| 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. |
- Lankreijer, Harry, et al.
(författare)
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Evaporation and storage of intercepted rain analysed by comparing two models applied to a boreal forest
- 1999
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 595-604
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Tidskriftsartikel (refereegranskat)abstract
- Rainfall and throughfall were measured during the summer of 1995. Rainfall interception is often simulated by a version of the well-known Rutter-Gash analytical model. In this study this model was compared to a model based on an exponential saturation equation. The concept of the ‘minimum method' for deriving canopy storage capacity and free throughfall coefficient by the Leyton-analysis, is compared to the concept of maximum storage capacity by reversing the models. Measured evaporation rate during rain events was found to be lower than simulated by the Penman equation using different known formulations for aerodynamic resistance. The concept of a high internal canopy resistance and decoupling of the canopy from the atmosphere should be analysed further in order to explain low evaporation during rainfall.
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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. |
- Lundberg, Angela
(författare)
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Interception evaporation : processes and measurement techniques
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In temperate regions, evaporation of water intercepted by the vegetation represents an important part of the evapotranspiration. A review of interception evaporation studies showed that knowledge about interception evaporation processes is limited and progress is hampered by technical difficulties. Existing snow and rain interception measurement methods were reviewed and evaluated with criteria that were established for the ideal method for interception evaporation process studies. No existing methods fulfil all criteria. Two new load-cell based devices for process studies are tested, one weighing-cut-tree technique for snow interception and a technique based on weighing of both net and gross precipitation (with new type of wind-shield) for rain interception. By comparing measured (weighed) and calculated (combination method with different ways to calculate the aerodynamic resistance) evaporation rates for wet intercepted snow it was shown that the most important factors for calculating the evaporation were the relative humidity, the aerodynamic resistance, the wind speed and the intercepted mass. Calculations of evaporation of intercepted snow with a water budget method showed an average evaporation of 0.24 mm/hour and a maximum evaporation of 3.9 mm/7 hours when snow canopy storage was measured with a gamma-ray attenuation system and throughfall with plastic-sheet net-rainfall gauges. Total snow interception evaporation was estimated to be of the order 200 mm/year in Scotland. Comparison with evaporation determined with a combination method (Penman) and two different aerodynamic resistances, the "standard" rain aerodynamic resistance ...
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| 5. |
- Lundberg, Angela
(författare)
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Laboratory calibration of TDR-probes for snow wetness measurements
- 1997
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Ingår i: Cold Regions Science and Technology. - 0165-232X .- 1872-7441. ; 25:3, s. 197-205
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Tidskriftsartikel (refereegranskat)abstract
- In situ registration of snow liquid water content for avalanche protection and snow-melt prognosis is much needed. The Time Domain Reflectometry (TDR) method is a possible method to meet this need. Laboratory comparison of the TDR-method with the dilution method shows that the TDR-method has the potential to register variations in snow liquid water content down to 1-2 vol.%. It should be possible to achieve continuous registration of snow-pack wetness with a spatial resolution down to approximately 5 cm with several sets of TDR-probes (mounted with 3 cm vertical and 5 cm horizontal distance) combined with a multiplexer and a storage unit. For application when changes in snow density are great (as with newly fallen snow) the method has to be combined with separate density determinations. The dielectric constant of night-time refrozen snow can be used for density determination.
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| 6. |
- Lundberg, Angela, et al.
(författare)
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New approach to the measurement of interception evaporation
- 1997
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Ingår i: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 14:5, s. 1023-1035
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Tidskriftsartikel (refereegranskat)abstract
- Evaporation of water intercepted by vegetation represents an important (sometimes major) part of evapotranspiration in temperate regions. Interception evaporation is an important process where insufficient measurement techniques hamper progress in knowledge and modeling. An ideal technique to study the interception evaporation process should monitor intercepted mass (and its vertical distribution) and interception loss with high accuracy (0.1 mm) and time resolution (1 min), and give correct area estimates. The method should be inexpensive, require minor supervision during extended periods, and work in dense forests. Net precipitation techniques, in which interception evaporation is determined from the difference between gross precipitation (measured with funnels) and throughfall (measured with funnels, troughs, or plastic sheet net-rainfall gauges) fulfill many of the requirements but usually have a too-low accuracy and time resolution for process studies. Precipitation measurements are normally affected by distortion of the wind field around gauges as well as by adhesive and evaporative losses. Throughfall measurements with precipitation funnels, troughs, or plastic sheet net-rainfall gauges, manually emptied or combined with tipping buckets, usually have too-low accuracy and time resolution for process studies and are impaired by adhesive losses. A new loadcell-based system to determine interception evaporation from gross and net precipitation is presented. A weighing gauge with minimal wind loss is used for precipitation, and weighing troughs are used for throughfall measurements. The weighing troughs minimize adhesive-loss errors and react instantaneously. Preliminary results with the method confirm that it can be used for process studies with a high accuracy (0.1 mm) and a high time resolution (1 min).
