bluuebunny/crossref_metadata_2025 huggingface dataset:
Order Selection of AR Models of Hydrologic Time Series
Authors: Padmanabhan G. , Rao A.R.
Published: May 2018

Recently several criteria have been proposed for selection of orders of AR models. One of these, namely the AIC has been used in hydrologic analysis. However, the results given by these criteria when observed hydrologic data are analyzed by using them have not been studied to determine the variability in the results obtained by these criteria.

Some of the more widely discussed of these criteria have been analyzed in this paper by using both hydrologic and nonhydrologic time series. The orders of AR models given by Akaike's information criterion, the final prediction error criterion, the criterion autoregression transfer function, Schwarz's criterion and the posterior probability criterion have been analyzed, and the results are discussed.

Although the results obtained by these criteria are often similar, the posterior probability criterion has been recommended for use because it is possible to estimate the probability of the order being correct by using it.
Areal Differentiation of Land and Lake Snowcover in a Small Sub-Arctic Drainage Basin
Authors: Adams W. P. , Roulet N. T.
Published: May 2018

A large, stratified, random sample is used to identify vegetation related patterns of snowcover and patterns of land and lake snowcover properties in the basin of Elizabeth Lake, Labrador, Canada. Spatial patterns of snow depth, density and water equivalent are discussed using analyses of variance and trend surface analyses. Statistically significant relationships are demonstrated between these properties and four distinctive “landscape units,” Tundra, Open Lichen Woodland, Close Lichen Woodland and Lake. Special features of a mid-winter lake snowcover are discussed including the amount and type of snow present and the spatial patterns in it. The survey system used appears to be useful for hydrologic work.
Areal Intensity-Duration-Frequency Curves for Short Term Rainfall Events in Lund
Authors: Niemczynowicz Janusz
Published: May 2018

»Design Storms«, which are usually derived from point intensity-duration-frequency (i-d-f) relationships, neither reflect the areal variations of the rainfall pattern nor the dynamics of the moving storms. Design storms derived from point i-d-f curves can, no longer be accepted as a reasonable rainfall input for simulation of runoff from large urban catchments. One possible way of improving the rainfall input for runoff simulationes is to develop areal i-d-f relationships and use them for deriving design storms. The raingauge network operating in Lund for the past four years provided the set of good quality data necessary for developing such relationships. This paper describes the procedure of developing point and areal i-d-f relationships and shows the differences between them. Statistically developed factors reducing point values to areal values for areas up to 25 sq.km for different durations and return periods of up to three years are given. Presented relationships will give more realistic design storms in comparison with design storms derived from point i-d-f curves. Observed spatial variations of the maximum rainfall intensities in Lund can probably be explained by the city's influence on the rainfall pattern. Orographic effects, earlier observed in Lund, can not be seen from the data used during this study. Since the used data set includes only short term, high intensity rainstorms, it can be concluded that long and low-intensity rainfalls are mainly responsible for orographic effects. This paper is the first part of a major work aiming at developing a stochastic model simulating a time series of the areal rainfall.
Methods for Calculating Areal Precipitation – Applied to the Suså-Catchment
Authors: Allerup Peter , Madsen Henning , Riis Jens
Published: May 2018

Experiments in the Suså-catchment aimed at the study of relations between precipitation patterns and field characteristics are used in calculations of areal precipitation. A statistical analysis of precipitation data is outlined. A new method – the TTSI method – of calculating areal precipitation is presented and discussed in relation to traditional methods. Along with areal precipitation estimates confidence bounds for areal precipitation values are determined.
Longitudinal Dispersion in a Stream Calculated by One Dimensional Numerical Model
Authors: Tjomsland Torulv
Published: May 2018

A one-dimensional numerical model was used to simulate water stage and dispersion of matter in a stream. The calculated results show quite good agreement with field measurements.
Kalman Filter Empirical Fitting on Monthly Rainfall-Runoff Responses
Authors: Mimikou Maria
Published: May 2018

The majority of the methods for fitting rainfall-runoff Kalman filters use the statistical characteristics of the input (rainfall) and the output (runoff) series according to the systems analysis and control theory methodology. The problem arising with these techniques is essentially related with their inabilityto describe variations of the basin runoff response when the relevant information concerning the causes of such variations is not contained into the input to the filter. The presented empirical method of fitting solves this problem by taking into account additional input information and improving the knowledge about the hydrological system, but without changing the simple two-variable structure of the filter. This information consists of the percentage of snowfall into the monthly basin rainfall, the magnitude of basin rainfall at month t in comparison to the rainfall at month t-1 (increasing, constant, or decreasing) and the amount of basin rainfall associated with basin's moisture and runoff characteristics. The Kalman filter empirically fitted on monthly rainfall-runoff responses of the Aliakmon and Acheloos river basins in Greece is shown to be more accurate and adequate than the automatically fitted filter by using conventional statistical methods. The filter has been used in hydroelectric energy studies of the area, where accurate runoff estimation from rainfall was necessary.
Sequential Generation of Short Time-Interval Rainfall Data
Authors: Srikanthan R. , McMahon T. A.
Published: May 2018

