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Papers by Thomas Skaugen

… Water Resources and …, 2005

... Editor: Authors: Eli Alfnes, Elin Langsholt, Thomas Skaugen and Hans-Christian Udnæs Print: N... more ... Editor: Authors: Eli Alfnes, Elin Langsholt, Thomas Skaugen and Hans-Christian Udnæs Print: NVE's printing office Number printed: 40 ... Oslo, January 2005 Svein Taksdal Acting director of the Hydrology Department Liss M. Andreassen Project leader Page 7. 6 Summary ...

Journal of Hydrology X, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Journal of Hydrology X, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

International Journal of River Basin Management, 2011

The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchmen... more The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchments of different scales. Daily runoff forecasts from these 117 catchments are regionalized for nationwide qualitative flood risk regionalization displayed in a map. The regionalization is performed without considering scale dependency in runoff response in different-sized catchments. We developed a simple catchment classification approach that allows

Hydrology Research

Based on downscaled daily precipitation values from the global climate model of the Max Planck In... more Based on downscaled daily precipitation values from the global climate model of the Max Planck Institute in Hamburg, time series of 20 years have been generated to describe the current climate of 1980- 1999 (control data) and the future climate of 2030-2049 (scenario data) for Norway. These time series serve as training data for the Randomised Bartlett-Lewis Rectangular Pulse Model (RBLRPM), a precipitation simulation model, and time series of 1000 years length have been generated to assess possible changes in the extreme precipitation regime due to climate change. The analysis of changes in extreme value patterns for annual and seasonal values in the scenario and control data sets shows tendencies towards increased extreme values and seasonal shifts for the scenario period. A general increase in mean and standard deviation of the extreme value sample and for values of 10 and 100 years return period is found, although the regional variability is significant. For some regions the inc...

A first version of an energy balance (EB) based snow melt model is formulated, based on previous ... more A first version of an energy balance (EB) based snow melt model is formulated, based on previous published work from several authors. The EB model uses only precipitation and air temperature as input, and the timeresolution is flexible (from hourly to daily). The algorithms of the EB model are presented in this technical research note, together with some evaluation results, where simulated and observed snow amounts and melt rates are compared. Comparison is also made between the EB approach and the basic degreeday approach to snow melt modelling.

For operational flood forecasting, the Norwegian Water Resources and Energy Administration (NVE) ... more For operational flood forecasting, the Norwegian Water Resources and Energy Administration (NVE) applies the conceptual HBV rainfall-runoff model for 117 catchments. The hydrological models are calibrated and run using an extensive meteorological grid data set providing daily temperature and precipitation data back to 1957 for entire Norway at 1x1 km grid resolution (seNorge grids). The daily temporal resolution is dictated by the resolution of historical meteorological data. However, since meteorological forecasts and runoff observations are also available at a much finer than a daily time-resolution (e.g. 6 hourly), and many hydrological extreme events happens at a temporal scale of less than daily, it is important to try to establish a historical dataset of meteorological input at a finer corresponding temporal resolution. We present a simple approach for the temporal disaggregation of the daily meteorological seNorge grids into 6-hour values by consulting a HIRLAM hindcast grid ...

Based on downscaled daily precipitation values from the global climate model of the Max Planck In... more Based on downscaled daily precipitation values from the global climate model of the Max Planck Institute in Hamburg, time series of 20 years have been generated to describe the current climate of 1980- 1999 (control data) and the future climate of 2030-2049 (scenario data) for Norway. These time series serve as training data for the Randomised Bartlett-Lewis Rectangular Pulse Model

