| 1. |
- Grelle, A., et al.
(författare)
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Evaporation components of a boreal forest : variations during the growing season
- 1997
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Ingår i: Journal of Hydrology. - 0022-1694 .- 1879-2707. ; 197:1-4, s. 70-87
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Tidskriftsartikel (refereegranskat)abstract
- To improve the understanding of interactions between the boreal forest and the climate system as a key issue for global climate change, the water budget of a mixed pine and spruce forest in central Sweden was estimated by measurements of the water flux components and the total evaporation flux during the period 16 May-31 October 1995. Total evaporation was measured using eddy correlation and the components were obtained using measurements of precipitation, throughfall, tree transpiration, and forest floor evaporation. On a daily basis, tree transpiration was the dominant evaporation component during the vegetation period. However, it could be efficiently blocked by a wet canopy associated with large interception evaporation. The accumulated total evaporation was 399 mm, transpiration was 243 mm, forest floor evaporation was 56 mm and interception evaporation was 74 mm. The accumulated sum of interception, transpiration, and floor evaporation was 51 mm larger than the actual measured total evaporation. This difference was mainly attributed to the fact that transpiration was measured in a rather dense 50-year-old stand while total evaporation represented the average conditions of older, roughly 100-year-old stands. To compare eddy-correlation measurements with small-scale measurements of evaporation components, a source area analysis was made to select the flux data that give the best representation of the investigated stand. Especially under stable atmospheric conditions the requirements for surface homogeneity were very high and extreme care had to be taken to be aware of the flux source areas. Canopy water storage was determined by two methods: by the water balance of the canopy, which gave a result of 3.3 mm; and by the so-called minimum method based on plots of throughfall versus precipitation, which gave a much lower value of 1.5 mm. Seasonal interception evaporation constituted 30% of the precipitation.
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| 2. |
- Halldin, Sven, et al.
(författare)
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NOPEX - a northern hemisphere climate processes land surface experiment
- 1998
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Ingår i: Journal of Hydrology. - 0022-1694 .- 1879-2707. ; 212-213, s. 172-187
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Tidskriftsartikel (refereegranskat)abstract
- The interface between land surfaces and the atmosphere is a key area in climate research, where lack of basic knowledge prevents us from reducing the considerable uncertainties about predicted changes. Boreal forests play an important, but not well known, role in the global hydrological and biogeochemical cycles. NOPEX (a NOrthern hemisphere climate Processes land surface EXperiment) is devoted to the study of land surface-atmosphere interaction in a northern European forest-dominated landscape. The main NOPEX region represents the southern edge of the boreal zone. It consists of a highly heterogeneous landscape, with forests, mires, agricultural land and lakes. A second study site, in northern Finland, representing the northern edge of the boreal zone, will be introduced into NOPEX in connection with its coming winter-time field activities. Field activities, dominating the initial phase of NOPEX, are conceived to strike a balance between the need to cover multi-annual observations and the resources required to carry out measurements covering all relevant spatial scales. The long-term data collection activities, the Continuous Climate Monitoring (CCM), form the backbone of the field programme. A suite of Concentrated Field Efforts (CFEs) covering periods of summer, spring and winter brings together scientists from more then 20 countries during month-long campaigns. CFEs have been carried out in May-June 1994 and April-July 1995. A third, winter-time CFE is planned for 1998-99. The System for Information in NOPEX (SINOP) is the database which forms a backbone for modelling and analysis work, dominating the second stage of NOPEX. A series of PhD courses are run in parallell to the research activities. Analysis and modelling are done in four interacting areas, including local-scale processes, meso-scale surface-atmosphere coupling and remote sensing techniques. The fourth area, regionalization methods, aims at bringing the previous three together in order to provide improved parameterization schemes for exchange of energy, momentum, water and CO2 between land and atmosphere in hydrological and meteorological models from the meso to the global scale.
