How come ETo from CIMIS is greater than Precipitation?
There is no direct connection between ETo calculation and Precipitation (P). ETo can be greater, less, or equal to P. ETo tells you the amount of evaporative demand that an atmosphere imposes on a given reference surface provided providing there is adequate moisture in the soil. If soil moisture is less than the atmospheric demand, then ETo has to be adjusted in the calculation of ETc. This adjustment is not done here because in irrigated agriculture one is not expected to stress the crops to the extent that the soil moisture is less than atmospheric demand. Generally speaking, factors such as clouds (during precipitation) may indirectly affect ETo by reducing solar radiation but cannot limit ETo to necessarily be less than P all the time. If rainfall is the only source of water, ETc (not ETo) should always be less or equal to P.
