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

Abstract

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 coefficient, scale effect. ResumenEfectos de las coberturas de sombreo de polietileno negro sobre la evaporación de pequeños embalses agrícolas La posibilidad de emplear coberturas de sombreo para reducir la evaporación en pequeños embalses de uso agrí-cola motivó el análisis de la incidencia de mallas de polietileno negro, tanto en la tasa de evaporación como en las variables microclimáticas implicadas, sobre un tanque evaporímetro Clase A. Durante el verano de 2003 se realizaron medidas de evaporación horaria en dos tanques evaporímetros localizados en Cartagena (España), así como de la temperatura y humedad del aire, la temperatura del agua, la radiación solar y la velocidad del viento. El primer tanque fue cubierto con mallas negras de polietileno, empleando dos configuraciones (simple y doble malla), mientras que el segundo permaneció descubierto durante la experimentación. Se analizaron los principales factores implicados en el proceso de evaporación (coeficiente de transferencia de masa y déficit de presión de vapor entre la superficie de agua y el aire), con especial incidencia en los cambios producidos respecto al tanque descubierto. Los resultados muestran una disminución de la tasa de evaporación diaria del 75 y 83% para las coberturas de simple y doble malla respectivamente. Se observó un importante efecto de recuperación de agua por condensación, que alcanzó el 14 y 21% de las pérdidas diarias por evaporación para las configuraciones de simple y doble malla respectivamente. Las conclusiones del estudio indican que: (i) las coberturas de sombreo de malla negra de polietileno suponen una solución eficiente para reducir las pé...

Estimating the evaporation from irrigation reservoirs of greenhouses using satellite imagery

“…The evaporation rates and effects of shading material obtained fell within the same order of magnitude as those obtained by in Murcia, which is an intensive agricultural zone with similar climatic conditions.…”

“…Figure 7 shows detail of the amount of evaporation from some irrigation reservoirs in January. Shading material extended over the open water bodies of irrigation reservoirs has a double effect, direct evaporation reduction and recovering of condensed water [2,3,33]. Both effects were joined and applied to the estimation of evaporation, using the middle point of the above-mentioned intervals.…”

“…There have been no previous studies about the exact determination of water loss via evaporation from the whole open irrigation reservoirs in an intensive agriculture zone. studied the effects of using shading plastics to cover a small irrigation reservoir on direct evaporation. This is one of the most important water loss from artificial on‐farm agricultural water reservoirs .…”

“…If the open water surface of an irrigation reservoir is covered with shade material, evaporation can be reduced by between 75% and 83% . If the lower reduction is applied, the evaporation from covered irrigation reservoirs is E cov = 384 mm.…”

“…There is a greenhouse surface of 7,074,500 m 2 and an open water bodies surface of 58,000 m 2 in the study area [16]. A total of 505 greenhouses and a similar number of open water bodies was counted.…”