i r a t i o n c a l c u l a t e d f r o m t h e a e r o d y n a m i c resistance f o r m o f t h e surface e n e r g y b a l a n c e was well c o r r e l a t e d w i t h l y s i m e t e r m e a s u r e m e n t s at all l o c a t i o n s . T h e errors using t h e surface e n e r g y b a l a n c e w e r e less t h a n 10% in all cases for full g r o u n d cover. T h e B a r t h o l i c - - N a m k e n - - W i e g a n d m e t h o d was m o r e closely c o u p l e d t o n e t radiation than canopy temperature. U n d e r partial c a n o p y cover, d i f f e r e n c e s b e t w e e n t h e t w o m o d e l s were a p p a r e n t . T h e Bartholic--Namken--Wiegand model overpredicted when the actual evapotranspiration was a b o v e 2 0 0 W m -2 b e c a u s e of its i n s e n s i t i v i t y to surface t e m p e r a t u r e . H o w e v e r , t h e surface e n e r g y b a l a n c e m o d e l e x h i b i t e d o n l y a slight o v e r p r e d i c t i o n a b o v e 200 W m -2 w h e n a w e i g h e d c o m p o s i t e surface t e m p e r a t u r e ( r e p r e s e n t a t i v e o f bare soil a n d c r o p t e m p e r a t u r e ) was used. This small o v e r p r e d i c t i o n c o u l d be o v e r c o m e b y c o n s i d e r i n g t h e soil h e a t flux t e r m . T h e r e was n o l o c a t i o n bias in t h e surface e n e r g y b a l a n c e m o d e l , w h i c h s h o w s t h a t it s h o u l d w o r k well a t o t h e r locations. INTRODUCTION

Evapotranspiration from a surface is a c o m p o n e n t of the partitioning of energy received

by that surface. This process

can be described

by the familiar

energy balance equation (Monteith, 1973) with expanded sensible and latent * C o n t r i b u t i o n f r o m t h e California A g r i c u l t u r a l E x p e r i m e n t S t a t i o n , P r o j e c t 3963-H, and the USDA-ARS.

42 heat terms, as

+ G

pCp

(Tc

--Ta) ra

+pCp [es(Tc)--ea] "y

ra + r c

(1)

where R n is the net radiation (Jm -2 s -1 ), G the soil heat flux (Jm -2 s -1 ), p C~ the volumetric heat capacity of air (Jkg -1 째C-1), Te the canopy temperature (째C), ra the aerodynamic resistance (sm -1 ), 7 the psychrometric constant (kPa 째C-1 ), es (Te) the saturated vapor pressure at Te (kPa), ea the actual vapor pressure (kPa), and re the canopy resistance to water vapor transfer (sm -1 ). This form of the energy balance equation has been often cited, but the difficulty in measuring canopy temperature had led to other approaches which decouple eq. 1 from a direct surface temperature measurement. The Penman--Monteith m e t h o d , which includes a canopy resistance term, can be used, but it is difficult to assess the canopy resistance changes with soil water status (Monteith, 1973). With the development of thermal infrared thermometers that are accurate and easily used, several investigators proposed the use of surface temperature in surface energy balance models to estimate evapotranspiration (Bartholic et al., 1970; Brown and Rosenberg, 1973; Jackson et al., 1977; Soer, 1980; and Sequin and Itier, 1983). These approaches have ranged from manipulation of eq. 1 to the use of empirical coefficients for deriving daily evapotranspiration from m i d d a y canopy measurements (Jackson et al., 1977). Although these approaches have been proposed only limited evaluations have been conducted. Stone and H o r t o n (1974) compared the techniques suggested by Bartholic et al. (1970) and Brown and Rosenberg (1973) against the more traditional methods (Bowen ratio (Bowen, 1926), Penman (1948), and van Bavel (1966)) and concluded that both approaches would have promise. The authors suggested that the Bartholic--Namken-Wiegand m e t h o d may be better to use because it does not require knowledge of the aerodynamic resistance which is a function of windspeed and the canopy aerodynamic properties. Verma et al. (1976), after extensive error analysis, concluded t h a t the aerodynamic resistance m e t h o d given by Brown-Rosenberg (1973) was more sensitive to errors in canopy temperature than errors in aerodynamic resistance. Blad and Rosenberg (1976) determined that either a resistance or mass transfer model utilizing remotely sensed canopy temperature would be useful in regional models. Other comparisons over limited conditions have been conducted by Heilman and Kanemasu (1976), Heilman et al. (1976), Soer (1980), Sumayao et al. (1980), and Hatfield et al. (1983). Soer (1980) found that the evapotranspiration estimated by an energy balance model agreed 70% of the time with water balance measurements in watersheds. He stated, however, t h a t regional estimation with remotely sensed canopy temperatures would allow for the standard error about the mean to approach zero and

