Aim of the study:
1. To verify the applicability of the Stachý formula for the mean low flow (SNQ) in small lowland rivers monitored by the National Hydrological Service; 2. To modify the Stachý formula for determining the mean low flow using the same catchment area parameters as in the original formula.

Material and methods:
The present study used recorded discharge data from the period 1991‒2020 in 91 river gauge stations monitored by the State Hydrological Service (IMGW-PIB), with catchment areas of up to 550 km². The analysis was conducted separately for two regions: 1) the northern part of Poland – encompassing the rivers of Pomorze (the Baltic Coast), the lower Wisła (Vistula), and the Mazury (Masurian Lake District); and 2) the central and southeastern part of the country – encompassing the lower Odra (Oder) River with the Warta and Noteć, the middle Wisła (Vistula) with the Narew River from Ostrołęka, and the San River. The number of river gauge stations in each area was 43 and 48, respectively.

Results and conclusions:
1. The application of Stachý’s original formula to determine the SNQ based on the catchment’s physiographic parameters indicates a match between the calculated values and the measured results, assessed by an R² coefficient of 68.5% in the northern part of the country, and 43.0% in the central and southeastern parts; 2. The application of the modified formula to determine the SNQ indicates a better match between the calculated values and the measured results in each of the considered areas, assessed by an R² coefficient of 81.6% and 58.3% in the aforementioned parts of Poland, respectively. 3. In the modified formulas, for both areas, one of the obtained exponents of the variable parameters was opposite to its counterpart in the original Stachý formula; namely, the SNQ value decreased in the northern area with an increase of the lake parameter, and in the central and south-eastern areas, with an increase of the stream gradient.