Analytical derivation of the Budyko curve based on rainfall
characteristics and a simple evaporation model
A. M. J. Gerrits,
1,2
H. H. G. Savenije,
1
E. J. M. Veling,
1
and L. Pfister
2
Received 24 July 2008; revised 18 November 2008; accepted 23 January 2009; published 3 April 2009.
[1] The Budyko curve is often used to estimate the actual evaporation as a function of the
aridity index in a catchment. Different empirical equations exist to describe this relationship;
however , these equations have very limited physical background. The model concept
presented in this paper is physically based and uses only measurable parameters. It makes
use of two types of evaporation: interception and transpiration. It assumes that interception can
be modeled as a threshold process on a daily time scale. If multiplied with the rainfall
distribution function, integrated, and multiplied with the expected number of rain days per
month, the monthly interception is obtained. In a similar way, the monthly interception can be
upscaled to annual interception. Analogous to the interception process, transpiration can
be modeled as a threshold process at a monthly time scale and can be upscaled by integration
and multiplication with the expected number of rain months. The expected rain days per month
are modeled in two ways: as a fixed proportion of the monthly rainfall and as a power
function based on Markov properties of rainfall. The latter is solved numerically. It appears
that on an annual basis the analytical model does not differ much from the numerical solution.
Hence, the analytical model is used and applied on 10 locations in different climates.
This paper shows that the empirical Budyko curve can be constructed on the basis of
measurable parameters representing evaporation threshold values and the expected
number of rain days and rain months and, in addition, a monthly moisture
carryover amount for semiarid zones.
Citation: Gerrits, A. M. J., H. H. G. Savenije, E. J. M. Veling, and L. Pfister (2009), Analytical derivation of the Budyko curve based
on rainfall characteristics and a simple evaporation model, Water Resour. Res., 45, W04403, doi:10.1029/2008WR007308.
1. Introduction
[2] In water resources modeling the Budyko curve is
often used to simulate evaporation as a function of an
aridity index in a simple supply-demand framework. In
some locations of the world, annual e vaporation may
approach annual precipitation. This occurs if there is always
sufficient energy available to evaporate the precipitation.
Such locations are moisture constrained. In other locations,
annual evaporation may approach potential evaporation .
This happens if the available energy is less than the required
energy to evaporate the annual precipitation. These loca-
tions are energy constrained. Depending on the dryness of
the climate, either the available water or the available
energy is the limiting factor.
[
3] The Budyko curve is based on two balance equations:
the water balance and the energy balance [Arora, 2002]:
dS
dt
¼ P E Q ð1Þ
R
n
¼ rlE þ H þG ð2Þ
where S is the water storage, P the precipitation, E actual
evaporation, Q the catchment runoff, R
n
the net radiation, l
the latent heat of vaporization, H the sensible heat flux, and
G the ground heat flux. On an annual time scale we can
assume that the water storage change is negligible (dS/dt =0)
and that the net ground heat flux approaches zero (G =0).
By dividing equation (2) by equation (1) we obtain with
P
a
= E
a
+ Q
a
where the subscript a indicates annual values:
R
n
P
a
¼
rlE
a
P
a
þ
H
P
a
ð3Þ
If we successively define the annual potential evaporation as
rlE
p
= R
n
(where Arora [2002] interprets potential
evaporation as all energy being converted into evaporation
and none in heating) and define the Bowen ratio as B
r
= H/rlE
a
we obtain
E
p
P
a
¼
E
a
P
a
þ
B
r
E
a
P
a
¼ f ¼
E
a
P
a
1 þ B
r
ðÞ
ð4Þ
with f the aridity index.
[
4] Since the Bowen ratio can also be expressed as a
function of the aridity index [Arora, 2002], equation (4) can
be rewritten as
E
a
P
a
¼
f
1 þ f fðÞ
¼ F fðÞ ð5Þ
1
Water Resources Section, Delft University of Technology, Delft,
Netherlands.
2
Department of Environment and Agro-biotechnologies, Centre de
Recherche Public-Gabriel Lippmann, Belvaux, Luxembourg.
Copyright 2009 by the American Geophysical Union.
0043-1397/09/2008WR007308
W04403
WATER RESOURCES RESEARCH, VOL. 45, W04403, doi:10.1029/2008WR007308, 2009
1of15
评论0
最新资源