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The National Institute of Hydrology (NIH) is a premier research Institute in India in the area of hydrology and water resources. The Institute was established in 1978 with the main objective of undertaking, aiding, promoting and coordinating systematic and scientific work in all aspects of hydrology. The Institute has its headquarters at Roorkee (Uttarakhand), four regional centres at Belagavi, Jammu, Kakinada and Bhopal and two centres for flood management studies at Guwahati and Patna. The Institute is well equipped to carry out computer, laboratory and field oriented studies. This article presents an overview of research activities being undertaken by the Institute.

The use of electrical resistivity tomography (ERT; non-intrusive geophysical technique) was assessed to identify the hydrogeological conditions at a surface water/groundwater test site in the southern Black Forest, Germany. A total of 111 ERT transects were measured, which adopted electrode spacings from 0·5 to 5 m as well as using either Wenner or dipole-dipole electrode arrays. The resulting two-dimensional (2D) electrical resistivity distributions are related to the structure and water content of the subsurface. The images were interpreted with respect to previous classical hillslope hydrological investigations within the same research basin using both tracer methods and groundwater level observations. A raster-grid survey provided a quasi 3D resistivity pattern of the floodplain. Strong structural heterogeneity of the subsurface could be demonstrated, and (non)connectivities between surface and subsurface bodies were mapped. Through the spatial identification of likely flow pathways and source areas of runoff, the deep groundwater within the steeper valley slope seems to be much more connected to runoff generation processes within the valley floodplain than commonly credited in such environmental circumstances. Further, there appears to be no direct link between subsurface water-bodies adjacent to the stream channel. Deep groundwater sources are also able to contribute towards streamflow from exfiltration at the edge of the floodplain as well as through the saturated areas overlying the floodplain itself. Such exfiltrated water then moves towards the stream as channelized surface flow. These findings support previous tracer investigations which showed that groundwater largely dominates the storm hydrograph of the stream, but the source areas of this component were unclear without geophysical measurements. The work highlighted the importance of using information from previous, complementary hydrochemical and hydrometric research campaigns to better interpret the ERT measurements. On the other hand, the ERT can provide a better spatial understanding of existing hydrochemical and hydrometric data. Copyright © 2009 John Wiley & Sons, Ltd.

is the combined outcome of both the 2001 EGS ses-sion on the ‘Water balance of mountain basins’ in Nice and the 2002 Annual Meeting of the Ger-man Hydrological Working Group in Berlin. A variety of hydrological and meteorological research top-ics presented at these meetings focused on water resources, modelling, parameterization, remote sens-ing, snow and ice melt, evaporation and transpiration. Case studies concentrated especially on moun-tain areas, which ranged in altitude from 1500 to 6000 m, and on a variety of regions, such asthe Himalayas, Karakorum, Hindukush, Yemen Mountain Massif, Norwegian Scandes and Alps. Theanalysis of water balance components was conducted at many different temporal and spatial scales,ranging from annual and seasonal to daily and hourly, and from entire basins to single lysimetricanalyses.The current discussion about climatic–hydrologic change and bio-adaptability reflects the necessityfor more comprehensive and more detailed knowledge about water balance in mountain basins. Sincemountain watersheds are source areas for large rivers, an understanding of the water balance is invalu-able. This should include a variety of components, such as water input, water storage, water loss, andwater transfer potentials on steep, heterogeneous surfaces. Many of the associated processes are insuf-ficiently investigated, although they are of major significance for both hydrologists and water resourcemanagers.AIMIt is the aim of this special issue to present a diverse range of topics and research areas in order to revealthe wide spectrum of research topics and methods on water balance studies in mountain basins. These stud-ies demonstrate both the success and the emerging difficulties and shortcomings in different research fields.Examples from the developed and from the developing world not only promote comparisons, but also newideas about the availability and transferability of methods.

This paper reports some of the achievements in hydrological research associated with the Mackenzie GEWEX Study (MAGS). MAGS is a multifaceted study of the energy and water cycle in the Mackenzie River Basin, north-western Canada, and emphasizes cold-region processes and modelling. It pursues methodologies of scaling-up process studies to a large river basin that has few measurement sites. This methodology involves new developments, adapting mid-latitude algorithms to the high latitude setting and promoting the use of remote sensing tools for scaling-up. Intensive hydrological process studies have been concentrated at a number of sites. These have been chosen to represent different biophysical facets of the Mackenzie River Basin. They include northern basins in open tundra and at tree-line, a western basin representing the hilly and mountainous western side of the basin, central basin sites of wetlands, Precambrian Shield terrain and the Mackenzie Basin counterpart of the large Laurentian Great Lakes. The most southerly site represents a mixed-wood boreal forest site that is undergoing land-use change as a result of forest harvesting. Regional community model analysis indicates that processes of lee cyclogenesis result in differential input of precipitation across the basin. Significant results centre on models of snow accumulation featuring the role of blowing snow, intercepted snow and snowmelt. These models emphasize the importance of snow patchiness to speed of melt. Snowmelt infiltration into frozen soils is important in controlling runoff, and tundra microlandforms play a unique role in determining the surface runoff characteristics. On the western margins of the basin, there is a very significant difference in the hydrological behaviour of slopes of different aspects as a result of differences in permafrost and vegetation. In the central basin, a deeper understanding of components of the water budget is being gained through stable isotope analysis of wetland waters.

The Niger Delta at the southernmost part of Nigeria, is a large and ecologically sensitive region. Natural and anthropogenic activities, such as subsidence, sea-level rise, upstream dams and reservoirs, dredging and overabstraction of groundwater, disturb this equilibrium, leading to adjustments in ecological conditions and boundaries. Ecological changes affecting large areas cannot be explained by local observation. Relating observed effects to causes of environmental behaviour therefore requires a regionalized data collection, that provides a broader perspective of the issues involved with resultant ease in the identification of common problems and formulation of effective solutions.

