Analysis of Power Characteristics for Sap Flow, Soil Moisture, and Soil Water Potential Sensors in Wireless Sensor Networking Systems

While wireless sensor networks (WSNs) become more popular as a tool for scientists and engineers in environmental monitoring, the energy efficiency of WSN's primary unit, the mote, is increasingly critical. Motes perform communication and processing functions and also take measurements using limited onboard battery power. For environmental monitoring deployment, sampling rates need to be assigned to motes such that useful information can be extracted over both spatial and temporal scales. The data sampling rate, which is intimately related to data transmission rate, plays an important role in the mote's battery life. Moreover, while it has been noted that mote operation continues under less than optimal power supply levels, data collected during this time undergoes attenuation from the desired measurement. That is, low battery conditions of motes could significantly affect the accuracy of sensed data. This paper investigates the above two issues to achieve extended battery life for individual wireless motes while maintaining sampling adequacy and reliability in transmitting accurate data. We first study how different sensor characteristics, such as different types of sap flow sensors, can affect motes' sampling and transmission rate. Then, we examine the effect of sensing attenuation due to low battery power on sap flow, soil moisture, and soil water potential measurements. A simple regression model was found to correct data for both the sap flow and soil water potential sensors sampled on under-powered motes. By operating under less than optimal power levels, this effectively increases motes' battery life for reliable data collection from wireless sensor networks.