Climate Based Irrigation Scheduling
Reference Evapotranspiration

Climate based irrigation scheduling is determined using climate data to calculate reference evapotranspiration  (Eto) or the water loss from either a grass or alfalfa cover.

The standard equations for calculating reference evapotranspiration are Peman, Penma-Moneith, Hargrave-Samani, the later only requiring air temperature data, whereas the Peman equations requires solar radiation, air temperature and humidity, and wind speed. See a presentation by Dr. Howell for additional information about reference evapotranspiration.

Figure 1 Daily potential evapotranspiration referenced to grass for the chile growing season

Reference evapotranspiration values when Campbell weather stations are located in New Mexico can be acquired from http://weather.nmsu.edu/map/map.htm  

Select the station from the list  by clicking on the number or use  the map. If you click on the name, information about the site were the climate station is located will appear.  Click on the autodata botton and then change the data to the desired time period. Select either metric or english units.  Select the crop you want climate data and calculated Gdd. If you want the data to paste the  into the irrigation spreadsheet hit retrieve, and the climate data will be retrieved. If you want the calculated evapotranspiration, select Generate Crop Coefficient and ET button  

Reference evapotranspiration can also be calculated based on temperature data and Samani's reference evapotranspiration equation.