I want to build an automatic garden/lawn watering system based on a small computer. The system is connected to the Internet and is able to learn the current weather conditions and the short-term forecast. What I need for this project is the ability to determine the soil moisture content using only the readily available weather data.
One may argue that a set of humidity sensors buried into the ground would be the best choice, however the cost of such sensors is prohibitive for a small gardening project (around $300 each). Custom made sensors are cheap and easy to build, but the accuracy is low and it degrades over time.
I found a Chinese study but that is of limited help due to its large scale. Does anyone have a clue about how to calculate the moisture content?