In the recent years, climate variation and urbanization can be considered as the major factors affecting the runoff in rivers, which could eventually lead to increased flood risks, reduced ground water potential, availability of drinking water and so on. The study focuses upon the quantitative analysis of impact contributed by major climatic variations (rainfall and Potential evapotranspiration) and urbanization (Land Use and Land Cover changes) on the runoff for the study area. The Karamana River Basin (KRB) has been chosen as the study area. To quantify the impact of climatic variations and urbanization, combination of statistical impact analysis methods like Pettit’s test, Double Mass Curve (DMC) method and hydrological benefit calculations have been applied. The study utilized the rainfall, Potential Evapo-Transpiration (PET) and runoff data for the study area collected over a period of 2010–2021, to carry out the combined statistical impact analysis. The PET values can be calculated from the collected temperature data using Hargreaves equation. From the Pettit’s test, the year 2016 has been identified as the change point year for (a) rainfall—runoff and (b) PET—runoff. From the identified change point year, DMC has been established for (a) cumulative rainfall—cumulative runoff and (b) cumulative PET—cumulative runoff. Then hydrological benefit calculations have been carried out, from the entities calculated using DMC. The results of the study indicate that the changes in land use patterns have a more significant impact on runoff than the climate variations.