Observations in the Nabebuta and Kaioku districts show that the deep artesian water is not contaminated by the sea water, despite of the saline damage in the ground surface owing to the saline water percolation through the earth dam. The artesian heads in those areas are measured to be always lower than the sea level. These results suggest the existence of the impermeable layer between the aquifer and the sea water. Tidal changes in the water levels of four artesian wells are observed and are found the results corresponding with theoretically presented by the author under the assumption that the tidal effect on the artesian head is only due to the fluctuations in the loading weight upon the submarine aquifer. Some of the values which represent the characteristics of the aquifer are computed in using the analytical treatment from the observed results.

Summary The Cathedral Peak Forest Influences Research Station was established in Natal in 1938 to determine the effect of exotic conifer plantations on water supplies. The major investigation is the planting to within one chain of any streams, of six catchments with Pinus patula at eight year intervals. One catchment is to be protected from burning and grazing for an indefinite period to determine the trend of plant succession and the effect on water supplies of the various stages of the succession. An eighth catchment is to be grazed intermittently for five years followed by complete aforestation. The ninth catchment is to be completely planted to Pinus patula and no margins will be left open next to streams. Discharge from all experimental catchments is continuously recorded by means of clockwork instruments. The situation, topography, geology, soils, vegetation and climate of the area are described and mention is made of the nature of the streams and the more important characteristics of the catchments...

Intercepted rainfall may be evaporated during or after the rain event. Intercepted rain is generally determined as the difference between rainfall measurements outside and inside the forest. Such measurements are often used to discriminate between water storage and evaporation during rain as well. Two well-accepted methods underestimate water storage by a factor two as compared to direct observations. The underestimation of storage is compensated by an overestimation of evaporation during rain by a factor of three. The direct observations of water storage and evaporation appear to agree with previous direct observations. Thus, it is concluded that these observations are representative Also, our results based on methods using only rainfall measurements inside and outside the forest appear to agree with previous results, This would result in the conclusion that the common methods systematically underestimate water storage and overestimate evaporation during rain. Indeed, the systematic errors can be explained by the neglect of drainage before saturation. Water storage is better simulated assuming an exponential saturation of a larger storage capacity. A smaller evaporation can be simulated using an appropriate resistance to vapour transport. The observations in dense coniferous forest showed water storage to be the dominant process in rainfall interception, but this conclusion should not be generalized to other forests and climates. Direct observations of water storage and evaporation are recommended to build a realistic set of parameters for rainfall interception studies of the main vegetation types. (C) 1998 Elsevier Science B.V. All rights reserved.

This paper reports the effects of altering an aspen forest cover in Utah on evapotranspiration losses, overland flow, erosion, and mantle storage deficits during three successive growing seasons. These data, together with supplemental measures of winter precipitation and estimates of evaporation from snow, provided a basis for estimating amounts of water available for stream flow. Removal of aspen trees, leaving the herbaceous understory and litter undisturbed, reduced evapotranspiration losses and increased the amount of water available for stream flow by about four inches without seriously increasing overland flow or soil erosion during summer rains. Removal of the remaining herbaceous cover further reduced evapotranspiration losses and increased the amount of water available to streams by an additional four inches but resulted in an undesirable increase in summer rainfall runoff and soil loss.

High-elevation grasslands worldwide provide essential hydrological services including water provision, flow regulation, and erosion control. Despite their importance, hydrological research of grasslands in montane regions is usually scarce and disperse, limiting the capacity to improve water resource management. We present a systematic literature review of the hydrological function of high Andean grasslands under conserved, degraded, and restored conditions in ecosystems situated above the tree line in the tropical Andes (paramos, punas, and jalcas). Most hydrological research on these grasslands has been developed in paramos (92%), especially in Ecuador, while research in punas is scarce (6%) despite being the largest grassland extent in the region. For paramos, published literature highlights the importance of conserving grasslands to facilitate water infiltration to soils, which in turn reduces erosive processes. Water-vegetation relations for conserved paramos are well understood, indicating that about 50% of water inputs return to the atmosphere via evapotranspiration, but knowledge about hydrological functions of conserved punas and jalcas is virtually non-existent. Under changing land use, afforestation of grassland ecosystems with exotic tree species, especially pines, reduces soil water storage as well as water yield and flow regulation capacity. Impacts of grazing and agriculture on the hydrological function of paramo grasslands strongly depend on historical land management and current land use practices and are not generalizable. Short-term restoration studies indicate that more than two years are necessary to recover the hydrological function of degraded grasslands, therefore medium and long-term studies are required to determine efficient restoration periods. These knowledge gaps limit the ability to extrapolate and regionalize findings. Future directions aimed to fill them are proposed, and methods successfully used to investigate the hydrology of high Andean grasslands are highlighted. This research not only enlightens what is known about the hydrology of high Andean grasslands, but also seeks to guide future hydrological evaluations to fill identified geographical and topical knowledge gaps precluding improved management of water resources in the tropical Andes.

