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    Critical droughts extracted from dry periods
    (Springer, 2024) Cavus, Yonca; Aksoy, Hafzullah
    Differently from common definitions, we distinguish droughts from dry periods in this study. From the dry periods, we extract subperiods of various durations changing from one month to the dry period length. Each subperiod is called a drought, and the most severe drought (with maximum severity or intensity) of each duration is defined critical drought. These newly defined concepts of this study were tested based on the 12-month timescale standardized precipitation index calculated from monthly precipitation data of two meteorological stations in Seyhan River Basin, Turkey. Frequency analysis was applied on the critical drought intensity for each drought duration separately. Results showed that statistical characteristics (duration, severity, intensity and frequency) of dry periods and critical droughts were different from each other. Critical droughts were more intense than the dry periods and their intensity increased as their duration decreased. The Generalized Extreme Value was confirmed as the best-fit probability distribution function of the critical drought intensity in majority. By extracting droughts from the dry periods instead of taking a dry period as one single drought, high number of short and intense critical droughts were deciphered that are masked otherwise within dry periods. These frequent droughts may have higher negative impact on water-dependent economic sectors and society if their timing of occurrence coincides with the season when the water demand is peaking. Therefore, extraction of critical droughts from dry periods is a substantial achievement of this study as the negative impact of drought is often related to its intensity along with its severity and duration.
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    Discussion of Drought assessment in a south Mediterranean transboundary catchment
    (Taylor & Francis Ltd, 2022) Aksoy, Hafzullah; Cavus, Yonca
    [Abstract Not Available]
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    Drought intensity-duration-frequency curves based on deficit in precipitation and streamflow for water resources management
    (Copernicus Gesellschaft Mbh, 2023) Cavus, Yonca; Stahl, Kerstin; Aksoy, Hafzullah
    Drought estimates in terms of physically measurable variables such as precipitation deficit or streamflow deficit are key knowledge for an effective water management. How these deficits vary with the drought event severity indicated by commonly used standardized indices is often unclear. Drought severity calculated from the drought index does not necessarily correspond to the same amount of deficit in precipitation or streamflow at different regions, and it is different for each month in the same region. We investigate drought to remove this disadvantage of the index-based drought intensity-duration-frequency (IDF) curves and develop IDF curves in terms of the associated deficit. In order to study the variation of deficits, we use the link between precipitation and streamflow and the associated indices, the Standardized Precipitation Index (SPI) and the Standardized Streamflow Index (SSI). More specifically, the analysis relies on frequency analysis combined with the total probability theorem applied to the critical drought severity. The critical drought has varying durations, and it is extracted from dry periods. IDF curves in terms of precipitation and streamflow deficits for the most severe drought of each drought duration in each year are then subject to comparison of statistical characteristics of droughts for different return periods. Precipitation and streamflow data from two catchments, the Seyhan River (Turkiye) and the Kocher River (Germany), provide examples for two climatically and hydrologically different cases. A comparison of the two cases allows a similar method to be tested in different hydrological conditions. We found that precipitation and streamflow deficits vary systematically, reflecting seasonality and the magnitude of precipitation and streamflow characteristics of the catchments. Deficits change from one month to another at a given station. Higher precipitation deficits were observed in winter months compared to summer months. Additionally, we assessed observed past major droughts experienced in both catchments on the IDF curves, which show that the major droughts have return periods at the order of 100 years at short durations. This coincides with the observation in the catchments and shows the applicability of the IDF curves. The IDF curves can be considered a tool for using in a range of specific activities of agriculture, ecology, industry, energy and water supply, etc. This is particularly important to end users and decision-makers to act against the drought quickly and precisely in a more physically understandable manner.
