In a wireless sensor network (WSN), interesting events are reported to the sink by the sensors in a distributed manner. Applications running at the sink require certain reliability in terms of events per unit time to be able to run satisfactorily. Individual report from sensor nodes is not important, but collective reports from sensor nodes of a region of interest of an application are crucial. Thus, the application event rate can be split across different sensor nodes so as to optimize the usage of scarce resources in WSN such as battery power and memory. In this paper, we propose a transport protocol which provides the desired event reliability to the application, by distributing the load at a sensor among its children based on their residual energies and average MAC layer data rate. The event rate distribution happens in such a way that the application at the sink gets its required event rate and the overall energy consumption of nodes is minimized. This protocol can be used for any MAC protocol as long as the average MAC data rate is known. We take the example of two MAC protocols, Slotted CSMA and Probabilistic TDMA. We derive a method for computing average MAC data rate for these two protocols and then show, using simulations, that our transport protocol performs close to optimal.