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    Critical Drought Intensity-Duration-Frequency Curves Based On Total Probability Theorem-Coupled Frequency Analysis
    (International Association of Hydrological Sciences [Associate Organisation], 2021) Çavuş, Yonca; Aksoy, Hafzullah; Çetin, Mahmut; Eriş, Ebru; Burgan, Halil İbrahim; Yıldırım, Işılsu; Sivapalan, Murugesu
    A methodology using the standardised precipitation index is proposed to develop critical drought intensity-duration-frequency (IDF) curves. We define dry periods within which we recognise droughts of different durations. The most severe drought for each drought duration in each year is called the critical drought. The total probability theorem-coupled frequency analysis is used to determine the bestfit probability distribution function of drought severity, which is then converted to intensity. The generalised extreme value probability distribution function is found to best fit the critical drought severity. The methodology is implemented using monthly precipitation data for a meteorological station in Turkey. The critical drought intensity decreases linearly with increasing drought duration, whereas the return period increases exponentially when the drought becomes more severe. The site-specific IDF curves furnished with an empirical relationship between the intensity and return period allow one to characterise the drought not by an index-based intensity but by its return period. This kind of presentation is physically easier to nderstand, in particular for stakeholders and decision makers in practice.
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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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    Revisiting Major Dry Periods by Rolling Time Series Analysis for Human-Water Relevance in Drought
    (SPRINGER, 2022) Çavuş, Yonca; Stahl, Kerstin; Aksoy, Hafzullah
    Drought is increasingly gaining importance for society, humans, and the environment. It is analyzed commonly by the use of available hydroclimatic or hydrologic data with little in depth consideration of specifc major dry periods experienced over a region. Also, it is not a common practice to assess the probability of drought categories with a rolling time series and hence the changing knowledge for operational drought monitoring. A combination of such quantitative analysis with a comprehensive qualitative assessment of drought as a human-water relation aimed to fll this gap performing a case study in the Seyhan River Basin, Turkey. Six major dry periods were identifed from the precipitation time series of 19 meteorological stations. Major dry periods were analyzed by rolling time series and full time series, and they were also analyzed individually. A major dry period could be impor tant in terms of its duration while another in terms of its severity or intensity, and each with its own impact on the human-water relations that can be infuential on the drought mitigation, management and governance. Signifcantly higher probabilities were calculated for extreme droughts with the use of individual major dry periods. An important outcome from the study is that drought is underestimated in practice with the sole use of the whole data record.
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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.