Sven Halldin

Modelling Hydrological Consequences of Climate Change—Progress and Challenges

The simulation of hydrological consequences of climate change has received increasing attention from the hydrology and land-surface modelling communities. There have been many studies of climate-change effects on hydrology and water resources which usually consist of three steps: (1) use of general circulation models (GCMs) to provide future global climate scenarios under the effect of increasing greenhouse gases, (2) use of downscaling techniques (both nested regional climate models, RCMs, and statistical methods) for “downscaling” the GCM output to the scales compatible with hydrological models, and (3) use of hydrologic models to simulate the effects of climate change on hydrological regimes at various scales. Great progress has been achieved in all three steps during the past few years, however, large uncertainties still exist in every stage of such study. This paper first reviews the present achievements in this field and then discusses the challenges for future studies of the hydrological impacts of climate change.

Energy, Water and Carbon Exchange in a Boreal Forest Landscape - NOPEX Experiences

The role of the land surface in controlling climate is still underestimated and access to information from the boreal-forest zone is instrumental to improve this situation. This motivated the organ ...

Errors in Net Radiometry: Comparison and Evaluation of Six Radiometer Designs

Abstract Net radiation is a fundamental variable in meteorology, but net radiometers have not been used extensively since the 1960s because of what is commonly considered as unreliable function. This study was conducted to determine whether this problem was caused by design, by calibration, or if some more fundamental problem was associated with net radiometry. Comparative measurements between six different types of net radiometers, calibrated in two wares were carried out under different climatic conditions. Calibration factors obtained from shadowing varied between 75% and 129% of the manufacturers. The range was 85%–161% for calibrations relative to reference measurements of individual radiation components. Differences between the reference and individual meters were between 6% and 20%. After correction for different responsivities for longwave and shortwave radiation, the differences decreased markedly, especially in nighttime, except for two meters that had bad cosine responses. Three meters showed ...

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.

Turbulent exchange above a pine forest. I: Fluxes and gradients

Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jädra»s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under ‘ideal conditions’ (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding non-dimensional potential temperature gradient (equivalent to kh/kw) is found to be unity for (z − d)/Lv less than about −0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large.

Water-use efficiency of willow : variation with season, humidity and biomass allocation

Information on the water-use efficiency (WUE) of a vegetation cover improves understanding of the interrelationship between the water and carbon cycles, and enables hydrological practices to be related to agricultural and silvicultural planning and management. This study determined seasonal and climatic variations of the WUE of a fertilized and irrigated short-rotation stand of Salix viminalis L. on a clay soil. The WUE was determined as the ratio of above-ground production to transpiration or, alternatively, to transpiration divided by the saturation vapour pressure deficit. Growth was estimated from a combination of destructive and non-destructive measurements for 10 day periods during the growing seasons of 1986 and 1988. Daily transpiration was estimated using a physically based evaporation model, tuned against energy-balance/Bowen-ratio measurements of total stand evaporation. Nutrients were adequate and climate conditions were similar in both years. In spite of irrigation soil-water deficits developed during midsummer and affected growth rates in different ways: in 1986, both stem and leaf growth decreased, while in 1988 only stem growth decreased. Exceptionally high stem growth rates, twice the total potential growth rates, were recorded after the drought of 1988. They were probably caused by root-allocated assimilates that were sent above-ground after the drought. In both years, stem growth ceased 2–3 weeks after the leaf area had reached its maximum. Since light and temperature were still sufficient to maintain assimilation, all growth presumably took place below ground towards the end of the season. Changes in root-shoot allocation caused large variations in the WUE in 1988. The WUE, weighted by the saturation vapour pressure deficit, was fairly constant in 1986. In both years, the WUE was correlated with the vapour pressure deficit. Towards the end of both growing seasons, when all assimilates were sent below ground, the WUE decreased rapidly to zero. The total WUE, estimated as the seasonally accumulated above-ground production divided by accumulated transpiration, was 4.1 g kg−1 in 1986 and 5.5 g kg−1 in 1988, which is relatively high in comparison with other species.

Regional water balance modelling in the NOPEX area: development and application of monthly water balance models

Abstract One of the main purposes of a water balance study is to evaluate the net available water resources, both on the surface and in the subsurface. Water balance models that simulate hydrographs of river flow on the basis of available meteorological data would be a valuable tool in the hands of the planners and designers of water resources systems. In this paper, a set of simple monthly snow and water balance models has been developed and applied to regional water balance studies in the NOPEX area. The models require as input monthly areal precipitation, monthly long-term average potential evapotranspiration and monthly mean air temperature. The model outputs are monthly river flow and other water balance components, such as actual evapotranspiration, slow and fast components of river flow, snow accumulation and melting. The results suggest that the proposed model structure is suitable for water balance study purposes in seasonally snow-covered catchments located in the region.

Canopy transpiration from a boreal forest in Sweden during a dry year

Abstract Estimation of areal evapotranspiration is crucial for the parameterization of the soil-vegetation-atmosphere interface in climate models and for the assessment of land-use changes on water resources. Present knowledge on how areal forest evapotranspiration depends on forest species composition and age is insufficient. In this study, transpiration of 50- and 100-year-old coniferous stands was estimated on the basis of sap-flow measurements on 24 trees, 12 in each stand. The measured samples represented the size distribution of Pinus sylvestris and Picea abies trees. Daily canopy transpiration (EQ) was scaled from individual tree flow rates using the quotients of stem circumferences of the sample trees to those of the stands. EQ was used in a rearranged Penman equation to deduce a potential canopy conductance, valid for non-limiting soil-water conditions, from a period when soil-water storage was not limiting transpiration. This enabled quantification of the seasonal transpiration deficit, which in both stands reached at least one fifth of the total potential transpiration over the growth season. The estimated fluxes of EQ were low with a maximum daily value of about 2.8 mm in the 50-year-old stand. For dry-weather days, EQ was well correlated to daily sums of stand evapotranspiration estimated from eddy-correlation measurements. Responses to drought were species specific. Transpiration in pines from the 50-year-old stand was less affected by drought relative to spruce or older pine trees, which was also reflected by stem increment during the season.

Evaporation of intercepted snow : analysis of governing factors

Insufficient understanding of winter hydrology conditions still hampers progress in predicting springtime discharge. The least known term in the winter water balance is evaporation, particularly of ...

NOPEX—a northern hemisphere climate processes land surface experiment

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 chan ...