An area-dependent wind function for estimating open water evaporation using land-based meteorological data
We propose a generally applicable formula for estimating evaporation rate from open water bodies which utilizes readily available land-based meteorological data. We follow the well-known aerodynamic approach in which evaporation rate is modelled as the product of a vapour pressure deficit between the water surface and upwind air and a wind function which depends on wind speed. Importantly, such a technique should account for the size of a given water body as evaporation rate is known to decrease with distance downwind from the shore due to the increasing entrainment of water vapour. Our area-dependent relationship is based on a compilation of site-specific wind functions reported for individual water bodies covering a large range of sizes from 0.07m2 to 33.5km2. The proposed wind function (mmd?1kPa?1) is f(U2)=(2.36+1.67U2)A?0.05 where U2 is wind speed (ms?1) measured over land at a height of 2m and A is the surface area of the water body (m2). The areal dependence of our relationship was found to be similar to that proposed in other experimental and theoretical studies. Our proposed area-dependent wind function might be expected to be a best estimate in the absence of more specific site related information. From the scatter of reported site-specific wind functions around our proposed relationship, we develop an estimate of its uncertainty. We also explore the propagation of this wind function uncertainty through aerodynamic and combination type evaporation estimation techniques. We use Rushy Billabong, a 5ha pond in Australia, as a case study and show how uncertainty can be reduced by more than 60% by using a combination equation approach.
wind function open water evaporation water body wind speed uncertainty pond lake