## 5 Network Costs
### Deployment Costs  
The deployment costs of a WSN include all the physical components necessary for operation (e.g., hardware, sensors, batteries, and enclosures). Table 7 presents a categorical breakdown of the network's physical costs associated with the network deployment up to May 2014. While growth and change in technology and industry produce fluctuating prices of products, the costs presented in the table are meant to show relative costs of the network rather than an absolute. It can be seen that over half of the physical costs of the network are associated with the WSN hardware, including the network gateway, motes, data acquisition boards, and antennas. The total cost of this network of 52 nodes is approximately $31,500, including the costs of the 149 external environmental sensors. For the network's nodes, the difference between the costs of relay nodes and regular nodes with external environmental sensors is about $330 to $700, respectively.  
The sensor costs of a WSN deployment will depend on the nature of the study. In this work, soil moisture and/or sap flow sensors were installed at 35 out of the 52 nodes. The WSN operates on rechargeable NiMH AA batteries, which have a nominal voltage around 1.2 V, such that three batteries in series are used to power the more. Following about two years of active charging and discharging, the batteries no longer hold appreciable charge and must be replaced. Over 700 batteries were originally purchased (approximately $2.2 each) to maintain over 150 batteries that are used in the motes of the WSN. The power costs are a combination of replacing old batteries as they go out of use and the cost of electricity to charge them. In addition to the batteries required for powering the motes, which also power the external soil moisture sensors via the voltage excitation output on the MDA300, ancillary 12 V batteries were used for powering the external sap flow sensors.  
The estimated area covered by Sites 2 through 5 is 5000 m\({}^{2}\) (using GIS polygon coverage). Assuming the \"sensed\" area for each regular node (_i.e._, soil moisture and/or sap flow sensors) to be around 6 m\({}^{2}\) (due to the heterogeneous nature of the site), the density of actual measurements to coverage area is about 4% (for 35 regular nodes). Based on the deployment costs, the WSN costs $150 m\({}^{-2}\) of sensed area. Including the results presented in Table 6, the approximate cost of the sensed area, adjusted for labor and maintenance, is $184 m\({}^{-2}\) y\({}^{-1}\). While these costs may not be uniformly scalable across disciplines or with network size (e.g., larger WSNs may require additional gateways), it serves as a point of reference for comparison with other data collection technologies (e.g., data loggers).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Description** & **Cost** \\ \hline Hardware & Wireless motes, antennas, gateway, _etc._ & \$15,940 \\ External Sensors & Soil moisture and sap flow. & \$12,600 \\ Power & Batteries (AA, D, 12 V). & \$1500 \\ Enclosures & Waterproof boxes, insulation and desiccants. & \$1400 \\ Mounting & PVC pipe, wiring, nuts, bolts and screws. & \$60 \\
**Total** & **Cumulative cost.** & \$31,500 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of the ASWP WSN physical costs.