Morphometric analyzes support an understanding of the physical characteristics of hydrographic basins and are commonly used to estimate hydrological variables. This work aims to compare the geomorphological characteristics and the time of concentration (Tc) in two watersheds: upper part of the Jundiaí river (139.3 km2) and Piraí river (209 km2) belonging to the Jundiaí river basin, state of São Paulo. To calculate the Tc, empirical equations and monitored hydrological data were applied.

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... the US Army Corps of Engineers, Ven te Chow, and Kirpich Tc formulas, the Piraí watershed had the highest Tc, with a mean value of 6.29 h with a maximum difference between the three formulas of approximately 27%. The mean Tc for the Jundiaí watershed was 5.18 h, with a 25% variation between the minimum and maximum calculated values. Applying the hydrological method to the data collected between 2018 and 2021 from the São Paulo State Flood Alert System (SAISP) fluviometric and pluviometric stations, the minimum and maximum calculated values were 6.83 h and 23.33 h, respectively, with mean of 14.4 h. We observed higher values for the Piraí watershed, which has a larger drainage area, with Tc varying from 17 h to 49 h and mean of 36.7 h. Note that the average Tc for the Piraí watershed was almost 2.5 times higher than that of the Jundiaí watershed, despite the almost equal values of slope and length of the thalweg, calculated from the empirical equations. These equations based on morphometric parameters underestimated the Tc compared with the values calculated from the hydrological method, presenting 82% and 64% of error, respectively, for the Piraí watershed and the upper part of the Jundiaí watershed.