Effect of black polyethylene shade covers on the evaporation rate of agricultural reservoirs

The potential use of shade covers to reduce evaporation from agricultural reservoirs motivated this study on the effect of black polyethylene shade on the evaporation rate from a small water body (Class-A pan) and of its driving variables. Evaporation was measured hourly in two pans during the summer in Cartagena (Spain), along with the measurements of air temperature and humidity, water temperature, solar radiation and wind speed. The first pan was uncovered whereas the second pan was covered with black polyethylene shade as either a single or double-layer. The main factors influencing reduced evaporation (mass transfer coefficient and surface-to-air vapour pressure deficit) were analyzed, focusing on the changes in the uncovered pan. In the shaded pan there was a decrease in daily evaporation of 75 and 83% for single and double-layer shade respectively. Condensation on the shade was considerable and was 14 and 21% of the daily evaporation losses for the single and double-layer shade respectively. It was concluded that (i) black polyethylene shade appears to be an efficient way to reduce evaporative loss from agricultural reservoirs, and (ii) an economic analysis of their implementation under the current scarce water supply, for agriculture, in southern Spain justified their use. Additional key words: class-A pan, condensation, economic viability, farm dams, mass transfer coeff icient, scale effect.