THE TERRITORIAL METHOD OF FORECASTING THE LOW–FLOW DISCHARGE AND ASSESSMENT OF STREAMFLOW DEFICIT OCCASION OF SOUTHERN UKRAINE RIVERS

Abstract

The paper investigates the features of low–flow discharge formation in the rivers of Southern Ukraine under current climate change conditions. It examines a territorial method for short–term forecasting of low–flow discharges and substantiates approaches to the spatial assessment of streamflow deficit and its probability of occurrence during the contemporary river water–bearing period. The relevance of the study is driven by the escalating water resource scarcity, the increasing frequency of hydrological droughts, and the urgent need for operational low–flow forecasting to ensure effective water resource management in the region. The objective of the study is to improve the territorial method for forecasting mean decade (10– day) water discharges during the summer–autumn and winter low–flow periods (by extending the hydrological observation series, refining seasonal territorial relationships, and extending the temporal boundaries of the summer hydrological season), as well as to develop approaches for the spatial assessment of streamflow deficit and its probability of occurrence in the rivers of Southern Ukraine. It is established that under current climate change conditions in Southern Ukraine, there is a downward trend in river low–flow discharge, which is associated with rising air temperatures and shifts in atmospheric precipitation patterns. This leads to an intensification of water resource scarcity and an increased risk of hydrological drought development. The study utilizes long–term observation series of water discharges in the river basins of the Southern Bug, Lower Dnieper, and rivers of the North–Western Black Sea region for the period 1980–2020 when improving the forecasting methodology and in the period until 2025 – to test the methodology on independent data. The forecast method used is based on calculating the characteristics of water runoff depletion during the low water period in the form of regional relationships between initial and forecast runoff modules, statistical evaluation of the forecasting technique, and cartographic analysis of the spatial distribution of forecast characteristics were performed. To evaluate the probability of streamflow deficit occurrence, empirical curves of low–flow discharge probability of exceedance were constructed. The findings indicate that the territorial method ensures satisfactory quality of short– term forecasts for mean decade water discharges, with a quality criterion ratio of S/σ = 0,71–0,82 and an admissible error confidence level of 74–87%. The continuation of the low–flow runoff phase in the contemporary climatic period has been detected, particularly for the rivers of the North–Western Black Sea region, where the drying up of small watercourses is observed in certain seasons. Spatial analysis of forecast runoff modules and their probability of exceedance allowed for the identification of areas at the highest risk of hydrological drought development. The practical significance of the obtained results lies in the possibility of their application in the operations of hydrometeorological and water management organizations, water use planning, assessment of the ecological status of river basins, and the development of climate change adaptation measures.