Modelling surface resistance from climatic variables?
Isabel Alves
*
, Luis Santos Pereira
Department of Agricultural Engineering, Technical University of Lisbon,
Tapada da Ajuda, 1399 Lisbon, Portugal
Accepted 15 February 1999
Abstract
For the Penman±Monteith equation to be used to predict crop evapotranspiration in a one-step
approach, methodologies for determining surface resistance (r
s
) must be available. One usual
approach to the modelling of r
s
is to compute it by inverting the Penman±Monteith equation and
then relate it to the most important environmental variables (radiation, temperature, vapour pressure
deficit) using the multiplicative model of Jarvis. In this paper, some results obtained for lettuce are
presented to illustrate the pitfalls of this approach. It is shown that the same environmental variables
and the same functional forms that are used in the Jarvis model are already considered when
calculating r
s
as the residual term. One cannot thus expect to get a better insight on the behaviour of
r
s
with the multiplicative model. Also, as r
s
includes information on the transport conditions inside
the canopy and thus, is dependent on wind speed (or, indirectly, on the aerodynamic resistance),
procedures that only contemplate stomatal functioning may be not adequate. The interactions
between r
s
and latent heat flux are also discussed and indicate that future studies should be focused
on the determinism and quantification of the energy partitioning. # 2000 Elsevier Science B.V. All
rights reserved.
Keywords: Evapotranspiration; Penman±Monteith equation; Aerodynamic resistance; Surface resistance
1. Introduction
Knowledge of crop evapotranspiration is necessary in many different situations. In
particular, planning and management both at the project and farm level rely on accurate
estimates of crop water needs. Penman (1948) was the first to combine the energy balance
with the expressions that describe heat fluxes to derive a method to estimate the vapour
Agricultural Water Management 42 (2000) 371±385
* Corresponding author. Tel.: +351-1-363-8161; fax: +351-1-362-1575
E-mail address: isabelmalves@isa.utl.pt (I. Alves)
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0378-3774(99)00041-4