Comparative studies on the water evaporation rate from a wetted surface and that from a free water surface

To perform this comparison, two identical ponds were constructed, internally measuring 116×116×22 cm. In one of the ponds, white towels were stretched over a densely perforated PVC panel supported by pieces of waterproof polystyrene to make the towels afloat on the water surface of the pond. The other pond was simply filled with water, and the surface of water was kept exposed to the ambient. The rates of water evaporation from both ponds to the ambient under a wide range of climatic conditions were recorded and correlated with wind velocity, water temperature, air temperature and relative humidity. The best linear fit to the measured data (using the least squares method) has indicated that the rate of water evaporation from water bodies to the ambient air is not a simple and direct proportional function of the difference in vapor pressure of the water at its temperature and that at the air dew point, as many investigators claimed in the past. In this experiment, it was found that the water evaporation rate is proportional to the exponent of this difference in the power of 0.82 when the evaporation is from a free water surface, and 0.7 when water evaporates from the wetted towels. A comparison of water evaporation rates from both wetted cloth surface and free water surface has shown that when wind velocity across the water surfaces is very low, the rate of water evaporation from the wetted towels’ surface is greater than that from the free water surface. When wind velocity is higher, this is reversed, and the evaporation rate from the free water surface was higher.