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| 3. |
- Lundberg, Angela, et al.
(författare)
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Evaporation of intercepted snow : measurement and modelling
- 1998
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Ingår i: Journal of Hydrology. - 0022-1694 .- 1879-2707. ; 206:3-4, s. 151-163
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Tidskriftsartikel (refereegranskat)abstract
- Snow storage on a coniferous forest canopy was measured using γ-ray attenuation and tree weighing systems, along with measurements of throughfall, using two plastic sheet net rainfall gauges. Meteorological parameters were measured with an automatic weather station. Estimates of evaporation of intercepted snow show an average rate of 0.24 mm h-1 and a maximum cumulative total of 3.9 mm in 7 h. Comparison with evaporation determined by a combination method with two different estimates of aerodynamic resistance (the "standard" rain aerodynamic resistance raL and a snow aerodynamic resistance raS-an order of magnitude larger than raL) showed that raL overestimated the evaporation by a factor of 2.6, whereas raS gave fair agreement with the measured evaporation. A multilayer model may be needed to take into account the variations of latent heat source area. Using the long-term measurements of the weight of snow on a single tree the total interception evaporation was estimated to be of the order 200 mm year-1.
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| 4. |
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| 5. |
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| 6. |
- Bendz, David, et al.
(författare)
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Evaporation from an active, uncovered landfill
- 1996
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 182:1-4, s. 143-155
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Tidskriftsartikel (refereegranskat)abstract
- The effect on evaporation of the presence of biodegradation heat in an uncovered biocell under operational conditions is elucidated. The contribution of the heat of biodegradation was investigated by measuring the temperature gradient in the top layer of a landfill. The evaporation was calculated by combining the energy budget with an expression for sensible heat flux, and taking the atmospheric stability into account by introducing the similarity theory of Monin and Obukhov. It was found that the biological heat enhanced the net energy flux and the actual evaporation by 20% and 10%, respectively.
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| 7. |
- Berndtsson, Ronny, et al.
(författare)
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Soil water and temperature patterns in an arid desert dune sand
- 1996
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 185:1-4, s. 221-240
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Tidskriftsartikel (refereegranskat)abstract
- Under arid natural conditions, soil water content governs and limits the number and size of perennial plant species. Thus, plant-available soil water is the main constraint for sustainable control of desert encroachment. To evaluate possibilities for re-vegetation of bare sand surfaces, soil water and temperature patterns for typical sand dunes in a desert climate were investigated. Bare and vegetated soil transects were selected for observation of soil water content and temperature. The investigated soil transects covered crest-to-crest spacings (about 60 m horizontally and 15 m vertically) in a shifting sand dune area. Observations were made at Shapotou field research station bordering the Tengger Desert in Northwestern China. The paper presents two-dimensional properties of soil water content (0.1-3.0 m depth) and temperature (0-1.0 m depth) before and after rainfall. Rainfall (15-22 mm) affected soil water distribution down to 1.5-2.0 m and temperature distribution down to 1.0 m. Soil water appeared to be transported through the apparently highly pervious and homogeneous sand along the dune slopes. High water contents and, thus, infiltration occurred mainly at the non-sloping parts, i.e. the dune crests and bottoms. Rainfall changed the temperature patterns from a mainly horizontally layered appearance before the rainfall to increasingly vertically shaped patterns.
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| 8. |
- Kawamura, Akira, et al.
