Zonal aspects of the influence of forest cover change on runoff in northern river basins of Central Siberia

Background Assessment of the reasons for the ambiguous influence of forests on the structure of the water balance is the subject of heated debate among forest hydrologists. Influencing the components of total evaporation, forest vegetation makes a significant contribution to the process of runoff formation, but this process has specific features in different geographical zones. The issues of the influence of forest vegetation on river runoff in the zonal aspect have not been sufficiently studied. Results Based on the analysis of the dependence of river runoff on forest cover, using the example of nine catchments located in the forest-tundra, northern and middle taiga of Northern Eurasia, it is shown that the share of forest cover in the total catchment area (percentage of forest cover, FCP) has different effects on runoff formation. Numerical experiments with the developed empirical models have shown that an increase in forest cover in the catchment area in northern latitudes contributes to an increase in runoff, while in the southern direction (in the middle taiga) extensive woody cover of catchments “works” to reduce runoff. The effectiveness of geographical zonality in regards to the influence of forests on runoff is more pronounced in the forest-tundra zone than in the zones of northern and middle taiga. Conclusion The study of this problem allowed us to analyze various aspects of the hydrological role of forests, and to show that forest ecosystems, depending on environmental conditions and the spatial distribution of forest cover, can transform water regimes in different ways. Despite the fact that the process of river runoff formation is controlled by many factors, such as temperature conditions, precipitation regime, geomorphology and the presence of permafrost, the models obtained allow us to reveal general trends in the dependence of the annual river runoff on the percentage of forest cover, at the level of catchments. The results obtained are consistent with the concept of geographic determinism, which explains the contradictions that exist in assessing the hydrological role of forests in various geographical and climatic conditions. The results of the study may serve as the basis for regulation of the forest cover of northern Eurasian river basins in order to obtain the desired hydrological effect depending on environmental and economic conditions.

Assessment of runoff characteristics changes of the left bank tributaries of the Upper Dniester under warming conditions

The inflow of water from the left-bank tributaries of the Upper Dniester is an important component of the runoff formation for the whole river. The study of changes in their water content in the context of global warming is quite urgent and agrees with the provisions of the strategic programme for the Dniester Basin for 2021-2035 aimed at ‘climate change mitigation’ and ‘promoting the principles of sustainable water management’. The paper's objective is to carry out a research into the changes in climatic factors and runoff characteristics that occurred on the left bank of the Upper Dniester in the early 21st century. The research was performed on the basis of hydrometeorological data for the period of 1945-2018. The main research methods include the regression analysis and the method of differential integral curves. It is found that, within the area under study, there is an increase in average annual air temperatures against the background of constant or insignificantly increasing amounts of annual precipitation, thus creating unsatisfactory conditions for runoff formation. It is established that, during the months of cold period, there is an increase in air temperature and that since 1989 the frequency of cases, when the temperature crosses the range of positive values, keeps increasing. 1989 is a turning point in the chronological course of average annual air temperatures; a positive phase of long-term fluctuations starts since that year. With regard to annual precipitation fluctuations from the mid-60s of the last century up to 2013, a positive phase was also identified; within this phase there was a short period of insufficient moisture (1981-1996). It was found that the response of runoff characteristics to climate change was not the same. Maximum runoff during the spring flood is the most sensitive to global warming, since such warming has worsened the conditions for accumulation of water reserves in the snow cover. The transition of maximum runoff fluctuations to the low-water phase took place in 1981. The average long-term decrease in the maximum water flow rates during the spring flood for the period of 1950-2018 amounts to -16.9%. Unlike maximum runoff, the characteristics of the annual and minimum runoff changed gradually during the winter and summer low-water periods and the final transition to the low-water phase of fluctuations occurred only in 2009-2011. Certain inertia of changes in the water resources of Podillya rivers under the warming conditions is caused by the subsurface component making a high contribution to the annual runoff formation (60%). Significant portion of the groundwater supply ‘mitigated’ the effects of surface runoff loss during spring floods. The research allowed establishing the occurrence of statistically significant negative trends in the annual and minimum runoff fluctuations since 1998. If continue to preserve, the identified climate change trends will decrease the inflow of water from the Podillya rivers to the main Dniester River.

A METHOD FOR SHORT-RANGE FORECASTING OF WATER DISCHARGE FOR THE KAMA RIVER BASIN BASED ON THE HBV MODEL

The experience of constructing a method for short-range forecasting of water discharge in the Kama River basin is described. The forecast method is based on the HBV-96 conceptual model of runoff formation in a watershed with optimized parameters, as well as on the algorithm for the correction of operational forecasts. It is shown that if the runoff formation model parameters are optimized and the forecast correction algorithm is applied, the model simulates variations in water discharge at gaging stations with high efficiency and can be used for operational short-range hydrological forecasting and for the evaluation of the hazard of expected hydrological conditions on the rivers. The implementation of the forecasting method allows obtaining water discharge forecasts for gaging stations in the Kama River basin with a lead time up to 3 days using meteorological forecasts with a corresponding lead time.