Telephone: 22 95 95 95 Telefax: 22 95 90 00 Internet: www.nve.no

Hydrological Processes, 2014

ABSTRACT A new parameter parsimonious rainfall-runoff model, the Distance Distribution Dynamics (... more ABSTRACT A new parameter parsimonious rainfall-runoff model, the Distance Distribution Dynamics (DDD) model, is used to simulate hydrological times series at ungauged sites in the Lygne basin in Norway. The model parameters were estimated as functions of catchment characteristics determined by GIS. The multiple regression equations relating catchment characteristics and model parameters were trained from 84 calibrated catchments located all over Norway and all model parameters showed significant correlations with catchment characteristics. The significant correlation coefficients (with p- value < 0.05) ranged from 0.22-0.55. The suitability of DDD for predictions in ungauged basins was tested for 17 catchments not used to estimate the multiple regression equations. For 10 of the 17 catchments, deviations in Nash-Suthcliffe Efficiency (NSE) criteria between the calibrated and regionalised model were less than 0.1 and for two calibrated catchments within the Lygne basin the deviations were less than 0.08. The median NSE for the regionalised DDD for the 17 catchments, for two time series was 0.66 and 0.72. Deviations in NSE between calibrated and regionalised models are well explained by the deviations between calibrated and regressed parameters describing spatial snow distribution and snowmelt, respectively. The quality of the simulated runoff series for the ungauged sites in the Lygne basin was assessed by comparing flow indices describing high, medium and low flow estimated from observed runoff at the 17 catchments and for the simulated runoff series. The indices estimated for the simulated series were generally well within the ranges defined by the 17 observed series. This article is protected by copyright. All rights reserved.

Recent years have witnessed a growing interest in Arctic and high-mountain regions. Above the nat... more Recent years have witnessed a growing interest in Arctic and high-mountain regions. Above the natural tree line Norway is characterised by a modern Arctic environment, and the modern southern boundary for Scandinavian permafrost is located in the mountains of Southern Norway. Permafrost and seasonal frost are considered key components of the cryosphere, and the climate-permafrost relation has acquired added importance with the increasing awareness and concern of rising temperatures. CRYOLINK aims at improving knowledge on past and present ground temperatures, seasonal frost, permafrost distribution and related periglacial processes in Southern Norway and adjoining regions of the North Atlantic region (Greenland and Iceland), by addressing the fundamental problem of heat transfer between the atmosphere and the ground surface. Methodologically, the project develops functional thermal offset models linking air temperatures to ground and permafrost temperatures through seasonal surface ...

Ideally, hydrological models should be built from equations parameterised from observed catchment... more Ideally, hydrological models should be built from equations parameterised from observed catchment characteristics and data. This state of affairs may never be reached, but a governing principle in hydrological modelling should be to keep the number of calibration parameters to a minimum. A reduced number of parameters to be calibrated, while maintaining the accuracy and detail required by modern hydrological models, will reduce parameter and model structure uncertainty and improve model diagnostics. The dynamics of runoff for small catchments are derived from the distribution of distances from points in the catchments to the nearest stream in a catchment. This distribution is unique for each catchment and can be determined from a geographical information system (GIS). The distribution of distances, will, when a celerity of (subsurface) flow is introduced, provide a distribution of travel times, or a unit hydrograph (UH). For spatially varying levels of saturation deficit we have dif...

Journal of Hydrometeorology, 2013

ABSTRACT This paper presents a simple approach for the temporal disaggregation from daily to 3-ho... more ABSTRACT This paper presents a simple approach for the temporal disaggregation from daily to 3-hourly observed gridded temperature and precipitation (1 3 1km2) on the national scale. The intended use of the disaggregated 3-hourly data is to recalibrate the hydrological model currently used by the Norwegian Water Resources and Energy Directorate (NVE) for daily flood forecasting. By adapting the hydrological model to a 3-hourly temporal scale, the flood forecasting can benefit from available meteorological forecasts with finer temporal resolution and can better represent critical events of short duration and at small spatial scales. By consulting the temporal patterns of a High-Resolution Limited-Area Model (HIRLAM) hindcast series for northern Europe with an hourly temporal and a 0.18 spatial resolution, existing daily 1 3 1km2 grids for temperature and precipitation covering all of Norway (the seNorge data) were disaggregated into 3-hourly values for the time period September 1957 to December 2010. For the period 2000–05, the disaggregated 3-hourly temperature and precipitation data are validated against observed values from five meteorological stations and against 3-hourly data from the HIRLAMhindcast and daily seNorge data simply split into eight fractions. The results show that the disaggregated data perform best with anomaly correlation coefficients between 0.89 and 0.92 for temperature. With regard to precipitation, the disaggregated data also provide the highest correlations and the lowest errors. In addition, the disaggregated data prove to be best in estimating intervals without precipitation and tend to be most appropriate in estimating extreme precipitation with low occurrence probability (,20%).