Looking at natural hazards and risks, such as landslides, debris flow and floods, we see a broad range of science fields involved: from earth science to engineering science, to management, policy and social science. Although research on natural hazards and risks contains mono-disciplinary aspects, it is best characterized by its multi-disciplinary nature. Moreover, the link to society has always been a very important driver in natural hazards research, pushed by the need for reliable prediction models to be implemented into early warning systems as well as by the required complete understanding of the physical mechanisms for the design of mitigation works. If we focus on landslide and debris flows as in this special issue of HESS, we will see impressive progress in the geotechnical and slope stability modelling needed for hazard analysis. Similarly, the risk management aspects, such as hazard mapping and risk assessment, have received much attention. Hydrology is an important aspect of landslide and debris flow assessment. Precipitation and snowmelt water infiltration, leading to local pore water pressure increase and/or matric suction decrease, is amongst the most common triggers of landslides. Fundamental knowledge about underlying processes affecting this infiltration process, such as macropore and fissure flow, water repellency, soil structure, soil–plant–atmosphere interactions as well as the effects of land use practices (e.g. deforestation, terracing, grazing), has strongly improved in the last decade or so, with a clear focus on more detailed knowledge of hydrological process dynamics. The hydrological process understanding progressed rapidly under pressure of societal needs such as prediction of discharge generation and contaminant transport, to name a few. Although hydrology research is very strongly linked to natural hazards, such as landslides and debris flows, this improved hydrological knowledge has found its way into the landslide community rather modestly. In particular, the incorporation of hydrological processes into large-scale models is still incomplete and their application to landslide prediction limited. Landslide research tends to be more focused on novel methods to include spatial data and on the practical applicability of, e.g. landslide triggering modelling and statistical analyses for regional hazard and risk assessment. However, without stating that these fields have been fully exploited, we see that in our quantitative landslide and debris flow modelling the inclusion of increased process knowledge seems to lag behind. This in depth process understanding needs to be incorporated in our technical predictions in order to improve the reliability of early warning systems, mitigation works and landslide zonation. This special issue aims to present innovative hydrological research applied to landslide studies to improve the understanding of the spatio-temporal patterns of slope movement mechanisms induced by precipitation. The initiative struck a sympathetic note, as many colleagues were facing these challenges. The topic of hydrology and landslides is finding more and more space in hydrological research, as witnessed by the number of relevant papers published, in special issues, e.g. on hydrology of unstable clay shales (Bogaard et al., 2012) and through the organization of workshops and special conference sessions, such as six EGU sessions on hydrology and landslides, three editions of the Italian Workshop on Landslides mainly dedicated to landslide hydrology (www.iwl.unina2.it), and special sessions at IAEG2014. Therefore, this Special Issue discusses the representation of

L'étude des activités hydrologiques menées au cours des cinquante dernières années dans les pays francophones de l'Afrique sub-saharienne apporte un précieux éclairage sur les difficultés rencontrées, les modes de collaboration adoptés et l'ampleur des défis à relever dans le domaine de l'eau à l'aube du troisième millénaire.Ceci est l'occasion de tracer les étapes marquantes de l'acquisition de la " connaissance hydrologique " depuis l'ère pionnière des années 50 jusqu'aux programmes scientifiques les plus récents faisant appel à des technologies avancées. Cette rétrospective permet également d'apprécier l'évolution des modes de partenariat entre acteurs scientifiques, techniques et économiques du Nord et du Sud.Ces travaux et recherches ainsi entrepris et développés montrent à la fois l'importance du savoir acquis et la nécessité de poursuivre les études en cours. Ils dressent aussi un constat encourageant quant à l'avenir des projets régionaux africains en hydrologie grâce au renforcement des relations scientifiques entre pays de la région., This article describes a study of hydrological research carried out in the French-speaking countries of sub-Saharan Africa over the past fifty years, by all the parties concerned.The work sheds useful light on the difficulties encountered, the types of co-operation adopted, and the scale of the challenge now facing the region in the matter of water resources, on the eve of the third millennium.The first stage in the acquisition of hydrological knowledge in the region was the pioneer period of the 1950s, when many different kinds of water demand were emerging very fast but hydrological information was almost nil. Hydrometric networks incorporating 1,500 instrument stations had to be rapidly set up for an overall survey of water resources. 200 representative or experimental catchments were equipped. The data obtained from the stations provided practical answers, meeting the operational needs of development, though in many cases the data acquired were used again, later on, for pure research purposes. The data gathering was a huge task, carried out on a range of different geographical scales. To achieve it, hydrologists had to develop appropriate methodologies for the region's tough climate and difficult field conditions; they also had to train highly motivated, seasoned hydrologists.During the sixties, the first hydrological monographs on the major river basins were completed,, rue Lafayette - research was conducted on regimes and ten-year flood peaks, and findings on the small catchments were collated. All this provided a first sketch of the region's main hydrological features.Two major upheavals marked the end of the 1960s: a long, disastrous drought and the information technology revolution.The scale of the hydropluviometric deficits recorded over several decades, amounting in some catchments to 30%-40%, led scientists to wonder how stable water resources really are and whether the notion of "normal" in hydrology or rainfall has any validity. The question was especially relevant because the design of all major African water engineering schemes completed around 1960-65 was necessarily influenced by the flow rates recorded in the previous, wet period. Changes in ground surface states and the environment in general, under the impact of climate change and increasing human activity respectively, made a more multidisciplinary approach to the hydrological cycle indispensable. Information technology also had a major impact, as the many numerical data gathered could at last be properly analysed and exploited. User-accessible reference databases were developed and processing software packages like Hydrom and Pluviom were widely distributed. It was now possible to model the relations between rainfall and discharges at a detailed scale; modelling improved steadily, first with the introduction of fine-mesh models, then of coupled surface water-groundwater models.As regards water engineering work, flood routing models were significantly improved for the Niger and Senegal rivers, and dam operation simulation models made spectacular strides. Of course, data processing greatly facilitated the use of statistical laws for everything to do with engineering hydrology.For several decades, metrology made little progress; but the instruments in use were robust and easy to use, and so well suited to local conditions. Not until the early 1980s was there significant technological progress in sensor technology; meanwhile civilian satellites made their debut and were used from the outset to facilitate hydrological data gathering, verification and transmission. Two applications illustrate this use locally. One is the Hydroniger project, involving eight countries bordering on the Niger and designed to produce an operational, real-time hydrological forecasting system on the river basin; the other is the Onchocerciasis Control Program, with WHO. In this program, discharge data in only slightly delayed time has made it possible to calculate the right dose of insecticide to add to infected rivers. Over the past ten or fifteen years, hydrological research has considerably extended its scope, to include spatialised hydrological parameters, soil/water/plant/atmosphere relations, soil erosion and conservation, geochemistry, hydrochemistry, etc. Meanwhile, sub-Saharan Africa has not been left out of the international organisations' programs involving operational or pure research hydrology, and it is making its contribution to the great challenges that face us with regard to water resources and management. Examples are the WMO WHYCOS project to establish permanent monitoring systems; the AOC-FRIEND program, a regional research project, which has been growing in strength; and the Hapex-Sahel experiment, designed to provide more information on the thermal and moisture characteristics of soil and vegetation, for integration into general atmospheric circulation models.Forms of collaboration among the partners involved in hydrology in this part of Africa have naturally changed over the course of fifty years. In the beginning, demand from local technical services set up a particular form of partnership with hydrology experts from the North. After independence, scientific and technical co-operation developed along with a policy for training management-level staff to build up national hydrology services. In many countries, however, the national services only gradually took full charge of their hydrometric networks, with technical assistance continuing for a long time. During the seventies and eighties, most countries acquired scientific or technical infrastructures that played host to hydrologists from the North for joint research programs and studies, with gradually increasing involvement by African water research scientists.After long years of work gathering, interpreting and applying data, the hydrological characteristics of this part of Africa have been established and the groundwork for a sound scientific partnership between Northern and Southern partners has been laid. Many of the region's water problems are linked to the difficulty of meeting constantly rising demand from fluctuating water resources; unrelenting effort is needed to resolve these problems.In this regard, there are promising signs for the new millennium: the growing community of African hydrologists is increasingly active in major international programs; links have been forged among those involved in Africa's various regional programs; and there are now thriving scientific associations like the Association of African Hydrologists.

