Agricultural Water Reservoirs Research Articles

Efficiency of shading materials in reducing evaporation from free water surfaces

The aim of this study was to characterize the influence of various shading materials on the evaporation rate from a Class-A pan and on its driving variables. Hourly evaporation measurements from two pans, one uncovered and the other covered with different types of shading meshes, were carried out under summer conditions, along with measurements of air temperature and humidity, water temperature, solar global radiation and wind speed. Single and double-layered polyethylene meshes of different colours and a single layered aluminized mesh were tested. The aerodynamic and shading effects responsible for the evaporation reduction, as well as the additional processes of condensation and water recovery, were analyzed and discussed. In all cases, shading the pan induced a significant decrease of the daily evaporation rate, ranging from 50% for the aluminized screen to near 80% for the coloured-polyethylene meshes. The latter materials combined an efficient aerodynamic effect (decrease of about 70% of the mass transfer coefficient) with a strong diminution of surface-to-air water vapour gradient. The condensation over the meshes was far for being negligible for the black polyethylene meshes, reaching up to 15–20% of the daily evaporation losses. It was concluded that the choice of the shading material should be based on criteria related to porosity and optical properties, but also to the capacity of the material in enhancing the condensation process during the night and collecting the condensed water. The extrapolation of the results to the case of agricultural water reservoirs was discussed, and an economic analysis was carried out to assess the viability of shading covers under the conditions of water use and pricing prevailing in Southern Spain.

Experimental assessment of shade-cloth covers on agricultural reservoirs for irrigation in south-eastern Spain

Agricultural water reservoirs (AWRs) are commonly used to guarantee water supply throughout the whole irrigation season in arid and semiarid areas. An important fraction of the total stored water is lost through evaporation, substantially decreasing overall irrigation efficiency. In this study, the effects of suspended shade cloth covers (SSCCs) on reducing evaporation and on the quality of the stored water have been experimentally assessed. To this aim, an AWR located in south-eastern Spain was monitored during two consecutive years. During the first year, the AWR remained uncovered, while during the second year it was covered with a black polyethylene SSCC. The evaporation rate, the water temperature profile and the driving meteorological variables determining evaporation were measured to assess the technical viability of the cover. Evaporation measurements for covered conditions were compared with estimations obtained from an openwater evaporation model which was validated during the first year of experimentation. A reduction close to 85% in the evaporation rate was found. To assess the effects on water quality, water samples and measurements with a multiparametric instrument were monthly collected during the two-year experimental period. Electrical conductivity, chlorophyll concentration and turbidity were measured using this equipment. Results indicate that the reduction in solar radiation (1% transmission through the cover) dramatically reduced the photosynthetic activity; hence algal bloom was highly limited. Finally, the main benefits and costs associated with the cover installation were identified in order to analyse economic viability under different scenarios representative of the current irrigated farming situation in south-eastern Spain.