Irrigation management is all about determining when and how much water to apply for irrigation. If we know how much water is used by plants, we can calculate how much water to put back in the soil to avoid plant water stress (how much to irrigate).
We can use a simple model called “soil water budget” to account for all the inputs and outputs and calculate soil water deficit, which would tell us how much water we can put in the soil, or how far we are from the dry and wet thresholds. One of the most important outputs in this model is Evapotranspiration (ET) or crop water use.
Evapotranspiration is a rate (e.g. inch per day) at which soil and plants release water into the atmosphere. ET is the sum of water loss through the processes of evaporation and transpiration. Evaporation is the water loss from either soil or plant surfaces. Transpiration occurs when water vapor moves out of tiny pores on plant leaf surface (stomata).
ET is driven by weather factors (solar radiation, temperature, humidity, wind) that determine the drying demand of the atmosphere. ET is usually estimated using complex mathematical equations and data from local weather data.
Atmometer and Rain Gauge
If there is no weather station near your field or you don’t have the budget to buy and install one, you can use an instrument called atmometer to directly estimate ET.
An atmometer mimics plants and like a mini-weather station can provide reference ET at a reasonable cost and with little effort. The value of ET is shown on a tube in front of a ruler on the atmometer. Growers or consultants can use an ET gauge to monitor how crop water based on their changing local weather.
Precipitation is another important input to the soil water budget. You can also install a rain gauge to collect this additional data and use it in your irrigation planning.
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