In the Federal Republic of Germany (West) at the present time 14 forest hydrological research projects with 34 individual catchments are being maintained. In some cases measurements have been extended to several sub-catchments. Mostly, the approaches of ungaged paired-watershed studies or single-watershed studies are used. Only in the Forest Hydrological Research Area of Krofdorf was the procedure of a watershed calibration with two control catchments and two planned experimental catchments carried out for 10 years and subsequently, during the winter of 1982–1983, reductions of forest cover density by cuttings were started. In connection with forest decline caused by air pollution and its effects on amount, timing and quality of discharge there is an increasing demand to use sound long-term quantitative and qualitative precipitation and streamflow measurements of forest catchments such as ‘benchmark-catchments’. However, for this purpose the procedures of a climatic calibration and/or watershed modelling must be realised.

The aim of the study was to analyze the long-term variability of the average annual water runoff of the rivers of the Pripyat basin within Ukraine and to assess its calculated characteristics in the high-water and low-water phases of water content.The research of the cyclical nature of long-term fluctuations of river water flow, which contributes to the establishment of long-term dynamics of water content and potential (forecast) changes – is a topical issue of modern hydrological research. To describe the long-term variability of river water flow and its structure (cycles and phases of water content) the most effective method is stochastic, based on mathematical statistics, the theory of random variables and functions, probability theory. Autocorrelation analysis, various statistical criteria (homogeneity, series, series lengths), total and difference integral curves, probability theory, correlations, statistical estimation of probable errors, etc. were used to identify stochastic regularities of long-term variability. According to the results of the study of long-term variability of the average annual water runoff of the rivers of the Pripyat basin within Ukraine, it is established that cycles with periods of 29±2 years have high reliability and indicate stability of periods of low (10±2 years) and high water content. 17±2 years). According to the identified stochastic patterns, it is assumed that by 2025-26 it is necessary to expect the continuation of the low-water phase of water, then with the duration of 16-17 years the high-water phase will begin and from 2044-45 there will be low water again until 2055-56. According to the proposed regression equations between the average annual water discharge for a long-term period and their average values during the high-water and low-water phases of water content (with very significant approximation coefficients) and the obtained transition coefficients, it is possible to establish the calculated characteristics of the average annual flow of water of various availability in the high-water and low-water phases of water content, thereby giving their forecast estimates.

Simulating streamflow under both high-and low-flows is required for versatile eco-hydrological modeling. Typical streamflow simulators require hydrological data such as river geometry and observed river discharge/water level as upstream/downstream boundary conditions. However, these are not always available in data-sparse regions. Furthermore, because of the potential inaccuracy of digital elevation model (DEM) data around water surfaces, this data has not generally been utilized in streamflow simulations. Therefore, this study explores the potential applicability of DEM data to extract river cross-sections, focusing on the upstream Hiikawa River, Japan. A 1-D streamflow simulation was performed using river cross-sections extracted from a 5 m LiDAR DEM and the observed dam discharge from 2018 to 2020 as the upstream boundary condition. The simulated water depths with Manning’s roughness coefficients of 0.03 to 0.05 m–1/3 s reproduce the observation results with Nash-Sutcliffe coefficients of 0.91–0.97 for the whole period and 0.60–0.97 for a flood event. The accurate results for both low and high flows were considered to reflect the reasonable representations of the river cross-section. Finally, the velocity-based suitability index for Ayu (P. altivelis) was evaluated. We demonstrate applicability and several possible limitations of DEM data for eco-hydrological modeling of data-scarce rivers.

