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Precipitation and soil moisture deficits are often used to characterize the nature of drought. Extended periods of an extreme meteorological event (precipitation deficits) result in a hydrological drought because of streamflow, groundwater and reservoir level reduction. The water deficits have an enormous impact on socioeconomic status and development. The percentile values in the range of Q70–Q95 are commonly adopted as the low threshold for perennial rivers (Kjeldsen et al., 2000). In this study, we quantified the critical dry up period (CDP) and streamflow severity due to extreme meteorological events over a set of 10 watershed located in South Korea. The TPHM, GR4H, and CAT hydrological models were calibrated against daily streamflow data using the Shuffled Complex Evolution algorithm considering similar objective functions. The CAT platform is used to set up the three hydrological models (Kim and Jang, 2017) which have been tested in several basins (Birhanu et al., 2018; Jang et al., 2016; Miller et al., 2014). We selected the Q95 of the flow duration curve compiled using daily streamflows of each watershed as a low threshold level to detect the CDP and seasonal streamflow deviations. The analysis revealed the CDP vary from 30 to 90 days and the streamflow deviations from the Q95 also responded differently in the wet and dry season. In addition, the streamflow deviations increased with increasing duration of precipitation and soil moisture deficits. The result will provide information to water managers, policymakers and other stakeholders regarding the duration and severity of streamflow to extreme meteorological events. Acknowledgments: This research is supported by the Research Program (20190101-001) of Korea Institute of Civil Engineering & Building Technology and by the Korea Environmental Industry & Technology Institute (KEITI) grant funded by the Ministry of Environment (Grant 18AWMP-B083066-05).ReferencesBirhanu, D., Kim, H., Jang, C. and Park, S.: Does the Complexity of Evapotranspiration and Hydrological Models Enhance Robustness?, Sustainability, 10(8), 2837, 2018.Kjeldsen, T. R., Lundorf, A., and Dan, R.: Use of two component exponential distribution in partial duration modeling of hydrological droughts in Zimbabwean rivers, Hydrological Sciences Journal, 45, 285–298, 2000.Kim, H.-J. and Jang, C.-H.: Catchment Hydrologic Cycle Assessment Tool - A User Guide, Korea Institute of Civil Engineering and Building Technology, Korea, 2017.Jang, C. H., Kim, H. J., Ahn, S. R. and Kim, S. J.: Assessment of hydrological changes in a river basin as affected by climate change and water management practices, by using the cat model, Irrigation and Drainage, 65, 26–35, 2016.Miller, J. D., Kim, H., Kjeldsen, T. R., Packman, J., Grebby, S. and Dearden, R.: Assessing the impact of urbanization on storm runoff in a peri-urban catchment using historical change in impervious cover, Journal of Hydrology, 515, 59–70, 2014. [ABSTRACT FROM AUTHOR]

Recently, there has been a growing interest in extreme drought in the hydrological impact assessment due to climate change, but most studies are either drought index based on weather information or development of drought prediction systems to monitor the status. Therefore, it is not enough to evaluate water resource availability that reflect watershed characteristics. In order to evaluate and predict the precise water resource availability during drought, it is necessary to evaluate the characteristics of the hydrological environment considering the artificial water use in the watershed. CAT (Catchment hydrologic cycle Assessment Tool, Kim et. al, 2017) was used to evaluate the characteristics of the hydrological environment. CAT provides tools for analyzing artificial water use such as water supply, water reuse and groundwater intake. In this study, the water resource availability of Boryeong-Dam watershed (163.6 ㎢), which is known to be an extreme drought region, was evaluated considering the complicated agricultural water use in the upstream region. Rainfall and observed inflow data were used from 1999 to 2016, and daily climate data such as mean temperature, relative humidity, wind speed, and duration of sunshine were used. Comparing the simulated runoff with the observed runoff in the upstream area of Boryeong-Dam watershed, the model efficiency was improved by more than 10 % compared with the existing simulated runoff without consideration of the agricultural water use. This is the result before parameter calibration. If the detailed water use such as groundwater intake is taken into consideration, the model efficiency will be improved more.AcknowledgementsThis research is supported by the Research Program (20190101-001) of Korea Institute of Civil Engineering & Building Technology and by the Korea Environmental Industry & Technology Institute (KEITI) grant funded by the Ministry of Environment (Grant 18AWMP-B083066-05). [ABSTRACT FROM AUTHOR]

