There is an increasing use of high stature plastic tunnels (macro-tunnels) to grow high value crops, such as raspberries, blueberries, vegetables and flowers. This is even in relatively frost free environment, such as coastal California. More commonly tunnels are used in colder climates to produce early season crops. But along the California coast there is increasing use because of other benefits, such as improved production and reduced disease. There is estimated to be about 11,000 acres in tunnels in Santa Barbara County and even more in Ventura.
A recent, unpublished study by Mike Cahn et al with UCCE in Monterey County evaluated water use by raspberries in tunnels. They found that pan evaporation was reduced by 18% in the tunnels over the season compared to open-field grown raspberries. Also, less water was applied for the inside trial than the adjacent outside trial. Even with the reduction in applied water the soil moisture remained higher inside the tunnels than outside. The canopy was larger earlier inside the tunnel than outside even though there evapotranspiration was lower inside the tunnels. The main components of transpiration are altered in tunnels. There is less radiation because of the interference of the plastic, less wind, higher humidity, despite the warmer temperatures.
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