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In numerous applications of wireless sensor networks (WSN), the reliability of the data collected by sensors is cast as specific QoS requirements expressed in terms of the minimum number of sensors needed to perform various tasks. Designing a long-lived sensor network with reliable performance has always been challenging due to the modest nonrenewable energy budget of individual sensors. In such a context, energy-unaware task management protocols may result in uneven expenditure of sensor energy by assigning uneven workloads to sensors. This, in turn, often translates into reduced sensor density around those heavily loaded sensors and may, eventually, lead to the creation of energy holes that partition the network into disconnected islands. To avoid these problems and to promote network longevity, we propose two energy-aware task management protocols: our first protocol is centralized, while the second one is fully distributed. The proposed protocols assign tasks to sensors based on their remaining energy so that energy expenditure among neighboring sensors is almost even. We compare the reliable lifetime of the network achieved by assigning tasks to sensors using the proposed protocols against optimal task assignment and also against energy-unaware protocols. Extensive simulation results have revealed that the performance of the proposed protocols is very close to that of the optimal task assignment. Furthermore, our simulation has shown that the proposed protocols can increase the functional longevity of the network by about 16 percent.