When I need to determine an alternative equation for estimating reference evapotranspiration for local with missing data, using meteorological data, from official weather stations, this means that I assume that the humidity is missing and I ignore it and estimate it according to the equations recommended by (FAO Irrigation and Drainage Paper No. 56 Chapter 3) and then determine suitable an alternative equation for estimating reference evapotranspiration, and then also determine the evapotranspiration from the standard equation (FAO Penman-Monteith equation 56) to performance evaluation with an alternative equation.
I also assume for temperature is missing and I ignore it and estimate it according to the equations and determine an alternative equation for estimating reference evapotranspiration, and so on for all climatological parameters.
Or what is the basic rule in determining alternative equations to calculate the reference evapotranspiration in places that are missing data, using meteorological data?
Because all those who define alternative equations for estimating reference evapotranspiration rely on data from agro-meteorological stations.
The FAO recommends hargreaves samani equation (HG) is an alternative method to compute PET (ETo) in case the penman monteith equation can't be used for any reason
You need some temperature and humidity information, even a daily average, even some long term climatic value from the records, something.
If you are seriously considering calculating ETo without both then the final value will be worthless.
It's 2021, and there exist free global daily datasets for these variables. You should be able to find something somewhere.
You can use formulas that require one or two parameters to estimate evapotranspiration, such as the Hargreaves-samani equation.
Also, if you need to use the Penman method, I think the ERA5 data is very similar to the station data. You can validate this data with multi-station observational data and use ERA5 data if the comparison is acceptable.
This is as suggested. I'm not sure.
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