Methodological approaches to assessing the risks of water resources depletion in the context of climate change (using plain territories of Ukraine as an example)

The relevance of the work is determined by the necessity of quantitative assessment of the risks of water resources changes occurring in the early 21st century due to the greenhouse effect and corresponding climate changes. To evaluate climate risks the authors used a probabilistic approach which involves multiplying the indicator of water resource reduction by the probability of such event occurrence. Reduction of water resources caused by warming characterizes their "depletion". The study utilized the results of calculations of the average long-term annual "climate" runoff, determined through the "climate-runoff" model based on meteorological data of the averaged model trajectory of the EURO-CORDEX project for the climate change scenarios RCP4.5 and RCP8.5. The calculation period extends from 2021 to 2050, while the baseline period continues from the beginning of observations until 1989. The comparison of the calculated climatic runoff and the baseline runoff was performed across meteorological stations. The assessment of the changes was carried out by calculating the relative deviation between the baseline and calculated runoff values. The probability of the "depletion" stage for water resources was determined as the ratio of the number of meteorological stations (grid nodes) where changes reached certain magnitudes to the total number of stations considered. The threshold indicators for the degree of depletion of water resources included changes exceeding 10% (statistically significant changes in water resources), 50% (water resources destruction), and 70% (irreversible destruction of water resources). The objective of the study consists in developing criteria for calculating climate risks and establishing the risks of formation of a tense, critical, and catastrophic state of water resources, taking Ukraine as an example. A detailed examination of the intervals of changes in water resources showed that the highest risk of their depletion over the period from 2020 to 2050 will be observed in the range from -20 to -40% (tense state) for the RCP4.5 scenario and from -30 to -60% (tense state) for the RCP8.5 scenario. It was established that the climate risks of depletion and irreversible depletion of water resources for the territory of Ukraine from 2020 to 2050 remain quite small, which is confirmed by the low probability of their occurrence. However, the risk values increase by 1.5 to 2.0 times for the RCP8.5 scenario compared to the RCP4.5 scenario. Additionally, for calculating the climate risks, it was proposed to use a relative area subject to depletion as an indirect indicator of losses (a larger area implies higher costs of restoration). This approach allowed for having more differentiated risk assessments. The developed methodology for assessing the climate risks of water resources depletion in the context of global warming can be applied for various models and countries.