EXPERIMENTAL AND THEORETICAL DETERMINATION OF THE DURATION OF SURFACE RUNOFF FORMATION

There are theoretical and experimental results of the study of the duration of surface runoff formation presented. The prospects of application of this method are defined at designing of sanitary technical measures on highways of public use and artificial constructions. There is prospective direction of increase of efficiency of sewerage rain network designing and local treatment facilities on highways defined. The issue of increasing the accuracy of hydraulic calculation has been studied. The main advantages of implementation of modern drainage solutions are investigated. It is proposed to use a monogram to determine the duration of the formation of surface concentration, in accordance with climatic characteristics.

Influence of autumn soil moisture on erosion-accumulation processes in the Krasnoyarsk forest-steppe

The results of field observations of erosion-accumulation processes caused by meltwater in the northern zone of the Krasnoyarsk forest-steppe in 2009-2017 are discussed. Hydrometeorological conditions of melt runoff formation during the study period and their influence on the values of washout and accumulation are analyzed. Data on snow cover, soil moisture in the pre-winter period, meltwater flow to the soil surface, and spring and autumn runoff are compared. Attention is focused on the autumn soil moisture. The correlation between the values of washout and preceding autumn moisture is revealed. Other things being equal, among all hydrometeorological factors on the territory of the Krasnoyarsk forest-steppe, pre-winter soil moisture has a significant impact on the intensity of erosion-accumulation processes. Keywords: soil washout, erosion, autumn moisture, hydrometeorological factors, runoff, snow cover, humidity, freezing, water permeability

Hydrological analysis of runoff generation in a forested mountain catchment using a nested multi-tracer approach

<p>Rising concentrations of dissolved organic carbon (DOC) in inland waters are observed and investigated intensely in the last decades. The development of adaptive measures requires the forecasting of DOC-exports from catchments. Since DOC is exported from river catchments along hydrological pathways it is evident that the investigation of runoff generation, retention and travel times along flow paths are important to quantify DOC-loads and to develop a forecast model.</p><p>To gain comprehensive insights in runoff formation and DOC export in a small forested catchment in the Bavarian Forest National Park we apply a nested multi-tracer approach, combining experimental and analytical methods with the aim to develop a hydrological forecast model which is able to reproduce the dominant mobilization- and export processes of DOC in forested mountain catchments. The use of multiple tracers combines different approaches to determine source areas, flow paths and retention times of runoff water in catchments. Stable isotopes (d2H, d18O) are suitable as natural tracers to estimate contributions from precipitation to stream discharge. With the additional use of geochemical tracers (e.g. DOC, SiO2) contributions from groundwater and the organic and mineral soil horizons can be estimated. Combined with a nested approach these analyses can be conducted on different spatial scales, enabling the development of scalable prognostic models of runoff formation in catchments.</p><p>To complement the limited information from historic data sets we instrumented two hill transects to observe lateral contributions from hill slopes and to investigate potential preferential flow paths. Water samples from stream-, soil-, ground- and precipitation water were collected during two flood events and analysed for stable isotopes and chemical compositions. To support the nested approach, the sampling sites were chosen at strategical sites within the catchment, including the instrumented hill transects and the stream network from the creek to the catchment outlet.</p><p>Preliminary results of stable isotope analysis show, that after dry periods nearly no event water seems to contribute to runoff formation, whereas after wet periods the proportions can be up to 40 %. A strongly delayed reaction of the groundwater was observed which suggests that deep groundwater is not contributing to stream flow, but a possible mobilization of pre-event water in the riparian zone was observed as a response to precipitation events.</p><p>A likely major source of DOC is in the organic soil horizons due to storage and degradation of organic material. This is supported by higher DOC-concentrations in the soil water from these horizons. In how far residence times, precipitation intensities and flow paths activation from different source areas influence concentration peaks of DOC in the stream will be analysed in the next steps.</p><p>The results of the recent field campaign help to identify the dominant processes of runoff generation and DOC mobilization on different temporal and spatial scales and for different antecedent system states. The data and insights gained from the field campaign will be used to develop and calibrate process models for hypothesis testing and further analyses to eventually develop a forecast model for DOC mobilization.</p>

Modeling impact of GLOF on the the Baksan River runoff (the Central Caucasus, Russia)

