Antti Korpela, Hille Rautkoski, Jorma Koskinen, Heikki Pajari, Mika Vähä-Nissi, Laura Alakukku, Hannu Mikkola, Juuso Tuure, Henri Vuollekoski 2017-02-22 23:40:20
Plastic mulch films are used increasingly in agriculture and horticulture, with the main uses being prevention of weed growth, control of soil temperature and moisture, and prevention of soil erosion. Through the use of plastic mulch films, farmers can improve both the quantity and quality of their yields and decrease their use of pesticides and irrigation water. A supplementary function of mulch films could be dew collection for use with cultivated plants. Dew collection with mulch films may be viable, especially in arid and semiarid regions where irrigation water is limited, the price of water is high, and dew events are frequent. As exposed surfaces, such as plant leaves, mulch film, and roofs, cool by radiating heat toward the sky in the evening, at night, and in the early morning, moisture in the ambient air condenses on the cooled surfaces, forming dew droplets. For dew to condense, the temperature of the exposed surface must be equal to or colder than the dew point temperature of the ambient air. Radiative cooling, and thus dew formation, is favored by clear skies. The process of cooling is greatly retarded by cloudiness, haze, and dust in the air. The formation of dew on exposed surfaces also depends on the moisture of the ambient air and the wind speed at the surface of the collector. In general, high humidity, calm winds, clear sky, and relatively warm ambient air result in high dew formation. In the Negev desert of Israel, dew occurs about 200 times per year and can produce the equivalent of 30 mm of precipitation annually. The highest reported dewfall on an artificial surface is 0.6 mm per night. In arid zones, the amount of dewfall can exceed that of rainfall and can even be the sole source of liquid water for plants. An Israeli company, Tal-Ya Water Technologies Ltd., markets funnel-shaped, rigid plastic mulch trays that can collect both rain and dew water. According to the manufacturer, the trays save considerable amounts of irrigation water through dew and rain water collection, reduce soil water evaporation, and reduce water use by weeds. However, continuous plastic mulch films are not yet used for dew harvesting. Continuous mulch films are laid on the soil from reels, with typical lengths of 500 to 1600 m and widths of 1.2 to 1.6 m. Conventionally, the mulch films are laid on flat soil, or they are laid on a raised soil bed to give a tighter fit to the ground. To collect dew water for plants, the mulch film should be laid on the soil so that the condensed water drains toward the plant roots. Therefore, the seedlings should be planted at the bottom of a dike that’s covered by the mulch film. The inclined edges of the film direct condensed dew water and rainwater toward the roots. The ability of the mulch film to harvest dew could probably be improved by the addition of certain minerals, such as TiO2 and BaSO4, to the mulch film. Such fillers increase the emissivity of plastic films in the IR region, thereby enhancing the radiative cooling of the film at night. Another way to increase the dew harvesting of mulch films is to transform the films’ hydrophilicity. Hydrophilicity of mulch films is an advantage because less energy is needed for the nucleation of water vapor on hydrophilic surfaces than on hydrophobic surfaces. Some researchers have considered whether the dew collection efficiency on solid surfaces could be enhanced by mimicking natural surface compositions and structures. The honeycomb-like micro-ornamentation of the scales on some desert lizards enhances condensation of water on the scales, especially toward the lizard’s mouth. Similar micro-ornamentation could possibly enhance the water condensation efficiency of plastic mulch films and could even help to funnel the condensed water to the plant roots. The development of mulch films for dew collection is an inspiring challenge for researchers and product development. VTT Technical Research Center of Finland, in cooperation with the Departments of Agricultural Sciences and Physics at the University of Helsinki, is examining dew water harvesting using mulch films. Our studies include pilot-scale manufacturing of surface-modified mulch films in the laboratory and testing of their dew collection efficiency in laboratory and field conditions. Antti Korpela, Senior Scientist (Antti.Korpela@vtt.fi), Hille Rautkoski, Senior Scientist, Jorma Koskinen, Senior Scientist, Heikki Pajari, Senior Scientist, and Mika Vähä- Nissi, Principal Scientist, VTT Technical Research Center of Finland, Espoo, Finland; ASABE member Laura Alakukku, Professor, Hannu Mikkola, University Lecturer, and Juuso Tuure, Doctoral Student, Department of Agricultural Sciences, University of Helsinki, Finland; and Henri Vuollekoski, Postdoctoral Researcher, Department of Physics, University of Helsinki, Finland.
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