Hourly and six-minute interval models were developed to stochastically generate sequences of rainfall depths. The models were applied to data recorded at twelve rainfall stations in Australia,including several from the arid zone. At each station, several replicates of rainfall sequences of length equal to the historical record were generated. Based on a range of parameters covering short rainfall bursts up to annual totals, the models were shown to be performing satisfactorily.
Dynamic Properties of Rainfall in Lund
Authors: Niemczynowicz Janusz , Dahlblom Peter
Published: May 2018

The shape of a runoff hydrograph depends on the dynamic properties of the rainfall. The possible range of impact of the rainfall movement on runoff generating process can only be judged if the rainfall volocity and direction are known in statistical terms. This paper gives the statistical characteristic of the rainfall movement in the city of Lund. The probability distribution of rainfall velocities and relative frequencies of the rainfall direction are given, based on the analysis of 400 rainfall events observed in the city of Lund. A simple program for determination of the velocities and the directions of storm movement has been developed and tested. Results show that there are two main storm directions in Lund: the first, generally oriented to the East, is prevalent throughout the year except summer; the second, oriented to the North-North-East, occurs primarily during summer months. During summer rainfall velocity is lower and intensity higher than during the rest of the year. Rainfall velocity is lognormally distributed. A strong correlation between wind movement parameters at 700 mb height and storm velocity and direction was observed.
Investigation of the Influence of Rainfall Movement on Runoff Hydrograph
Authors: Niemczynowicz Janusz
Published: May 2018

Work reported here consists of two parts: the first part describes studies performed on a conceptual catchment, the second part, presented as a separate paper, describes studies on real catchments in the city of Lund (Niemczynowicz 1984b).

In this paper we try to find the general shape of the relation between storm movement parameters like storm duration, intensity, velocity and direction, and its influence on peak discharge. This influence is called a “directional bias”. Different factors affecting the magnitude of directional bias are described. The relations between rainfall characteristics and the magnitude of the directional bias are shown for a conceptual catchment.

The most important conclusion is that the maximal directional bias can theoretically reach values of several hundred percent, but this can happen only in a catchment with specially designed linear geometry. Maximal directional bias can be expected for storms moving downstream the catchment with velocity equal to the average flow velocity. Maximal directional bias occurs for storms with short duration giving low peak discharge, which are not important for design purposes.
Investigation of the Influence of Rainfall Movement on Runoff Hydrograph
Authors: Niemczynowcz Janusz
Published: May 2018

Work presented here consists of two parts: the first part, presented before as a separate paper (Niemczynowicz 1984b), described studies performed on conceptual catchment, the second part presented here describes studies made on real catchments in the city of Lund.

The scope of the paper is to use previously calculated statistical characteristics of rainfall movement in Lund for quantitative description of the influence of storm movement on runoff modeled on real catchments in Lund. The relations between rainfall characteristics and the magnitude of directional bias are shown. The special interest in this study is to locate the combination of factors which maximize the effect of directional bias. Finally, by combination of rainfall movement statistics with modeled runoff, the probability distribution of directional bias for differently oriented catchments in Lund is given.

The most important conclusion is that the maximal directional bias observed for real catchments is much smaller than for the conceptual catchment, but still can reach significant values. The maximal bias occurs when the rainfall movement is parallel to direction to conduits and when rain velocity is close to average flow velocity. The highest values of directional bias occur for storms with short durations which give low peak flow discharge without practical importance. Still, the phenomenon of the directional bias can be one of the reasons why frequency of the rainfall does not follow frequency of runoff.
Runoff Pattern and Peak Flows from Moving Block Rains Based on a Linear Time - Area Curve
Authors: Jensen Mogens
Published: May 2018

One of the most outstanding problems within urban drainage is to give a statistically correct description of the runoff effect from areal and temporal distributed rain storms. Due to the large number of parameters for catchments and storms involved in such a description it is important to derive dominating dimensionless quantities by combining the parameters.

The paper deals with dimensionless peak flow calculations using moving block rains. Two important dimensionless parameters p1 and p2 (see Eqs. (7), (8) and (14)) and their influence on peak flow is found in a deterministic way by applying the time area model (TAM) to a linear time-area curve. Dimensionless peak flows are found as functions of p1 and p2 and results are compared with computations from the Storm Water Management Model (SWMM) version 2. TAM overestimated the peak flows by appr. 5% of the full flow capacity relative to SWMM on an average of 26 comparisons. The standard deviation is appr. 10%.

