hello:
I have a problem about the instruction of ETSZ .In the instruction there is two outputs ,the one is ET,and the other is solar radiation.But I don't know what the solar radiation
refer to ?Thanks!
Solar radiation is the value of the Rs parameter (the solar radiation measurement in mJ/m2). It is included to help the user monitor whether or not the solar radiation sensor is functioning correctly.
Regards,
Dana
The two values coming from the ETsz instruction is the potential evapotranspiration value in millimeters and a "clear sky solar radiation value" - Rso in units of megajoules/m². Rso is what you would have in terms of incoming solar radiation if you had perfectly clear skies with no aerosols in the atmosphere for the entire hour.
Part of coming up with the ETsz value is figuring out whether the present solar conditions are overcast or not. To figure this out the datalogger calculates what the Rso value would be based on latitude, longitude, elevation, and time of year and compares it to what is presently being measured. Changing the datalogger clock to Daylights Savings Time can subtly effect both the ETsz and Rso values.
The Rso value is included as part of the ETsz equation to help the user determine the quality of their data and if their solar radiation sensor is drifting and needs calibration. Their are a couple of "gotchas" to using the Rso value.
1. Days with a mix of blue sky and clouds can increase the amount of solar radiation hitting the sensor. Both sunlight and reflected light from clouds are impacting the sensor potentially giving a higher measured solar radiation value in comparison to the Rso value.
2. Not everyone views "blue skies" the same way. There are days that I would swear are perfectly clear but in reality have some high altitude dust or aerosols that cause the measured solar values to be less than the calculated Rso value.
To use Rso you have to know something about that day. Rso can be a great tool to catch drifting sensors. I like to store the measured hourly solar radiation values and plot them against Rso. Keep in mind that solar radiation makes up about 80% of the equation - it's the primary source of energy.