Wetlands are hydrologically sensitive ecosystems to the impact of anthropic actions and natural changes. At the same time, there is a close relationship between the functioning of wetlands and their capacity to provide services to human well-being and other ecosystems, so that certain modifications of the hydrology of wetlands have a significant negative impact on the human societies of their environment. Knowledge of hydrological functioning and the hydrodynamic and hydrochemical characteristics of surface and underground water of wetlands is essential to achieve sustainable water management, preserve the environment and evaluate the response of the environment to the impacts produced by human actions and natural changes. The Ozama Wetlands National Park (OWNP) is one of the most important in the Dominican Republic and around 344,000 people live in their surrounding area. The water balance of the OWNP is being studied by means of different methods to quantify the atmospheric, superficial and subterranean input and output water flows, and to identify which are the main mechanisms controlling the wetlands hydrology, its state of functionality (natural or altered) and the main factors that can induce changes in their hydrological regime. The preliminary results of the quantification of the main surface water inputs and outputs to the OWNP by current meter in the months of April (wet season) and December 2018 (dry season) are presented. In the Ozama River, the main river of the basin, input flows to OWNP were measured to amount 7 and 3 m³/s respectively (both months). In the Yabacao River, the second stream contributing to the OWNP, 6 and 7 m³/s were measured for the same months. The only surface outlet of the OWNP is the Ozama river. The output flows measured in both months were 17 and 19 m³/s, respectively. The difference between incoming and outgoing measured flows reveals the existence of significant contributions to the wetlands of the OWNP not measured, which must be quantified.