# Energy-Efficient Wireless Communication Strategy for Precision Agriculture Irrigation Control
## 2 Precision Irrigation Control System
### Networked Control System  
Efficient irrigation management requires real-time soil moisture measurements to capture temporal and spatial variations in the crop root zone. However, obtaining representative and accurate readings over large crop areas is a challenging task since the soil hydraulic properties and spatial heterogeneity heavily influence the moisture and crop rooting patterns, among other variables [39]. Moisture sensors use electromagnetic principles to measure the water content; furthermore, to obtain accurate values, soil-specific calibration is required before installation.  
The sensor 10 HS (METER Group Inc., Pullman, WA, USA) is a low-cost VWC sensor with a measurement volume range of 1 dm\({}^{3}\), and according to [40] measurement error can be reduced from 5% to 1% if off-line calibration is conducted previous to installation. The experimental field consists of one hectare of pecan crop (_Carya illinotiensis_) divided into four irrigation areas of 25 m \(\times\) 100 m, as illustrated in Figure 1. Each area contains approximately 45 productive trees watered with a micro-sprinkler irrigation system. The field is located in northern Mexico (Chihuahua city), which is an arid region with very low annual rainfall.  
As shown in Figure 2, the networked control system comprised one sensor node and one actuator node for each irrigation area; there was one weather node and a single controller for the entire experimental field. The WSN was implemented over the IEEE 802.15.4 standard, which is the basis for the Zigbee communication protocol [41]. Zigbee has become a popular protocol due to its low cost, low power consumption, and small communication packet size. Using a client-server communication schema, each sensor node sends soil moisture data to the controller node, the controller uses the weather and soil moisture information to determine if irrigation is needed, and then the control message is sent to the corresponding actuator node.  
Each sensing node includes three 10 HS sensors buried in the shallow level of the pecan effective root zone (60 cms), with a horizontal separation of 3 m among them; the three readings' average value provides a representative soil moisture measurement for the irrigation area. The sensors were previously calibrated for the specific soil texture (silty clay loam) according to the method described by [42]. An irrigation electrovalve activates and deactivates field watering for each actuator node, and a flow sensor measures the water consumption. The weather node includes a solar radiation sensor (PYR sensor from METER Group Inc., USA), a wind velocity sensor (Davis Cup sensor from METER Group Inc., USA), and a temperature-humidity sensor (VP4 sensor from METER Group Inc., USA) to calculate the reference evapotranspiration for the crop field.  
These three types of nodes were built upon the Arduino MKR board (Arduino, USA), which includes a 32-bit microprocessor and a 12-bit analog-to-digital converter. The board was energized with a 3.3-V power supply, and it was capable of reducing the current consumption from 20 mA in operation mode to 0.57 mA in standby mode. The controller node was implemented with a Raspberry Pi 3 Model B (Raspberry Pi Foundation, Cambridge, UK) based on an ARM Cortex-A53 64-bit processor. The controller ran four simultaneous control tasks to implement the four loops of the irrigation system.