Finally some perspectives are presented in relation to a general statistical runoff description of the influence of areal and temporal rain storms. The possibility is discussed of achieving such a description by combining existing knowledge within 1) synthetic point rainfall design storms, 2) areal rain depth reduction factors, 3) shape of moving storm cell isohyetographs and 4) distribution of storm direction.
Groundwater in Norwegian River Plains
Authors: Stang Ole
Published: May 2018

Existing conflicts or problems concerning groundwater in Norwegian river plains are outlined. Emphasis is placed on the effects of river regulation. Two methods of determining aquifer diffusivity are proposed. The methods require continuous recordings of river stage and groundwater level at the time of a flood peak or during periodic fluctuations of river stage.

The methods are applied to data from two Norwegian river plain aquifers and consistent results for aquifer diffusivity are obtained.

It is concluded that the proposed solutions represent a feasible and competitive way of obtaining aquifer parameters in Norwegian river plains.
Runoff Models – Do They Tell What Actually Happens!
Authors: Erichsen Bredo , Nordseth Kjell
Published: May 2018

All scientific work performed so far to find the causality of the acidification of many Norwegian open waters, have – considerably – increased our knowledge about precipitation and soil water chemistry. But has it improved our understanding of what happens to the water itself – from being rain – or meltwater – to reappearing into the nearest stream channel? Most hydrochemical models so far have been based on too generalized and too simplified concepts of hillslope runoff genesis. It won't mend matters, however, that the last 20-30 years' of investigation within international hillslope hydrology has rendered conflicting results. Most analytical hydrologists today are, thus, anxious to have at least one conceptural model with a correct description of the runoff processes within a natural hillslope. This article tries to open up for a discussion of the relevance of the most adequate conceptual models in hillslope hydrology. Snowmelt runoff is emphasized and based upon our own experience from small forested watersheds in SE-Norway.
Simulation of the Impact of Combined Sewer Overflow on Rivers
Authors: Johansen Niels Bent , Hvitved-Jacobsen Thorkild , Harremoës Poul
Published: May 2018

Combined sewer overflows can in some cases cause critical oxygen concentrations in small rivers. The traditional design criteria for the overflow structures is not specificly related to this effect. By combination of a long historical rain series, a runoff model and a river quality model, it has been possible to achieve operational design diagrams for overflow structures. The diagrams are statistically based and relate the independent parameters for the catchment, the overflow structure and the river to the effect in question, the oxygen concentration.
Deep Groundwater Contribution to a Small Stream
Authors: Calles Ulla Maria
Published: May 2018

The contribution of surface water and groundwater were studied in a small stream in the central part of Sweden using electrical conductivity of water as a tracer. The portion of deep groundwater increases in the downstream direction from approx. 5 % to 20 %.
Passive and Active Microwave Studies of Wet Snowpack Properties
Authors: Chang A. T. C. , Foster J. L. , Owe M. , Hall D. K. , Rango A.
Published: May 2018

Microwave signatures have been found to be related to variations in snow conditions found on the earth's surface. Most of these observations have been obtained by passive microwave radiometry. In general, inverse relationships between microwave brightness temperature (TB) and snow depth were observed for dry snowpacks. The results from truck-mounted scatterometers indicated that the backscattering cross sections from snowpacks increased with snow depths, also in dry snow conditions. The reported aircraft mission was the first trial in which simultaneous active and passive microwave measurements were made over a wet snowpack. The test site was located in the Colorado Rocky Mountains. The results from this experiment suggest that microwave techniques using both radiometers and scatterometers may be useful in determining snow water equivalent even when the snowpack is wet.
Snowmelt Runoff Models for Operational Forecasts
Authors: Martinec J.
Published: May 2018

Remote sensing is changing the approach in snowmelt runoff modelling. Instead of a simulated snow cover, the areal extent of the real snow cover can be periodically evaluated. Adaptation of depletion curves of the snow coverage for real time forecasts is outlined.
Application Limits for the Kinematic Wave Approximation
Authors: Hager Willi H. , Hager Kurt
Published: May 2018

Flows with predominate flow direction are governed by the de Saint Venant flow equations. The kinematic wave approach retains the mass balance but considers pseudo-uniform flow conditions instead of the full momentum balance. This approximation is particularly well-suited for overland runoff processes. The zero-inertia approach may be regarded as an intermediate formulation which retains the effect of surface slope but neglects dynamical flow properties. This investigation considers in detail the differences between the three of the aforementioned approaches under pseudo-steady flow conditions. The result indicates application limits of the two simplified wave theories when compared to the de Saint Venant flow equations and may be regarded as a useful decisive criterion.
Improving the Reliability of Catchment Mass Input: Output Estimations from Sub-Optimal Data in Areas of Urban Growth
Authors: Prowse Colin W.
Published: May 2018

