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 open- water 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.