Temporal and Spatial Changes of Runoff Regime in the Yellow River Basin from 1956 to 2017

The Yellow River is one of the major rivers with severe runoff declines in China, but there are significant differences in runoff changes in the upper and lower reaches of the basin and among different tributaries. However, the characteristic of runoff change and its spatial heterogeneity are not well understood in the whole basin. In this paper, 48 hydrological stations located in the mainstream and major tributaries were selected, and the meteorological and runoff data from 1956 to 2017 were collected. The multi-year and intra-year changes in runoff were analyzed, and then the attribution of climate change and human activity to runoff change was quantified by the climate elasticity coefficients. The results showed that: (1) in the past 60 years, the runoff of the Yellow River showed a serious decrease trend of −8.25 mm/10a. Moreover, most tributaries decreased significantly in runoff with a rate of −1.42 mm/10a to −28.99 mm/10a; (2) for the whole basin, the contribution of climate change and human activity to runoff changes was 13% and 87%, respectively. Moreover, the contribution of the two factors varied considerably in different tributaries. Finally, focusing on different runoff regime and socioeconomic characteristics, this study provided corresponding water resources adaptive management suggestions.

Runoff regime after heavy rainfall events in view of changing climate in a beech stand at the LTER-CWN site “Klausenleopoldsdorf”

<p>The Austrian Research Infrastructure LTER-CWN (Long-Term Ecosystem Research Infrastructure for Carbon, Water and Nitrogen) aims for measuring extreme events in high temporal resolution. Within the framework of this project a measuring weir was installed near Klausen-Leopoldsdorf (Lower Austria) in order to collect high-resolution data of stream-water quantity and quality. The measuring weir is located in the western part of the „Wienerwald“, the north-eastern edge of the Alps, at about 475m a.s.l. Especially in the year 2020 this area showed humid weather conditions with an annual precipitation of 904mm. The observed catchment has an area of about 46 hectares. The dominating soil types in the catchment are Planosoils and Stagnosols. The observations at the weir with a time resolution of 5 minutes started in February 2019. The plot was set up for recordings of carbon (C), nitrogen (N) and water fluxes theparameters TOC-N, DOC-N, NO<sub>3,</sub> water level, water temperature, electrical conductivity, turbidity and organic matter values being measured. To answer one of the main research issues - the impact of heavy rainfall events on the runoff regime of a catchment within a dense beech forest in relation to the soil, specific time, the influence of interception and corresponding water level in the observed river - a water level sensor (OTT) and a multifunction spectrolyzer (S:CAN) were installed at the weir. During the measuring period 2019/2020 11 heavy rainfall events (corresponding to more than 20mm daily precipitation sum) were recorded. Due to the small catchment area the average time interval between heavy rainfall events and the corresponding increase of the water level at the measuring weir is about 2 hours. The time and intensity of the rainfall event together with the level of soil moisture before the precipitation event are the key factors for the amount of runoff. Additionally, other measured parameters like the turbidity or the electrical conductivity of the water correspond very well with the amount of runoff. Data with such a high time resolution will help to get a better understanding of extreme events and the consequences of these events in respect to climate change.</p>

MONITORING OF FLOODING EVENTS ON THE RIVERS OF THE NORTH-WESTERN CAUCASUS IN 2014–2020

The rivers of the Northwestern Caucasus between Anapa and Tuapse are characterized by a flood runoff regime. Floods leading to material damage occur on the rivers of the region every year. After the 2012 flood, a network of automatic level gauges is developing in the region, recording water levels every 10 minutes. During the expeditions of the SSC RAS in the winter period 2019-2020 the analysis of the installation sites of 69 level gauges was carried out. The features of level gauges influencing the collection of observations are identified: the installation options, floodplain vegetation, the features of the channel, floodplain and river valley. The analysis of the accumulated for 2014–2020 database of level observations on rivers with catchment areas from 1.4 km2 to 1245 km2 was carried out. 34 778 periods of growth, 70 135 periods of stagnation, 39 485 periods of decline and 42 032 gaps were identified. Of these, 3398 flood situations were considered. More than 100 flood events were recorded on 11 level gauges: on the rivers Abin, Shebsh, Psekups, Pshish (3 gauges), Nechepsukho, Dzhubga, Vulan, Pshada and Mezyb. Less than 10 flood situations were recorded on 14 level gauges. To increase the efficiency of the network of automatic level gauges, it is necessary to expand it to the watersheds between rivers Abin and Khable, as well as on Pshish on the northern macroslope and on the watersheds of Pshada, Nechepsukho and Tuapse on the southern macroslope.

Which Aspects of Hydrological Regime in Mid-Latitude Montane Basins Are Affected by Climate Change?

This study analyzed the long-term alterations in runoff regime, seasonality and variability in headwater montane basins in Central Europe in response to the manifestations of climate change. We tested the common hypotheses on climate change effects on surface runoff dynamics in the Central Europe region, assuming that (i) recent climate warming will result in shifts in the seasonality of runoff, (ii) the runoff balance will remain without significant changes and (iii) that higher variability in runoff can be expected. The analyses were done on eight montane catchments in four mid-latitude mountain ranges in Central Europe, based on the uninterrupted time series of daily discharge observations from 1952 to 2018. We used 33 indicators of hydrologic alteration (IHA), 34 indicators of environmental flow components, the baseflow index, the calculation of surplus and deficit volumes and the frequency of peak and low flows. Homogeneity testing using Buishand, Pettitt and SNHT tests was applied to test the response of the hydrological alteration indicators to climate warming. We have proved the significant shifts in runoff seasonality, coinciding with the timing of the air temperature rise, marked by earlier snowmelt, followed by a decline in spring flows and a prolonged period of low flows. There was detected a rise in the baseflow index across the mountain ranges. Unlike the common hypotheses, the expected rise of runoff variability and frequency of peak flows was not demonstrated. However, we have identified a significant change of the flood hydrographs, tending to steeper shape with shorter recessing limbs as a sign of rising inner dynamics of flood events in montane catchments.

