**Figure 6.**

*Closed-loop control system.*

models are derived from the real-time dynamics of a system. A combination of physics-based and data-driven models is a gray-box model. Here a mechanistic model is developed from physics and during operation data from the system is used to update the model depending on the dynamics in the plant environment. Under closed-loop irrigation, irrigation scheduling decisions are made by micro-controllers by comparing a current state with the desired state. A closed-loop control algorithm helps to initiate an action of the actuators. **Figure 6** is a schematic presentation of a closed-loop control system.

Closed-loop control is further divided into intelligent, optimal, and linear control strategies. With advances in computing power, smart irrigation systems are able to make decisions in real-time, depending on the prevailing environment of the plant. A detailed explanation of closed-loop irrigation strategies can be found in [14, 20, 53, 54].