43 t h a t this would make the estimates comparable with those from a Penman model, with both utilizing the same meteorological data base. Heilman et al. (1976) used remotely sensed canopy temperatures and found the models agreed very well after the remotely sensed canopy temperatures were corrected for atmospheric attenuation between aircraft and ground. This experiment was implemented with the objective of evaluating evapotranspiration models which use remotely sensed canopy temperature as an input. Our report addresses the performance of the models compared to lysimeter evapotranspiration for various crops and locations in the United States. MATERIALS AND METHODS

Description and equations The energy balance of a surface given in eq. 1 (Monteith, 1973) can be rewritten as

hE = Rn + G

pCp

(Tc--Ta)

(2)

where hE is the latent heat of vaporization (J kg -1 ). Sumayao et al. (1980) and Hatfield et al. (1983) showed that when the canopy was cooler than air hE would be larger than net radiation. In this m e t h o d we calculate r a from ra = [ln

(z--d)/z o] 2/k2u

(3)

with z being the height (m) of observation of air temperature, windspeed, and vapor pressure above the crop surface, d the displacement height (m), zg the roughness length (m), k yon Karmans constant (0.40), and u the windspeed (m s -1 ). Following Monteith's (1973) correction for stability, we corrected the aerodynamic resistance as rac = r a

(

n(z--d)g(Tc--Ta))' Tu 2

(4)

where g is the acceleration of gravity ( 9 . 8 m s -2) and T the absolute temperature (K), taken as the mean of the canopy and air temperatures. Monteith (1973) suggested that a value of 5 for n would be appropriate for field conditions. The roughness lengths and displacement heights for the crops in this study are given in Table I. Bartholic et al. (1970) rearranged eq. 1 to obtain the following equation

Rn+G

E = 1+3"

(Ta - - T c )

[es(Ta)--es(Tc)]

(5)

> 1 > > 1 > ~

0 0

O 0

0 oO

.o ~0 C'xl

•

Cxl

~'~

C'-1

.N N M

O~ O~

0 0

~ 0 ~ 0

~ 0 ~

oo 0

~'~

c~ x

O~ ~ 0 0 0

0 ~ 0

~ 0 O~ 0 0

~ 0

0 0

O 0

0 0

~ 0 ~ 0

~ ~

0 ~ 0 ~

0 0 0 ~

"~"

