This model uses the Hargreaves-Samani equation to estimate evapotranspiration (ET0) using only min and max daily temperature time series and latitude inputs. This method is good for sites that lack measured radiation data. Note that because this function is empirical, you should calibrate to actual measured data if possible. This model is intended to run on a 1-day time step.
Reference: Hargreaves-samani.pdf
Comments
2 comments
Hello Jason,
Thanks for your lesson today regarding evaporation which is one of the Water Management webinars series. I just wanted to follow up with you on Daily extraterrestrial radiation ('RadEx' element in model) especially when Latitude is greater than 67 degree (I was not a questioner). Such so, arccos(x) in Omega element could not be calculated when Lat is greater than 67 degree because of the limitation of x range which is between -1 and +1. In order to get daily extraterrestrial radiation where Lat is greater than 67 degree, I prefer to use the 15th day of the month value instead of using 'DayofYear' in 'Delta' element and GoldSim could interpolate/estimate daily 'RadEx'.
Here is the reference table and see Table 2.6 and there are some comments above and below the table regarding Ra. For example, 'For the winter months in latitudes greater than 55° (N or S), the equations for Ra have limited validity. Reference should be made to the Smithsonian Tables to assess possible deviations'
http://www.fao.org/3/X0490E/x0490e0j.htm#annex%202.%20meteorological%20tables
As well as it could be implemented in 'Reference ET model' or other models. I think it's only for very high latitude situations and just wanted to let you know my thoughts.
Thanks in advance for your taking your time to consider this.
Regards,
Homin Kim
Homin Kim,
Thank you for the reference to that table. I incorporated this into the model so it will be used when the latitude is out of range.
-Jason
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