Crop water use will vary with climate, weather patterns and the stage of development of the crop, as shown in the graph below.
There are four main factors which drive crop water usage:
The first factor is Solar Radiation or sunlight.
Sunlight hitting a plant causes photosynthesis to occur which increases transpiration of water from the plant. This is affected by the slope of the land, so that northerly slopes have more plant transpiration in the southern hemisphere in winter. This is one of the driving principles behind the microclimates concept discussed in other articles on Informed Farmers.
The second factor is humidity or the amount of water vapour in the air.
In a humid climate, plants use less water than at the same temperature in a dry environment. In a humid environment, the air surrounding the plant already has a high water vapour content and is less able to suck out moisture from the plants leaves.
The third factor driving crop water use is wind speed .
As wind speed increases, evapotranspiration from the plant increases as the wind removes the water vapour that the plant is transpiring. As the water vapour is removed, more water vapour can be transpired. Plants can literally transpire themselves to death in extreme circumstances.
Rainforest trees do not possess any mechanisms to shut down the stomata on the undersides of their leaves whereas desert plants do have efficient devices to prevent excessive transpiration such as thick waxy leaf coatings.
The fourth factor driving crop water use is temperature.
Hotter temperatures increase the rate of transpiration, and are usually linked to increased sunlight. Increasing temperature increases crop water use.
All four factors (solar radiation, humidity, wind speed and temperature) are inter-related with only humidity acting negatively on crop water use.