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| 7. |
- Lundberg, Angela, et al.
(författare)
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Radar snow surveys : influence of snow density
- 1998
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Ingår i: XX Nordic Hydrological Conference. - Helsinki : Nordic Association for Hydrology. - 9521103221 ; , s. 679-
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Konferensbidrag (refereegranskat)
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| 8. |
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| 9. |
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| 10. |
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| 11. |
- Lundberg, Angela
(författare)
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Water budget analysis : a tool to minimize leachate production
- 1998
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Ingår i: Hydrology in the humid tropic environment. - : IAHS Press. ; , s. 15-25
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Konferensbidrag (refereegranskat)abstract
- Uncontrolled leakage of polluted leachate from landfills is the largest long-term hazard of environment disturbance from landfills. Large efforts should be made to control the leakage. Depending on the type of waste, different water transport regimes are desirable. For municipal waste with a large fraction of organic matter, high water transport through the landfill is advantageous during landfill operation time. Large water transport benefits biological decomposition and the attendance time can be reduced. For coal and peat ashes that produce leachate where we lack practical applicable methods today to reduce the environmentally hazardous concentrations, the water flux through the deposit has to be small to minimize leachate production from such deposits. For tailings with a large sulphide content, oxidation (producing acid) can be reduced by using a saturated cover-layer. Actions undertaken must be of such sustainability that the landfills then can be left without supervision. Water budget analysis is a useful tool when tailoring water flow through landfills. Effects of location (recharge/discharge area), bottom and top liners, surface drains, capillary barrier and vegetation covers are discussed.
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| 12. |
- Lundin, Lars-Christer, et al.
(författare)
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Continuous long-term measurements of soil-plant-atmosphere variables at a forest site
- 1999
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 53-73
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Tidskriftsartikel (refereegranskat)abstract
- It is a major challenge in modem science to decrease the uncertainty in predictions of global climate change. One of the largest uncertainties in present-day global climate models resides with the understanding of processes in the soil-vegetation-atmosphere-transfer (SVAT) system. Continuous, long-term data are needed in order to correctly quantify balances of water, energy and CO2 in this system and to correctly model it. It is the objective of this paper to demonstrate how a combined system of existing sensor, computer, and network technologies could be set up to provide continuous and reliable long-term SVAT-process data from a forested site under almost all environmental conditions. The Central Tower Site (CTS) system was set up in 1993-1994 in a 25 m high boreal forest growing on a highly heterogeneous till soil with a high content of stones and blocks. It has successfully monitored relevant states and fluxes in the system, such as atmospheric fluxes of momentum, heat, water vapour and CO2, atmospheric profiles of temperature, water vapour, CO2, short-and long-wave radiation, heat storage in soil and trees, sap-dow and a variety of ecophysiological properties, soil-water contents and tensions, and groundwater levels, rainfall and throughfall. System uptime has been more than 90% for most of its components during the first 5 years of operation. Results from the first 5 years of operation include e.g., budgets for energy, water and CO2, information on important but rarely occurring events such as evaporation from snow-covered canopies, and reactions of the forest to extreme drought. The carbon budget shows that the forest may be a sink of carbon although it is still growing. The completeness of the data has made it possible to test the internal consistency of SVAT models. The pioneering set-up at the CTS has been adopted by a large number of SVAT-monitoring sites around the world. Questions concerning tower maintenance, long-term calibration plans, maintenance of sensors and data-collection system, and continuous development of the computer network to keep it up to date are, however, only partly of interest as a research project in itself. It is thus difficult to get it funded from usual research-funding agencies. The full value of data generated by the CTS system can best be appreciated after a decade or more of continuous operation. Main uses of the data would be to evaluate how SVAT models handle the natural variability of climate conditions, quantification of water. carbon and energy budgets during various weather conditions, rind development of new parameterisation schemes in global and regional climate models.
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