Abstract. The purpose of this study was to assess long-term impacts of climate and land use change on a catchment runoff and dam overtopping dam reliability. Long hydrological time series (30 years) from six rainfall stations and one stream flow stations were analysed. A methodology combining common statistical methods with hydrological modelling was adopted in order to distinguish between the effects of climate and land use change and to present probabilistic assessment of overtopping reliability of the selected earth-fill dam. It is important to ensure that extreme meteorologically induced flood rises do not exceed dam crest level. Considering climate change factor, intensity-duration-frequency curves of the catchment were updated. In addition, in consistency with the areal development plan, year of 2020 was targeted to evaluate the effect of land use changes on the generation of storm runoff. Accordingly, compared with current imperviousness it was found that the areal imperviousness will be increased up to 4.5 % by the year of 2030. Step-by-step procedures were carried out in tandem to evaluate the hydrological performance of the spillway capacity in light of an extreme storm event of PMP / PMF magnitude. The HEC-HMS was applied to transform the PMPs to PMFs and estimate the outflows and corresponding flood rises over the crest level for all durations. A conventional reservoir routing procedure with modified technique was then carried out for all PMP / PMF durations; i.e., 1 to 120 h. Three cases of different floods were performed where the last case represented the most severe flood on the dam reservoir. The simulations indicated that the flood rises for all durations were lower than the embankment crest level. Although results showed marginally adequate capacity to allow safe passage of flood water of PMP / PMF magnitude, the dam safety in hydrological aspect was assured.