A number of rainfall-runoff models have been developed for hydraulic and hydrological engineering with an emphasis on reproducing river discharge time series. Physically-based rainfall-runoff models have recently reached a certain level of achievement following the advancement of computers and the development of various geographical and meteorological datasets. However, it has been pointed out that the current physically-based models do not properly reflect observed hillslope water dynamics. The present paper proposes a methodology to examine the capability of a depth-discharge constitutive equation for physically-based rainfall-runoff modelling to simulate hillslope water dynamics. An application of the methodology suggested that 1) the targeted constitutive equation was capable of representing the depth-discharge relationship on hillslopes under the assumed conditions, 2) the runoff simulations with the constitutive equation described hillslope water flows, at least in the downward direction, and 3) there was a possibility that the parameters in the constitutive equation was determined from the internal structure of hillslope water dynamics.

Understanding of flood dynamics forms the basis for the leading water resource management and flood risk mitigation practices. In particular, accurate prediction of river flow during massive flood events and capturing the hysteretic behavior of river stage-discharge are among the key interests in hydrological research. The literature demonstrates that data-driven models are significant in identifying complex and hidden relationships among dependent variables, without considering explicit physical schemes. In this regard, we aim to discover the extent to which data-driven models can recognize the hidden relationships among different hydrological variables, in order to generate accurate predictions of the river flow. A secondary aim involves the detection of whether data-driven models can digest the internal features of training inputs to extrapolate severe flood records beyond the training domain. To achieve these aims, we developed a recurrent neural network (RNN) model of two hidden layers to capture the hidden relationships among the inputs, and investigated the model's predictive capability using quantitative and qualitative analyses. The quantitative analysis comprised of a comparison between model predictions, and another set of precise independent records obtained through an advanced hydroacoustic system for reference. A qualitative approach was adopted to visualize the hysteretic behavior of the stage-discharge relations of the model records, with the high-resolution records of the hydroacoustic system. The findings display the potential of data-driven models for accurately predicting river flow. Consequently, the qualitative analysis revealed moderate correlations of stage-discharge loops as compared to the reference records. Additionally, the model was tested against severe destructive flood records generated from the East Asian monsoon and tropical cyclones. Its findings suggest that data-driven models cannot extrapolate new features beyond their training dataset. Overall, this study discusses the competence of RNNs in providing reliable and accurate river flow predictions during floods.

Dissolved gases, including noble gases, are versatile environmental tracers. Historically, the application of dissolved (noble) gases as tracers in hydrology used to be limited because their measurement required expensive, laboratory-based instrumentation operated by highly trained personnel. Here, we highlight recent advances in mobile mass spectrometry (MS) methods for noble and other dissolved gases, which enable low cost, high-throughput, real-time measurements. We also present applications using mobile MS to quantify hydrological and biogeochemical processes in groundwater and surface waters and to assess hazards and risks to aquatic environments. Finally, we indicate potential future applications of these instruments to enhance hydrological research.

In hydrological research, flood events can be analyzed by flood hydrograph coincidence. The duration of the flood hydrograph is a key variable to calculate the flood hydrograph coincidence risk probability and determining whether flood hydrograph coincidence occurs, while the actual duration of the flood hydrograph is neglected in most of existing related research. This paper creatively proposes a novel method to analyze the flood hydrograph coincidence risk probability by establishing a five-dimensional joint distribution of flood volumes, durations and interval time for two hydrologic stations. More specifically, taking the annual maximum flood of the upper Yangtze River and input from Dongting Lake as an example, the Pearson Type III and the mixed von Mises distributions were used to establish the marginal distribution of flood volumes, flood duration and interval time. Subsequently, the five-dimensional joint distribution based on vine copula was established to analyze the flood hydrograph coincidence risk probability. The results were verified by comparison with a historical flood sequence, which show that during 1951–2002, the hydrograph coincidence probabilities corresponding to its flood event coincidence volumes of 2.00 × 1011 m3, 4.00 × 1011 m3, and 6.00 × 1011 m3 are 0.213, 0.123, and 0.049, respectively. It has provided theoretical support for flood control safety and risk management in the middle and lower Yangtze River. This study also demonstrates the significant beneficial role of regulation by the Three Gorges Water Conservancy Project in mitigating flood risk of the Yangtze River. The hydrograph coincidence probability corresponding to its flood event coincidence volume of 2.00 × 1011 m3 has decreased by 0.141.

The rapid increase in heavy precipitation flooding events highlights the need for efficient flood forecasting techniques to facilitate flood hydrological research and effective flood management by civic bodies. The current study aims to develop a near-real-time flood forecasting framework by integrating a 3-way coupled hydrodynamic flood model framework with numerical weather modelling based rainfall forecasts. The proposed framework has been demonstrated over Mumbai city in India, which is subjected to flooding every year during the monsoon months. A fine-resolution atmospheric simulation with the Weather Research and Forecasting (WRF) model has been performed for rainfall forecasts, which serve as an input to the flood model. To access the impact of urbanization on rainfall extremes, three scenarios are considered in the WRF simulations, i.e., WRF model: (1) without Urban canopy model (WRF-NoUCM), (2) coupled with a single-layer Urban canopy model (WRF-SUCM), and (3) coupled with a multi-layer Urban canopy model (WRF-MUCM). Further, a three-way coupled flood model has been developed where the MIKE 11 model (streamflow) with the drainage network (stormwater drains) and the MIKE 21 model (overland flow) have been considered for flood inundation and subsequently hazard mapping. In addition, the tidal elevation is provided along the coastline in the model setup. The flood maps developed by three WRF forecasted rainfall scenarios have been compared with that of the maps developed with observed rainfall. The extent to which the scenarios have been able to imitate the pattern and extent of flooding generated by observed rainfall has been investigated to decide the best scenario to be adapted in the comprehensive flood forecasting network. This state-of-art flood forecasting approach may be implemented in other flood-prone coastal regions as a major non-structural flood management strategy to reduce flood risk and vulnerabilities for the people dwelling in those regions.

