ESTIMATING SURFACE FLUXES IN IRRIGATED AREAS WITH SCINTILLOMETERS

Abstract

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.