Journal of Geophysical Research, 1996

The analysis and modeling of extreme areal precipitation is carried out focusing the attention on... more The analysis and modeling of extreme areal precipitation is carried out focusing the attention on extreme areal precipitation events rather than extreme point precipitation. The data is daily precipitation values from southern Norway. By investigating the spatial coverage of the fractional area (fraction of the considered catchment) where it rains over a certain intensity threshold, it was found, for a range of rain intensities, that the ratio between two fractional areas of consecutive thresholds is independent of the actual value of the rain intensity and the size of the considered catchment. The mean areal precipitation is seen as the sum of the fractional thresholded rainy areas for all intensity thresholds. By the properties of independence of the consecutive area ratios, it is possible to simulate fractional thresholded rainy areas, and thus compute the mean areal rainfall. The mean and the standard deviation of simulated series of extreme areal precipitation for five catchments of different size (from 48000 to 468 km 2) are in very good agreement with those of the observed. This is also the case for the observed and simulated values of the fractional thresholded rainy areas. The mean values of the spatial coverage of thresholded areas for mesoscale convective complexes in the United States are found to be well modeled by the proposed method. 26,287 26,288 SKAUGEN ET AL.: FREQUENCY OF EXTREME AREAL PRECIPITATION

International Journal of River Basin Management, 2011

The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchmen... more The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchments of different scales. Daily runoff forecasts from these 117 catchments are regionalized for nationwide qualitative flood risk regionalization displayed in a map. The regionalization is performed without considering scale dependency in runoff response in different-sized catchments. We developed a simple catchment classification approach that allows

Hydrology Research, 2012

ABSTRACT Trends in time series of snow water equivalent (SWE) are analyzed for different time per... more ABSTRACT Trends in time series of snow water equivalent (SWE) are analyzed for different time periods and as a function of elevation. Since 1914, hydropower companies have measured snow at the expected snow maximum. Most of the measurements are located at high elevations, 700–1,700 meters above sea level (masl), providing a unique dataset in areas poorly covered by meteorological stations. For single stations, significant positive and negative trends are found using the Mann–Kendall trend test. The trends depend on region, observation period and elevation. In southern Norway negative trends are found for sites below 1,350 masl for the periods 1931–1960 and 1991–2009, and below 850 masl for the period 1961–1990. Above these elevations, positive trends are found. For the entire period, 1931–2009, positive trends are found for stations located above 850 masl. These findings can be explained by varying trends in precipitation and temperature in winter and spring. For central and northern Norway, the pattern in trends is similar, although less pronounced. High and low values of the North Atlantic Oscillation (NAO) index correlate differently with SWE. High NAO index is positively correlated with SWE at high elevations and negatively at low elevations. Low NAO index is positively correlated with SWE at all elevations.

Hydrology and Earth System Sciences, 2003

A simulation exercise has been performed to study the temporal development of snow covered area a... more A simulation exercise has been performed to study the temporal development of snow covered area and the spatial distribution of snow-water equivalent (SWE). Special consideration has been paid to how the properties of the spatial statistical distribution of SWE change as a response to accumulation and ablation events. A distributed rainfall-runoff model at resolution 1 × 1 km 2 has been run with time series of precipitation and temperature fields of the same spatial resolution derived from the atmospheric model HIRLAM. The precipitation fields are disaggregated and the temperature fields are interpolated. Time series of the spatial distribution of snow-water equivalent and snow-covered area for three seasons for a catchment in Norway is generated. The catchment is of size 3085 km 2 and two rectangular sub-areas of 484 km 2 are located within the larger catchment. The results show that the shape of the spatial distribution of SWE for all three areas changes during winter. The distribution is very skewed at the start of the accumulation season but then the skew decreases and, as the ablation season sets in, the spatial distribution again becomes more skewed with a maximum near the end of the ablation season. For one of the sub-areas, a consistently more skewed distribution of SWE is found, related to higher variability in precipitation. This indicates that observed differences in the spatial distribution of snow between alpine and forested areas can result from differences in the spatial variability of precipitation. The results obtained from the simulation exercise are consistent with modelling the spatial distribution of SWE as summations of a gamma distributed variable.

