Geologista

I've had a look around and didn't find anything on this so thought I would ask. I need a simple formula that relates evaporation at a free surface to the air temperature at the free surface. I'm trying to model the effects of rainfall on landslip movement but need to adjust the rainfall values to include evaporation to see if the effect is significant. I have found many varying formulae online and in journals but they all include data that I just don't have. They require complex variables based on vegetation type for example and I don't need anything as accurate as that, what I need more is a crude relationship. The data I have is precipitation and air temperature. I think I can get dew point and humidity data as well if that is helpful? I know this is a bit vague but any help would be appreciated even if it's a new direction to look in. Thanks