I am doing exactly this, and found this thread trying to work the problem.
I have an Acurite temperature/humidity sensor that transmits on 433Mhz. With RTL-SDR and OpenHab, I can connect the sensor to my sprinkler schedule.
I am converting the RH values to gH2O/m^3. In direct sunlight, I'm seeing 140F, and the response of the sensor doesn't seem to be very reliable.
I'm not interested in accuracy, mainly repeatability. I can find the threshold myself to determine when the lawn needs watering. In my case, I'm using the sensor fasted to the bottom of a plastic show box. It's in the yard open side down.
I'm toying with the idea of a 2D array with threshold values for watering. humidity by temperature.