Wireless sensor networks are widely used for monitoring different processes, including agriculture, in order to reach sustainability. One of the keys to sustainable crops is water saving. In particular, saving water is extremely important in arid and semiarid regions. In those regions, citrus trees are cultivated, and drip irrigation is used to save water. In this paper, we propose a smart irrigation system for citrus trees using a WSN. We describe the employed sensors and nodes for this proposal. Next, we present the proposed architecture and the operational algorithms for the nodes. Moreover, we designed different algorithms for fault tolerance and energy saving functionalities. The energy saving algorithm is based on the relevance of the gathered data, which is analyzed in order to consider whether the information should be forwarded or not. A TPC-based protocol is proposed to perform the communication among the nodes of our system. In addition, we present different simulations of the proposed system. Particularly, we show the consumed bandwidth and the remaining energy in the different nodes. Finally, we test different energy configurations to evaluate the network lifetime and the remaining energy when the first node depletes its energy.