## 1 Introduction  
A wireless sensor network is composed of a large number of tiny sensor nodes [1]. The main task of a wireless sensor node is to sense and collect data from a certain region, process them and transmit them to a sink node where further processing on the collected data can be performed [2]. Sensor nodes are small-scale and cost effective devices with limited capabilities. Wireless sensor networks generally contain thousands of sensor nodes, which are randomly deployed to a field. The sensor nodes are powered by batteries and controlled remotely. For most applications, it is impossible to recharge or replace the batteries of sensor nodes after deployment [3]. Depending on the application of sensor networks, certain routing protocols are required in order to establish the communication among sensor nodes and the sink nodes [4]. Wireless sensor networks consume their limited energy for collecting data, performing calculations and routing the received data. Nevertheless, in most applications, each sensor node is expected to last for a long time [5]. For this reason, both efficient use of energy and efficient routing schemes are highly important in sensor networks [3]. Various routing protocols have been proposed for conventional wireless _ad hoc_ networks. But these protocols are not fully suited to the unique features and application requirements of sensor networks [6]. Hence, new routing algorithms for sensor networks are also proposed in the literature. Routing algorithms for _ad-hoc_ networks are typically classified as either proactive or reactive [7]. Similarly, routing protocols for sensor networks can be categorized as proactive and reactive as well. Proactive routing protocols will have routing paths established/determined from all nodes to sink all the time. Hence they are very suitable for continuous monitoring applications where all parts of a region are to be monitored all the time. In such applications, all sensor nodes sense and send data to the sink node periodically. LEACH (Low Energy Adaptive Clustering Hierarchy) [8] is a good approximation of a proactive network protocol. Reactive routing protocols do not have to establish paths from all nodes to the sink all the time. They may establish paths when required. For example, the Directed Diffusion [9] routing protocol establishes paths from some sensor nodes to the sink upon sending of queries from sink to nodes. This type of routing protocols can be used for event-driven or query-driven sensor network applications. These applications can also be called as reactive applications. In such applications, data is sent from one or more sensor nodes when there are certain events happened or when queries are sent. Not all nodes have data to send to the sink at each period of time. Event though the communication and routing can be reactive, in such applications nodes may need to sense their environment continuously to detect suddenly happening events. For example, the TEEN (Threshold-sensitive Energy Efficient sensor Network) protocol [10] has been developed specifically for such networks. While proactive algorithms are convenient for applications that require periodical observations, reactive algorithms are convenient for applications where sudden changes are considered to be important. In this paper we follow a hybrid approach and propose two routing protocols that are a combination of the proactive and reactive approaches. Our protocols are designed for reactive emergency applications. Hence, we do not transport data to the sink node all the time. We transfer with some certain rules, so in this sense it is reactive. However, since we are dealing with emergency applications, when there is a need, the data has to be transported to the sink node as soon as possible. For this reason we should have the routing paths to be established earlier, proactively. Therefore, our approach has both features. It is proactively building the routing paths, but using them reactively.