Hydrological Sciences Journal, 2010

... 487976 Thomas Skaugen a * & Jess Andersen a pages 676-686. ... In Norway, the national fl... more ... 487976 Thomas Skaugen a * & Jess Andersen a pages 676-686. ... In Norway, the national flood forecasting service is based on simulations and forecasts from 117 catchments, using the Swedish hydrological model, the HBV model (Bergström, 19922. Bergström, S. 1992. ...

Hydrological Processes, 2013

ABSTRACT A guiding principle in hydrological modelling should be to keep the number of calibratio... more ABSTRACT A guiding principle in hydrological modelling should be to keep the number of calibration parameters to a minimum. A reduced number of parameters to be calibrated, while maintaining the accuracy and detail required by modern hydrological models, will reduce parameter and model structure uncertainty and improve model diagnostics. In this study, the dynamics of runoff are derived from the distribution of distances from points in the catchments to the nearest stream. This distribution is unique for each catchment and can be determined from a geographical information system. The distribution of distances, will, when a celerity of (subsurface) flow is introduced, provide a distribution of travel times, or a unit hydrograph (UH). For spatially varying levels of saturation deficit, we have different celerities and, hence, different UHs. Runoff is derived from the superposition of the different UHs. This study shows how celerities can be estimated if we assume that recession events represent the combined UHs for different levels of saturation deficit. A new soil moisture routine which estimates saturated and unsaturated volumes of subsurface water and with only one parameter to calibrate is included in the new model. The performance of the new model is compared with that of the Swedish HBV model and is found to perform equally well for eight Norwegian catchments although the number of parameters to be calibrated in the module concerning soil moisture and runoff dynamics is reduced from seven in the HBV model to one in the new model. It is also shown that the new model has a more realistic representation of the subsurface hydrology. Copyright © 2013 John Wiley & Sons, Ltd.

… Water Resources and …, 2005

... Editor: Authors: Eli Alfnes, Elin Langsholt, Thomas Skaugen and Hans-Christian Udnæs Print: N... more ... Editor: Authors: Eli Alfnes, Elin Langsholt, Thomas Skaugen and Hans-Christian Udnæs Print: NVE's printing office Number printed: 40 ... Oslo, January 2005 Svein Taksdal Acting director of the Hydrology Department Liss M. Andreassen Project leader Page 7. 6 Summary ...

Journal of Hydrology X, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Journal of Hydrology X, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

International Journal of River Basin Management, 2011

The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchmen... more The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchments of different scales. Daily runoff forecasts from these 117 catchments are regionalized for nationwide qualitative flood risk regionalization displayed in a map. The regionalization is performed without considering scale dependency in runoff response in different-sized catchments. We developed a simple catchment classification approach that allows

Hydrology Research

Based on downscaled daily precipitation values from the global climate model of the Max Planck In... more Based on downscaled daily precipitation values from the global climate model of the Max Planck Institute in Hamburg, time series of 20 years have been generated to describe the current climate of 1980- 1999 (control data) and the future climate of 2030-2049 (scenario data) for Norway. These time series serve as training data for the Randomised Bartlett-Lewis Rectangular Pulse Model (RBLRPM), a precipitation simulation model, and time series of 1000 years length have been generated to assess possible changes in the extreme precipitation regime due to climate change. The analysis of changes in extreme value patterns for annual and seasonal values in the scenario and control data sets shows tendencies towards increased extreme values and seasonal shifts for the scenario period. A general increase in mean and standard deviation of the extreme value sample and for values of 10 and 100 years return period is found, although the regional variability is significant. For some regions the inc...

A first version of an energy balance (EB) based snow melt model is formulated, based on previous ... more A first version of an energy balance (EB) based snow melt model is formulated, based on previous published work from several authors. The EB model uses only precipitation and air temperature as input, and the timeresolution is flexible (from hourly to daily). The algorithms of the EB model are presented in this technical research note, together with some evaluation results, where simulated and observed snow amounts and melt rates are compared. Comparison is also made between the EB approach and the basic degreeday approach to snow melt modelling.

