Regional hydrological analysis was conducted using data from the catchments located in the area of karst basin of the Kupa river. The study included data from 32 hydrological profiles on 20 watercourses down to Kamanje on Kupa, Stative Donje on Dobra, Mrzlo polje on Mrežnica and Veljun on Korana profiles. Catchment area to specified profiles is approximately 5450 km2. The processed hydrological data are the average daily flows and the average, maximum and minimum annual flows. In addition to hydrological data, the daily rainfall data collected on climatological stations Parg, Ogulin and Karlovac were processed. The period of analysis is from 1981 to 2012, and the results were compared with the results of previous studies that analyzed the period from 1951 to 1980. The comparison of trends in mean annual flows, which also justifies the chosen period of analysis, has shown that in the earlier period from 1951 to 1980 eight time-sets showed a declining trend, and 11 time-sets showed growing trend, while in the analyzed period from 1981 to 2012, 20 time-sets showed a declining trend, and 12 time-sets showed growing trend. The maximum annual flows in the earlier period consisted of seven time-sets showing a declining trend and 12 time-sets showing the growing trend, while in the period of this analysis 27 time-sets showed a declining trend and five time-sets showed a growing trend. The minimum annual flows in the earlier period analysis consisted of seven time-sets showing a declining trend and 12 time-sets showing the growing trend, and in the period of this analysis, 15 time-sets have shown a declining trend, and 17 time-sets have shown a growing trend. The analysis of homogeneity of average, maximum and minimum annual flow for the 1981 to 2012 period in relation to the 1951 to 1980 period, shows that majority of non-homogeneous time-sets were present with average annual flows, while the majority of time-sets of maximum and minimum annual flows were homogeneous. The homogeneity of average annual flows confirms the assumption of a drier period, which occurred in the early 1980s and continues to this day. In this regional hydrological analysis, the following was conducted: 10 two-parameter models’ relationships of the average annual flow characteristic parameters, 72 two-parameter models’ relationships of the maximum annual flow characteristic parameters, together with the parameters of the maximum flow rate and the volume of different return periods and 10 two-parameter models’ relationships of the minimum annual flow characteristic parameters. Theoretical hydrographs of 10-, 100- and 1000-year return period along the Kupa river flow were defined. Two-parameters’ models that can be used in practice were selected. Although the analysis of multi-parameter relationships produced two possible three-parameter relationships, they are not recommended for use because of the very wide confidence interval, regardless of the correlation coefficient high value. Spectral analysis of the flows and precipitation shows that during the period of analysis from 1981 to 2012, extremes of flows and precipitation were strengthened and more pointed out, but the total amount of precipitation and flows declined, compared to the previous period from 1951 to 1980.