E. Kellner

Dynamics of biogenic gas bubbles in peat: Potential effects on water storage and peat deformation

[1] Dynamics of biogenic bubbles in peat soils were studied at a field site in southern Quebec, Canada. The maximum gas content measured in this study varied spatially with a maximum seasonal increase in volumetric gas content of 0.15. The size of changes in total gas content of a 1 m deep profile was comparable to the seasonal water storage change. Changes in bubble volume in the saturated zone alter the water table level and, consequently, the water content in the unsaturated zone and the apparent water budget. In highly compressible soils (and floating root mats), buoyancy forces from bubbles also cause relations between the surface and the water table to change. These effects cannot be omitted in modeling the hydrology of peatlands. Our results indicate a great spatial variability of trapped bubbles. Using pressure transducers sealed to the surface, we found pressure deviations indicating small areas closed off by bubbles clogging the pores. The hydrological influence of these areas may be considerable as they may restrict or deflect water flows. Open pipe piezometers did not show these pressure deviations, possibly because the closed zones were too small to influence the head in pipes or because of less amount of gas close to the pipe screen.

Continuous long-term measurements of soil-plant-atmosphere variables at a forest site

It is a major challenge in modern 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-vegetationatmosphere-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-flow 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 researchfunding agencies.

Test of a modified Shuttleworth-Wallace estimate of boreal forest evaporation

A modified version of the two-layer Shuttleworth‐Wallace approach was used to simulate evaporation from a mixed 50‐ 100 years old boreal forest stand in central Sweden. It was tested against eddy correlation flux measurements from the Northern hemisphere climate Processes land-surface Experiment (NOPEX). The soil regulation on canopy resistance was taken into account when estimating transpiration. The generality of the model performance was attained by using model parameters that represented typical boreal forest stand and not the specific stands at the site. The analysis was done for two growing seasons, 1994 and 1995 having quite different weather conditions. Despite the rough parameterisation, the model performance was good. Problems regarding representativity of the measured data used for the model validation are discussed. It was concluded that the model could accurately simulate evaporation components on a seasonal time scale and is suitable to be incorporated as a submodel in larger forest ecosystem models. # 1999 Elsevier Science B.V. All rights reserved.

New Approach to the Measurement of Interception Evaporation

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...

Excess resistance of bog surfaces in central Sweden

Typical bogs in boreal forest zone can be characterized by hummock and hollow micro-topography and sparsely vegetated surfaces, thus, are quite distinct from other types of wetlands. Micrometeorological measurements were carried out in central Sweden at two bogs in different summer seasons. The data analysis aimed at obtaining good estimates for the roughness length z(ou) and the kB(-1) factor. The roughness length for wind speed (z(ou)) was estimated to be 2 cm. A good surface temperature estimate is crucial for kB-1 calculations. The surface temperature of such a heterogeneous surface showed high spatial variability, the differences between its minimum and maximum values reaching 10 K during daytime. However, an infrared thermometer averaging over a circle of a few meters still gave an acceptable result. The kB-1 factor was determined through the roughness sublayer Stanton number and the corresponding drag coefficient and was dependent on the roughness Reynolds number (Re-o). It took the form: kB(-1) = 1.58Re(o)(0.25) - 3.4. Within considerable scatter of data, a constant kB(-1) = 3.2 was also acceptable. This result lies in between the bluff-rough and vegetated surface cases. (Less)