Vapour-pressure deficit the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa. In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. To compute the VPD, we need the ambient (greenhouse) air
Water in California million tons. About one-third of that, around 700,000 tons, went to China, Japan took about the same amount and Saudi Arabia bought 5,000 tons. Alfalfa farmers pay about $70 an acre-foot, in Los Angeles that same amount of water is worth $1000 per acre-foot. In 2012, California exported 575,000 tons of alfalfa to China, for $586 million. Other common crop water use, if using all irrigated water: fruits and nuts with 34% of water use and 45% of revenue, field crops with 14% of water and 4% of revenue, pasture forage with 11% of water use and 1% of revenue,