Parameterizing the effect of a wind shelter on evaporation from small water bodies

The potential use of windbreaks to reduce evaporation from small agricultural reservoirs motivated the development of a simple evaporation model that includes the effect of wind shelters. The shelter effect is parameterized by averaging the integral of the horizontal velocity deficit curve over the length of the water body. This parameterization, termed the "shelter index," ranges between 0 and 1, representing no shelter to complete shelter, respectively. The results of a two-dimensional aerodynamic model that solves the disturbed flow field and evolving microclimate over the water body guided the development of a Dalton-type evaporation model, modified to include the shelter parameterization. The modified Dalton expression summarized the results of the aerodynamic model to a high degree of accuracy (R(2) = 0.988). Because the shelter parameterization requires knowledge of the horizontal velocity profile, an approximation of the shelter index that can easily be estimated from physical windbreak characteristics (height, porosity) is presented. In addition, a simple approximation based exclusively on upwind meteorological information is presented for estimation of surface humidity. The evaporation model using the approximations for the shelter index and surface humidity showed excellent agreement (R(2) = 0.875) with measured evaporation data from a variety of small wind-sheltered water bodies at two sites in the agricultural districts of Western Australia. The evaporation model and approximations have the advantage of requiring only routinely available meteorological information and information on windbreak physical characteristics that can be estimated a priori. It is therefore an excellent design tool for water resource managers to evaluate the efficiency of a wind shelter in reducing evaporation or for coupling with hydrodynamic models.