A device for determining the real evapotranspiration of a vegetated surface of a soil includes a porous evaporator, at least one portion of which has resistance to water vapor flow depending on a parameter, the porous evaporator having an upper surface exposed to the atmosphere, a tank of liquid water underlying the porous evaporator to which the tank is connected by a suction tube, and provided with a meter for measuring water level, a sensor for measuring the humidity value of the soil, and a CPU controlling the parameter on which the resistance to the water vapor flow through the porous evaporator depends, depending on the humidity value of the soil detected by the sensor.