Automatic hydrograph separation approach provides possibility to look at less-studied characteristics of water regime

<p>The study presents an approach to automatic river hydrograph separation and analysis implemented in GrWat open source package for R programming language. In the proposed scheme of hydrograph separation, river hydrograph is separated into base and quick flow. For plain rivers quick flow is further separated into seasonal snowmelt flood quick flow; rain quick flow and thaw quick flow. For mountainous rivers seasonal snowmelt flood quick flow component is divided into “basic snowmelt flood” component and overlapping rain floods. Base and quick runoff is separated by a critical gradient. Flash-floods are separated from the seasonal snowmelt wave by critical values of air temperature and precipitation on the event for the plain rivers and using a critical gradient concept for mountainous rivers. More than 30 characteristics of river runoff regime are calculated for each water resource year: characteristics of annual and seasonal runoff, contribution of each genetic component, characteristics of maximum runoff, n-day minimum discharges and dates when they are observed. Additionally, more than 50 characteristics of each flash-flood are calculated:  characteristics of shape, volume, timing of flash-floods, the values of meteorological parameters that bring about different types of floods. The presented approach to automatic river hydrograph separation and analysis was tested on 45 plain rivers in the European part of Russia in different climatic zones and on 10 mountainous rivers in the North Caucasus. The result of application provides a possibility for analyzing previously unstudied characteristics of river runoff regime and its climate-related transformation on the European part of Russia.</p><p>The study was supported by the Russian Science Foundation grant No. 19-77-10032</p>

CHANGES OF FLOOD-FORMING FEATURES OF NORTH-WEST CAUCASUS BASINS IN 2000–2014

The rivers of the North West Caucasus are characterized by a flood runoff regime. Floods leading to material damage occur on the rivers of the region every year. However, in the XXI century. the development of both automatic monitoring systems and the capabilities of mathematical modeling of hydrological processes make it possible to fill this gap. After the flood of 2012, a network of automatic level gauges is developing in the region, recording water levels every 10 minutes. Analysis of the accumulated archive of observations allows us to determine the features of the formation and passage of floods on the rivers of the region. Among the features of catchments that determine the transformation of precipitation into runoff in the catchment, vegetation and soil cover are distinguished. When it rains in a multi-layer forest, each layer is sequentially moistened from top to bottom. After the vegetation has been moistened, the precipitated water reaches the soil surface and some of it infiltrates into deeper soil horizons up to the waterproof layers, where it replenishes the groundwater lenses. The vegetation cover of the North West Caucasus in the foothill part is represented mainly by plowed steppes with ravine forests, and in the mountainous part – almost entirely by forests. From west to east of the region, mainly oak forests give way to beech and hornbeam forests with a higher water retention. High forest cover and widespread in the region gray, brown forest and soddy-calcareous soils have a water-holding capacity sufficient to absorb intense precipitation. A large proportion of anthropogenic landscapes in the western part of the region causes a lower capacity for absorbing precipitation and, accordingly, a greater flood hazard. Further decrease in forest cover and development of the region will increase the flood hazard.

Temporal variation of water discharges in the lower course of the Danube River across the area from Reni to Izmail under the influence of natural and anthropogenic factors

The Danube River mouth is a main source for fresh water used for water supply and irrigation purposes in the arid southern part of Ukraine. In addition, the water of the mouth is used for filling the Danube Lakes. Climate change and numerous social and economic factors reduce the area and the water level of the Danube Lakes and increase their salinity. Under these circumstances the water exchange between the Danube River and the lakes is a very important process which allows maintenance of the water–salt balance of the latter. Such water exchange massively depends on the Danube River regime. The paper presents research of temporal variation of typical water discharges in the lower course of the Danube River associated with assessment of natural and anthropogenic factors affecting river runoff fluctuations. Time series of annual average water discharge at the water gauging stations of such cities as Reni and Izmail were considered uniform for the selected periods. They cover the period of conventionally natural river runoff regime (1840–1920), the period of the least altered river runoff regime (1921–1960) and the most altered river runoff regime under a heavy anthropogenic influence (1961–1989). The analysis of the impact of climate changes on the river runoff allowed introduction of a new, modern period of the Danube River hydrological regime (1990–2015). It is established that the series of maximum and minimum water discharges are characterized by the lack of their uniformity from the beginning of the period of initial influence of hydraulic engineering structures on the hydrological regime (since 1961). The analysis of the aggregate multi-year series (1840–2015) of average annual, maximum and minimum water discharges of the Danube River across its length from Reni to Izmail showed the presence of positive trends. At the same time the period of climatic changes (after 1989) is characterized by a less intensive growth of maximum water discharges. The research resulted in establishing the periodicity and synchronism of water discharge fluctuations in the lower course of the Danube River from Reni to Izmail within the territory of Ukraine. In particular, the study showed for the first time that from the 2000s onwards the redistribution of river runoff between the Danube River branches led to unsynchronization of minimum river runoff in different parts of the Danube Delta. The research shows that comprehensive analysis of the Danube River hydrological regime under modern climatic conditions is required to ensure efficient engineering regulation of the Danube Lakes filling pattern. Such regulation, in its turn, will ensure that the water supply and irrigation purposes are achieved.