This article has been examined maximum runoff of the rivers of the Crimean Mountains. The rivers flow through the western and eastern part of the northern slope Crimean Mountains, and on its southern coast. The largest of them: Belbek, Alma, Salgir, Su-Indol and others. To characterize the maximum runoff of rain floods (the layers of rain floods and maximum discharge of water) on the rivers of the Crimean Mountains were used materials of observations for long-term period (from the beginning of observations to 2010 inclusive) on 54 of streamflow station with using a the so-called «operator» model for maximum runoff formation.

The Danube River mouth is a main source for fresh water used for water supply and irrigation purposes in the arid southern part of Ukraine. In addition, the water of the mouth is used for filling the Danube Lakes. Climate change and numerous social and economic factors reduce the area and the water level of the Danube Lakes and increase their salinity. Under these circumstances the water exchange between the Danube River and the lakes is a very important process which allows maintenance of the water–salt balance of the latter. Such water exchange massively depends on the Danube River regime. The paper presents research of temporal variation of typical water discharges in the lower course of the Danube River associated with assessment of natural and anthropogenic factors affecting river runoff fluctuations. Time series of annual average water discharge at the water gauging stations of such cities as Reni and Izmail were considered uniform for the selected periods. They cover the period of conventionally natural river runoff regime (1840–1920), the period of the least altered river runoff regime (1921–1960) and the most altered river runoff regime under a heavy anthropogenic influence (1961–1989). The analysis of the impact of climate changes on the river runoff allowed introduction of a new, modern period of the Danube River hydrological regime (1990–2015). It is established that the series of maximum and minimum water discharges are characterized by the lack of their uniformity from the beginning of the period of initial influence of hydraulic engineering structures on the hydrological regime (since 1961). The analysis of the aggregate multi-year series (1840–2015) of average annual, maximum and minimum water discharges of the Danube River across its length from Reni to Izmail showed the presence of positive trends. At the same time the period of climatic changes (after 1989) is characterized by a less intensive growth of maximum water discharges. The research resulted in establishing the periodicity and synchronism of water discharge fluctuations in the lower course of the Danube River from Reni to Izmail within the territory of Ukraine. In particular, the study showed for the first time that from the 2000s onwards the redistribution of river runoff between the Danube River branches led to unsynchronization of minimum river runoff in different parts of the Danube Delta. The research shows that comprehensive analysis of the Danube River hydrological regime under modern climatic conditions is required to ensure efficient engineering regulation of the Danube Lakes filling pattern. Such regulation, in its turn, will ensure that the water supply and irrigation purposes are achieved.

In the conditions of modern changes of climate both in Ukraine and in the world on the whole, probability of the extreme hydrological phenomena rises substantially, in particular, catastrophic floods of different origin. The analysis of flood distribution on the territory of Ukraine shows that an overwhelming amount of destructive floods happened in Ukrainian Carpathians, where they are the characteristic feature of the hydrological mode of the rivers. Floods which are observed on the rivers of Crimean Mountains also, for individual years, had catastrophic character. For determination of water discharge calculations in the period of floods on the mountain rivers of Ukraine authors offer a scientific-methodical base which is based on modern scientific achievements in area of theoretical and applied hydrology. Fundamentally new in the proposed model is the separation of factors of forming and transformation of flood flow on slopes and in a river-bed network. The method of numeral solution of task was developed for nonmeasureable descriptions of floods and it found practical application. As a result, the numerical values of the main components of the slope inflow are obtained: the layers of runoff for the flood of 1% probability and the duration of the slope inflow, which are generalized over the territory, taking into account the influence of intrazonal and azonal factors (forestation, mean height of the catchments and karst) and coefficient of time nonuniformity of slope inflow, which are averaged within the studied regions. The process of transformation of slope inflow into channel runoff is described with the help of transformation functions that take into account the time of channel run-up and flood plain regulation. The accuracy of determining the maximum modules of the runoff of 1% probability of exceeding depends on the accuracy of the initial information, which makes it possible to recommend the proposed methodology for practical use in order to increase the level of hydraulic engineering projects and water resources management on the mountain territories of Crimea and Carpathians.

The maximal surface runoff from territory of the Crimean Mountains is represented as a runoff of small rivers that flow through the western and eastern part of the northern slope and from the southern coast. The materials from 54 water gauging stations (WGS) were used to characterize the maximum runoff during rain and meltwater-rain floods on the rivers in the Crimean Mountains. A modified reduction structure of a calculation formula was used for valuation of the maximal runoff of different origin flash flood for rivers at the Mountainous Crimea. The main parameters of the proposed model are summarized as dependencies on the average height of the catchments and generalized in the form of a map. It is also possible to use the second variant of the suggested method taking into account the factor of underlying surface is introduced. Comparison of the calculated values of maximal runoff shows good convergence with both the initial information, and the largest values in the observation period.