o 0

0 0

?T ? ?T 71

4.a

6 o

e', 0

45 This a p p r o x i m a t i o n sets t h e s u r f a c e a n d air at t h e s a t u r a t i o n v a p o r pressure w h i c h limits t h e e q u a t i o n to p o t e n t i a l e v a p o t r a n s p i r a t i o n f r o m an infinitely w e t surface. EXPERIMENTAL PROCEDURES D u r i n g t h e s u m m e r o f 1 9 8 0 , an e x p e r i m e n t was c o n d u c t e d at several l o c a t i o n s in t h e w e s t e r n half o f t h e U n i t e d States. T h e l o c a t i o n s a n d c r o p i n f o r m a t i o n are given in T a b l e I. T h e s e sites w e r e c h o s e n b e c a u s e t h e y had l y s i m e t e r s w i t h s u f f i c i e n t a c c u r a c y t o give a reliable h o u r l y m e a s u r e o f e v a p o t r a n s p i r a t i o n . T a b l e II describes t h e l y s i m e t e r s at each l o c a t i o n . In all cases e x c e p t T e m p l e t h e f e t c h r e q u i r e m e n t s w e r e m e t f o r these m e a s u r e m e n t s a n d at T e m p l e t h e l y s i m e t e r was s u r r o u n d e d b y taller corn. T h e sites m o n i t o r e d also c o v e r e d a w i d e r a n g e in l a t i t u d e , c l i m a t e a n d crops, w h i c h was d e e m e d n e c e s s a r y t o achieve o u r e x p e r i m e n t a l objectives. At each e x p e r i m e n t a l site, m e a s u r e m e n t s w e r e m a d e o f c r o p h e i g h t and t h e values f o r r o u g h n e s s length a n d d i s p l a c e m e n t height w e r e d e t e r m i n e d f r o m t y p i c a l values r e p o r t e d in t h e l i t e r a t u r e . F o r t h o s e l o c a t i o n s w i t h less t h a n c o m p l e t e g r o u n d c o v e r t h e r o u g h n e s s length was a s s u m e d to be larger t h a n r e p o r t e d values as suggested b y V e r m a a n d Barfield ( 1 9 7 9 ) a n d H a t f i e l d ( 1 9 8 2 , u n p u b l i s h e d data). A t each l o c a t i o n the e x p e r i m e n t a l p r o c e d u r e was t h e s a m e , including t h e f r e q u e n c y o f o b s e r v a t i o n . M e t e o r o l o g i c a l variables w e r e r e c o r d e d at onem i n u t e intervals b y a c o m p u t e r - c o n t r o l l e d d a t a a c q u i s i t i o n s y s t e m a n d t h e d a t a r e c o r d e d o n t o flexible discs. T h e s y s t e m was h o u s e d in a m o b i l e van w h i c h was p a r k e d n e a r t h e l y s i m e t e r at e a c h l o c a t i o n . O b s e r v a t i o n s r e c o r d e d o n t h e r o o f o f t h e van w e r e global solar r a d i a t i o n a n d l o n g w a v e r a d i a t i o n f r o m t h e a t m o s p h e r e . T h e p y r a n o m e t e r was an E p p l e y PSP p y r a n o m e t e r w i t h a W G 2 9 5 filter. L o n g w a v e r a d i a t i o n f r o m t h e a t m o s p h e r e was m e a s u r e d w i t h a Swissteco p y r a d i o m e t e r w i t h a b l a c k b o d y cup. P o s i t i o n e d o v e r t h e l y s i m e t e r was an i n v e r t e d E p p l e y p y r a n o m e t e r , a F r i t s c h e n m i n i a t u r e net r a d i o m e t e r , an a s p i r a t e d d r y - b u l b a n d w e t - b u l b t h e r m i s t o r , a n d Gill lightchopper anemometer. The pyranometer and net radiometer were p o s i t i o n e d 5 0 c m a b o v e t h e u p p e r s u r f a c e o f t h e c a n o p y and, w h e r e a p p l i c a b l e , o v e r t h e p l a n t r o w . T h e a s p i r a t e d dry- a n d w e t - b u l b t h e r m i s t o r s and anemometers were positioned 30cm and 130cm above the upper surface o f t h e c a n o p y . T h e s e i n s t r u m e n t s w e r e a t t a c h e d to t h e d a t a a c q u i s i t i o n s y s t e m b y a m u l t i c o n d u c t o r shielded cable a n d w e r e also s a m p l e d once per minute. The data were reduced and quality controlled through r o u t i n e s d e s c r i b e d b y H a t f i e l d et al. ( 1 9 8 1 ) a n d r e p o r t e d as i n t e g r a t e d h o u r l y values. C a n o p y t e m p e r a t u r e s w e r e m e a s u r e d o v e r t h e l y s i m e t e r w i t h an i n f r a r e d t h e r m o m e t e r w i t h a 4 째 f o v a n d 8 - - 1 4 p m w a v e b a n d . This u n i t has an a c c u r a c y o f + 0 . 5 째 C a n d a r e s o l u t i o n o f 0.1째C. M e a s u r e m e n t s w e r e m a d e o f t h e c r o p f r o m e a c h c a r d i n a l d i r e c t i o n a t a b o u t a 30 째 angle f r o m t h e

