Data-based analysis of drought propagation

Abstract

In order to better understand the drought, it is crucial to investigate its different types and how it evolves, which is known as drought propagation. This study provides an in-depth investigation of the mechanisms behind drought propagation, focusing on four key issues: (i) intermittency as an indicator of hydrological drought, (ii) identification of drought events, (iii) drought evolution, and (iv) propagation from meteorological drought to hydrological drought. The study utilizes a comprehensive data set from three river basins in western Turkey, namely Kucuk Menderes, Gediz, and Ergene, as robust case studies. After introduction in Chapter 1, the study begins with a literature review on drought in Chapter 2, which leads to the formulation of research questions. The study area and data are presented in Chapter 3. Chapter 4 presents the analysis of intermittency as an indicator of hydrological drought. The dataset from the three river basins is used to explore the link between intermittency in the streamflow and groundwater. It is seen that there has been an increase in water demand in the river basins, and the only solution for balancing the water demand and supply is the groundwater, which is already being overused. The overuse of groundwater causes the water table to descend, streamflow to decrease, and the intermittency ratio of streamflow to increase, resulting in a vicious cycle that leads to a no-water future if no action is taken. This part of the study introduces the concept of intermittency cycle and suggests the intermittency as an indicator of drought in streamflow and groundwater. Chapter 5 introduces a new method to identify drought events by eliminating short and weak wet periods between subsequent droughts. The method was applied in Gediz River Basin to examine the average duration and intensity of drought events across the basin. It was found that drought duration and intensity do not vary greatly over the basin, but the minor variation could be important depending on the practical problem being addressed. The focus of Chapter 6 is on the drought evolution referring to the transition between different drought classes. A method was proposed to calculate drought class probability, expected residence time, and expected initial transition time using precipitation data. Additionally, two new concepts are introduced to improve the proposed method. Results show that the probability of drought occurrence decreases as the drought severity increases, and the expected residence time in each drought class decreases as the drought severity increases from the near normal class to the severe drought class. Results also highlight the importance of considering the expected first transition time, with the longest time observed in the extreme drought class indicating the persistency of this particular class of drought. These findings can be useful in developing preventive measures against drought. Chapter 7 introduces the concept of Formative Drought Rate (FDR) as a potential prediction tool for drought propagation, and tests it on precipitation and streamflow data from three river basins in Turkey. Results show that the FDR is capable of providing prior information for the prediction of drought propagation, which is valuable for drought mitigation. The comparison with Drought Propagation Rate (DPR) from literature supports the use of FDR as a start-up tool, needing further testing in different basins to increase its reliability. Overall, this study introduces new tools and methods to understand drought propagation mechanism, with each chapter presenting the details of the tools and the methods through various case studies. The insights gained from this study can be applied in drought mitigation efforts in any region facing similar challenges. Further research and case studies can improve the tools and the methods and expand our knowledge on drought propagation. The insights gained from this study can suggest new avenues for the future research. The FDR is an open issue for further development. It could be applied to different data sets that represent different types of droughts, such as the groundwater level, to gain a better understanding of the propagation between droughts in precipitation, streamflow, and groundwater.