Henrik Søgaard

Rapid response of greenhouse gas emission to early spring thaw in a subarctic mire as shown by micrometeorological techniques

Most studies of soil/atmosphere greenhouse gas exchange in Arctic and Sub-Arctic wetlands have been conducted by the use of small scale chamber techniques during the growing season. To improve the knowledge about the processes in the transition period from winter to growth season, an experiment is presented here showing measurements of CH 4 , CO 2 and H 2 O using both chambers (only CH 4 ) and eddy correlation technique from the thaw period in early spring and during mid summer. The emphasis is on a comparison between eddy correlation and chamber measurements of methane fluxes during spring thawing in a subarctic mire near Abisko, northern Sweden. Methane exchange as measured by the two techniques is compared and evaluated in relation to temperature variations and atmospheric conductance. During the thaw period, integrated daily net fluxes of CH 4 flux showed emission rates increasing from 2.6 mg m -2 d -1 to 22.5 mg m -2 d -1 within four days; the later rate corresponding to approximately 25% of the mid-summer flux. A profound diurnal cycle was observed in the release of methane, emphasising the importance of continuous measurements when calculating integrated fluxes.

High‐Arctic Soil CO2 and CH4 Production Controlled by Temperature, Water, Freezing and Snow

Soil gas production processes, mainly anaerobic or aerobic soil respiration, drive major gas fluxes across the soil-atmosphere interface. Carbon dioxide (CO2) effluxes, an efflux which in most ecosystems is a result of both autotrophic and heterotrophic respiration, in particular have received international attention. The importance of both CO2 and methane (CH4) fluxes are emphasised in the Arctic because of the large amount of soil organic carbon stored in terrestrial ecosystems and changes in uptake and release due to climate changes. This chapter focuses on controls on spatial and temporal trends in subsurface CO2 and CH4 production as well as on transport and release of gases from the soil observed in the valley Zackenbergdalen. A dominance of near-surface temperatures controlling both spatial and seasonal trends is shown based on data obtained using closed chamber and eddy-correlation techniques as well as in manipulated field plots and in controlled incubation experiments. Despite variable temperature sensitivities reported, most data can be fairly well fitted to exponential temperature-dependent equations. The water content (at wet sites linked to the depth to the water table) is a second major factor regulating soil respiration processes, but the effect is quite different in contrasting vegetation types. Dry heath sites are shown to be periodically water limited during the growing season and respond therefore with high respiration rates when watered. In contrast, water saturated conditions during most of the growing season in the fen areas hinder the availability of oxygen, resulting in both CO2 and CH4 production. Thus, water table drawdown results in decreasing CH4 effluxes but increasing CO2 effluxes. Additional controls on gas production are shown to be related to the availability of substrate and plant productivity. Subsurface gas production will produce partial and total pressure gradient causing gas transport, which in well-drained soils is mainly controlled by diffusion, whereas gas advection, bubbles and transport through roots and stems may be important in more saturated soils. Bursts of CO2 gas have been observed during spring thaw and confirmed in controlled soil thawing experiments. Field observations as well as experimental work suggest that such bursts represent partly on-going soil respiration and a physical release of gas produced during the winter. The importance of winter soil respiration is emphasised because of the fact that microbial respiration in Zackenberg samples is noted down to a least -18 degrees C. Hence, the importance of winter respiration and burst events in relation to seasonal and future climate trends requires more than just summer measurements. For example, the autumn period seems important as snowfall prior to low air temperature may insulate the soil, keeping soil temperatures high. This will extend the period of high soil respiration rates and thereby increase the importance of the winter period for the annual carbon balance. Because of the complexity of factors controlling subsurface gas production, we conclude that different parts of the landscape will respond quite differently to the same climate changes as well as that short-term effects are likely to be different from long-term effects.

ET FORSØG PÅ EN KLIMATISK- HYDROLOGISK REGIONSINDDELING AF HOLSTEINSBORG KOMMUNE (SISIMIUT)

Hasholt, B. & H. Sogaard 1978: Et forsog pa en klimatisk-hydrologisk regionsinddeling af Holsteinsborg kommune (Sisimut). Geografisk Tidsskrift 77: 72–92. Kobenhavn juni 1,1978. The Holsteinsborg municipality (Sisimut) has been divided into climatic and hydrological regions. The investigation is of a preliminary character due to the limited possibilities for field work. The regionalization is based upon selected climatological and hydrological parameters; these were primarily snow cover (precipitation), potential evapotranspiration, run-off, and temperature. Measurements of water quality have been used as indicator of the aridity in the area.

