| 1. |
- 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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| 2. |
- 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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| 3. |
- Lundberg, Angela, et al.
(författare)
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Snow interception evaporation : review of measurement techniques, processes, and models
- 2001
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Ingår i: Journal of Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 70:1-4, s. 117-133
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Tidskriftsartikel (refereegranskat)abstract
- A global warming, primarily affecting wintertime conditions at high latitudes will influence the functioning of the boreal forest. The least known term of the winter water-balance equation is evaporation of snow intercepted in forest canopies. Several investigations stress the importance of snow-interception evaporation in coniferous forests and evaporation fractions of gross precipitation as large as 0.2-0.5 have been observed by investigators in Scotland, Canada, and Japan. Evaporation rates as high as 0.56 mm hm1 are reported. The largest differences between the rain and snow interception evaporation processes are the differences in storage. Snow storage (both mass and duration) is often an order of magnitude larger than that for rain. Snow interception changes the canopy albedo although some studies indicate the opposite. Process knowledge is limited because of measurement difficulties but it is known that canopy closure, aerodynamic resistance (ra), and vapour-pressure deficit are important factors. Existing formulations of ra as function of storage location and age cannot fully explain observed differences in evaporation rates. Operationalhydrology and weather models, and GCMs describe snow interception in a very simplified way and might benefit from incorporation of more realistic schemes.
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| 4. |
- Lundberg, Angela, et al.
(författare)
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Snow measurement techniques for land-surface-atmosphere exchange studies in boreal landscapes
- 2001
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Ingår i: Journal of Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 70:1-4, s. 215-230
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Tidskriftsartikel (refereegranskat)abstract
- Snow has been studied widely in hydrology for many decades whereas recent meteorological interest in snow is caused by increased emphasis on high latitudes and wintertime in climate-change research as well as by the need to improve weather-forecast models during these conditions. Ground-based measurements of snow properties are needed both to improve understanding of surface-atmosphere exchange processes and to provide ground truth to new remote-sensing algorithms. This justifies a review of techniques to measure snow in combination with establishment of criteria for the suitability of the methods for process studies. This review assesses the state-of-art in ground-based snow-measurement techniques in the end of the 1990s in view of their accuracy, time resolution, possibility to automate, practicality and suitability in different terrain. Methods for snow-pack water equivalent, depth, density, growth, quality, liquid-water content and water leaving the snow pack are reviewed. Synoptic snow measurements in Fennoscandian countries are widely varying and there is no single standard on which process-related studies can build. A long-term, continuous monitoring of mass and energy properties of a snow cover requires a combination of point-measurement techniques. Areally representative values of snow properties can be achieved through a combination of automatically collected point data with repeated manual, areally covering measurements, remote-sensing data and digital elevation models, preferably in a GIS framework.
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| 5. |
- 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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