seaurchin:
I am making a soil moisture sensor , I had a spare DHT11 , so I attached that too, I am wondering that soil will get drier faster if the rate of evaporation is high , is it possible to calculate the rate of evaporation when if we know the temperature/humidity and what else will be required ?
The DHT11 probably lacks of the precision need for that type of measurement.
The Davis Instruments Vantage Pro weather stations family is capable to offer this derived variable.
I would recommend have a look on this in page 48: Vantage Pro2 Console Manual
This document from Davis Instruments can be useful too:
DERIVED VARIABLES IN DAVIS WEATHER PRODUCTS
Also:
Measuring Evapotranspiration
Remote Sensing of Evapotranspiration
Good luck!