A background area for a rural and urban basin

SILVANA ARREGHINI; LAURA DE CABO; RAFAEL SEOANE; NICOLÁS TOMAZIN; ROBERTO SERAFINI; ALICIA FABRIZIO DE IORIO

Foz de Iguazú, Brasil

Congreso; VII IAHS Scientific Assembly; 2005

International Association of Hydrological Sciences

Water quality changes quickly, even in background areas, as a consequence of itsinterdependence with discharge. In Argentina there are numerous basins that lack joint rainfalland discharge measurement data in order to estimate runoff. Consequently, the parameters ofhydrological models must be indirectly estimated. The aim of this study is to identify abackground area for a temperate and a lowland river (Reconquista River, Buenos Airesprovince, Argentina) with scarce hydrometeorological continuous data and to describe thewater quality dynamics in the background area with an ecohydrological focus. We propose amethodology based in discharge estimation through instantaneous unit hydrograph model(IUH) and chemical and physical measurements in stream water under different hydrologicalconditions.According to agricultural land use and low population density, high basin of Reconquistastream might be considered a background area. Compared with the rest of sites of theReconquista river, the water quality at Durazno stream is the nearest to that from naturalsources. The oxygen, ammonium, nitrate, o-phosphate and particulate organic carbon medianconcentrations allowed to confirm Durazno stream as a background area.Flow values estimated by the IUH model were compared with indirect techniques valuesestimated in situ. The results of both methods showed a good agreement. The analysis of thehydrographs denotes the sensitivity of the shape of the estimated hydrographs for the range ofCurve Number values values considered.Nitrates, ammonium, major ions and conductivity were significantly lower during high flowconditions. Dissolved organic carbon (DOC) and E2/E3 ratio increased suggest a large inputof carbon by runoff, and an increase of fulvic/humic acids ratio. In the receding limb of thehydrograph, nitrate concentration was lower than during discharge peak. This variation wasthe result of rainwater diluting in the ditch. DOC increased and E2/E3 ratio did not change,this allows to suggest that the composition of DOC did not change but its concentrationincreased in the water of ditches. During dry periods, chlorides, other anion concentrationsand conductivity were similar to groundwater, showing the main source of stream water inbase flow conditions.The methodology applied in this study also can be used in others lowland temperate riverswith similar geomorphological, edaphic and lithological characteristics. Due the impossibilityto define baseline areas in surface waters, the knowledge of the background water quality isessential to understand pollution trends and anthropogenic impacts on the rivers.