Evapotranspiration ¯uxes over mixed vegetation
areas measured from large aperture scintillometer
, Lalith Chandrapala
Arnold F. Moene
International Water Management Institute, P.O. Box 2075, Colombo, Sri Lanka
Meteorological Department, Bauddhaloka Mawatha, Colombo 7, Sri Lanka
Meteorology and Air Quality Group, Wageningen University and Research Center,
Duivendaal 2, 6701 AP Wageningen, The Netherlands
Routine measurement of spatially averaged surface ¯uxes of sensible heat (H) in river basins is
now feasible. These ¯uxes, when combined with net radiation estimates, can be used to derive areally
averaged actual evapotranspiration (ET). The scintillation method is shown to be promising method
for estimating areally averaged sensible heat ¯uxes. The large aperture scintillometer (LAS) is an
optical device used to monitor ¯uctuations in refractive index of the turbulent atmosphere over a
relatively large area. The study reported here has estimated ET ¯uxes for an area of mixed vegetation
at Horana, a ®eld site about 40 km southeast of Colombo, Sri Lanka. ET estimates derived from the
scintillometer and net radiometer were compared with estimates obtained from a remote sensing
based surface energy balance algorithm for land (SEBAL). The SEBAL estimating of ET were
derived using NOAA satellite images without any a priori calibration. The average deviation of ET
estimates between SEBAL and LAS for 10-day periods was 17%. However, this deviation fell to 1%
when monthly estimates were considered. This suggests regional ET values derived from remote
sensing are reasonable estimates, however, the LAS was used in only one agro-ecosystem to validate
the SEBAL model. The LAS is a low cost alternative to other methods of estimating heat ¯uxes for
use in basin scale water use studies.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Scintillometer; Sensible heat ¯ux; Energy balance; Evaporative fraction; Evapotranspiration
Agricultural Water Management 58 (2003) 109±122
Corresponding author. Tel.: 94-178-7404; fax: 94-178-6854.
E-mail addresses: firstname.lastname@example.org (H.M. Hemakumara), email@example.com (L. Chandrapala).
0378-3774/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0378-3774(02)00131-2