  Microwave remote sensors mounted on center pivot irrigation systems provide a
feasible approach to obtain soil moisture information, in the form of water
content maps, for the implementation of closed-loop irrigation. Major
challenges such as significant time delays in the soil moisture measurements,
the inability of the sensors to provide soil moisture information in instances
where the center pivot is stationary, and the inability of the sensors to
provide soil moisture information in the root zone reduce the usability of the
water content maps in the effective implementation of closed-loop irrigation.
In this paper, we seek to address the aforementioned challenges and
consequently describe a water content map construction procedure that is
suitable for the implementation of closed-loop irrigation. Firstly, we propose
the cylindrical coordinates version of the Richards equation (field model)
which naturally models fields equipped with a center pivot irrigation system.
Secondly, measurements obtained from the microwave sensors are assimilated into
the field model using the extended Kalman filter to form an information fusion
system, which will provide frequent soil moisture estimates and predictions in
the form of moisture content maps. The utility of the proposed information
fusion system is first investigated with simulated microwave sensor
measurements. The information fusion system is then applied to a real
large-scale agriculture field where we demonstrate the its ability to address
the challenges. Three performance evaluation criteria are used to validate the
soil moisture estimates and predictions provided by the proposed information
fusion system.