The Marcell Experimental Forest (MEF) in northern Minnesota, USA, with hydrological research and monitoring of peatland catchments in a low-topographic relief landscape, contrasts with the mountainous terrain that typifies most research catchments. Six research catchments were instrumented and hydrological and meteorological monitoring was initiated during 1960. Paired-catchment studies, which started during 1969, have been used to assess land management and environmental change effects on forests, water availability, and biogeochemistry. Over the decades, the research and collaborations have proliferated to include new monitoring and ecosystem experiments. We provide an overview of available datasets and access information for hydrological and meteorological data. Data on streamflow, water table elevation, precipitation, snow, ground frost, air temperature, soil moisture, upland runoff, and water chemistry are discoverable with associated metadata and are archived through several Web-based, community repositories. The research programme is ongoing and we anticipate updates on an annual or more frequent basis. Additionally, we aim to release other physical, chemical, and isotopic measurements associated with long-term catchment monitoring and studies at the MEF.

In the first section of the chapter, the overview of Mongolian hydrological research is introduced. The geographical background of the surface water is introduced in the second part, describing the morphological, hydrological and ecological characteristics of rivers and lakes, including the description of the genetic lake types. Detailed analysis is given on the Tuul and Ulz River, Lake Khuvsgul, and Shargaljuut mineral spa. The surface water body, such as rivers, streams and lakes cover 10,560 sq. km or 0.67% of the total territory. Mongolia is divided into three hydrological basins, such as Northern Arctic Ocean Basin, Pacific Ocean Basin and Central Asian Internal Drainage Basin, in the Central and Eastern Asia. The total surface water resource of Mongolia is estimated at 599 km3/year. The main water resource is stored in lakes (500 km3/year) and glaciers (62.9 km3/year). Water resource in the rivers is shared only 5.8% of the total surface water resources, that is, 34.6 km3/year. The third section introduces groundwater resources and its geographical distribution. The amount of water resources in the renewable groundwater (i.e., groundwater with a smaller residence time that can be replenished relatively quickly) is estimated at 10.8 km3/year. Groundwater is the main source for drinking, agricultural and industrial usage in Mongolia.

This chapter provides a holistic view of the hydrology of the Murray–Darling Basin (MDB) within which local hydrological systems and issues can be contextualised. Surprisingly, despite many decades of hydrological research, this is the first time the hydrology of the whole Basin has been described, including its natural hydrological connectivity, the impacts of Basin developments, and efforts to restore the Basin's water regimes. An understanding of Basin hydrology is fundamentally important for Basin management because it shapes many of the important natural processes and associated benefits and threats for humans. The MDB's hydrology is typical of an inland river Basin in an arid climate having low catchment streamflow yields and high interannual variability in streamflows. High transmission losses through long lowland rivers mean that many of the Basin's rivers flow intermittently, becoming longitudinally disconnected during cease-to-flow periods. Floods move slowly along the Basin's long, low-gradient rivers and spread out to inundate large floodplain areas in a low relief landscape. Connections between the surface waters and groundwater follow generally predictable sequences producing gaining and losing reaches along each of the major tributaries. These lateral, longitudinal, and vertical hydrological connections are fundamental to the Basin's freshwater ecosystems. Since irrigation developments began in the 1850s, the development of both surface and groundwater water resources, combined with flood management schemes, has dramatically modified the Basin's hydrology and patterns of connectivity. By the 1980s, it was clear that Basin development had led to major environmental decline, and this led to efforts to restore environmental flow regimes. The last four decades have seen some progress in increasing the volume of water protected for the environment throughout the MDB. This has led to improvements in environmental flow regimes and the possibility of preserving important hydrological connections through the Basin and dependent freshwater ecosystems.

Eco-hydrology of hydrophyte forests and wet meadows are very important question in changing climate. Hydrological extremes can cause in these habitats droughts, intense rainfall events and floods. In this study, we investigated a riparian alder forest and its edge, and a neighbouring meadow in Hidegvíz Valley experimental catchment to compare different surface cover type hydrological and botanical characteristics. The research has conducted in 2018-2019 hydrological year.Throughout the botanical study, a list has been made of the various plant species – trees, bushes, and herbs - of the elder woodland and its edge, and the grassland, in each vegetation period. The classifications of the habitats were made by the Á-NÉR system, which is a Hungarian classification system for Hungarian habitats.The hydrological research was focused on three important factors in the one-year period: precipitation, the changes of the groundwater levels, and the soil moisture values. We summarised the monthly data, and from all this, we calculated an annual water balance graph. This graph showed us a correlation between hydrological extremes and soil moisture value changes.The research was supported by the “EFOP-3.6.1-16-2016-00018 – Improving the role of research+development+innovation in the higher education through institutional developments assisting intelligent specialization in Sopron and Szombathely” project.

