## 1 Introduction  
During the past few years, the development of commercial and research platforms for wireless sensor networks (WSNs) has gained an increasing interest in a broad range of new scientific research and applications. WSN technologies provide high resolution spatial and temporal data at a declining cost per unit area, thus making them an affordable and practical option for many researchers [1]. Additionally, since WSNs are commonly deployed for various scientific applications outside of the computer science and networking fields, researchers require solutions in which they do not have to directly handle the complexity of WSN systems. As a result, off-the-shelf commercial solutions are preferable if collaborations with WSN researchers are not possible. The adoption of WSNs presents new challenges for scientists and engineers who require high-quality data and optimal network behavior. As an _in situ_ mechanism for data collection, WSNs are deployed in multiple scenarios where outdoor locations represent a critical challenge. Indeed, simulation and laboratory methods are unable to capture the complexity of outdoor environments (e.g., forests, oceans, mountains, or glaciers), which significantly affect WSN operations and maintenance. Consequently, experimental deployments are essential to study and analyze WSN performance and its maintenance characteristics under these harsh conditions. Related experiments have shown unexpected behaviors and observations in outdoor deployments compared to indoor conditions [1; 2; 3; 4]. To better understand these situations, previous studies have tested large-scale networks [2; 3; 5; 6]. However, the majority of those large-scale outdoor WSNs had rather short experiment durations, lasting from several days to a few months, mostly due to power constraints. This paper presents the experimental study of an environmental multi-hop WSN that has been in operation for more than four years. The WSN experiment, named _ASWP testbed_, is deployed in a forested nature reserve of the Audubon Society of Western Pennsylvania (ASWP). One of the initial motivations for this deployment was to explore the feasibility of using WSNs for collecting reliable long-term hydrological data to investigate impacts of vegetation heterogeneity and soil properties on the status and trends of soil moisture and transpiration. Over the course of this study, the ASWP testbed presented various challenges to WSN performance including extreme weather, wildlife, human interactions, and location restrictions for node placement. Our study is focused on the deployment of a long-term multi-hop WSN testbed that employs both _off-the-shelf commercial_ and _research-oriented open source_ networking software with TinyOS-based WSN platforms. The main objectives are twofold: first, we analyze the network performance and operation of the WSN. This analysis is conducted over specific time periods (_i.e._, one year and six months) during the network deployment time, considering long-term network dynamics (e.g., unstable links, node failures, node relocations, battery replacements) that affect the overall network performance in the outdoor deployment environment. Second, we analyze the network costs, including the deployment, labor, and maintenance costs of the WSN. Multi-hop WSNs pose additional challenges on their routing protocols, which are responsible for dynamically establishing efficient routes towards the sink node(s). The evaluation and direct comparison of routing protocols in WSNs remains an open research problem, knowing that it is challenging to guarantee the same wireless channel conditions for multiple WSN evaluations [7, 8]. Moreover, concurrent implementations of different protocol stacks require more resources from highly constrained WSN motes (e.g., 4 KB of RAM in MicaZ motes [9]). These resources are often limited even for the implementation of a WSN application that uses a single protocol stack.