With the PAR Lite (2), can I derive the crop evaporation from the net radiation?
There is something as a “reference crop evaporation” or “actual evaporation” that can be derived from net radiation (solar plus thermal) together with surface temperature, soil and vegetation data. Unfortunately we don™t have the information how to do it. There is a simpler approach in which the expected evaporation is coupled to the total dose of global radiation as measured by a horizontal pyranometer. If the metoffice gives such figures for your area it should be of importance to know the relation between PAR intensity and total global irradiance for at least a solar spectrum at air mass 1.5 (solar elevation 53°). This relation is for: Clear sky: 681 W/m² total, 308 W/m² in the 400 – 700 nm band 1408 µmol/s.mÂ² PAR Light cloud cover: 200 W/mÂ² total, 109 W/mÂ² in the 400 – 700 nm band 493 Âµmol/s.mÂ² PAR Be aware that many green leaves are highly reflective for near IR but absorb strongly in the 400 – 700 nm band (PAR region). For evaporation only W/mÂ² counts and for photosynthesis