Water content virtual sensor for tomatoes in coconut coir substrate for irrigation control design

The main objective of this work has been the development of a virtual sensor (VS) based on the water balance dynamics of tomato crops in greenhouses, using coconut coir substrate as an example in this case. Such sensors are used to provide a viable and economical alternative to expensive or impractical sensors. The final virtual sensor is the result of combining: crop growth, where the water is distributed in structural and non-structural plant biomass (storage); the substrate, which is considered to be composed of a single layer; and the water loss caused by transpiration and drainage. In this work, two ECH2O-EC-5 sensors (Decagon Devices) were calibrated and then used to determine the substrate water content; and a microlysimeter was installed to continuously sample the water supplied, the drainage, and the crop water loss values. The VS took into account the water supplied, the amount of water in three stores (the substrate, the root, and the aerial part of the crop), the climate, and the water loss. The water dynamic was determined by system identification techniques and by physical virtual sensors, which considered the water balance from a holistic point of view – as a sub-model for a customizable interface between crop growth and the plant ecosystem. The influence of both the crop and the climate conditions on the water balance was analysed and the virtual sensors were evaluated giving good final results.