Evaporation measurements at kilometre scales determined using two-wavelength scintillometry

Reliable measurements of the land surface evaporation are required for hydrological modelling and water resource management, as well as for boundary layer modelling and numerical weather prediction. Catchment models and satellite estimates usually operate at large grid scales of at least 1 km2. To validate these models, and measure catchment water balances, requires up-scaled measurements that must average land surface variability, in particular, the wide range and mixture of vegetation types and soil moisture conditions that are often found at such scales. Traditional methods are difficult to apply at these scales, invoking many replications across the different land-surface types. Two-wavelength scintillometry allows the determination of evaporation over paths of many kilometres, without the disadvantages of in situ sensors. Low maintenance transceivers were installed at both ends of a 2.4 km path over mixed agriculture on chalk downland at Sheepdrove Organic Farm, West Berkshire, UK. Infra-red and millimetre-wave beams sensed the path-averaged turbulent changes of temperature and moisture (scintillations) and Monin-Obukhov Similarity Theory was applied to determine the evaporation. Results show that large scale evaporation can be reliably determined by scintillometry, under most conditions. Recommendations for deployment and the limitations of scintillometry to determine evaporation are discussed.