46 horizontal. Where the ground cover was not complete, nadir views of the soil between the rows were made along with temperatures of individual plant leaves. These measurements were made at half-hourly intervals, from 15 min before sunrise to 15 min after sunset. In the analysis procedure, the canopy temperatures were averaged to match the hourly meteorological values. In cases where incomplete ground cover was present composite scene temperature was determined by computing a weighed average based on the fraction of bare soil relative to the crop cover and their respective temperatures. This could be improved by monitoring composite scene temperature with a nadir looking angle infrared thermometer. Lysimeter values were recorded at the same half-hourly intervals as canopy temperatures. These data were then plotted and smoothed with a three-term running average from which hourly values of evapotranspiration could be determined. It was felt that this time resolution would be adequate for all lysimeters and would insure the greatest degree of precision. The meteorological, canopy temperature, and evapotranspiration data formed the data set from which all subsequent analyses were conducted. Evapotranspiration was calculated from eq. 2 with the inclusion of eq. 4 for the surface energy balance and eq. 5 for the Bartholic--Namken-Wiegand method. The Penman--Monteith equation was given as

ET =

(Rn + G) + pC, (es (Ta) --ea)/ra A+7

(6)

where A is the slope of the saturation vapor pressure curve (kPa 째C-1 ) and was used with a canopy resistance term of zero in the calculations. This m e t h o d provided an estimate of potential evapotranspiration for our study. RESULTS AND DISCUSSION For each location the Bartholic--Namken--Wiegand model (eq. 5), the surface energy balance model (eq. 2), and Penman--Monteith combination equation (eq. 6), formulas for the estimation of potential evapotranspiration, were calculated with the hourly data set. The hourly estimates were then compared with measured evapotranspiration values for each location. On each graph the integrated daily total of evapotranspiration is given as a point of reference for the reader. Only one representative day is shown for each site. Diurnal trends of net radiation, measured evapotranspiration and the calculations of potential and actual evapotranspiration are shown in Fig. 1 for alfalfa at Kimberly. These results were for a clear day with moderate windspeeds, and the evapotranspiration values were quite high. The Bartholic--Namken--Wiegand model underestimated actual evapotranspiration values and the other models t h r o u g h o u t the day. Evapotranspiration estimated by the surface energy balance (eq. 2) closely followed the

IO00 E

800

I I I I I I KIMBERLY-DAY 2 0 8 - NET RADIATION -- ____ LYSIMETE R ET

I I I ALFALFA

DArLY TOTAL ET = 8 4 r a m

O.--O PENMAN

6OO

E3--.O S - N - W ~SURFACE ENERGY BALANCE

0 a

~I000 i 800 600 400 2O0 20 0 'E

TIME ( HOURSI

[

LINCOLN-DAY 1 7 9 --NET RADIATION .... LYSIMETERET ~--OB-N-W ~,----E~SURFACE ENERGY BALANCE N

200

]

[

]

SOYBEAN OAJLy TOTAL ET'38mm

PENMAN

I 6

TIME

P8 ~

(HOURS)

Fig. 1. Diurnal trend o f net radiation and measured evapotranspiration for alfalfa and calculated evapotranspiration by the surface energy balance, Bartholic--Namken--Wiegand and Penman--MonLeith combination m o d e l for Kimberly, Idaho. Fig. 2. Diurnal trend o f net radiation and measured evapotranspiration for soybeans and calculated evapotranspiration by the surface energy balance method, Bartholic-Namken--Wiegand model, and Penman--Monteith combination model for Lincoln, Nebraska.

