SUGITA Michiaki

Tsukuba Repository

Estimation of Large Scale Evaporation by a
Complementary Relationship with a simple ABL
Model (Session 5: Modeling studies)
著者 SUGITA Michiaki
journal or
publication title
Bulletin of the Terrestrial Environment
Research Center, the University of Tsukuba,
Proceedings of the International Workshop on
GAME-AAN/Radiation
volume 1
number 別冊
page range 91-93
year 2001-03
URL http://doi.org/10.15068/00147195
Estimation of Large Scale Evaporation by a 
Complementary Relationship with a simple ABL Model 
Michiaki Sugita 
Institute of Geoscience, University ofTsukuba, Tsukuba, Ibaraki 305-8571, Japan 
1. Introduction 
Estimation of evaporation from the land 
surface is stil a difficult job, particularly in 
an area where soil moisture decreases 
frequently to a point to force the stomatal 
closure of vegetation and thus to reduce 
evaporation even when the atmospheric 
condition requires and allows more 
evaporation. Traditionally there have been 
two ways to incorporate this efect. The 
first approach uses some kind of soil 
moisture information and uses that 
information to reduce the potential 
evaporation calculated from commonly 
available meteorological data. The main 
difficulty of this approach is that it is not 
easy to obtain any soil moisture information 
to begin with, especially when one deals with 
the regional evaporation. Even when soil 
moisture data are available, they do not 
necessarily represent the whole region well 
since soil moisture is known to vary quite 
largely both vertically and horizontally. 
Another approach that avoids the use of soil 
moisture data is to treat the soil moisture as 
one of the variable to be predicted within a 
model; thus not only the atmosphere but also 
upper soil column are included within the 
model. This is becoming a common practice 
in some of the regional and global circulation 
models. Unfortunately, the accuracy of the 
predicted fluxes and soil moisture are stil 
not satisfactory, even though a constant 
progress has been made to improve the 
products. Also, itis usually necessary to 
increase the complexity of a model to 
enhance the accuracy, while more 
complicated models tend to have a problem 
of "equifinality" in the calibration (Franks et 
al, 1997); because of the complex nature of 
these models, it is difficult to make a 
calibration of needed parameters and there 
are many possible combination of optimal 
parameters that produce an equally good 
result. Thus it may not always be clear if 
the calibrated parameters are universal and 
one can expect a reasonable result with them 
outside the area where the calibration took 
place. 
Sugita et . al.(2001) presented a 
simple model which has a potential to avoid 
the problems outlined above to estimate 
large scale evaporation even under drying 
surface conditions without the knowledge of 
soil moisture. In this paper, a brief outline 
of the model and the results of the model 
application wil be presented together with 
possible usages of the method to derive a 
continental scale evaporation map. 
2. Method 
The idea of a simple procedure to estimate 
evaporation estimation without soil moisture 
data has been accomplished by employing 
the complementary relationship (Bouchet, 
1963) as the main framework of the model. 
In essence, it can be expressed by. 
ら+E = 7x Epo (1) 
where EP is the potential evaporation which 
would take place under the prevailing 
atmospheric conditions with the available 
energy being the only limiting factor, Eis the 
actual regional evaporation, and 11=2.0. 
When E=Ep, it is denoted by EP0 and this is 
the case when there is plenty of available 
water and there is no plant physiological 
control to the water transfer from the soil 
into the atmosphere. The main hypothesis of 
Bouchet (1963) was that when evaporation 
E decreases for some reason below EP0 while 
the available energy remains the same, the 
potential evaporation EP is increased by 
exactly that amount, which yields the 
complementary relation between E and EP. 
The rationale behind this argument is that the 
energy that becomes available due to the 
decrease of E will be used to raise the air 
temperature through the sensible heat flux, 
and this should dry the air and result in the 
increase in EP. Thus if the appropriate 
forms for EP and EP0 can be specified, E 
should be able to be evaluated without the 
knowledge of soil moisture and vegetation 
status. 
However, recent studies have shown 
that the hypothesis is not strictly valid (e.g., 
McNaughton and Spriggs, 1989; Lhomme 
1997, Sugita et al. 2001). Thus an 
alternative method was proposed in Sugita et 
al. (2001) in which the ri value is treated as 
unlmown variable and is determined thourgh 
iteration in (1). This "relaxed 
complementary relationship" can be applied 
to the data sets to estimate regional 
evaporation stil without the knowledge of 
soil moisture condition. 
For E and E P poevaluation, the 