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    Frequency curves of high and low flows in intermittent river basins for hydrological analysis and hydraulic design
    (Springer, 2024) Sarigil, Gokhan; Aksoy, Hafzullah; Cavus, Yonca; Eris, Ebru
    Upper and lower percentiles of Flow Duration Curves (FDCs) of daily streamflow data were investigated to develop frequency curves. Upper percentiles with exceedance probability of 1, 5 and 10% (Q1, Q5, Q10) were used for high flows, and lower percentiles with non-exceedance probability of 10, 5 and 1% (Q90, Q95, Q99) for low flows. Median value (Q50) was covered to represent the average conditions of streamflow. A mixed frequency analysis based on the total probability theorem taking zero values into account was applied for the lower percentiles of FDC. Case studies were performed for three intermittent Streamflow Gauging Stations (SGSs) from Kucuk Menderes River Basin in western Turkey. An overall assessment of results shows that the best-fit probability distribution function does not change from one SGS to another considerably for low flows while each SGS has its own probability distribution function for high flows. Upper and lower percentiles, and median value were calculated at various return periods by using the identified probability distribution functions. The calculated values were plotted in the form of frequency curves of high flow percentiles and low flow percentiles. The frequency curves have a practically significant potential use in hydrological analysis, water resources management and hydraulic design under high and low flow conditions. They are yet open to further development for regionalization and their applicability can be extended to ungauged sites in river basins.
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    Spatial characterization of drought through CHIRPS and a station-based dataset in the Eastern Mediterranean
    (Copernicus Publications, 2024) Orieschnig, Christina; Cavus, Yonca
    Drought is a natural hazard which occurs in all climatic zones and affects different sectors, such as irrigation, energy, water supply, and ecology. Monitoring and predicting drought are pressing challenges, as drought is becoming more common and severe owing to the impacts of climate change and increased climatic variability. However, in many areas of the globe, the temporal and spatial characterization of droughts and drought severities are hindered by a lack of reliable, locally-measured long-term data and unevenly distributed, erratic meteorological stations. In this situation, remote sensing datasets such as Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS) can offer valuable insights into long-term developments and the spatial characteristics of droughts. Nonetheless, it is often uncertain to which extent data such as CHIRPS succeeds in representing local dynamics and how this varies between geographical regions and climate zones. In this analysis, we aim to evaluate spatial drought conditions over the Seyhan River basin in Turkey in the Eastern Mediterranean. Using the Standardized Precipitation Index (SPI) as a drought index, the applicability of CHIRPS as a long-term satellite precipitation product for drought monitoring is investigated. We compare two spatial representations of the SPI: one derived on a per-pixel basis from CHIRPS data since 1981, the other based on data from 19 meteorological stations scattered across the basin, which was spatialized using inverse distance weighted interpolation (IDW). Our results offer insights into the relative accuracy of CHIRPS data and avenues towards optimizing the quality of spatial drought characterization. Copyright: © 2024 Christina Orieschnig.
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    Spatiotemporal analysis of meteorological drought over Kucuk Menderes River Basin in the Aegean Region of Turkey
    (Springer Wien, 2020) Eris, Ebru; Cavus, Yonca; Aksoy, Hafzullah; Burgan, Halil Ibrahim; Aksu, Hakan; Boyacioglu, Hulya
    Meteorological drought is analyzed both in time and space by using drought indices based on site-specific precipitation and temperature data. Departure of precipitation from its normal called in this study as the Dimensionless Precipitation Anomaly Index (DPAI) is used at annual scale, while the Standardized Precipitation Index (SPI) using precipitation and the Standardized Precipitation Evapotranspiration Index (SPEI) using precipitation and temperature are considered at monthly time scale. Five meteorological stations over Kucuk Menderes River Basin in the western part of Turkey are selected for the case study. Results are presented in the forms of drought index time series, drought risk graphs, and drought severity maps. The prolonged severe historical dry periods of the river basin are correctly identified by the drought indices. It is seen that simple tools such as the drought indices used in this study based on easily available meteorological data could explain temporal variability at a site or spatial variability over a river basin. They are therefore important not only for the scientific community dealing with drought as a research problem but also for decision-makers, stakeholders, and end-users making practice about the drought through water allocation studies and drought management plans.