| 1. |
- Westerström, Göran, et al.
(författare)
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An investigation of snowmelt runoff on experimental plots in Lulea, Sweden
- 2000
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Ingår i: Hydrological Processes. - 0885-6087 .- 1099-1085. ; 14:10, s. 1869-1885
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Tidskriftsartikel (refereegranskat)abstract
- Empirical characteristics of snowmelt runoff are derived from observations made during snowmelt in a six-year period from 1980 to 1985 on three experimental plots and three plates located on the campus of the Lulea University of Technology in Lulea, Sweden. The plots had asphalt, gravel and grass surfaces. The plates were of different designs with one having the bottom cut out so that it was more like a frame. With the assumption that the asphalt surface of the plots was impervious, infiltration of meltwater into gravel and grass surfaces was deduced. Unlike rainfall infiltration, the graph of snowmelt infiltration rate resembled a flow hydrograph, with a distinct rise, a peak and a distinct recession. A strong linear relationship between the snowmelt runoff hydrograph peak and the snowmelt amount was found, which explained more than 90% of the variability in the snowmelt peak. This is in contrast with rainfall runoff where the relationship between runoff peak and volume is decidedly nonlinear. Hourly snowmelt runoff peak and daily snowmelt amount were found to exhibit nearly constant skew and follow approximately a Gumbel frequency distribution.
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| 2. |
- Beldring, Stein, et al.
(författare)
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Kinematic wave approximations to hillslope hydrological processes in tills
- 2000
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Ingår i: Hydrological Processes. - 0885-6087 .- 1099-1085. ; 14:4, s. 727-745
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Tidskriftsartikel (refereegranskat)abstract
- This work has been carried out within the framework of NOPEX - a NOrthern hemisphere climate Processes land surface EXperiment. Its purpose is to describe the spatial variability of groundwater levels and soil moisture content and their influence on runoff generation in small catchments in a landscape dominated by boreal forest and till soils, which is characteristic for the Nordic countries. Kinematic wave approximations have been used to describe saturated subsurface flow and saturation overland flow in hillslopes with a thin soil layer overlying a relatively impermeable bedrock. Simultaneous analyses of catchment runoff, groundwater-table depths and soil moisture in the unsaturated zone have been performed by including the kinematic wave descriptions in a precipitation-runoff model. The results have been compared with observed hydrographs and spatial patterns of groundwater levels and soil moisture content in two small experimental catchments. Results from this study indicate that is reasonable to apply the same parameter set when describing hydrological processes in computational elements with similar characteristics at a scale of about 1 km(2) in the NOPEX area.
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| 3. |
- Gustafsson, David, et al.
(författare)
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Factors governing the formation and persistance of layers in a sub-alpine snowpack
- 2004
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 18:7, s. 1165-1183
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Tidskriftsartikel (refereegranskat)abstract
- The layered structure of a snowpack has a great effect on several important physical processes, such as water movement, reflection of solar radiation or avalanche release. Our aim was to investigate what factors are most important with respect to the formation and persistence of distinct layers in a subalpine environment. We used a physically based numerical one-dimensional model to simulate the development of a snowpack on a subalpine meadow in central Switzerland during one winter season (1998-99). A thorough model validation was based on extensive measurement data including meteorological and snow physical parameters. The model simulated the snow water equivalent and the depth of the snowpack as well as the energy balance accurately. The observed strong layering of the snowpack, however, was not reproduced satisfactorily. In a sensitivity analysis, we tested different model options and parameter settings significant for the formation of snow layers. The neglection of effects of snow microstructure on the compaction rate, and the current description of the water redistribution inside the snowpack, which disregard capillary barrier effects, preferential flow and lateral water flow, were the major limitations for a more realistic simulation of the snowpack layering.
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| 4. |
- Lundberg, Angela, et al.
(författare)
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Estimating winter evaporation in boreal forests with operational snow course data
- 2003
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 17:8, s. 1479-1493
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Tidskriftsartikel (refereegranskat)abstract
- Snow course measurements from 11 sites located in eastern and northern Finland were used to estimate the total interception evaporation of a winter season for different forest categories. We categorized the sites based on forest density and tree species. Results showed that interception loss from gross precipitation increased with forest density and approached 30% for a forest with the highest density class. Interception loss for the most common forest density class was 11%. Interception losses were slightly larger in spruce forests than in pine, deciduous, or mixed forests. We provide suggestions as to how to design snow surveys to estimate wintertime interception evaporation better. Rough terrain and transition zones between forest and open areas should be avoided. Since evaporation fraction was strongly dependent on tree crown characteristics, snow course data should include direct estimates of canopy closure. Qualitative observations made by different observers should be given a reference frame to ensure comparability of records from different sites.
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| 5. |
- Lundberg, Angela, et al.