Penman-type equation 
E 
~(Rn-G) y 
+ EA (2) PE ＝ 
~+r f △ +r 
was used, inwhich~is the slope of the 
saturation water vapor pressure at 
temperature九and'Yis the psychrometric 
constant defined asy = cPp I (0.622x£) 
where cP is the specific heat of air, pis the air 
pressure, and f is the latent heat for 
vaporization. EA isa drying power of the air 
and can be evaluated by (3). 
EA" ku,I'(q* -q) 1[f :0:。 ) — 甲 ,[z-,。)r(J) 
where u. isthe friction velocity, p isthe air 
density, k is the von Karman's constant 
(=0.4) *. , q is the saturation specific humidity 
and q isthe specific humidity, z isthe height 
above the ground, ~。 is the zero-plane 
displacement height, z0v is the scalar 
roughness for water vapor,'P孔sthe stability 
correction function for water vapor and L is
the Obukhov length. Actual measurements 
of (q*-q) is used to determine EP while (q *-
q) values simulated by an ABL model with 
surface resistance rs1=0 are used to predict 
Epo. A modified ABL model (Sugita et al., 
200 I) of Lhome (1997) can be used for this 
purpose. 
3. Model Validation 
3.1 HEIFE data set 
The idea to estimate E with (I)ー(3)with the 
inputs from the CBL model was explored 
with the data set obtained during the Heife 
River Field Experiment (HEIFE) (Mitsuta, 
1988) whose major observations took place 
in 1990-1992 in the Hexi Corridor area 
between the Qilian Mountains and the 
Longshou Mountains in Gansu Province of 
China. A typical annual rainfall is as small as 
100 mm and the land surfaces are covered 
primarily with sand and stony deserts with 
some dotted oases and irrigated farm lands. 
In the present study, the data set of surface 
fluxes and related variables produced from 
measurements at a desert station located at 
100°10'E and 39°23'N with altitude of 
1400 m above a surface of fine sand was 
mainly used. The data set of Mitsuta et al. 
(1995) were mainly used in the analysis. 
Figures 1 and 2 give the comparison 
between measured daytime evaporation and 
corresponding estimated values. Figure 1 is
the result of assumed value of ri=2. 0while 
Figure 2 came from the procedure outlined 
above without the assumption. Clearly, the 
procedure gave better agreement of the 
evaporation. 
3.2 GAME/AAN data sets 
Similar analysis can be made with the data 
set obtaind from GAME/ AAN project. In 
view of the experimental area of HEIFE, 
priorities should be placed to those data sets 
taken in vastly different regions. The 
candidate should include several data sets in 
Thailand which give opportunity to test the 
usefulness of the method in a very 
contrasting conditions of dry/wet season 
encountered in the same areas. Also data 
sets from cold regions should give important 
insights into not only the model applicability 
over these areas but also the land-
atmosphere interaction problems in general. 
References 
Bouchet, R.J., Evapotranspiration reelle et 
potentielle, signification climatique, IAHS 
Publ. No. 62, 134-142, 1963. 
Franks, S.W., K.J.Beven, P.F.Quinn, and 
I.R. Wright, On the sensitivity of soil-
vegetation-atmoshpere transfer schemes: 
equifinality and the problem of 
robustcalibration, Agric. For. Meteorol., 86, 
63-75, 1997. 
Lhomme, J.-P., An examination of the 
Priestley-Taylor equation using a convective 
boundary layer model, Water Resour. Res., 
33, 2571-2578, 1997. 
McNaughton, KG. and T.W. Spriggs, An 
evaluation of the Priestley and Taylor 
equation and the complementary relationship 
using results from a mixed-layer model of 
the convective boundary layer. IAHS Puhl., 
no.177, 89-104, 1989. 
Mitsuta, 
program 
process. 
Y., Sino-Japanese cooperational 
on the atmosphere-land surface 
Tenki, 35, 501-505, 1988. 
Mitsuta, Y., I.Tamagawa, K.Sahashi, and 
I.Wang, Estimation of annual evaporation 
from the Linze desert during HEIFE, J. 
Meteorol. Soc. Japan, 73, 967-974, 1995. 
Sugita, M., J.Usui, I.Tamagawa, and I. 
Kaihotsu, Complementary relationship with 
a convective boundary layer model to 
estimate regional evaporation, Water 
Resources Research, 37-2, 353-365, 2001. 
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250 
150 
5゜ ゜
-50 I 
-50 
。 ?
daytime mean 
Daytime moan (Rainy day) 
~ 
50 
LE (!J'./m勾
Fig. 1 Comparison between observed latent heat 
flux LE. and model predicted values LE with 
ri=2.0. The straight line indicates 1 to1 
agreement while the dotted line gives the slope 
of the regression equation of the form y=ax. 
? ?
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250 
150 
50 
-50 
150 250 
。 ?
da;<,me mean 
D● ;lime mean (Rainy day) 
． 
? ? ?
? ?
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?、
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．?
-50 50 150 250 
LE (W/m2J 
Fig. 2 Same as Fig. lbut model predication was 
carried out without the assumption of ri=2.0 


Estimation of Large Scale Evaporation by a Complementary Relationship with a simple ABL Model (Session 5: Modeling studies)

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