A new mass input:output formula is presented which makes full use of available data which are commonly sub-optimal for mass input:output analysis. Results indicate that some useful information on wet and dry fallout may be obtained from bulk precipitation data only, and that the new formula provides generally more reliable results for catchment chemical mass export under various conditions than other simpler formulae. Care is taken not to assume rates of natural denudation from the results in the light of an inability clearly to separate non-denudational from denudational components of mass export, and natural from man-induced components of denudation.
The Paths of the Suspended Particulate Inorganic and Organic Matter in a Small Urban Estuary: The Aarhus Harbour Estuary
Authors: Kronvang K. B. , Christiansen C.
Published: May 2018

The nverine supply of suspended inorganic and organic sediment, its temporal and spatial distribution and its exchange with the bay, were estimated for an urban estuary (Aarhus Harbour Estuary, Denmark) during 1983-84. The river supplies high levels of particulate matter to the estuary. Export from the estuary averages one fifth of the river introduced particulate matter resulting in the accumulation of terrigenous material in the estuary.

River discharge determines whether the high depletion of particulate matter in the upper estuary follows an exponential(flocculation) or a lineary curve (dilution). Preferential deposition of organic as opposed to inorganic matter in the upper estuary depletes the particulate matter of organic matter and associated pollutants.

The estuary is divided into three depositional zones identified on the basis of sediment activity, grain-size and organic content. A sediment budget is precented for the estuary. Good agreement is shown between the actually dredged sediment quantity and the mass to sedimentation from the budget.
Attempting Flow Forecasts of the Indus River, Pakistan Using Remotely Sensed Snow Cover Data
Authors: Makhdoom M.T.A. , Solomon S.I.
Published: May 2018

Recent developments in collection and interpretation of remotely sensed back radiation data from the earth surface obtained by satellites has made it possible to readily delineate areas covered by snow on the earth surface. In view of this, attempts have been intensified during the last decade to generate statistical relationships between the snow covered area in a basin and the corresponding runoff during the snow-melt period. However, examination of such relationships developed specifically for forecasting purposes in the Indus River basin in Pakistan indicates that, given the current data and operational constraints, their usefulness is very limited. The need for improving collection of remotely sensed data on snow to obtain information on snow density and other auxiliary information is emphasized.
Laboratory Experiments on Solute Transport in Non-Homogeneous Porous Media
Authors: Refsgaard Anders
Published: May 2018

Solute transport in groundwater is a process which has become of major importance during the last decades due to increasing contamination of ground water. This process usually occurs in a medium heterogeneous with respect to hydraulic conductivity and porosity, properties that affect the dispersion of the solutes.

The present paper describes an experimental investigation of the solute transport process in heterogeneous porous media, especially the connection between the statistical properties of their hydraulic conductivity distributions and the dispersion parameters governing the spreading of the solutes. The experimental results are compared to theoretical solutions derived for the same case of a solute pulse in an average uniform flow through a heterogeneous porous medium. Generally there is good agreement between the theory and the experiments.

In field applications this means that the dispersion parameters can be more readily determined from the soil properties. Furthermore, the deviations between dispersivities determined in laboratory columns and dispersitivies found under field conditions can be explained quantitatively by the differencies in the length scales and in the variances of the hydraulic conductivity distributions.
WATBAL
Authors: Knudsen J. , Thomsen A. , Refsgaard J. Chr
Published: May 2018

A semi-distributed, physically based hydrological modelling system, WATBAL, which accounts for the entire land phase of the hydrological cycle is described. As compared to the two alternative hydrological model types, i.e. the traditional lumped, conceptual rainfall runoff models (STANFORD model type) and the complex, fully distributed, physically based model (SHE model type) WATBAL represents an intermediate approach.

In the model, primary attention is given to the hydrological processes at the root zone level through a distributed, physically based approach whereas the groundwater processes are simulated in less details by use of a lumped, conceptual approach. This approach allows WATBAL to utilize spatially distributed input data to account for the spatial and temporal variability of meteorological conditions, vegetation and soil properties. Thus WATBAL can e.g. utilize digital satellite information as input data.

WATBAL has primarily been designed as a tool for predicting the runoff from ungauged catchments and for assessing the hydrological effects of land use changes. The capability of the model for simulating ungauged catchments is tested using results from a recent feasibility study for medium size dams in Zimbabwe.
An Evaluation of the Importance of Transmissivity, Head and Concentration Data for Contaminant Transport Modelling
Authors: Van Rooy Dirk
Published: May 2018