lysimeter values throughout the day. The differences between the surface energy balance and the measured values for Kimberly were largest on this day, although they were generally less than 10%. Estimates from the Penman--Monteith approach provide an estimate of potential evapotranspiration and are shown for the benefit of the reader. Since the purpose of the study is to compare actual evapotranspiration with the canopy temperature methods, the estimates of potential evapotranspiration will not be discussed in the remainder of the paper. In comparing evapotranspiration estimated from the measurement of net radiation directly or from the calculation of net radiation components, differences were always less than 5%; there appeared to be no bias in the differences. For this reason, only the calculation of net radiation, as used by Soer (1980) in his evaluations, will be discussed for the remainder of the paper. In regional applications a direct measure of net radiation may not be feasible and for this reason it was felt this approach was closer to our objectives. For the full ground cover alfalfa at Kimberly, the evapotranspiration was substantially reduced. This is illustrated by Fig. 2 for soybeans at Lincoln. Data for this day show that the conditions were generally clear with some clouds in the afternoon. Of more interest is the behavior of the measured evapotranspiration rates and the two models. The Bartholic--Namken-Wiegand model tracks the net radiation curve very closely but greatly overestimates the measured evapotranspiration rates while the surface energy balance method more closely approximates the measured values throughout the day. In the partial canopy-cover cases at Lincoln (Fig. 2), Fargo, and Brawley (Fig. 3), the surface temperature had to be adjusted to approximate the

Z I000

'IE 8 0 0

I I I - D AI Y Ir55 [ COTTON I I [ BRAWLEY ~ -'--

NET RAD4ATION LYSIMETER E T

DAILY TOTAL ET ; 4 7 r a m

oZ 800

SURFACE

--NET ------

/,--,~:

ENERGY BALANCE

600

]

ST PAUL DAY 83

600

4OO

400

200

2OO

ALFAL;A

R~Ct~TIO~ uYSIMETER ET

0~ILY TOTAL E T : 78mm

~URRACE ENERGY

B&L~'WCE

HPENM~N

r2 14 TIME (HOURS)

12 14 TIME (HOURS)

Fig. 3. Diurnal trend of net radiation, measured evapotranspiration and calculated evapotranspiration from the surface energy balance, Bartholic--Namken--Wiegand, and Penman--Monteith combination models for Brawley, California. Fig. 4. Net radiation, measured evapotranspiration and calculated evapotranspiration by the surface energy balance, Bartholic--Namken--Wiegand and Penman--Monteith combination models for alfalfa at St. Paul, Minnesota. co mp o s ite scene t em pe r at ur e . This was done by averaging the soil surface area o f the lysimeter t h e y represented. Using only crop c a n o p y temperatures, which were m u c h cooler than the soil surface, the predicted evapotranspiration values were t oo high because the m odel assumes that the entire surface area is at this t e m p e r a t u r e . This adjustment was made for each location before the calculation of evapotranspiration with any of the meth o d s . Soil heat flux was n o t measured in this study and would have to be a c c o u n t e d for if the m odel is to be applicable over large areas. The deletion of this c o m p o n e n t does not appear to i nt roduce a large error, but it does i n t r o d u c e a bias in the data. Composite scene c a n o p y temperatures would be o f mo r e use than individual plant or leaf t em perat ures in the estimation of evapotranspiration. Alfalfa evapotranspiration at St. Paul exhibited a condition in which the measured values were larger than the estimated values by the Bartholic-Namken--Wiegand m e t h o d (Fig. 4). The measured and calculated values from the surface energy balance m e t h o d agreed very closely. For this day the measured evapotranspiration was above net radiation for the early and late part o f the day and below net radiation during midday. At all times the surface energy balance model responded to the environmental conditions to estimate the measured evapotranspiration values very closely. At Temple, the estimated evapotranspiration with the surface energy balance agreed very closely with measured values (Fig. 5). As was f o u n d in the o t h e r locations the Bartholic--Namken--Wiegand m e t h o d closely followed the net radiation curves. This effect is also evident at Davis (Fig. 6) where the measured values and those predicted by the surface energy balance agreed very closely t h r o u g h o u t the day while the Bartholic--Namken--Wiegand m e t h o d , in responding p r e d o m i n a n t l y to net radiation, was n o t affected by the late a f t e r n o o n sea breeze which caused an increase in the evapotranspiration rate. This increase in evapotranspiration was affected by the

~ IOOO 'e

TEMPLE-DAY 167 GRAIN SORGHUM B oo

-NET RADIATION ~ ___ LYSIMETER EI

800

DAJLYTOTAL ET: 16ram

~--CB-N-W

600

.......