Understanding the urban hydrology of Oslo, Norway, is hampered considerably by a paucity of reliable data describing the urban runoff response to rainfall under varying degrees of urbanisation in different areas within the city. This knowledge is nevertheless critical for quantifying the potential benefits of low-impact flood mitigation measures, such as green roofs and rain beds. The modelling approach needed is akin to Predictions in Ungauged Basins in that parameter parsimonious models adaptable to local conditions are required and model 'calibration' involves the use of alternative data sources for building the hydrological model. In the SURF (Sustainable URban Flood management) project, we are applying a parsimonious rainfall-runoff model, DDD (Distance Distribution Dynamics), to quantify the water balance and runoff response in two contrasting urban catchments. One catchment is a typical residential area with detached houses and gardens, whereas the other is situated in an older part of Oslo, and is densely populated with blocks of flats and a park. The fraction of impermeable area (defined as roofs and roads) is 17% and 42%, respectively for the two catchments. In the DDD model, the distribution of distances from points in the catchment to the river network is an essential factor describing the runoff dynamics. As the natural drainage network is disrupted by urbanisation in both catchments, a GIS-derived surface flow network, with areal support such that it resembles the road network, serves as a proxy for estimating the distance distribution for the model. Runoff data for a partial calibration of the response to precipitation is derived from flow data from points within the combined sewer systems within the catchments. As information on sub-daily water usage is not available for the sewer points, a novel signal processing technique has been used to extract the wastewater flow from the runoff series. Preliminary modelling results indicate that the DDD model can successfully reproduce the residual runoff using these data sources, together with an hourly rainfall series, in a model calibration. The primary goal of the project is to identify the minimum level of data required for producing hydrological simulations useful for municipal authorities and planners. [ABSTRACT FROM AUTHOR]

The devastating flood of 1868 in the alps triggered a systematic study of the role of forests for the hydrology in Switzerland (and neighboured countries). As a direct consequence, the first longterm observations of runoff in small forested catchments were launched in 1903 in the Swiss Emmental (and have been run ever since). Several similar longterm investigations of runoff formation in other first-order catchments of the Swiss pre-alps – with different geology, forest compositions and rainfall regimes – followed in the course of the past century, among others the 50-year old hydrological observatory in the Alptal (central Switzerland).Looking back to the history of these old pillars of forest-hydrological research, it becomes obvious that they served much more than for satisfying pure academic curiosity. The legacy of correspondence from the archive tells us that the initiative and planning of new forest-hydrological observatories was closely related with political and forest-promoting interests. In the same way, the results of these studies have been interpreted and communicated in a way that sometimes exceeded pure scientific conclusions. In fact, all these data from more nearly 120 years illustrate a very differentiated (ambiguous) relationship between the presence (and shape) of the forest and the runoff response to heavy rainfall. In contrast, the public doctrine (disseminated by both researchers and stakeholders) was often simplified and biased towards a too positive effect of forests in mitigating floods. On the other hand, the complex (differentiated) insights from numerous specific analyses of students and researchers have often been disremembered.We state that this conclusion doesn't apply only to the present case of forest hydrology, but pose a challenge for other hydrological (or: environmental) topics as well. [ABSTRACT FROM AUTHOR]

The West African Sahel experienced a long drought from the 1970s to the 1990s during which runoff has paradoxically increased, as a response to human and climate-induced changes in surface conditions. Despite the vegetation recovery (re-greening) observed at regional scale over the past 30 years, surface runoff is still increasing, suggesting that the Sahelian eco-hydrological system passed a tipping point and is now trapped in a « high runoff » state. To study this hypothesis, we developed a system dynamics model incorporating vegetation-hydrology interactions at annual time scale. The model successfully reproduced the vegetation collapse and the increase of runoff-prone bare soil areas monitored over 65 years on a pilot site in Northern Mali. Our results confirmed the existence of a tipping point between alternative high/low runoff states at the small catchment scale. According to the model, a reverse shift to the pre-drought low runoff state is possible, but the conditions in which this shift would occur remain uncertain. The system trajectory presents a strong sensitivity to annual rainfall variability (amplitude and temporal structure). This study suggests that the increasing runoff in a re-greening environment is caused by the tipping of some areas to a high runoff/low vegetation state, illustrating how a regime shift in sub-systems can result in eco-hydrological changes at larger scale. The associated large-scale changes of the rainfall partitioning may alter evapo-transpiration and thus the surface-atmosphere feed-back. Those changes also bear strong environmental and socio-economic consequences, either adverse (increase of degraded areas to the detriment of agriculture, and increased flood risk) or beneficial (increased water resource in ponds and water tables). [ABSTRACT FROM AUTHOR]