For operational flood forecasting, the Norwegian Water Resources and Energy Administration (NVE) ... more For operational flood forecasting, the Norwegian Water Resources and Energy Administration (NVE) applies the conceptual HBV rainfall-runoff model for 117 catchments. The hydrological models are calibrated and run using an extensive meteorological grid data set providing daily temperature and precipitation data back to 1957 for entire Norway at 1x1 km grid resolution (seNorge grids). The daily temporal resolution is dictated by the resolution of historical meteorological data. However, since meteorological forecasts and runoff observations are also available at a much finer than a daily time-resolution (e.g. 6 hourly), and many hydrological extreme events happens at a temporal scale of less than daily, it is important to try to establish a historical dataset of meteorological input at a finer corresponding temporal resolution. We present a simple approach for the temporal disaggregation of the daily meteorological seNorge grids into 6-hour values by consulting a HIRLAM hindcast grid ...

Based on downscaled daily precipitation values from the global climate model of the Max Planck In... more Based on downscaled daily precipitation values from the global climate model of the Max Planck Institute in Hamburg, time series of 20 years have been generated to describe the current climate of 1980- 1999 (control data) and the future climate of 2030-2049 (scenario data) for Norway. These time series serve as training data for the Randomised Bartlett-Lewis Rectangular Pulse Model

Telephone: 22 95 95 95 Telefax: 22 95 90 00 Internet: www.nve.no

Hydrological Processes, 2014

ABSTRACT A new parameter parsimonious rainfall-runoff model, the Distance Distribution Dynamics (... more ABSTRACT A new parameter parsimonious rainfall-runoff model, the Distance Distribution Dynamics (DDD) model, is used to simulate hydrological times series at ungauged sites in the Lygne basin in Norway. The model parameters were estimated as functions of catchment characteristics determined by GIS. The multiple regression equations relating catchment characteristics and model parameters were trained from 84 calibrated catchments located all over Norway and all model parameters showed significant correlations with catchment characteristics. The significant correlation coefficients (with p- value < 0.05) ranged from 0.22-0.55. The suitability of DDD for predictions in ungauged basins was tested for 17 catchments not used to estimate the multiple regression equations. For 10 of the 17 catchments, deviations in Nash-Suthcliffe Efficiency (NSE) criteria between the calibrated and regionalised model were less than 0.1 and for two calibrated catchments within the Lygne basin the deviations were less than 0.08. The median NSE for the regionalised DDD for the 17 catchments, for two time series was 0.66 and 0.72. Deviations in NSE between calibrated and regionalised models are well explained by the deviations between calibrated and regressed parameters describing spatial snow distribution and snowmelt, respectively. The quality of the simulated runoff series for the ungauged sites in the Lygne basin was assessed by comparing flow indices describing high, medium and low flow estimated from observed runoff at the 17 catchments and for the simulated runoff series. The indices estimated for the simulated series were generally well within the ranges defined by the 17 observed series. This article is protected by copyright. All rights reserved.

Recent years have witnessed a growing interest in Arctic and high-mountain regions. Above the nat... more Recent years have witnessed a growing interest in Arctic and high-mountain regions. Above the natural tree line Norway is characterised by a modern Arctic environment, and the modern southern boundary for Scandinavian permafrost is located in the mountains of Southern Norway. Permafrost and seasonal frost are considered key components of the cryosphere, and the climate-permafrost relation has acquired added importance with the increasing awareness and concern of rising temperatures. CRYOLINK aims at improving knowledge on past and present ground temperatures, seasonal frost, permafrost distribution and related periglacial processes in Southern Norway and adjoining regions of the North Atlantic region (Greenland and Iceland), by addressing the fundamental problem of heat transfer between the atmosphere and the ground surface. Methodologically, the project develops functional thermal offset models linking air temperatures to ground and permafrost temperatures through seasonal surface ...

