Hydrometeorological application of a microwave link: 1. Evaporation

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Abstract
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 Cn2, the total available energy Rn − G, the wind velocity u, the effective average vegetation height h0, and the correlation coefficient between the temperature and humidity fluctuations rTQ). 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.

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