<p>The ongoing intensive deglaciation in high mountain areas is resulting in great instability of mountainous headwater regions, which could significantly extreme hydrological events In this research a model “chain” of hydrodynamic and runoff formation models is adopted to simulate a glacier lake outburst flood (GLOF) from Bashkara Lake, situated in headwater region of the Baksan River and its effect on the downstream.</p><p>Two-dimensional hydrodynamic model for the Adylsu River valley was developed, based on the STREAM_2D software (author V. Belikov). The ECOMAG runoff formation model (author Yu. Motovilov) for the entire Baksan River basin was adopted. The output flood hydrograph from the STREAM_2D model was set as additional input into the Baksan River runoff formation model in the upper reaches of the Adylsu River below Bashkara and Lapa Lakes.</p><p>Based on field surveys and remote sensing data, actual Bashkara Lake GLOF on September 1, 2017 was modelled. The GLOF event was triggered by extreme precipitation that caused overwetting of the dam and increase in the lake water level. The peak GLOF discharge according to modeling was estimated as 710 m<sup>3</sup>/s at the dambreak section and 320 m<sup>3</sup>/s at the Adylsu River mouth 40 minutes after the outburst. Two possible mechanisms for re-outburst of Bashkara Lake were taken into account: the rock avalanche impact, forming displacement waves, and the lake outburst due to increase in the water level, accompanied by expansion of the existing dam break. Under the rock avalanche scenario, there was no significant model response. Based on the results of modeling of the second re-outburst scenario, the maximum discharge of the outflow was estimated as 298 m<sup>3</sup>/s at the dambreak section and 101 m<sup>3</sup>/s in the Adylsu River mouth.</p><p>As a result of model chain application contribution of GLOFs and precipitation to an increase in peak discharge along the Baksan River was estimated. The actual outburst flood amounted to 45% and the precipitation - to 30% of the peak flow in the Baksan River at the mouth of the Adylsu river (10 km from the outburst site). In Tyrnyauz (40 km from the outburst site) the components of the outburst flood and precipitation were equalized, and in Zayukovo (70 km from the outburst site) the outburst flood contributed only about 20% to the peak flow, whereas precipitation - 44%.</p><p>Similar calculations were made for a potential re-outburst flood, taking into account expected climate changes with an increase in air temperatures by 2°С and an increase in precipitation by 10% in winter and decrease by 10% in summer. The maximum discharge of the re-outburst flood in the Adylsu river mouth according to modeling can be approximately 3 times less than discharge of the actual outburst on September 1, 2017 and can contribute up to 18% to peak discharge in the Baksan River at the confluence with the Adylsu river.</p><p>The Baksan River runoff formation model was developed under support of RFBR, project number 20-35-70024. The glaciation changes and climate impact scenarios analysis was funded by RFBR and the Royal Society of London (RS), project number 21-55-10003.</p>

Processes of runoff formation at the Putorana Plateau (Central Siberia, Russia)

<p>The Putorana Plateau is located in the North-West of the Central Siberian Plateau in the Krasnoyarsk Territory in permafrost zone. Some mountain peaks reach a height of 1400 - 1700 m. The plateau is composed of stepped canyons formed by the outpouring of a huge mass of red-hot basalts. The Putorana Plateau is the territory that is still unexplored in hydrological terms. Climate change contributes to an increase in the thickness of seasonal thawing, therefore, the conditions of runoff formation change.</p><p>The purpose of the work is to study the factors of runoff formation, including the research of geocryological conditions based on short-term expedition data of the State Hydrological Institute (St. Petersburg, Russia) collected in small catchments in 1988-1990.</p><p>The object of study is the catchment of the stream Dupkun (an area of 2.75 sq. km), which is located in the basin of the Kureyka river basin, the right tributary of Yenisei River in the southwestern part of the Putorana Plateau. The maximum height of the catchment is 1228 m, and the hydrological gauge is located at an altitude of 923 m. The average slope of the catchment area is 12°. The landscape is a moss-grass mountain tundra, and perennial snowfields are also formed.</p><p>The expedition studies in the period from July 19 to September 4, 1990 included the collection of hydrometeorological information, the determination of soil characteristics (moisture content, temperature, structure at different depths and landscapes), and the study of snow cover. The route studies were conducted to determine the characteristics of landscapes, vegetation and soils in the basins of the rivers Kureyka and Khantayka.</p><p>The data of the expedition studies were processed, digitized and systematized.  Based on the collected material, the water balance of the stream Dupkun was calculated. The presence of perennial snowfields has a significant impact on the formation of runoff. At the beginning of observations, the area of snowfields was 15 %, the average snow height was 2.6 m, and the average density was 0.7 g / cm<sup>3</sup>. At the end of observations, snowfields occupied 8% of the catchment area. For 20 days, the snowmelt depth was 38 mm, the precipitation reached 140 mm, and the runoff was 167 mm. The runoff coefficient is 0.89. During the entire observation period, the runoff reached 355 mm.</p><p>These observations are considerable value due to the lack of knowledge of the geocryological, landscape and hydrological conditions of the Putorana Plateau. Since there are practically no hydrological stations in this region that study the processes of flow formation, the collected data become even more unique. Extremely scarce data allowed to conduct the assessment and verification of the parameters of the hydrological model "Hydrograph". The developed set of the model parameters was used to simulate the river flow of tundra landscapes of the Putorana Plateau and assess its contribution to the formation of the water balance of the territory in the current climate.</p><p>The study was supported by the RFBR (project No. 19-55-80028).</p>