(författare)
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Real-time tracking of convective rainfall properties using a two-dimensional advection-diffusion model
- 1997
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Ingår i: Journal of Hydrology. - 0022-1694. ; 203:1-4, s. 109-118
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Tidskriftsartikel (refereegranskat)abstract
- There is a need to improve rainfall forecasting capabilities for small ungaged urban catchments to reduce flooding hazards and pollution release. For this purpose, information is required on small-scale and short-term convective cell behavior. We use a two-dimensional stochastic advection-diffusion model to parameterize the space-time rainfall intensity from convective rainfall. The rainfall intensity resulting from different separable components of the rain cell, such as apparent turbulent diffusion and development/decay of rainfall intensity, is quantified for 10 observed and, for southern Sweden, representative high-intensity rainfall events. This is done following a Lagrangian approach. It is shown the used model was able to respond to rapid changes in observed rainfall intensity in both space and time, thus giving a small average root-mean-square error for all 10 events (0.06 mm min-1). When dividing the total rainfall intensity into apparent turbulent diffusion and development/decay terms, respectively, it was shown that D(y,center) and γ(center) contribute approximately equally to the observed rainfall intensity. The D(x,center) is usually only half the value of D(y,center), thus indicating less intensity contribution from this term and that the general elliptical shape of rain cells are elongated in the direction of movement. The observations indicate that the cumulus stage represents half and the dissipating stage half of the total cell development, respectively. The results can be used as first choice of parameter values when modeling rain cell movement over ungaged areas and the presented methodology can be used to study the effects of different cell components on total rainfall intensity.
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| 9. |
- Klove, B., et al.
(författare)
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Runoff generation in a plough-drained cutover fen in Central Finland
- 1999
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Ingår i: Journal of Hydrology. - 0022-1694. ; 218:3-4, s. 157-168
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Tidskriftsartikel (refereegranskat)abstract
- The hydrology of a cutover fen was studied from May to October in 1995 and 1996. Rainfall equivalent depths were measured every 15 min and stream runoff was continuously monitored. Water table depths were measured bi-weekly during 1995 and continuously during 1996. Storm runoff was separated into different components; rain falling directly into channels and rapid groundwater response based on a contributing area method and on different electrical conductivities of rain-, ground- and streamwater. The results show three sources of storm runoff from cutover fens. Frequent small runoff peaks were caused by rain falling directly into channels. The amount of quick runoff increased when rain fell on wet soil, resulting in intermediate sized runoff peaks generated by rapid groundwater response. The largest runoff values were observed after prolonged rainfall when water from the upper part of the catchment flooded the fen. (C) 1999 Elsevier Science B.V. All rights reserved.
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| 10. |
- Persson, Magnus, et al.
(författare)
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Estimating transport parameters in an undisturbed soil column using time domain reflectometry and transfer function theory
- 1998
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Ingår i: Journal of Hydrology. - 0022-1694. ; 205:3-4, s. 232-247
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Tidskriftsartikel (refereegranskat)abstract
- Modelling solute transport in the unsaturated zone is a complex problem given the inherent soil heterogeneity. Using time domain reflectometry (TDR) as described in this paper can improve data quality to be used in predictive models. The TDR technique requires appropriate calibration if it is to be used for solute concentration measurements under constant and varying water content. Two ways of determining the parameters in the relationship between bulk soil electrical conductivity, σa, soil solution electrical conductivity, σw and volumetric water content, θ, were used. Among the two calibration methods tested here, the one which involved eluting the soil with several pore volumes of water of known electrical conductivity under transient flow conditions in an undisturbed soil column gave a linear σw−σa−θ relationship with depth dependent parameters. The other calibration method involved the use of disturbed soil under static conditions and resulted in a curvilinear σw−σa−θ relationship. The former calibration was subsequently used to determine solute concentrations in breakthrough experiments. Two solute transport experiments were carried out in the same undisturbed soil column; one for steady-state and one for transient flow condition. Parameters of the convective log normal transfer function (CLT) model were determined using time-integral-normalized resident concentrations. It was shown that the stochastic-convective approach described the experimental data well. Horizontal heterogeneity was more important during the transient experiment, where the topsoil was periodically ponded during a short period, than in the steady-state experiment, where unsaturated conditions were maintained throughout the experiment. This increased heterogeneity during transient flow was also reflected by the fact that preferential solute flow was only observed in the column's outlet under transient conditions.
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