The 3 basic data types of contaminant hydrology are examined by stochastic modelling of a groundwater contamination case. The stochastic transport model, which is of the Monte Carlo type, uses a numerical flow and transport model, and views transmissivity as a random autocorrelated field. A large set of transmissivity realisations is generated using the turning bands technique. Conditioning is done with regard to transmissivity, head and concentration observations. The unconditional approach assumes, explicitly, a stationary stochastic process of logtransmissivity. This is implicitly turned into a non-stationary process by the conditioning procedures. These use simple and universal kriging, and utilize the kriging uncertainties to determine subsets of realisations that are in agreement with the observations at a predefined confidence level. The approach followed allows quantification of the uncertainties of predicted head and concentration through space and time. Conditioning on head observations leaves large transport uncertainties. Conditioning on the transmissivity data has a more prominent effect. The single, most effective data type is the concentration data. Smallest transport uncertainties occur when all the data are simultaneously taken into account. The conditioning effect depends on the number and spatial configuration of the data. A trade-off between the stochastic and deterministic transport approach is suggested. In modelling terms this corresponds to a trade-off between advection and dispersion.
Simulation of Surface Runoff and Pipe Discharge from an Agricultural Soil in Nothern Sweden
Authors: Jansson Per-Erik , Gustafson Arne
Published: May 2018

In order to test the ability of a physically based water and heat model to predict surface runoff and pipe discharge, adaptations were made to an agricultural field in the north of Sweden. A five-year period was selected, including observations of meteorological data, frost in the soil and discharge. Basic model requirements on soil properties, i.e., the water retention curve and the saturated hydraulic conductivity, were available from a previous investigation. Unsaturated conductivity was estimated from the water retention curve and by assuming a substantial influence of macro pores in the subsoil. Snow properties and thermal soil properties were adjusted to obtain a reasonable agreement with observed frost depths for areas with barley and with grass leys. Surface runoff was the dominating part of the total runoff, especially during conditions of frozen soil. The simulated discharge agreed well with the general partitioning between surface runoff and pipe discharge but discrepancies occurred in their temporal patterns. A probable explanation of these discrepancies was that the model did not account for the enhanced spatial heterogeneity in water flow through snow and in partially frozen soil.
On the Applicability of Theoretical Expressions for Flow Rate into Perforated Draintubes
Authors: Panu U.S.
Published: May 2018

An assessment of the existing theoretical expressions of flow into perforated draintubes has been made in regard to their applicability in the estimation of flow rate into perforated drains under field conditions. A method based on the experimental results is proposed for extending the application of existing expressions to curved watertable conditions.
A Model for the Relationship between the Hydraulic Conductivity and Primary Sedimentary Structures of Till
Authors: Nyborg Mats R.
Published: May 2018

An investigation has been made of the relationships between saturated hydraulic conductivity, porosity and micro-structure of undisturbed lodgement till samples. 35 measurements with a constant head laboratory permeameter are the basis for the discussion. All the measurements were made on undisturbed till samples. In order to minimize the effects of soil forming processes and to have the most homogeneous conditions all sampling were made from the C-horizon. The samples have a size of about 300 cm3. Porosity data were derived from capillary pressure curves. A model for how flow direction and long-axis orientations of elongated grains relate to the saturated hydraulic conductivity is presented. For an unsorted sediment such factors as grain size are concluded to be of minor importance for the hydraulic conductivity. The structural properties seem to be a more important factor. This effect can be explained in two ways. Either due to directional relations between sorted lenses and bands which have higher permeability and the flow route through the sample. The other explanation is due to a more continuous pore pattern parallel to the grain orientation. The hydraulic conductivity takes on a directional property, being smaller in directions normal to the structural long-axis orientation than in directions parallel to the orientation. A study of the effective porosity Versus hydraulic conductivity exhibits weak correlation.
Estimation of Extreme Precipitation in Norway
Authors: Førland E.J. , Kristoffersen D.
Published: May 2018

Probable Maximum Precipitation (PMP) is an important parameter for estimation of Probable Maximum Flood. This paper describes results of PMP estimation by different methods, both meteorological and statistical. A survey of the highest recorded rainfall values in Norway is also presented.
Proposed Swedish Spillway Design Floods in Relation to Observations and Frequency Analysis
Authors: Bergström S. , Lindström G. , Sanner H.
Published: May 2018

Proposed guidelines for the computation of spillway design floods in Sweden are described. The guidelines are based on the transformation of extreme hydrological conditions to floods by use of conceptual hydrological models.

Floods computed according to the guidelines are compared to observations for both autumn conditions when rainfall dominates and for snowmelt conditions in spring. The highest floods are in the order of 60% of the proposed guidelines.

Comparisons with frequency analysis show that the computed floods are well beyond the 10,000 year flood according to the Gumbel and Lognormal 2 distribution functions, while the results according to Lognormal 3 are more uncertain.
Unsteady Flow to a Non-Penetrating Large Diameter Well in Extensive Aquifers
Authors: Şen Zekâi
Published: May 2018

