In the context of Ukraine's integration into the European Union and implementation of the main provisions of Directive 2007/60/EC which implies assessment of potential hydrological risks, long-term factors of their formation, in particular the effects of climate change and the trend of river water regime changes should be taken into consideration. With this in mind, given the presence of both current long-term tendencies to reduction of runoff layers (volumes) and maximum discharge of water of spring flood across the Ukrainian rivers there is an important task to identify, using the modern initial data, both the average long-term values of these characteristics and different probability of their exceedance probability. For the first time, the authors of the study implemented an operator model of runoff formation to determine the average long-term values of maximum water discharge of spring flood in the basin of the Pivdenny Buh using meteorological characteristics (snowpack and precipitation) and runoff coefficients as basic parameters. The model was applied to determine the maximum runoff modules of spring floods for the rivers with a wide range of catchment areas affected by different physical and geographical conditions within the Pivdenny Buh Basin. Application of the operator model allowed the authors of the article to calculate and summarize all input parameters of the calculation model, including those obtained from observational data (snowpack, precipitation) and those that can't be measured by the hydrometeorological network (runoff coefficient, temporal irregularity coefficient and duration of surface inflow of snowmelt and rain water, transformational function of the flood waves layering under the influence of channel lag, coefficient of channel and floodplain regulation) for the rivers of the Pivdenny Buh Basin. The verifying calculation related to determination of the average long-term values of the maximum modules of spring flood runoff using the operator model showed satisfactory concordance with the initial data and this allowed recommending it for practical application for the rivers of the Pivdenny Buh Basin, including those that haven't been studied from the hydrological perspective.
Introduction. In Ukraine, under the conditions of current climate variability, the water content of the rivers, including the period of spring flood, changes. Against the background of the general tendency of reduction of the runoff characteristics of the spring flood of the rivers in Ukraine, formation of catastrophic floods on rivers leading to flooding of territories and destruction of economic objects should not be ruled out. The research is aimed at solving an important scientific-applied and socio-economic problem of improving the water management in the basin of the country's large waterway, the Dnipro River, through probabilistic assessments of hydrological threats which include spring flood and its consequences – floods of various magnitudes. Therefore it is necessary to create a unified universal mathematical model for formation, calculation and long-term forecasting of water flood characteristics, and to implement it for the Dnipro Basin's rivers (including small ones which usually are not recorded in the data of runoff observations), taking into account the climate changes of the winter-spring period. The purpose consists in prognostic assessment of the hydrological regime characteristics of the Dnipro Basin's rivers during the most full-flowing phase of their water regime, the spring flood, for the purpose of spatial monitoring of the state of water objects during this period. The research offers the method of territorial long-term forecasts of the spring floods characteristics which is based on the theory of channel isochrones and consideration of the complex of runoff formation hydrometeorological factors with preliminary determination of the type of water content of spring flood. The spatial representation of forecast variables in cartographic form will allow estimation of the size of the water flood and its long-term repeatability throughout the entire basin and preparation of forecasts for separate rivers, regardless of the degree of their hydrological exploration. Results. As a result of application of the complex forecasting method of territorial long-term forecasts of the spring flood characteristics of the rivers (through the example of the Dnipro Basin within Ukrainian borders), typification of floods based on their water content was performed using a discriminant function apparatus which takes into account the probable combination of hydrometeorological factors of spring floods. In terms of regional dependencies forecast of runoff depths and maximum water flow of floods was conducted and probability of their occurrence in the long-term period was determined. Assessment of the forecast methodology provided satisfactory results, and, under the conditions of long-term tendency to decrease runoff depths and maximum water flow during the spring flood of the Dnipro Basin's rivers, it is proposed to specify the average long-term values of these variables which are basic ones when applying the method of territorial long-term forecasts of the maximum spring runoff of rivers. A spatial preliminary estimation of the size of spring floods within the Dnipro River Basin was also fulfilled. The authors completed implementation of the method of territorial long-term forecasts of spring flood characteristics using the data of the water flood of 2017-2018. Conclusion. In general, taking into account the obtained criteria of accuracy, the method can be considered effective and recommended for release of annual long-term forecasts of the rivers' spring flood characteristics at in operational units delivering forecasting hydrological services to respective consumers. Through the example of the spring flood of 2017-2018 the possibility of forecasting assessment of the hydrological regime characteristics of the Dnipro Basin's rivers was shown which allows determining the zones of increased river runoff over the spring period and increased threat of possible flooding of surrounding areas, especially when the floods with a rare probability of excess occur.
Introduction. The problem of long-term and prognostic assessment of the terms of spring floods on rivers is relevant in hydrological investigations. Due to climate warming, the flood tides shifted to early dates or the beginning of the calendar year. The purpose is to analyze the long-term passage of spring floods and to carry out their spatial mapping forecasting on the Dnieper (within Ukraine). Methods. Investigation of the dynamics of long-term changes in the timing of spring floods and the use of the forecast method based on the meteorological forecast of ten-day air temperature in the winter-spring season in the Dnieper basin. Results. The method of forecasting the dates of starting and passing the maximum water discharge (levels) is based on the regional equations of the forecast scheme and allows to make a preliminary estimation of flood terms each year, regardless of the availability of meteorological observations. Assessment of the forecasting methodology allows to recommend it as a consultation, and the earliness of forecast varies depending on the geographical location of the catchments and the current weather conditions of a particular spring. Conclusion. The methodology was implemented on the example of spring floods 2017-2018. The overall forecasts were satisfactory, but the quality of the forecasts was affected by the non-simultaneous terms of the spring flood formation in the Dnieper basin. The prognostic probabilities of the dates of spring flood allow characterizing the frequency of their occurrence over many years. On the example of the 2017-2018 spring flood, the rivers of the Dnieper basin found that, in a changing climate, floods can be shifted to earlier and later ones.
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