The water balance of a tropical urban watershed
In cities, natural hydrologic processes are drastically altered by human interventions and land use change, including the construction of systems for water storage, abstraction, distribution, use, collection, and discharge. The effects of this altered hydrology, such as the artificial linkages within and between catchments, contribute to unique patterns of water flow not observed in natural basins. Water balances have been constructed for several temperate cities, yet there is a lack of understanding of the inputs and outputs of water in tropical urban settings. Our objective was to develop a water budget for the Afluente Norte branch of the Juan Méndez stream, located in the Río Piedras district of San Juan, Puerto Rico. As part of a graduate course in environmental hydrology, we carried out an empirical study for one month during the dry season, in which we measured or estimated water inputs into (precipitation, potable water, infiltration from pipe leakage) and outputs from the watershed (evapotranspiration, wastewater, and stream discharge). Using the data from this study and the continuity equation, we calculated the water balance for this watershed. We also collected stream water quality data to evaluate whether or not wastewater leakage occurred during dry periods.
We calculated a total combined water input of 16,735 m3 and output of 21,007 m3, for a total change in storage of -4,272 m3. The average stream discharge value measured only differed by 0.04% from the derived discharge calculated from the combined sum of surface runoff, groundwater, and pipe leakage, indicating that our estimations for these quantities were highly accurate. We conclude that in this urban watershed during low rainfall periods, human engineered components of the hydrologic system dominate and contribute significantly to stream discharge. The contribution of rainfall to the water balance was minimal, representing 8% of the input and 12% of the output. In contrast, potable water comprised more than 60% of input flows, and treated wastewater represented 44% of output. Furthermore, leakages from the potable water and wastewater systems together accounted for almost half of the measured stream discharge. Water quality data confirm that during dry periods, the water of the Afluente Norte is infiltrated by wastewater. Groundwater discharge to the stream did occur, as suggested by the observed negative change in storage. These results align with findings from temperate urban watersheds, highlighting the central role of piped inflows and outflows in driving urban water balances.