Evapotranspiration Details

Evapotranspiration (ET) is a term that describes the transport of water from the ground surface and soil to the atmosphere. This includes processes like evaporation of liquid water and transpiration done by vegetation. It is an important part of the Water Cycle and knowing the amount of ET happening during the growing season is important for usage in modelling, irrigation, and crop management.

So lets 'observe' ET!

Directly measuring ET is not practical as you would have to somehow account for every molecule of water vapor and know its exact source to know if it contributed to the local ET. The most common approach to directly measuring ET is very expensive as it involves tracking air flow and moisture in 3 dimensions. Another approach is through the usage of a lysimeters, which attempt to keep an accounting of soil water.

So when all else fails, we can produce model estimates based on more commonly collected data. That is what the legacy ISU AgClimate Network and now ISU Soil Moisture Network has done. This page attempts to fully document the equations used and the application caveats of the data.

Estimating Evapotranspiration

The estimation techniques described should apply for both the legacy ISU AgClimate Network and the current ISU Soil Moisture Network.

Hourly estimates of ET are made by a set of equations found on the Campbell Scientific Data Logger. These hourly values are then summed to produce calendar day totals to avoid confusion with having slightly conflicting ET values for a given day.

The hourly ET is estimated by the ASCE Standardized Reference Evapotranspiration Equation , which assumes a reference tall crop (alfalfa). The equation is as follows, taken from the aforementioned reference.
Reference ET Equation

The terms are defined as:

Δ Slope of the vapor pressure-tempertaure curve, estimated by using the mean temperature for the day or hour. kPa °C-1
Rn Net Radition. This is the combination of received shortwave minus the amount reflected by the crop (albedo) and longwave transmitted by the soil. MJ m-2 d-1
G Ground Soil Heat Flux. MJ m-2 d-1


  1. Wikipedia's Evapotranspiration
  2. ASCE Standardized Reference ET
  3. FAO 56
  4. Python Implementation of FAO 56 and other equations