Ideally, hydrological models should be built from equations parameterised from observed catchment... more Ideally, hydrological models should be built from equations parameterised from observed catchment characteristics and data. This state of affairs may never be reached, but a governing principle in hydrological modelling should be to keep the number of calibration parameters to a minimum. A reduced number of parameters to be calibrated, while maintaining the accuracy and detail required by modern hydrological models, will reduce parameter and model structure uncertainty and improve model diagnostics. The dynamics of runoff for small catchments are derived from the distribution of distances from points in the catchments to the nearest stream in a catchment. This distribution is unique for each catchment and can be determined from a geographical information system (GIS). The distribution of distances, will, when a celerity of (subsurface) flow is introduced, provide a distribution of travel times, or a unit hydrograph (UH). For spatially varying levels of saturation deficit we have dif...

Journal of Hydrometeorology, 2013

ABSTRACT This paper presents a simple approach for the temporal disaggregation from daily to 3-ho... more ABSTRACT This paper presents a simple approach for the temporal disaggregation from daily to 3-hourly observed gridded temperature and precipitation (1 3 1km2) on the national scale. The intended use of the disaggregated 3-hourly data is to recalibrate the hydrological model currently used by the Norwegian Water Resources and Energy Directorate (NVE) for daily flood forecasting. By adapting the hydrological model to a 3-hourly temporal scale, the flood forecasting can benefit from available meteorological forecasts with finer temporal resolution and can better represent critical events of short duration and at small spatial scales. By consulting the temporal patterns of a High-Resolution Limited-Area Model (HIRLAM) hindcast series for northern Europe with an hourly temporal and a 0.18 spatial resolution, existing daily 1 3 1km2 grids for temperature and precipitation covering all of Norway (the seNorge data) were disaggregated into 3-hourly values for the time period September 1957 to December 2010. For the period 2000–05, the disaggregated 3-hourly temperature and precipitation data are validated against observed values from five meteorological stations and against 3-hourly data from the HIRLAMhindcast and daily seNorge data simply split into eight fractions. The results show that the disaggregated data perform best with anomaly correlation coefficients between 0.89 and 0.92 for temperature. With regard to precipitation, the disaggregated data also provide the highest correlations and the lowest errors. In addition, the disaggregated data prove to be best in estimating intervals without precipitation and tend to be most appropriate in estimating extreme precipitation with low occurrence probability (,20%).

Journal of Geophysical Research, 1996

The analysis and modeling of extreme areal precipitation is carried out focusing the attention on... more The analysis and modeling of extreme areal precipitation is carried out focusing the attention on extreme areal precipitation events rather than extreme point precipitation. The data is daily precipitation values from southern Norway. By investigating the spatial coverage of the fractional area (fraction of the considered catchment) where it rains over a certain intensity threshold, it was found, for a range of rain intensities, that the ratio between two fractional areas of consecutive thresholds is independent of the actual value of the rain intensity and the size of the considered catchment. The mean areal precipitation is seen as the sum of the fractional thresholded rainy areas for all intensity thresholds. By the properties of independence of the consecutive area ratios, it is possible to simulate fractional thresholded rainy areas, and thus compute the mean areal rainfall. The mean and the standard deviation of simulated series of extreme areal precipitation for five catchments of different size (from 48000 to 468 km 2) are in very good agreement with those of the observed. This is also the case for the observed and simulated values of the fractional thresholded rainy areas. The mean values of the spatial coverage of thresholded areas for mesoscale convective complexes in the United States are found to be well modeled by the proposed method. 26,287 26,288 SKAUGEN ET AL.: FREQUENCY OF EXTREME AREAL PRECIPITATION

International Journal of River Basin Management, 2011

The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchmen... more The national flood forecasting service in Norway applies the conceptual HBV model in 117 catchments of different scales. Daily runoff forecasts from these 117 catchments are regionalized for nationwide qualitative flood risk regionalization displayed in a map. The regionalization is performed without considering scale dependency in runoff response in different-sized catchments. We developed a simple catchment classification approach that allows

