Method of forecasting assessment of the maximum runoff characteristics of the Dnipro Basin's rivers in spring season

Abstract

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.