400 ~

! 6

[

]

-NET RADIATION - - - - LYSI~ETER ET

DAILEY TOTAL ET= 53ram

~---'~ SURFACE

SURFACE ENERGI

DAVIS-DAY 217 TOMATO

ENERGY

BALANCE ........... H p N

400

12 TIME

I 18

0 20

{HOURS)

TIME

{HOURS)

Fig. 5. Diurnal trend of net radiation, measured evapotranspiration, and calculated evapotranspiration from the surface energy balance, Bartholic--Namken--Wiegand, and Penman--Monteith combination models for Temple, Texas. Fig. 6. Diurnal trend of net radiation, measured evapotranspiration, and calculated evapotranspiration from the surface energy balance, Bartholic--Namken--Wiegand, and Penman--Monteith combination models for Davis, California.

increase in w i n d s p e e d w i t h o n l y a slight m o d e r a t i o n in air temperature and vapor pressure deficit. For all l o c a t i o n s w i t h at least 80% ground cover, h o u r l y measured evapotranspiration rates were compared against values calculated from the B a r t h o l i c - - N a m k e n - - W i e g a n d m e t h o d or the surface energy balance. For each h o u r the a e r o d y n a m i c resistance was stability corrected via eq. 4. The comparisons for the B a r t h o l i c - - N a m k e n - - W i e g a n d m o d e l are s h o w n in Fig. 7 for Kimberly, Temple, St. Paul, and Davis. There is generally a g o o d fit a b o u t t h e 1 : 1 line, but the standard error is almost 1 0 0 J m -~ s -~ . The predictions ',.o

-:S

I000

C800

600 ~-

x ~

_ -

/oo.; o "o x x$ e

400

•

F-

I000

SURFACE

NAMKEN -

.•

~ a_

800

600

_ ~.ot. x -

x ,x,

F"x

400

o Lu F-

O.Bl (ET)

i BALANCE

• KIMBERLY z o

•

I ENERGY

200

-- --~:~ O~ ~

P R F D I C T ED " 9 . 8 3 + 0 9 9 ( E T ) r 2 • 0 92

n = 85

a ~-

200 MEASURED

400

600

BOO

EVAPOTRANSPIRATION(Jm

I000

- 2 -I s )

200 MEASURED

400

600

800

EVAPOTRANSPIRATION

,ooo

( J m ' 2 s "1)

Fig. 7. Measured evapotranspiration from lysimeters at Kimberly, Indiana; Temple, Texas; St. Paul, Minnesota; and Davis, California, compared with calculated evapotranspiration by the Bartholic--Namken--Wiegand model. Fig. 8. Measured evapotranspiration from lysimeters at Kimberly, Indiana; Temple, Texas; St. Paul, Minnesota; and Davis, California compared to calculated evapotranspiration by the surface energy balance model.

50 could also be grouped by location -- at St. Paul and Kimberly the m odel ten d ed to underpredict, while at Temple and Davis there was an overprediction. Because net radiation is the main driving factor in this model, predictions of evapotranspiration are insensitive to c a n o p y t e m p e r a t u r e , as shown by Hatfield et al. (1983). Measured and predicted evapotranspiration by the surface energy balance (eq. 2) showed an e x t r e m e l y good fit for all of the locations with fullground cover (Fig. 8). T her e was not any location bias, and the fit was good for the entire range of data. Fr om the good agreement between predicted and measured values, the stability adjustment in aerodynam i c resistance appears adequate. The lack of location bias indicates t hat there is no need for a local wind or crop factor, which makes this m odel m ore widely applicable. The standard error about the line was a b o u t 5 0 J m -2 s -1 , which would be an acceptable error over the range of the data. There also was not an overestimation at the higher evapotranspiration rates, as was f o u n d by Heilman et al. (1976). These results show that either the surface energy balance model with c a n o p y t e m p e r a t u r e and net radiation, either directly measured or calculated from its c o m p o n e n t s , would provide a reliable and accurate estimate of t he evapotranspiration of a cropped surface. During the early stages of crop growth, ground cover is minimal for m o s t crops and in m a n y locations, full ground cover is not obtained. As seen in Table I, three locations we visited had only 15% ground cover, so we were able to evaluate the two evapotranspiration models for the partial c a n o p y cover condition. Results for the Bartholic--Namken--Wiegand m odel are given in Fig. 9. As shown in Fig. 2, this m odel overestimates the measured evapotranspiration rate at all values above 2 0 0 J m -2 s -1 . Below this level the measured values are very closely tied t o the net radiation. At early morning and late evening hours the evapotranspiration is closely tied to radiation availability and the soil and canopy t e m p e r a t u r e s are relatively close to air t e m p e r a t u r e . During midday, however, the evapotranspiration process over the entire surface is low or negligible for the soil as c o m p a r e d to the crop and the relative disagreement decreases as the fraction of soil cover decreases. Agreement between measured and calculated evapotranspiration by the surface energy balance m e t h o d , when composite surface t e m p e r a t u r e was used, was m u c h better than the Bartholic--Namken--Wiegand m e t h o d for partial c a n o p y cover (Fig. 10). When only crop c a n o p y t e m p e r a t u r e was used for these sites the slope was 1.7 with R 2 = 0.51. There is still considerable scatter a b o u t the 1 : 1 line, and the m odel tends to overpredict the measured amounts. This overprediction could be a c c o u n t e d for in eq. 2 if the soil heat flux term were included in the analysis. It is possible t hat this problem could be overcome by using the change in the soil surface t e m p e r a t u r e from one h o u r to the n e x t to a p p r o x i m a t e a change in heat storage. This aspect will require f u r t h e r investigation before the m odel can be applied t h r o u g h o u t a growing season. Measured com pos i t e t e m p e r a t u r e s would possibly improve the mo d el over the weighted average which was utilized in this study.

