# Energy-Efficient Wireless Sensor Networks for Precision Agriculture: A Review
## 3 Agriculture-Based Energy-Efficient Schemes in Literature
### Agriculture-Based Power Reduction Techniques
#### 3.1.4 Routing Protocol  
_Routing protocol_ introduces another power reduction scheme for agriculture WSNs to minimize the path between sensor nodes and sink node, thereby the power consumption of the WSN is reduced. Routing protocol can be performed via (i) sink mobility, (ii) multi-path routing, (iii) cluster architecture, and (iv) routing metric. _Sink mobility_ was adopted in previous research [99; 100]. [PERSON] et al. [99] proposed a novel mechanism that adopts unmanned aerial vehicles (UAV) to gather data from forest regions instead of classical sensing from nodes deployed in WSN. The proposed method enables the UAV to collect data from WSN in harsh terrain and transmit it to the base station situated far from the sensing area. Therefore, the multi-hop transmission between cluster heads can be completely avoided, and the communication range can be extended. _Multi-path routing_ has also been suggested by other scholars for agriculture WSNs [67; 68; 101]. Recently, [PERSON] et al. [67] proposed an agricultural field weather monitoring system based on the ZigBee wireless protocol. An advance energy-efficient routing protocol was implemented for WSN to automate irrigation management. The transmitter could modify its power based on the distance between the sensor node and base station to save energy. The results revealed that network lifetime increased to 1825 min. [PERSON] et al. [101] developed a WSN soil moisture mapping and monitoring method to provide information for irrigation scheduling. To conserve power, the sensor nodes relayed data to the BS every 15 min through the greatest energy-efficient route. _Cluster head_ method is considered by [PERSON] et al. [102] to extend the ZigBee WSN lifetime for agricultural application. In addition, the quality of the crop yields is improved and their cost is minimized. The research presented WSN coverage measurements in a mixed crop farmland. An adaptive energy consumption model for each sensor node was proposed and used to compute the energy consumption in the network. A deployment model with a cluster head and two antennas was also proposed and simulated to alleviate short network lifetime due to the vegetation attenuation of signals. This network deployment model extended the lifetime of the network by a factor of more than 20 compared with a deployment where cluster heads are not used. [PERSON] et al. [103] proposed a new routing protocol for WSN that can reduce the power consumption of sensor nodes through balanced clustering. This protocol was called \"equalized cluster head election routing protocol\" (ECHERP). ECHERP was implemented in Java language for 500 homogeneous nodes distributed in a 100 \(\times\) 100 m\({}^{2}\) land. The simulation results showed that the proposed protocol outperforms existing protocols in terms of power consumption. A potential application of this protocol is PA.  
Another type of routing protocol is called _routing metric_. [PERSON] et al. [104] used two routing metrics, namely, remained energy and expected transmission count, to compute the optimal routing path to the sink node that ensures low power consumption. \"A scalable context-aware objective function (SCAOF)\" is proposed by combining robustness-aware, energy-aware, resource-aware, and reliability-aware contexts that render to the compound routing metrics scheme. The experiment is conducted in a garden based on a testbed, which includes a uIPv6 protocol stack and contiki OS. The obtained results prove that SCAOF can convey the desired advantages on network lifespan prolonging, as well as high efficiency and reliability in different hardware testbeds and simulation scenarios. Table 5 shows the comparison of power reduction technique based on routing protocol schemes for previous research in agricultural applications. \begin{table}
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**Power Reduction Scheme** & \begin{tabular}{c} **Reference** \\ **Example** \\ \end{tabular}  &