Spatial and interannual variability of trace gas fluxes in a heterogeneous High Arctic landscape

Summertime measurements of CO2 and CH4 fluxes were carried out over a range of high-arctic ecosystem types in the valley Zackenbergdalen since 1996 using both chamber and eddy covariance methodology. The net ecosystem CO2 exchange and CH4 flux data presented reveal a high degree of inter-annual variability within the dominant vegetation types in the valley, but also show distinct differences between them. In particular, the wet and dry parts of the valley show distinct differences. In general, the wet parts of the valley, the fens dominated by white cotton grass Eriophorum scheuchzeri, show high productivity, also in comparison with other sites, whereas CO2 uptake rates in the white arctic bell heather Cassiope tetragona and mountain avens Dryas spp.-dominated heaths are much smaller. Also within the different ecosystem types, a high degree of spatial variability can be documented. The spatial variability both within and between ecosystem types is especially pronounced for the CH4 flux and can, at least partly, be related to differences in vegetation composition and water table level. The importance of the CH4 emission from the various ecosystem types is evaluated both in relation to carbon and greenhouse gas budgets. In both wet and drier ecosystem components, inter-annual variability seems best explained through differences in the amount and distribution of snow in spring and the length of the growing season. A large number of replicate chamber measurements carried out over various vegetation types in the valley are used to produce a synthesis of 10 years of flux data available on growing season carbon dynamics and CH4 emission patterns in the individual parts of this high-arctic ecosystem and relates the differences between the ecosystems found in Zackenbergdalen to comparable sites in the circumpolar North.

Hydrology and Transport of Sediment and Solutes at Zackenberg

Publisher Summary This chapter focuses on the hydrology together with sediment and solute transport in the Zackenberg area in relation to climate variability during 1995–2005. The hydrology of the Zackenberg catchment area has certain distinct characteristics that determine the distribution of water and the timing of runoff. Sediment and solute transport is strongly related to the surface runoff because the water erodes bed and banks along the watercourses and acts as a transport medium for both sediment and solutes. Sediment is made available for transport by mechanical erosion of frost action, by glacial erosion, and erosion by running water on surfaces and in stream beds and banks. Sediment sinks in the catchment are alluvial cones, inland deltas, and lake basins. The chapter discusses the amount and distribution of precipitation relevant to runoff. Point measurements of precipitation at the Zackenberg Research Station in the period 1995–2005 show a slight but significant decrease in the annual amounts. Elements of the water balance—such as precipitation and snowmelt, evapotranspiration, storage, and runoff—are discussed in the chapter.

Estimation of the surface energy balance in the Sahelian zone of Western Africa

Sogaard, Henrik: Estimation of the surface energy balance in the Sahelian zone of Western Africa. Geografisk Tidsskrift 88: 108- 115. Copenhagen 1988. In studies of desertification in the Sahelian zone of Western Africa an improved knowledge of the water and surface energy balance is recognized to be of major importance. Based on micro-climatological measurements collected during an ongoing field study in the northern part of Burkina Faso, a number of methods for deriving surface energy balance are examined. It is found that for the actual case, with profile measurements restricted to two levels above the surface, exact values of the sensible heat flux can accurately be obtained by applying non-dimensional gradients based on the Monin-Obukhov turbulence theory. Utilization of the results for deriving actual evapotranspiration from standard observations is demonstrated, and the paper finally discusses applicability of the results in satellite remote sensing.

Estimation of Surface Temperature and Rainfall in Ghana

Abstract Geografisk Tidsskrift, Danish Journal of Geography 97: 76–85, 1997. Daily Meteosat images for 1989 have been analysed for the purpose of mapping surface temperature and rainfall in Ghana. The images in the time-series are georeferenced and the maximum value composite (MVC) technique is applied to the temperature calibrated IR-channel images after a cloud screening of the images. The cloud covered areas are outlined and traced using the Meteosat visible channels. Stratiform clouds are detected using surface albedo while cumuliform clouds are detected based on a technique calculating the spatial variability within a 5 pixel by 5 pixel window around each point in the image. The cloud covered parts of the scene are then delimitated by comparing the normal albedo values and the spatial variance in albedo. The paper then discusses how the outlined procedure can be applied in an environmental monitoring context. For each of the months June to November 1989 the mean monthly rainfall amounts at 35 meteoro...

Estimation of the Spatial Variation in Evapotranspiration based on Landsat TM and NOAA Satellite Imagery

Sogaard, Henrik: Estimation of the Spatial Variation in Evapotranspiration based on Landsat TM and NOAA Satellite Imagery. Geografisk Tidsskrift 92: 80–85. Copenhagen 1992. During the last decade, the use of satellite imagery for monitoring water and energy budgets has gained increasing interest. Based on ground reference data collected above a barley field located near Viborg in Jutland, the so-called simplified relationship between net radiation, temperature, and evapotranspiration was examined. It was found that the difference between evapotranspiration and net radiation depends on both the temperature difference between the surface and the air, and the surface roughness. The derived relationship was applied for monitoring evapotranspiration on the basis of surface temperature measured by the NOAA- satellites. This paper will present this study and examine the resolution problem caused by the field size being less in area than the 1 km by 1 km covered by a single NOAA picture element. For a selected da...