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Fig. 9. Measured evapotranspiration from partial canopy cover crops on lysimeters at Brawley, California; Fargo, North Dakota; and Lincoln, Nebraska compared to calculated evapotranspiration by the Bartholic--Namken--Wiegand model. Fig. 10. Measured evapotranspiration from partial canopy cover crops on lysimeters at Brawley, California; Fargo, North Dakota; and Lincoln, Nebraska compared to calculated evapotranspiration by the surface energy balance model. SUMMARY AND CONCLUSIONS Canopy t e m p e r a t u r e as an input to surface energy balance models provides a m e t h o d for estimating actual evapotranspiration from a surface. Surface energy balance models pr oposed by Brown and Rosenberg (1973) and Soer (1980) p e r f o r m e d well for all locations and crops in this study. The best agreement b e t w e e n measured and predicted evapotranspiration rates was for full c a n o p y cover, although a composite surface t e m p e r a t u r e (soil and plant) improved the fit between predicted and actual evapotranspiration values. Diurnal plots of t he data showed t hat the Bartholic--Namken--Wiegand m e t h o d was driven mainly by net radiation and had a large error in estimating actual evapotranspiration, particularly in situations with partial c a n o p y cover. Inclusion of c a n o p y t e m p e r a t u r e with o t h e r meteorological data does n o t make a c o m p l e t e r e m o t e sensing approach but it does allow for dynamic coupling b e t w een the plant and atmosphere. Ground-based net radiation, air t e m p e r a t u r e and windspeed are still needed as model inputs. Any collection o f surface t e m p e r a t u r e f r om an air or spacecraft would represent an instantaneous evapotranspiration rate and this technique would have to be applied over a c o m p l e t e day to improve the utility of the i nform at i on for agricultural management. Jackson et al. (1983) proposed a m e t h o d based on latitude, time-of-day and time-of-year which adjusts one time-ofday measurements t o daily totals. This approach, along with an estimation of seasonal changes in the c a n o p y a e r o d y n a m i c properties, will have to be evaluated over a c o m p l e t e growing season to provide a rigorous test of the