Non-equilibrium groundwater movement equations are written explicitly for the continuity and flow laws in the case of a non-penetrating large diameter well discharging from extensively thick confined porous medium aquifers. The equations representing these laws are combined together with the Boltzmann transformation which leads to the specific discharge equation within the aquifer. Type curve expressions are obtained with convenient definitions of the well function and dimensionless time factor for a non-penetrating well. The forms of these curves on double logarithmic paper appear identical to the fully penetrating large diameter well type curves but with differences in the variables as well as the aquifer parameters. In the non-penetrating well case the significant aquifer parameters are the hydraulic conductivity and specific storage coefficient which are intact of the aquifer saturated thickness. The methodology developed herein is applied to the time-drawdown data from a non-penetrating well in Saudi Arabia. Aquifer parameter determinations can be obtained through a similar procedure to the Theis type curve usage.
Radar Signal Interpretation in Warm Season Rainstorms
Authors: Dalezios Nicolas R. , Kouwen Nicholas
Published: May 2018

There are several sources of error affecting the returned power of weather radar signals with an impact on the accuracy of radar rainfall measurements. In this study, several factors and sources of error are considered at the preprocessing stage of the raw radar reflectivities for surface rainfall estimation. In particular, deterministic corrective algorithms are developed and alternatively used for non-rainfall echoes, wind effect, signal attenuation and the Z-R relationship. These techniques are applied to a number of warm season rainstorms. A bivariate statistical objective analysis is used as a basis for comparing the different algorithms. Two error statistics are computed in order to assess the performance of each algorithm. The results indicate that the deterministic techniques constitute a necessary step in the ultimate improvement of surface rainfall estimates by weather radar.
A Net Radiation Model for Calculating Daily Snowmelt in Open Environments
Authors: Granger R. J. , Gray D. M.
Published: May 2018

Climatological data from three stations in western Canada are used to develop a model for estimating daily net radiation from station latitude, station altitude and cloud cover. Expressions describing: a) the seasonal variation in atmospheric transmissivity; b) the interaction between direct-beam and diffuse short-wave radiation and c) the effect of cloud cover on these components are presented.

The seasonal decrease in albedo of a shallow snowcover is approximated by three line segments. Albedo decreases at an average rate of 0.0061/day during premelt; at an average rate of about 0.071/day during melt and takes on a reasonably constant value of 0.17 when the ground is snowfree.

The exchange of long-wave radiation dominates the radiation balance of a complete snowcover. During these periods it is recommended that the net flux be calculated from observations of ambient air temperature and vapor pressure in standard expressions.
Sensitivity Analysis of Groundwater Flow
Authors: Bao Yung-Bing , Thunvik Roger
Published: May 2018

A numerical model developed for sensitivity analysis of groundwater flow is presented. Sensitivity analysis is a useful complementary aid for groundwater flow modelling to assess the importance of various governing flow parameters to the behaviour of any specific flow problem. Two different methods are considered: One is called the direct method and the other the adjoint method. In the direct method the sensitivity equations are obtained by directly differentiating the flow equations with respect to the parameters, while in the adjoint method they are obtained by a variational technique. The numerical method for solving the groundwater flow equation and the sensitivity equations are based on the Galerkin finite element method. The sensitivity model developed was applied to a simple flow problem, in which the sensitivity of the piezometric head as well as various flux performance measures to perturbations of the permeability of various layers of the flow domain were analyzed. The following performance measures were considered: The piezometric head, the Darcy flux at selected regions and the total influx into a tunnel system.
Rapid Flow Model with Lateral Inflow
Authors: Strupczewski W. G. , Kundzewicz Z. W.
Published: May 2018

The rapid flow model (equivalent to the distributed delayed Muskingum model) is an attractive linear flood routing model meeting the conflicting demands of physical significance and simplicity. After having relaxed the assumption of negligible lateral inflow, made in former studies, the impulse response of the above model with lateral inflow is derived in the analytical (series) form. The cumulants of the impulse response of the model are developed. Although the results presented were obtained for the spatially uniform distribution of lateral inflow, it is possible to solve it also for non-uniform lateral inflows, at the cost of additional algebra.
Parameter Determination for Chloride and Tritium Transport in Undisturbed Lysimeters during Steady Flow
Authors: Jacobsen Ole H. , Leij Feike J. , van Genuchten Martinus Th.
Published: May 2018

Breakthrough curves of Cl and 3H2O were obtained during steady unsaturated flow in five lysimeters containing an undisturbed coarse sand (Orthic Haplohumod). The experimental data were analyzed in terms of the classical two-parameter convection-dispersion equation and a four-parameter two-region type physical nonequilibrium solute transport model. Model parameters were obtained by both curve fitting and time moment analysis. The four-parameter model provided a much better fit to the data for three soil columns, but performed only slightly better for the two remaining columns. The retardation factor for Cl was about 10 % less than for 3H2O, indicating some anion exclusion. For the four-parameter model the average immobile water fraction was 0.14 and the Peclet numbers of the mobile region varied between 50 and 200. Time moments analysis proved to be a useful tool for quantifying the break through curve (BTC) although the moments were found to be sensitive to experimental scattering in the measured data at larger times. Also, fitted parameters described the experimental data better than moment generated parameter values.
Stability of River Flow Regimes
Authors: Krasovskaia I. , Gottschalk L.
Published: May 2018

