Evaluation of Canopy Temperature--evapotranspiration Models over Various Crops*

Hatf ie ld , J. L., Reg ina to , R. J. a n d Idso, S. B., 1984. Eva lua t ion of 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 mode l s over var ious crops. Agric. For. Meteorol . , 32: 41-53 . 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 emo te ly , o f fe r a m e t h o d of e s t ima t ing evapo t r a n s p i r a t i o n wi th surface energy ba lance models . E q u a t i o n s wh ich have been deve loped by o the r s have been eva lua ted on ly at a l imi ted n u m b e r o f l oca t ions and wi th a few crops. Our s t u d y was c o n d u c t e d at several loca t ions w i th weighing lys imete rs w i t h a var ie ty of c rops a r o u n d t he U n i t e d S ta tes : Brawley, CA; Temple , TX; Lincoln , NE; St. Paul, MN; Fargo , ND; Kimber ly , ID; and Davis, CA, to evalua te e v a p o t r a n s p i r a t i o n ut i l iz ing c a n o p y t e m p e r a t u r e as an i n p u t in to the surface energy balance. The resul ts show t h a t e v a p o t r a n s p i r a t i o n ca lcu la ted f rom the a e r o d y n a m i c res is tance fo rm of the surface energy ba lance was well co r re l a t ed w i th lys imete r m e a s u r e m e n t s at all loca t ions . The er rors us ing the surface energy ba lance were less t h a n 10% in all cases for full g round cover. 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 was m o r e closely coup led to ne t r ad i a t i on t h a n c a n o p y t e m p e r a t u r e . U n d e r par t ia l c a n o p y cover, d i f fe rences b e t w e e n the two mode l s were apparen t . 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 ove rp red i c t ed w h e n the ac tua l e v a p o t r a n s p i r a t i o n was above 200 W m -2 because of its insens i t iv i ty to surface t e m p e r a t u r e . However , t he surface energy ba lance m o d e l e x h i b i t e d on ly a sl ight ove rp r ed i c t i on above 200 W m -2 w h e n a weighed c o m p o s i t e surface t e m p e r a t u r e ( r ep resen ta t ive o f bare soil and c rop t e m p e r a t u r e ) was used. This small ove r p r ed i c t i on could be o v e r c o m e by cons ider ing the soil hea t f lux te rm. There was no loca t ion bias in t he surface energy ba lance mode l , wh ich shows t h a t i t s h o u l d work well a t o t h e r loca t ions . I N T R O D U C T I O N Evapotranspiration from a surface is a component of the partitioning of e n e r g y r e c e i v e d b y t h a t s u r f a c e . T h i s p r o c e s s c a n b e d e s c r i b e d b y t h e f a m i l i a r energy balance equation (Monteith, 1973) with expanded sensible and latent * C o n t r i b u t i o n f rom the Cal i fornia Agr icu l tu ra l E x p e r i m e n t S ta t ion , Pro jec t 3963-H, and t he USDA-ARS.