# A Networked Sensor System for the Analysis of Plot-Scale Hydrology
## 1 Introduction  
The sustainable condition of our freshwater resources partially depends on our understanding of the natural system in which it is cycled [1]. It has long been known that physically-based distributed hydrologic models require an understanding of the spatiotemporal variability of environmental data, which is difficult without an abundance of ground-based measurements for calibration and validation [2]. Soil moisture and transpiration play a fundamental role in the soil-atmosphere interactions and eco-hydrological processes. Moreover, the impacts of these and other hydrologicalparameters on regional hydrologic and climatologic conditions need permanent in situ measurements. Exploring the variability of soil moisture and transpiration at the plot scale and qualifying such measurements statistically can help improve estimates (including flux and storage components) of water budgets at the regional/watershed scale [3; 4; 5]. Ground-based measurements and monitoring of environmental variables have been impacted over the past decade by wireless sensor network (WSN) technology. Traditional data logging methods use cumbersome equipment, which is expensive to operate and inconvenient to maintain, that leads to its limited spatial coverage capabilities. Because of the high expense of sensors and data logging equipment, researchers are often forced to either forgo data loggers for high spatial density measurements with poor temporal resolutions (i.e., hand measurements) or obtain high temporal resolution at a limited number of strategically located data loggers. Small, inexpensive, wireless monitoring devices are pervading beyond networking and communications research fields. These devices are providing scalable, high resolution data at a declining cost [6; 7] and have found applications in a variety of environmental monitoring fields, including: habitat monitoring [8; 9; 10], microclimate monitoring [11; 12], seismology [13; 14], understory sunlight studies [15], agriculture [16; 17; 18; 19; 20; 21; 22; 23; 24], ecology [25; 26; 27; 28] and hydrology [29; 30; 31; 32; 33]. High-resolution sensor networks of plot-scale hydrology is a growing application for WSNs due, in part, to the increasing demand for calibrating and characterizing sub-grid variability of airborne and space-borne measurements [34; 35]. A long-term (over six years) WSN has been measuring edapnic (e.g., moisture, water potential and temperature) and arboreal (i.e., xylem sap flow) hydrological properties in a forested nature reserve at the Audubon Society of Western Pennsylvania (ASWP) [32]. The original motivation of the ASWP network was to determine the feasibility of using WSNs to continuously and reliably collect hydrological data under natural outdoor conditions. Following the successful deployment of the network, which has been running on TinyOS 2.1.2 [36] and CTP [37], comes a new stage of research, starting in 2015, aimed towards network expansion and improvement of data collection and processing. The novelty of this WSN study includes: (1) a data acquisition board design and software driver for integrating digital environmental sensors into the wireless hardware platform; (2) an energy efficient and balanced routing protocol CTP + EER [38]; and (3) a heterogeneous over-the-air mode-reprogramming tool. Furthermore, our WSN enables the study of the following: (1) assessment of the quality of the data collected from soil moisture, soil water potential and soil temperature sensors attached to WSN nodes using a specially designed sensor board; (2) retrieval of high quality sap flow measurements from our lab-made [39] Granier-style [40; 41] sap flow sensors; (3) the application of the collected data to: (a) estimate soil hydraulic properties; (b) calculate transpiration based on sap flow; and (c) explore spatiotemporal patterns of soil moisture and soil water potential; and (4) evaluation of the utility of WSNs for environmental monitoring applications.