One of the most important consequences of future climate change may be an alteration of the surface hydrological balance, including changes in flow regimes, i.e. seasonal distribution of flow and especially the time of occurrence of high/low flow, which is of vital importance for environmental and economic policies. Classification of flow regimes still has an important role for the analyses of hydrological response to climate change as well as for validating climate models on present climatic and hydrologic data, however, with some modifications in the methodology. In this paper an approach for flow regime classification is developed in this context. Different ways of flow regime classification are discussed. The stability of flow regimes is studied in relation to changes in mean annual temperature and precipitation. The analyses have shown that even rather small changes in these variables can cause changes in river flow regimes. Different patterns of response have been traced for different regions of the Nordic countries.
Landsat Thematic Mapper (TM) Derived Reflectance from a Mountainous Watershed during the Snow Melt Season
Authors: Winther Jan-Gunnar
Published: May 2018

Landsat-5 Thematic Mapper (TM) reflectances recorded i May 1989 from a mountainous catchment at Kvikne, Norway, are analyzed and compared with simultaneously measured in situ albedo. The satellite-registered shortwave snow albedo comparable to bare ground albedo and as low as 0.19 was found in areas where the snow was highly metamorphosed and heavily blackened by organic material. However, the contrast between snow and snow-free areas can be strongly improved by using a normalized TM Band 2-5 difference image. While TM Band 2 shows varying degrees of snow surface contamination within the study area, the normalized TM Band 2-5 difference image is not affected by impurities. Further, a normalized TM Band 2-4 difference image indicates that about one third of the variability in TM Band 4 is caused by effective snow grain size variations while the remaining is a result of surface contamination. Finally, the satellite reflectances have been corrected for atmospheric absorption and scattering present on the day of the satellite overpass.
Frequency of Extremes and its Relation to Climate Fluctuations
Authors: Krasovskaia I. , Gottschalk L.
Published: May 2018

Possible consequences of climate change concern both changes in long-term mean values of runoff and changes in frequency and magnitude of extreme runoff events. The physical safety of dams and protection against floods are not sensitive to the moderate changes in mean values but to the frequency and magnitude of extremes. This study presents the results of the analyses of the changes in the behavior of the extreme runoff values due to observed changes of temperature and precipitation. Statistical parameters of the magnitude of floods as well as their intensity have been studied. An attempt is also made to establish regional probability distribution curves for the frequencies of the extreme floods for different patterns of changes in the climatic variables considered.
A Theoretical Model of Thermal Bar Movement in a Circular Lake
Authors: Zilitinkevich S. S. , Malm J. G. B.
Published: May 2018

The present paper concentrates on the movement of the spring thermal bar with time, and a simple theoretical model is derived for the case of an axisymmetric circular lake. The lake is considered to consist of three different zones: 1) a nearshore stably stratified warm water zone, 2) the thermal bar zone of descending water with temperature approximately equal to that of maximum density, and 3) an offshore cold unstably stratified zone. The basis for the model is the heat budget equation, where horizontal dynamical heat exchange in the vicinity of the bar zone is taken into account. In comparison with a previous model (Zilitinkevich et al. 1991), describing an infinite wedge of water, the present model is more general and takes into account the effect of shore-line curvature. This effect is characterized by a non-dimensional number and causes a marked increase in the displacement rate of the bar at the final stage of its existence. Model predictions are also compared with field data taken from the literature.
Hydrological Model for the Tude Å Catchment
Authors: Christensen Steen
Published: May 2018

A numerical hydrological model has been developed for a 450 km2 Danish catchment using comprehensive field data. The model integrates a simple evapotranspiration model, a lumped flow model for a phreatic aquifer found in till, and a traditional two-dimensional groundwater model for a confined fluvio-glacial aquifer. A minimal but adequate number of model parameters were calibrated by trial and error to make the model fit 29-year time series of hydraulic head and stream runoff data.

By simulating a “semi-natural” hydrological situation unaffected by withdrawals it is demonstrated that groundwater development can change the water balance considerably. In the actual case withdrawals induce a 25% increase in leakage from the phreatic to the confined aquifer, and reduce stream base flow by up to 30% in normal years, and up to 35% in dry years. On the other hand the reduction in base flow is considerably smaller for the upper stream catchments.
On Currents and Vertical Mixing in Lake Ontario during Summer Stratification
Authors: Omstedt A. , Murthy C.R.
Published: May 2018

Currents and vertical mixing characteristics were investigated on the basis of time series of current meter and temperature data from a summer-stratified period in Lake Ontario. The experimental set up consisted of seven current meters distributed in one vertical line from 12 m below the surface to 1 m above the lake bottom at a total depth of 143 m. The period considered for the analysis was from June to September, 1991.

