Greenhouse technology for cultivation in arid and semi-arid regions

Greenhouses have expanded from Northern areas to other climatic regions in the world, including arid and semi-arid regions. The main limiting factors for greenhouse cultivation in such areas are the high ambient temperatures with low humidity (sometimes with high humidity in coastal areas), typical of these regions during almost all the year, and the availability of good quality water for irrigation and eventually for cooling. Therefore, the technological options for protected cultivation in these areas are quite dependent on the average maximum temperatures along the year. If the area has average maximum temperatures not higher than 30-35°C during the autumn, winter and spring months, a lower technology approach can be used, combining the use of simple plastic covered structures, with high natural ventilation capacity (i.e. retractable roof greenhouses or similar technological approaches), selective mobile shading screens (NIR reflection) and/or evaporative cooling, provided that quality water is available or can be produced. During the summer months, cultivation could be moved to highlands, with cooler night-time temperatures, using screenhouses combined with fogging (low or high pressure, depending on the water quality). If temperatures are higher than 30-35°C during the whole year, pad and fan combined with selective shading is possibly the best option, avoiding cultivation during the summer months, but limiting the size of the greenhouses. Other more sophisticated technologies have been proposed for these regions, which involve in many cases closing the greenhouse and using highly energy dependent active cooling systems and different ways of seawater desalination. The high energy required to cool actively the greenhouses makes it essential to study carefully the possibilities of using renewable energies (wind, solar, geothermal, etc.) to cover at least, a part of large energy demand, as well as to establish resource conserving climate control strategies. In many cases and for some crops under extreme environments, food security concerns and urbanization makes reasonable to consider the possibility of indoor cultivation (plant factories) as potential economic option if they are demonstrated as resource conserving and profitable technologies and production mechanism.