ESTIMATING SURFACE FLUXES IN IRRIGATED AREAS WITH SCINTILLOMETERS
Creators: name: Hartogensis, Oscar
name: De Bruin, Henk
name: Hartogensis, Oscar
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: Scintillometry has proven to be a good alternative method to obtain surface fluxes 
over heterogeneous areas over spatial scales of up to 10 km and in non-stationary 
conditions in the stable surface layer (see e.g. [1]). This study concerns agro-
hydrological scintillometer applications of estimating evaporation over homogeneous 
irrigated areas on a scale of 50 to 500 m. Two types of scintillometers will be 
considered, notably the displaced beam small aperture scintillometer (DBSAS) and the 
large aperture scintillometer (LAS) deployed in two field campaigns in Idaho, USA in 
1999 [2] and in the Yaqui Valley, Sonora, Mexico in 2000 [3].
The DBSAS and the LAS are optical instruments that consist of a transmitter and 
receiver. The receiver records intensity fluctuations of the light beam emitted by 
the transmitter, which are caused by refraction of the beam upon its passage through 
the turbulent surface layer. These intensity fluctuations are a measure of the 
structure parameter of temperature, CT2. The DBSAS obtains also the dissipation rate 
of turbulent kinetic energy, e, from the correlation between the two displaced 
beams. CT2 and e are related to the surface fluxes of heat, H, and momentum, t, by 
virtue of Monin-Obukhov similarity theory. For the LAS - that provides CT2 only - t 
is obtained from additional wind speed measurements and an estimate of the roughness 
length. Evapotranspiration can then be estimated from net radiation and the soil 
heat flux measurements.
In both field campaigns the irrigated agricultural area was surrounded by a desert. 
In these conditions dry, warm desert air can be advected over the cool evaporating 
surface by which sensible heat becomes negative and the water vapor deficit is 
increased, both enhancing evapotranspiration. As a result the surface layer is 
stably stratified and wind shear is the only turbulence generating mechanism. The 
DBSAS directly gives information on this process, the LAS does not.
We will outline the potential of scintillometers of obtaining principle turbulence 
parameters (CT2 and e) and fluxes of latent and sensible heat, and compare these 
with eddy covariance method based estimates for the two experiments. We will present 
evidence that scintillometers have advantages over the eddy covariance (EC) method 
in the often non-stationary stable surface layer, since they can be used for short 
flux-averaging periods as they average turbulence not only in time but also in 
space. Furthermore, scintillometers require less complex data processing and quality 
control procedures. Last, the transmitter and receiver of the instrument can be 
installed at the borders of the field by which the instrument does not interfere 
with the farmer’s activities in the field. 
Finally, the medium aperture scintillometer (MAS) will be discussed, which has an 
aperture size that includes features of both the DBSAS and the LAS. Result of MAS 
measurements over grass at Cabauw, Netherlands will be shown. 

References
[1] De Bruin, H.A.R.: 2002, ‘Introduction, renaissance of scintillometry’, Boundary-
Layer Meteorol. 105, 1-4. and the papers of this special issue on scintillometry.
[2] De Bruin, H.A.R., Hartogensis, O.K., Allen, R.G., and Kramer, J.W.J.L., 
2004: ‘Note on the Regional Advection Perturbations in an Irrigated Desert (RAPID) 
Experiment’, Theor. Appl. Climatol. 80, 143-152.
[3] Hoedjes, J.C.B., Zuurbier, R.M., and Watts, C.J.: 2002, ‘Large aperture 
scintillometer used over a homogeneous irrigated are partly affected by regional 
advection’, Boundary-Layer Meteorol. 105, 99-117.
[4] Hartogensis, O.K., De Bruin, H.A.R., Van De Wiel, B.J.H.: 2002, ‘Displaced-Beam 
Small Aperture Scintillometer Test. Part II: Cases-99 Stable Boundary-Layer 
Experiment’, Boundary-Layer Meteorol. 105, 149-176.