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How to Measure ET for Irrigation

Evapotranspiration (ET) is easy enough to understand but difficult to measure. Irrigators must be able to calculate the amount of water being lost to the atmosphere when it evaporates from the soil or plants or when plants release (transpire) it. For ET to occur, water must be present at the surface, there must be energy to convert water into water vapor, and something to transport the vapor away from the evaporating surface is required. The variables affecting the process are many, from weather conditions to the nature of the crop, from soil structure to maturity of the plants. Scientists have developed "models" -- sets of complex equations -- as well as control crops to measure ET.

Things You'll Need

  • Calculator
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Instructions

    • 1

      Find the air temperature in Celsius. The calculation is going to ask you to convert the temperature in Celsius to the Kelvin scale ( 0 = absolute zero) by adding 273.16.

    • 2

      Calculate the saturation vapor pressure, which is calculated from relative humidity and air temperature, with the equation: es = 0.6108 * exp(T * 17.27/ (T + 237.3))

    • 3

      Subtract the mean hourly vapor pressure deficit from "es" to find the vapor pressure deficit (VPD).

    • 4

      Find the relationship between the saturation vapor pressure and the air temperature curve with the following equation: DEL = (4099 * es)/(T + 237.3)^2.

    • 5

      Factor in the barometric pressure, (P = 101.3 - 0.0115 * Z + 5.44 * 10^-7 * Z^2 ), the psychrometer (relative humidity) constant (GAM = 0.000646 (1 + 0.000946*T), and the weighting function (W = DEL/(DEL + GAM).

    • 6

      Find the wind function (FU2). For the mean hourly net radiation (Rn) after nightfall, the wind function will be 0.125 plus 0.0439 times the mean hourly wind speed at two meters. The daytime Rn will be 0.030 plus 0.0576 times the mean hourly wind speed at two meters.

    • 7

      Calculate NR by converting Rn to mm with the equation R = Rn/(694.5 (1-0.000946*T)).

    • 8

      Find RET, which is the hourly rate of ETo with the equation RET = W*NR + (1-W)VPD * FU2. The daily ETo equals the sum of 24 hours RET (mm).

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