The currents showed pronounced oscillations with two significant kinetic energy peaks, one at about 17 hours due to inertial motions, and one at 10 days, probably due to meteorological forcing. The current shear in the hypolimnion was strong enough to overcome stability and generate turbulence (Richardson numbers below 0.25) and there was probably turbulence enough available to keep the matter (almost neutral buoyant particles) in the whole Nepheloid bottom layer in suspension. In the thermocline region the turbulence was mainly damped (Richardson numbers above 1), but some events with lower Richardson numbers were also calculated indicating increased mixing during these events. By analysing filtered and unfiltered current meter data it was found that the shear-generated turbulence in the hypolimnion was mainly due to the meteorologically forced currents. In the thermocline region, however, the vertical shear associated with the inertial oscillation had a greater impact on the mixing.
Statistical Estimates of PMP Values
Authors: Elíasson Jónas
Published: May 2018

The article discusses two statistical methods to estimate PMP values, the Hershfield and the NERC methods. Neither method offers any explanation why the PMP values can be calculated by the use of unbounded statistical distributions, but both methods include the use of envelope curves that are not independent of the region. Bounded data that fits an unbounded distribution must deviate from the distribution for high return periods and tend to a limiting value, and then there exists, a limiting reduced variate that can be used to find the PMP value. When the distribution is EV1, the limiting reduced variate can be defined by a mapping transformation, or by cutting off the distribution. It is shown that when Hershfield or NERC methods are used, the limiting reduced variate is included in the PMP values and can be separated from regional parameters. It is suggested that the limiting reduced variate, that depends solely on return period, may more easily be transferred between regions than the other parameters. This may be a great help in finding PMP values in regions where observations are not extensive enough to define limiting return periods with necessary certainty.

A case study with data from Iceland demonstrates, that using the limiting reduced variate, similarities emerge in the Icelandic data and the NERC PMP that justify the acceptance of the NERC method.
Application of the ETH Snow Model to Three Basins of Different Character in Central Europe
Authors: Hottelet Ch. , Blažková Š. , Bičík M.
Published: May 2018

The performance of a version of the HBV-ETH model is evaluated, taking into account various aspect classes in each elevation belt in three different types of catchments: an alpine basin, and two basins in Bohemia at much lower elevation, with a snowpack which is smaller and of shorter duration. In all three cases a reasonably good agreement between modelled and observed runoff and between modelled and observed snowpack was achieved. In the smaller of the Bohemian basins (1.87 km2, 775-885 m a.s.l., influenced by deforestation), the effect of the presence and age or absence of forest on the development of the snow-water equivalent was found to be as important as the effect of aspect and altitude.
Discharge Data Collection and Analysis Strategies in Low Flow Studies
Authors: Clausen B.
Published: May 2018

The objective of this paper is to determine the most appropriate data collection strategy and analysis techniques which should be used to assess the low flow regime of a catchment. The data used were: a) synchronous discharge measurements during low flow periods, and b) continuous daily flow records. The analyses based on both types of data were able to distinguish different low flow regimes within a 114 km2 Danish catchment. Despite the limited spatial variation in climate and geomorphology there was a high spatial variability in low flows caused by differences in the lithology of sediments. This demonstrates the difficulties in using simple indices of catchment geology in regional low flow estimation. The results highlight the benefits of using synchronous discharge measurements, both for estimating low flows at ungauged sites, and for understanding groundwater flow paths. Analyses of daily flow records from six gauging stations in the catchment showed that a baseflow index was more useful than the flow duration curve for classifying low flow regimes when only short records were available. The paper illustrates the importance of estimating the uncertainty of discharge measurements when interpreting low flow data.
Calculation of the Discharge through Øresund at the Drogden Sill by Measurements at two Fixed Stations
Authors: Jakobsen Flemming , Castejon Stéphane
Published: May 2018

The paper aims at relating the discharge through Øresund (the sound between Denmark and Sweden) to measured current and stratification at the two fixed stations Nordre Røse (St. 24) and Flinten SW (St. 21). The information on the discharge is gained from ship-based ADCP measurements carried out on board R/V Sensor. Close to the sill area, Saltholm island divides the flux into two parts: One part through the Drogden Channel (where St. 24 is located), and the other through the Flinten Channel (where St. 21 is located). The establishment of a relationship between discharge and current measurements at two fixed stations is based on three basic assumptions: 1) the stratification in the Drogden and Flinten channels is considered to be one- or two-layered; 2) the flow to be driven by a constant energy gradient across each channel; and 3) by simplifying the bathymetry of the cross-sections. The study of the water discharge through Øresund reveals a close relationship of hydrographic measurements (current profile, salinity profile and water level) at Nordre Røse (St. 24) and Flinten SW (St. 21). From the investigation an interface friction factor (fi/2) for the Drogden Channel is found to be 5.6 × 10−3. This value is applied for the Flinten channel analysis as well.