Nowadays, a large part of hydrological research is focussed on hydrological modelling, both to improve system understanding and to simulate future systems to support decision making. Although the necessary simplifications in hydrological models such as empirical formulas or spatial and temporal discretisation can result in deviations in model predictions, hydrological models often perform well due to model calibration. However, fundamental changes in system behaviour can occur that are not represented by the used model structure. These changes can therefore not be simulated and can result in deviating model results. We refer to this situation as ‘systemic change’. To detect systemic change, one can calibrate the model separately for different time periods, and evaluate whether thus-found parameter values change over time, which is an indication of systemic change (Verstegen et al., 2016). The aim of this study is to use this approach to detect possible systemic changes in the Rhine-Meuse basin when modelled with the PCR-GLOBWB hydrological model.PCR-GLOBWB is run for Rhine-Meuse basin for 1901-2010 at a daily time step with a 30 arcminute resolution, after which a brute force calibration is performed for five parameters (degree day factor, Manning’s roughness coefficient, soil thickness, saturated hydraulic conductivity and groundwater coefficient) using measured discharge data from the Global Runoff Data Centre (GRDC) at four locations in the catchment. To be able to identify the time stability of these parameters, the model is not only calibrated for the entire 1901-2010 period, but also for 10-year rolling calibration periods (i.e. 1901-1911, 1902-1912, 1903-1913, etc.). This results in a time series with 100 parameter values for each parameter, which is analysed for potential trends at the different calibration locations. First results indicate a decrease in the optimal parameter values for soil thickness and saturated hydraulic conductivity and an increase in the optimal parameter values for degree day factor and Manning’s roughness coefficient through time, especially in the upstream areas such as Basel. If the calibration is performed more downstream, for example at Lobith, the optimal parameter values are less variable through time.These results are used to determine the effect of potential systemic changes on the uncertainty of hydrological predictions by making three forecasts; one with stable parameter values and a stationary climate, one with time-variant parameter values and one with a future climate scenario. The last forecast enables comparing the magnitude of change caused by the potential time-variant parameters with the change caused by time-variant climatic forcing. This way, the study gives more insight in both the occurrence of systemic change and its potential consequences, which can contribute to a better understanding of the behaviour of hydrological models under changing conditions.ReferenceVerstegen, J. A., Karssenberg, D., van der Hilst, F., & Faaij, A. P. C. (2016). Detecting systemic change in a land use system by Bayesian data assimilation. Environmental Modelling & Software, 75, 424–438. https://doi.org/10.1016/j.envsoft.2015.02.013

Continental-scale hydrological research is becoming more important as climate variability and change, and anthropogenic impacts on groundwater, are increasing over large spatial and temporal scales. Groundwater quantities and flows are usually difficult to observe due to sparse or spatially limited monitoring networks. Thus, large-scale hydrological models are needed to provide continuous predictions of hydrological states and fluxes for water resource management. A large part of groundwater consumed comes from alluvial aquifers, which constitute valley fills of continental catchments. While the role of alluvial aquifers as a significant water store has been subject of many previous studies, the importance of the spatial extent and continuity of alluvial aquifers in the drainage characteristics of freshwater from the continental interior to the oceans is unclear. We present a high resolution (3km) hydrological model of continental Europe using ParFlow, a 3D, parallel groundwater and surface water flow model, which uses detailed hydrofacies information as input. We discuss the effect of spatial continuity and extent of alluvial aquifers on continental lateral groundwater flow and discharge to the oceans, water table depth, streamflow, and surface and subsurface storage. The results suggest that the alluvial valleys act as conduits that manage the drainage and retention of continental freshwater in sync with the atmospheric forcing. This dynamic equilibrium may be significantly disturbed by human interventions such as pumping and irrigation leading to a new equilibrium in terms of continental water quantity and also quality.

The rapidly expanding field of machine learning (ML) provides many methodological opportunities which match very well with the needs and challenges of hydrological research. Due to extended measurement networks, more frequent automatic measurements of hydrological variables, and not the least increasing use of remote sensing products, the era of big data surely has arrived in hydrology. Process-based models are usually developed for certain spatiotemporal scales, not fitting easily to the scope of the new datasets. Automatic methods that learn patterns and generalizations have been demonstrated to be superior in many applications. The chapter provides an overview of some of the most important machine learning algorithms which have been used in the hydrological literature. It will be shown that there is no single best method among them, but instead a spectrum of methods should be utilized, from highly flexible ones to more parsimonious learning methods, depending on the specific hydrological application, research question, and data availability. Most machine learning techniques require a calibration and a validation dataset for training. As these data are usually correlated in time and space, the problem of bias-variance tradeoff arises will be discussed as a simple example. The presentation of ML algorithms, roughly following chronological order, is discussed starting with artificial neural networks through support vector machines to gradient boosting machines. As data streams increase, these and other machine learning techniques will play an ever more important role in hydrology.

Soil moisture is a key issue for eco-hydrological research in arid and semi-arid regions, and is primarily concerned with water availability for vegetation. Shallow and deep soil moisture occurs according to the maximum infiltration depth. Soil moisture has three-dimensional characteristics: inter-layer variability, horizontal heterogeneity and temporal variability. Soil moisture is affected by various factors including terrain, soil characteristics, climate and vegetation, and the effects of these change with time (e.g., rainfall patterns) and space (e.g., soil depth). In arid and semi-arid regions, deep soil moisture is of particular importance to vegetation restoration and the evaluation of vegetation sustainability; however, accurate prediction of the spatial distribution of deep soil moisture in the Loess Plateau of China still faces numerous challenges. Therefore, future research should focus on the mechanisms, models and scale effects of soil moisture, particularly for deep soil moisture.

Understanding potential evapotranspiration (PET) changes under climate change is of great importance for hydrological research. The trends of PET changes and their driving forces were investigated in the Lancang (LRB) and Yarlung Zangbo (YRB) river basins, southwest China, using diagnosis graphs and the Mann-Kendall test. Analysis of variance was applied to examine the contribution of different climatic variables to PET. The results show that: (i) there was a statistically significant increase in PET in the period of 1957–2015 in the LRB, while it showed a markedly decreasing trend in the YRB; (ii) PET in both basins is fairly sensitive to wind speed, relative humidity, solar radiation and maximum air temperature, and the interactions between wind speed and relative humidity are also important; and (iii) the increase in PET in the LRB is due mainly to the increase in maximum air temperature and decrease in relative humidity, while declines in wind speed and solar radiation are the main reasons for the decrease in PET in the YRB.