52 model's limitations. These results indicate that the surface energy balance with canopy temperature i n p u t s is a n a c c u r a t e a n d r e l i a b l e m e t h o d o f obtaining actual evapotranspiration from a crop surface. ACKNOWLEDGEMENTS We express our gratitude to the following individuals for their cooperation a n d a s s i s t a n c e : C. E h l i g ( B r a w l e y ) , J. T. R i t c h i e ( T e m p l e ) , E. T. K a n e m a s u ( M a n h a t t a n ) , B. L . B l a d ( L i n c o l n ) , D. G. B a k e r ( S t . P a u l ) , L. J. B r u n ( F a r g o ) , J. K. A a s e ( S i d n e y , M o n t a n a ) , J. L . W r i g h t ( K i m b e r l y ) , a n d W. O. P r u i t t (Davis). Without the help of these individuals and their allowing us to use their facilities, this work would not have been possible. REFERENCES Bartholic, J. F., Namken, L. N. and Wiegand, C. L., 1970. Combination equations used to calculate evaporation and potential evaporation. USDA-ARS-Bull. 4 1 - - 1 7 0 . 1 4 pp. Blad, B. L. and Rosenberg, N. J., 1976. Evaluation of resistance and mass transport evapotranspiration models requiring canopy temperature data. Agron. J., 68: 764--769. Bowen, I. S., 1926. The ratio of heat losses by conduction and by evaporation from any water surface. Phys. Rev., 27: 779--787. Brown, K. W. and Rosenberg, N. J., 1973. A resistance model to predict evapotranspiration and its application to a sugar beet field. Agron. J., 65 : 341--347. Hatfield, J. L., 1983. Evapotranspiration obtained from remote sensing methods. In: D. E. Hillel (Editor), Advances in Irrigation, Vol. 2, Academic Press, New York, pp. 395--416. Hatfield, J. L., Smietana, P. J., Jr., Carroll, J. J., Flocchini, R. G. and Hamilton, R. H., 1981. Solar Energy: Implementation of a research and training site at Davis, California. Land, Air and Water Resources Paper 10005. University of California. 259 pp. Hatfield, J. L., Perrier, A. and Jackson, R. D., 1983. Estimation of evapotranspiration at one-time-of-day using remotely sensed surface temperature. Agric. Water Manage., 7: 341--350. Heilman, J. L. and Kanemasu, E. T., 1976. An evaluation of a resistance form of the energy balance to estimate evapotranspiration. Agron. J., 68: 607--611. Heilman, J. L., Kanemasu, E. T., Rosenberg, N. J. and Blad, B. L., 1976. Thermal scanner measurement of canopy temperatures to estimate evapotranspiration. Remote Sensing Environ., 5: 137--145. Jackson, R. D., Reginato, R. J. and Idso, S. B., 1977. Wheat canopy temperatures: a practical tool for evaluating water requirements. Water Resour. Res., 13: 651--656. Jackson, R. D., Hatfield, J. L., Reginato, R. J., Idso, S. B. and Pinter Jr., P. J., 1983. Estimation of daily evapotranspiration from one time of day measurements. Agric. Water Manage., 7 : 351--362. Monteith, J. L., 1973. Principles of Environmental Physics. American Elsevier, New York, 243 pp. Penman, H. L., 1948. Natural evaporation from open water, bare soil, and grass. Proc. R. Soc. London, Ser. A, 193: 120--145. Sequin, B. and Itier, B., 1983. Using midday surface temperature to estimate daily evaporation from satellite thermal IR data. Int. J. Remote Sensing., 4: 371--384. Soer, G. J. R., 1980. Estimation o f regional evapotranspiration and soil moisture conditions using remotely sensed crop surface temperatures. Remote Sensing Environ., 9: 27--45.

53 Stone, L. R. and Horton, M. L., 1974. Estimating evapotranspiration using canopy temperatures: field evaluation. Agron. J., 66: 450--454. Sumayao, C. R., Kanemasu, E. T. and Brakke, T. W., 1980. Using leaf temperature to assess evapotranspiration and advection. Agric. Meteorol., 22: 153--166. Van Bavel, C. H. M., 1966. Potential evaporation: the combination concept and its experimental verification. Water Resour. Res., 2 : 455--467. Verma, S. B. and Barfield, B. J., 1979. Aerial and crop resistances affecting energy transport. In: B. J. Barfield and J. F. Gerber (Eds.), Modification of the Aerial Environment of Crops. ASAE, pp. 230--248. Verma, S. B., Rosenberg, N. J., Blad, B. L. and Baradas, M. W., 1976. Resistance--energy method for predicting evapotranspiration: Determination of boundary layer resistance and evaluation of error effects. Agron. J., 68: 776--782.