Hydrology Research, 2012

ABSTRACT Trends in time series of snow water equivalent (SWE) are analyzed for different time per... more ABSTRACT Trends in time series of snow water equivalent (SWE) are analyzed for different time periods and as a function of elevation. Since 1914, hydropower companies have measured snow at the expected snow maximum. Most of the measurements are located at high elevations, 700–1,700 meters above sea level (masl), providing a unique dataset in areas poorly covered by meteorological stations. For single stations, significant positive and negative trends are found using the Mann–Kendall trend test. The trends depend on region, observation period and elevation. In southern Norway negative trends are found for sites below 1,350 masl for the periods 1931–1960 and 1991–2009, and below 850 masl for the period 1961–1990. Above these elevations, positive trends are found. For the entire period, 1931–2009, positive trends are found for stations located above 850 masl. These findings can be explained by varying trends in precipitation and temperature in winter and spring. For central and northern Norway, the pattern in trends is similar, although less pronounced. High and low values of the North Atlantic Oscillation (NAO) index correlate differently with SWE. High NAO index is positively correlated with SWE at high elevations and negatively at low elevations. Low NAO index is positively correlated with SWE at all elevations.

Hydrology and Earth System Sciences, 2003

A simulation exercise has been performed to study the temporal development of snow covered area a... more A simulation exercise has been performed to study the temporal development of snow covered area and the spatial distribution of snow-water equivalent (SWE). Special consideration has been paid to how the properties of the spatial statistical distribution of SWE change as a response to accumulation and ablation events. A distributed rainfall-runoff model at resolution 1 × 1 km 2 has been run with time series of precipitation and temperature fields of the same spatial resolution derived from the atmospheric model HIRLAM. The precipitation fields are disaggregated and the temperature fields are interpolated. Time series of the spatial distribution of snow-water equivalent and snow-covered area for three seasons for a catchment in Norway is generated. The catchment is of size 3085 km 2 and two rectangular sub-areas of 484 km 2 are located within the larger catchment. The results show that the shape of the spatial distribution of SWE for all three areas changes during winter. The distribution is very skewed at the start of the accumulation season but then the skew decreases and, as the ablation season sets in, the spatial distribution again becomes more skewed with a maximum near the end of the ablation season. For one of the sub-areas, a consistently more skewed distribution of SWE is found, related to higher variability in precipitation. This indicates that observed differences in the spatial distribution of snow between alpine and forested areas can result from differences in the spatial variability of precipitation. The results obtained from the simulation exercise are consistent with modelling the spatial distribution of SWE as summations of a gamma distributed variable.

Hydrological Sciences Journal, 2010

... 487976 Thomas Skaugen a * & Jess Andersen a pages 676-686. ... In Norway, the national fl... more ... 487976 Thomas Skaugen a * & Jess Andersen a pages 676-686. ... In Norway, the national flood forecasting service is based on simulations and forecasts from 117 catchments, using the Swedish hydrological model, the HBV model (Bergström, 19922. Bergström, S. 1992. ...

Hydrological Processes, 2013

ABSTRACT A guiding principle in hydrological modelling should be to keep the number of calibratio... more ABSTRACT A guiding principle in hydrological modelling should be to keep the number of calibration parameters to a minimum. A reduced number of parameters to be calibrated, while maintaining the accuracy and detail required by modern hydrological models, will reduce parameter and model structure uncertainty and improve model diagnostics. In this study, the dynamics of runoff are derived from the distribution of distances from points in the catchments to the nearest stream. This distribution is unique for each catchment and can be determined from a geographical information system. The distribution of distances, will, when a celerity of (subsurface) flow is introduced, provide a distribution of travel times, or a unit hydrograph (UH). For spatially varying levels of saturation deficit, we have different celerities and, hence, different UHs. Runoff is derived from the superposition of the different UHs. This study shows how celerities can be estimated if we assume that recession events represent the combined UHs for different levels of saturation deficit. A new soil moisture routine which estimates saturated and unsaturated volumes of subsurface water and with only one parameter to calibrate is included in the new model. The performance of the new model is compared with that of the Swedish HBV model and is found to perform equally well for eight Norwegian catchments although the number of parameters to be calibrated in the module concerning soil moisture and runoff dynamics is reduced from seven in the HBV model to one in the new model. It is also shown that the new model has a more realistic representation of the subsurface hydrology. Copyright © 2013 John Wiley & Sons, Ltd.