Hydrometeorological application of a microwave link: 1. Evaporation

[1] A method to estimate areal evaporation using a microwave link (radio wave scintillometer) in combination with an energy budget constraint is proposed. This radio wave scintillometry-energy budget method (RWS-EBM) is evaluated for its applicability in different meteorological conditions and for its sensitivity to various variables (the structure parameter of the refractive index of air C_n², the total available energy R_n - G, the wind velocity u, the effective average vegetation height h ₀, and the correlation coefficient between the temperature and humidity fluctuations r_TQ). The method is shown to be best suited for use in wet to moderately dry conditions, where the latent heat flux is at least a third of the total available energy (i.e., Bowen ratio ≤2). It is important to accurately measure the total available energy and the wind velocity as the RWS-EBM is most sensitive to these variables. The Flevoland field experiment has provided the data, obtained with a 27-GHz radio wave scintillometer (over 2.2 km), a large-aperture scintillometer (also 2.2 km), and four eddy covariance systems, which are used to test the RWS-EBM. Comparing 92 daytime measurements (30-min intervals) of the evaporation estimated using the RWS-EBM to that determined in alternative manners (eddy covariance and two-wavelength scintillometry) leads to the conclusion that the method provides consistent estimates (coefficient of determination r² = 0.85 in both cases) under relatively wet conditions.