The aim of hydrological research has always been, and should still be to improve our understanding of hydrologic processes. Catchment hydrological response has the potential to enhance understanding of these processes. Regionalisation of hydrological response is important in identifying watersheds with similar catchment hydrological response, thus making predictions in ungauged and poorly gauged basins to be possible. This study establishes the hydrological response of 122 catchments in Swaziland's trans -boundary basins on the basis of validated WR90 hydro-meteorological time series data. The flow duration indices (FDIs: Q10, Q50, Q90) were determined and standardised by the mean monthly streamflow, and the runoff ratio was calculated as a method to determine how catchments respond to precipitation inputs. The runoff ratio and FDIs were then plotted against aridity indices, and the plot shows that the runoff ratio is consistent with the aridity index, except for a few suspected anomalies emanating from values of Q50 and Q90. The findings indicate that the hydrological response of the catchments is diverse and somewhat characterised by regions. The diversity and the regions are associated with climate regime and topography controls. It is anticipated that the potential of using hydrological response to gain more understanding of catchments behaviour will improve and further rigorous exploration may be required.

In 2018, 70 years have passed since the beginning of observations at the Kolyma Water-Balance Station (KWBS), a unique scientific research hydrological and permafrost catchment. The volume and duration (50 continuous years) of hydrometeorological standard and experimental data, characterizing the natural conditions and processes occurring in mountainous permafrost conditions, significantly exceed any counterparts elsewhere in the world. The data are representative of mountainous territory of the North-East of Russia. In 1997, the station was terminated, thereby leaving Russia without operating research watersheds in the permafrost zone. This paper describes the dataset containing the series of daily runoff from 10 watersheds with an area from 0.27 to 21.3 km2, precipitation, meteorological observations, evaporation from soil and snow, snow surveys, soil thaw and freeze depths, and soil temperature for the period 1948–1997. It also highlights the main historical stages of the station's existence, its work and scientific significance, and outlines the prospects for its future, where the Kolyma Water-Balance Station could be restored to the status of a scientific research watershed and become a valuable international centre for hydrological research in permafrost. The data are available at https://doi.org/10.1594/PANGAEA.881731.

Intensity of action and frequency of high and low pressure barometric fields of large and medium scale have great influence on dynamics of pluviometric regime. On their action depend characteristics of thermic regime, relative humidity, cloudiness and windiness, which directly affect the precipitation. Concerning that the water balance is difference between the inflow and evaporation, it comes that precipitations do not have just special role, but they are also the most important factor of the Una river regime. Disposition and precipitation structure are the river regime's essential assumptions, so at the same annual height of precipitation, we have more water in rivers in the year with more precipitations during the colder period. Then we have less evaporation so the inflow from the basin is bigger. The amount and disposition of the precipitation in the Una River basin is analyzed in this paper in order to define the pluviometric regime of this area. Pluviometric regime greatly influences the Una river regime and water balance, what was the reason for this analysis and further hydrological research in the Una River basin.

Under the background of global climate change, drought is causing devastating impacts on the balance of the regional water resources system. Hydrological drought assessment is critical for drought prevention and water resources management. However, in China to assess hydrological drought at national scale is still challenging basically because of the difficulty of obtaining runoff data. In this study, we used the state-of-the-art passive microwave remote sensing techniques in river runoff modelling and thus assessed hydrological drought in Mainland China in 1996–2016. Specifically, 79 typical hydrological stations in 9 major basins were selected to simulate river runoff using the M/C signal method based on a high-resolution passive microwave bright temperature dataset. The standardized runoff index (SRI) was calculated for the spatial and temporal patterns of hydrological drought. Results show that passive microwave remote sensing can provide an effective way for runoff modelling as 92.4% and 59.5% of the selected 79 stations had the Pearson correlation coefficient (R) and the Nash-Sutcliffe efficiency coefficient (NS) scores greater than 0.5. Especially in areas located on Qinghai-Tibet Plateau in the Inland and the Southwest River Basin, the performance of the M/C signal method is quite outstanding. Further analysis indicates that stations with small rivers in the plateau areas with sparse vegetation tend to have better simulated results, which are usually located in drought-prone regions. Hydrological drought assessment shows that 30 out of the 79 stations present significant increasing trends in SRI-3 and 18 indicate significant decreasing trends. The duration and severity of droughts in the non-permanent dry areas of the Hai River Basin, the middle reaches of the Yangtze River Basin and the Southwest of China were found out to be more frequent and severe than other regions. This work can provide guidance for extending the applications of remote sensing data in drought assessment and other hydrological research.

During hydrological research in a Chilean swamp forest, we noted a pattern of higher streamflows close to midday and lower ones close to midnight, the opposite of an evapotranspiration (Et)-driven cycle. We analyzed this diurnal streamflow signal (DSS), which appeared mid-spring (in the growing season). The end of this DSS coincided with a sustained rain event in autumn, which deeply affected stream and meteorological variables. A survey along the stream revealed that the DSS maximum and minimum values appeared 6 and 4 hours earlier, respectively, at headwaters located in the mountain forests/ plantations than at the control point in the swamp forest. Et in the swamp forest was higher in the morning and in the late afternoon, but this process could not influence the groundwater stage. Trees in the mountain headwaters reached their maximum Ets in the early morning and/or close to midday. Our results suggest that the DSS is a wave that moves from forests high in the mountains towards lowland areas, where Et is decoupled from the DSS. This signal delay seems to convert the link between streamflow and Et in an apparent, but spurious positive relationship. It also highlights the role of landscape heterogeneity in shaping hydrological processes.