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Hydrologic Analysis for Nutrient and Sediment Loading Determinations

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Watershed Management to Meet Water Quality Standards and Emerging TMDL (Total Maximum Daily Load) Proceedings of the Third Conference 5-9 March 2005 (Atlanta, Georgia USA) Publication Date 5 March 2005  701P0105.(doi:10.13031/2013.18076)
Authors:   Luis R. Pérez, G. Suárez, D. Sotomayor and G. Martínez

The Ro Grande de Arecibo (RGA) is a 44,000 ha watershed located in the central, northern region of Puerto Rico with elevations ranging from 50 to 1300 mmsl, average slope is 38 percent. The watershed is the focal point for a mega project that supplies 73 million gallons per day of potable water to the metropolitan area of San Juan with 1.5 million people. A total of 34,000 acre-feet of storage reservoirs are located within the watershed. The Total Maximum Daily Load (TMDL) program of the USEPA requires a specific analysis of hydrological processes within a watershed to quantify contribution of stakeholders to nutrient and sediment loading. This paper presents a methodology developed to allocate nutrient and sediment loading to gauged and un-gauged sub-watersheds within the original large basin. Five sub-watersheds were delineated within the RGA with different but homogeneous landuse patterns. Two of them were instrumented with automatic water and sediment samplers and flow meters which controls sampling during storms events and track the rising and recessing limb of the hydrographs. The remaining three sub-watersheds provided grab samples mostly during low or intermediate flow events. Samples were removed and transported for analysis to the laboratory within a 24-hr period. Samples were analyzed for total and dissolved phosphorous (TP and DP), total nitrogen (TN), and suspended sediment. Samples were taken throughout calendar year 2003. Nutrient and sediment loading was estimated using an expression of the form separating base flow loading from runoff from intense tropical rainfall events. Loading of TP ranges from 0.58 to 2.29 kg ha-1 yr-1; DP from 0.65 to 2.25 kg ha-1 yr-1 and TN from 1.06 to 23.34 kg ha-1 yr-1. Procedures to allocate loading to landuse will be presented.

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