Influence of the spatial distribution of rain gauges on SWAT model calibration in two coastal river basins in Paraná State, Brazil

ABSTRACT The availability and treatment of rainfall data and its impact on SWAT calibration were evaluated in two watersheds on the coast of Paraná: Nhundiaquara and Guaraguaçu. The tested scenarios combined internal and external stations and also tested harmonization using a double mass curve. Performance was quantified at daily and monthly scales. In the Nhundiaquara (1975–1983), increasing the number of internal stations improved performance. For 1997–2005, the inclusion of an external station without harmonization introduced bias and degraded the model fit; the double mass curve removed the multiplicative bias and recovered performance. In the Guaraguaçu (2008–2009), the inclusion of a neighboring station yielded modest gains; correction was neutral or slightly negative, suggesting prior compatibility between the data series over the short timeframe. It is concluded that expanding the internal network and harmonizing external data series reduces biases and stabilizes calibration, especially in environments with strong orographic control, and that the monthly scale is more robust for evaluation.