(författare)
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Snow accumulation in forests from ground and remote sensing data
- 2004
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 18:10, s. 1941-1955
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Tidskriftsartikel (refereegranskat)abstract
- Winter-forest processes affect global and local climates. The interception-sublimation fraction (F) of snowfall in forests is a substantial part of the winter water budget (up to 40%). Climate, weather-forecast and hydrological modellers incorporate increasingly realistic surface schemes into their models, and algorithms describing snow accumulation and snow-interception sublimation are now finding their way into these schemes. Spatially variable data for calibration and verification of wintertime dynamics therefore are needed for such modelling schemes. The value of F was determined from snow courses in open and forested areas in Hokkaido, Japan. The value of F was related to species and canopy-structure measures such as closure, sky-view fraction (SVF) and leaf-area index (LAI). Forest structure was deduced from fish-eye photographs. The value of F showed a strong linear correlation to structure: F = 0·44 - 0·6 × SVF for SVF < 0·72 and F = 0 for SVF > 0·72, and F = 0·11 LAI. These relationships seemed valid for evergreen conifers, larch trees, alder, birch and mixed deciduous stands. Forest snow accumulation (SF) could be estimated from snowfall in open fields (So) and to LAI according to SF = So (1 - 0·11 LAI) as well as from SVF according to SF = So (0·56 + 0·6 SVF) for SVF < 0·72. The value of SF was equal to So for SVF values above 0·72. The value of sky-view fraction was correlated to the normalized difference snow index (NDSI) using a Landsat-TM image for observation plots exceeding 1 ha. Variables F and SF were related to NDSI for these plots according to: F = -0·37NDSI + 0·29 and SF = So (0·81 + 0·37NDSI). These relationships are somewhat hypothetical because plot-size limitation only allowed one sparse-forest observation of NDSI to be used. There is, therefore, a need to confirm these relationships with further studies.
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| 6. |
- Merabtene, T, et al.
(författare)
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Risk assessment for optimal drought management of an integrated water resources system using a genetic algorithm
- 2002
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Ingår i: Hydrological Processes. - : Wiley. - 1099-1085 .- 0885-6087. ; 16:11, s. 2189-2208
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Tidskriftsartikel (refereegranskat)abstract
- A decision support system (DSS) is developed and applied to assess the susceptibility of water supply systems to droughts, and to aid decision-makers in determining optimal supply strategies. The DSS integrates three fundamental modules for water resources management: (1) a real time rainfall-runoff forecasting model enhanced by Kalman filtering; (2) a water demand forecast model; and (3) a reservoir operation model. Simulation and optimization procedures for the reservoir operation model are based on risk analysis to evaluate the system performance and to derive the most appropriate supply strategy of minimum risk, for the designed operating conditions. The optimization technique, based on genetic algorithms, introduces two new and distinct features, with the aim of minimizing the risks of drought damage and improving the convergence of the model toward practical solutions. Firstly, risk-based measures of system performance, termed reliability, resiliency and vulnerability, are combined into a global risk index, referred to as the drought risk index (DRI). The DRI, formulated as a weighted function of the risk measures, serves as the objective function to be minimized during the search for the optimal operation. Secondly, in the genetic algorithm search, each new generation of water supply solutions is created from solutions with risk levels clustered inside a defined 'acceptable risk space'. In other words, the convergence of the algorithm is improved by retaining only those solutions with DRI values smaller than the maximum acceptable risk. As a case study, the DSS is applied to the water resources system in Fukuoka City, western Japan. The DSS is believed to be an efficient tool for the assessment of a sequence of water supply scenarios, leading to the improved utilization of existing water resources during drought.
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| 7. |
- Seibert, J., et al.
(författare)
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Simulating interactions between saturated and unsaturated storage in a conceptual runoff model
- 2003
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 17:2, s. 379-390
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Tidskriftsartikel (refereegranskat)abstract
- There is an increasing demand for modelling the fluxes of chemical constituents at the catchment scale. Conceptual runoff models provide a basis for such modelling tasks provided that they capture the essential hydrological processes. However, most conceptual models do not fully address interactions between unsaturated and saturated storage. This can lead to unrealistic simulations for watersheds with shallow groundwater, where a large part of the soil volume can contribute to both the unsaturated and the saturated storage, depending on groundwater levels. Adding a small amount of water to the saturated storage will cause a significant amount of water stored in the unsaturated zone to change its status to ‘saturated’. The maximum volume of the unsaturated storage also decreases with rising groundwater levels, i.e. increasing saturated storage. In this study, a new model concept was proposed in which special emphasis was put on the interaction between saturated and unsaturated storage. The total storage was divided into two compartments, representing saturated and unsaturated storage, with a boundary moving up and down in response to the water budget of the compartments. Groundwater dynamics show a distinct pattern along the hillslope studied. Groundwater levels in an area close to the stream had dynamics similar to runoff, whereas levels further upslope responded to rainfall with a delay. To represent these differences in the model, the hillslope was subdivided into a riparian and an upslope reservoir. The performance of the new model was compared with that of simpler model variants without spatial differentiation and with or without the new formulation allowing for interactions between unsaturated and saturated storage. The new model approach provided the best results for simulating both runoff and groundwater dynamics. The subdivision of the hillslope accounted for most of the performance increase. To test the model structure further, 18O concentrations in the stream were simulated and compared with measured values.
