Evapotranspiration is the combination of the amount of water lost through both evaporation from the soil and by evaporation and/or transpiration from plants.
The amount of water lost from evapotranspiration from a field is about the same as that lost from a swimming pool or pond. These losses increase with increases in sunshine intensity, air temperature and wind speed and also with decreases in air humidity.
The amount of water being lost from evapotranspiration can be calculated from measurements of the amounts of water evaporating from large pans of water such as the Class A pans used by the Bureau of Meteorology throughout Australia.
The table below gives the Evaporation rates in mm per day for some localities around Australia as measured in Class A Open Pan evaporimeters. In practice losses from paddocks run at about 80% of these figures.
Evaporation from many farming situations can be a lot less because:
- the use of Mulches greatly reduce evaporation from the soil.
- Less than full plant coverage of the ground by some crops means fewer leaves to transpire water.
- As a soil dries out, plants are less able to extract water so they transpire less and less.
If we want to maintain pasture (or lawn) during a dry spell by irrigation, then we need to replace about 25% of the evaporation on a daily basis. If we want to get some productive growth from the pastures, we need to replace more like 50%.
If we watered vegetables or fruit trees at only 25% of the daily evaporation losses, we would not get much marketable produce from the plants. If the crop covers all of the ground, then we would need about 70% of the evaporation to be replaced before we got adequate production from that crop.
If a fruit tree crop is mulched and also covers only 20% of the area we could get away with only providing 30% of the evaporated water.
