Agricultural and Forest Meteorology 102 2000 113–124
Estimating daytime latent heat flux and evapotranspiration in Jamaica
D. Amarakoon
∗
, A. Chen, P. McLean
Department of Physics, University of the West Indies, Kingston 7, West Indies, Jamaica Received 20 July 1999; received in revised form 6 January 2000; accepted 14 January 2000
Abstract
The suitability of the two-parameter scheme of De Bruin and Holtslag 1982 to estimate daytime latent heat flux and evapotranspiration in Jamaica was examined using meteorological data collected over fields of short grass. The study covered
three different periods, January to February of 1994, January to February of 1990, and August to September of 1989. The fields were irrigated regularly three times a week to provide enough water to evaporate. The daytime 07:00–18:00 hours
20 min and hourly latent heat flux values, average evaporation for the earlier mentioned three periods and evaporation on individual days were estimated using De Bruin and Holtslag scheme for two situations. One was with the parameter α
′
= 0.95
and β=20 W m
− 2
and the other was with α
′
= 1.05 and β=20 W m
− 2
. The analysis of the latent heat flux and evaporation led to the following results. i The values of latent heat flux and evaporation estimated using α
′
= 0.95 and β=20 W m
− 2
compare well with the corresponding measured values for time steps of the order of 20 min to several days. ii The level of agreement
between estimated and measured hourly values of latent heat flux seen in our work was very close to that reported in the study of De Bruin and Holtslag, implying the consistency of the skill of that scheme at higher average latent heat flux levels
in Jamaica. iii The choice of 1.05 for α
′
applies better for our data. These results indicate that the scheme of De Bruin and Holtslag with α
′
= 0.95 and β=20 W m
− 2
can satisfactorily be used to estimate daytime latent heat flux and evaporation from well watered grass surfaces in regions having environmental conditions similar to those encountered in Jamaica, but appears
better with α
′
= 1.05. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Grass; Bowen ratio; Latent heat flux; Evaporation; Radiation; Priestly–Taylor parameter
1. Introduction
Estimation of surface latent heat flux and hence ac- tual evaporation evapotranspiration from surfaces is
important for purposes such as regional water bal- ance studies, field irrigation practice, description of
the atmospheric boundary layer including stability, and weather forecasting. Many schemes are available to
accomplish the estimation Brutsaert, 1982. However, little work has been done on evaporation in the tropics
and all the schemes available have been developed in
∗
Corresponding author. Fax: +1-876-977-1595. E-mail address:
amarakuwimona.edu.jm D. Amarakoon
temperate climates. The lack of information on evap- oration in the tropics leads to situations where avail-
able schemes have to be used to estimate evaporation without any prior knowledge of their applicability to
tropical environmental conditions. This can cause er- rors in the estimates.
Three schemes Penman, 1948; Priestley and Tay- lor, 1972; De Bruin and Holtslag, 1982 need to be
mentioned. The schemes Penman, 1948; Priestley and Taylor, 1972 are important as they are widely used
in hydrology because of their sound analytical basis, and also they have resulted in other schemes e.g.
Brutsaert and Stricker, 1979; De Bruin and Holtslag, 1982 that are of practical value. The scheme of De
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 0 5 - 2
114 D. Amarakoon et al. Agricultural and Forest Meteorology 102 2000 113–124
Bruin and Holtslag 1982 is a simple empirical mod- ification of Priestley and Taylor 1972 scheme, and
it appears useful in estimating surface latent heat flux from surfaces with short crops when the surface is
supplied with enough water to evaporate. The prac- tical advantage of this scheme is the fact that only a
few measurable input parameters are required to es- timate latent heat flux apart from the two empirical
parameters α
′
and β. The results of De Bruin and Holtslag 1982 have shown that the scheme has the
same skill as Penman–Monteith approach Monteith, 1965; Brutsaert, 1982. Penman–Monteih approach
Monteith, 1965; Brutsaert, 1982 is a more complete scheme, but requires a relatively large number of input
parameters.
The objective of our study was to examine the ap- plicability of De Bruin and Holtslag 1982 scheme
for estimating latent heat flux and evaporation in Ja- maica, which has a tropical climate. The scheme was
developed in the Netherlands, which has a temperate climate and appear to have the potential for practical
applications. If its applicability can be verified under tropical environmental conditions, it may be a useful
tool, especially in agricultural meteorology, for esti- mating latent heat flux and hence evaporation from
surfaces covered with short vegetation and irrigated regularly. For testing De Bruin and Holtslag 1982
scheme, we estimated the quantities stated below using that scheme and the data collected over fields
with grass during the periods January to February of 1994, January to February of 1990 and August to
September of 1989. The quantities estimated were daytime 07:00–18:00 hours values of i 20 min and
hourly latent heat flux, ii average evaporation for the three periods of data collection and iii evaporation
on individual days. The estimated values were then compared with corresponding measured values using
linear regression analysis and calculating percentage deviations where appropriate.
2. Consideration of schemes