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| 8. |
- Singh, P, et al.
(författare)
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Hydrological sensitivity of a large Himalayan basin to climate change
- 2004
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Ingår i: Hydrological Processes. - : Wiley. - 1099-1085 .- 0885-6087. ; 18:13, s. 2363-2385
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Tidskriftsartikel (refereegranskat)abstract
- The present study sets out to investigate the sensitivity of water availability to climate change for a large western Himalayan river (the Satluj River basin with an area of 22 275 km(2) and elevation range of 500 to 7000 m), which receives contributions from rain, snow and glacier melt runoff. About 65% of the basin area is covered with snow during winter, which reduces to about 11% after the ablation period. After having calibrated a conceptual hydrological model to provide accurate simulations of observed stream flow, the hydrological response of the basin was simulated using different climatic scenarios over a period of 9 years. Adopted plausible climate scenarios included three temperature scenarios (T + 1, T + 2, T + 3degreesC) and four rainfall scenarios (P - 10, P - 5, P + 5 and P + 10%). The effect of climate change was studied on snowmelt and rainfall contribution runoff, and total stream flow. Under warmer climate, a typical feature of the study basin was found to be reduction in melt from the lower part of the basin owing to a reduction in snow covered area and shortening of the summer melting season, and, in contrast, an increase in the melt from the glacierized part owing to larger melt and an extended ablation period. Thus, on the basin scale, reduction in melt from the lower part was counteracted by the increase from melt from upper part of the basin, resulting in a decrease in the magnitude of change in annual melt runoff. The impact of climate change was found to be more prominent on seasonal rather than annual water availability. Reduction of water availability during the summer period, which contributes about 60% to the annual flow, may have severe implications on the water resources of the region, because demand of water for irrigation, hydropower and other usage is at its peak at this time. Copyright (C) 2004 John Wiley Sons, Ltd.
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| 9. |
- Stahli, M., et al.
(författare)
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Soil frost effects on soil water and runoff dynamics along a boreal transect : 2. Simulations
- 2001
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 15:6, s. 927-941
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Tidskriftsartikel (refereegranskat)abstract
- A physically based SVAT-model was rested with soil and snow physical measurements, as well as runoff data fr om an 8600 m(2) catchment in northern Sweden in order to quantify the influence of soil frost on spring snowmelt runoff in a moderately sloped, boreal forest. The model was run as an array of connected profiles cascading to the brook. For three winter seasons (1995-98) it was able to predict the onset and total accumulation of the runoff with satisfactory accuracy. Surface runoff was identified as only a minor fraction of the total runoff occurring during short periods in connection with ice blocking of the water-conducting pores. Little surface runoff, though, does not mean that soil frost is unimportant for spring runoff. Simulations without frost routines systematically underestimated the total accumulated runoff. The possibility of major frost effects appearing in response to specific combinations of weather conditions were also tested. Different scenarios of critical initial conditions for the winter, e.g, high water saturation and delayed snow accumulation leading to an increased frost penetration, were tested. These showed that under special circumstances there is potential for increased spring runoff due to soil frost.
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| 10. |
- Wilk, Julie, et al.
(författare)
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Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand
- 2001
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 15:14, s. 2729-2748
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Tidskriftsartikel (refereegranskat)abstract
- Small-scale experiments have demonstrated that forest clearance leads to an increase in water yield, but it is unclear if this result holds for larger river basins (> 1000 km(2)). No widespread changes in rainfall totals and patterns were found in the 12100 km(2) Nam Pong catchment in northeast Thailand between 1957 and 1995, despite a reduction in the area classified as forest from 80% to 27% in the last three decades. Neither were any detectable changes found in any other water balance terms nor in the dynamics of the recession at the end of the rainy season. When a hydrological model calibrated against data from the period before the deforestation was applied for the last years of the study period (1987-1995), runoff generation was however underestimated by approximately 15%, indicating increased runoff generation after the deforestation. However, this was mainly due to the hydrological response during one single year in the first period, when the Q/P ratio was very low. When excluding this year, neither analysis based on the hydrological model could reveal any significant change of the water balance due to the deforestation. More detailed land-use analysis revealed that shade trees were left on agricultural plots as well as a number of abandoned areas where secondary growth can be expected, which is believed to account for the results.
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