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When measuring the energy balance at the earth’s surface using the Eddy covariance technique, the obtained budgets seldom produce a closed energy balance. The measurements often miss some of the energy fluxes. A possible reason is the neglect of non-turbulent surface fluxes of latent heat and sensible heat, i.e. advective fluxes of these quantities. We present estimates of advective latent and sensible heat fluxes for three different sites across Europe based on the ADVEX dataset. The obtained horizontal and vertical advective fluxes were site-specific and characterized by large scatter. In relative terms, the data indicated that the sensible heat budget was less affected by advection than the latent heat budget during nighttime; this is because vertical turbulent latent heat fluxes were very small or close to zero during the night. The results further showed that the additional energy gain by sensible heat advection might have triggered enhanced evaporation for two sites during nighttime. Accounting for advective fluxes improved the energy balance closure for one of the three ADVEX sites. However, the energy balance closure of the other two sites did not improve overall. A comparison with energy balance residuals (energy missed by the measurements without accounting for advection) indicated a large influence of systematic errors. An inspection of the energy balance for the sloped site of the ADVEX dataset underlined the necessity of slope-parallel measurement of radiation.

The algorithm for and results of a newly developed multivariate non-parametric model, the Euclidean distance model (EDM), for the hourly disaggregation of daily climate data are presented here. The EDM is a resampling method based on the assumption that the day to be disaggregated has already occurred once in the past. The Euclidean distance (ED) serves as a measure of similarity to select the most similar day from historical records. EDM is designed to disaggregate daily means/sums of several climate elements at once, here temperature (T), precipitation (P), sunshine duration (SD), relative humidity (rH), and wind speed (WS), while conserving physical consistency over all disaggregated elements. Since weather conditions and hence the diurnal cycles of climate elements depend on the weather pattern, a selection approach including objective weather patterns (OWP) was developed. The OWP serve as an additional criterion to filter the most similar day. For a case study, EDM was applied to the daily climate data of the stations Dresden and Fichtelberg (Saxony, Germany). The EDM results agree well with the observed data, maintaining their statistics. Hourly results fit better for climate elements with homogenous diurnal cycles, e.g., T with very high correlations of up to 0.99. In contrast, the hourly results of the SD and the WS provide correlations up to 0.79. EDM tends to overestimate heavy precipitation rates, e.g., by up to 15% for Dresden and 26% for Fichtelberg, potentially due to, e.g., the smaller data pool for such events, and the equal-weighted impact of P in the ED calculation. The OWPs lead to somewhat improved results for all climate elements in terms of similar climate conditions of the basic stations. Finally, the performance of EDM is compared with the disaggregation tool MELODIST (Förster et al. 2015). Both tools deliver comparable and well corresponding results. All analyses of the generated hourly data show that EDM is a very robust and flexible model that can be applied to any climate station. Since EDM can disaggregate daily data of climate projections, future research should address whether the model is capable to respect and (re)produce future climate trends. Further, possible improvements by including the flow direction and future OWPs should be investigated, also with regard to reduce the overestimation of heavy rainfall rates.

This study investigates the trends of precipitation and temperature extremes for the historical observations (1961–1990) and future period (2061–2090) in the Jhelum River Basin. Future trends are estimated by using ensemble mean of three general circulation models under RCP4.5 and RCP8.5. Therefore, statistical downscaling model has been used to downscale the future precipitation and temperature. A total of 15 precipitation and temperature indices were calculated using the RClimdex package. Man-Kendall and Sen’s slope tests were used to detect the trends in climate extreme indices. Overall, the results of study indicate that there were significant changes in precipitation and temperature patterns as well as in the climate extremes in the basin for both observed as well as projected climate. Generally, more warming and increase in precipitation were observed, which increases from RCP4.5 to RCP8.5. For all the stations, increasing trends were found for both precipitation and temperature for twenty-first century at a 95% significance level. The frequency of warm days (TX90p), warm nights (TN90p), and summer days (SU25) showed significant increasing trends, alternatively the number of cold nights (TN10p) and cold days (TX10p) exhibited opposite behaviors. In addition, an increasing trend of warmest day (TXx) and coldest day (TNn) was observed. Our analysis also reveals that the number of very wet days (R90p) and heavy precipitation days (R10 mm) will likely increase in the future. Meanwhile, the Max 1-day (RX1-day) and 5-day (RX5-day) precipitation indices showed increasing trends at most of the stations of basin. The results of the study is of potential benefit for decision-makers to develop basin wide appropriate mitigation and adaptation measures to combat climate change and its consequences.

The eddy-covariance (EC) technique is used to determine mass and energy fluxes between the Earth’s surface and the lower atmosphere at high temporal resolution. Despite the frequent and successful use of the EC technique at terrestrial sites, its application over water surfaces is rare. We present one season of EC measurements conducted on the Rappbode Reservoir, Germany’s largest drinking water reservoir. A floating observation platform in the centre of the reservoir is used for observations of fluxes that were unaffected by surrounding land surfaces and therefore representative of the actual water–atmosphere exchange. The temporal patterns of sensible heat flux are inverted compared to land sites, since the maxima and the minima occur at night and day respectively. The latent heat flux and the evaporation are unexpectedly low for a site where evaporation is not limited by the availability of water. The daily totals in summer and autumn are only 50% and 75% of the potential evaporation assessed by the FAO grass-reference evaporation, respectively. Measurement uncertainties and the effects of the energy balance closure are ruled out as potential factors, so that low values appear to be a general feature of large water surfaces. The observed carbon dioxide fluxes are characterized by distinctive diurnal variations in a typical range for lakes and reservoirs. However, the methane fluxes are low compared to other inland waters.

In this study, statistical downscaling models were used to project possible future patterns of precipitation and temperature in the Jhelum River basin shared by Pakistan and India. In-situ meteorological data were used to downscale precipitation and temperature using different General Circulation Models (i.e., CanESM2, BCC-CSM1–1, and MICROC5) relative to baseline (1961–1990) under the Representative Concentration Pathway (RCP) scenarios RCP4.5 and RCP8.5. The downscaling models used were the Statistical Downscaling Model (SDSM), which uses multiple linear regression and weather generator methods, and the Long Ashton Research Station Weather Generator (LARS-WG), which uses weather generators. The results showed that the SDSM performance was slightly better than that of LARS-WG during validation and that the representation of the simulated mean monthly precipitation was more correct than that of monthly precipitation. The results also revealed that BCC-CSM1–1 performed better than CanESM2 and MICROC5 in the study region. The future annual mean temperature and precipitation are expected to rise under both RCP scenarios. The changes in the annual mean temperature and precipitation with LARS-WG were relatively higher than those with SDSM. Out of four seasons, winter and autumn are expected to be more diverse with regard to precipitation changes. However, although both models yielded non-identical results, it is certain that the basin will face a hotter climate in the future.

Taylor’s frozen turbulence hypothesis states that “standard” eddy-covariance measurements of fluxes at a fixed location can replace a spatial ensemble of instantaneous values at multiple locations. For testing this hypothesis, a unique turbulence measurement set-up was used for two measurement campaigns over desert (Namibia) and grassland (Germany) in 2012. This “Eddy Matrix” combined nine ultrasonic anemometer–thermometers and 17 thermocouples in a 10 m $$\times $$ 10 m regular grid with 2.5-m grid distance. The instantaneous buoyancy flux derived from the spatial eddy covariance of the Eddy Matrix was highly variable in time (from $$-0.3$$ to 1 m K s $$^{-1}$$ ). However, the 10-min average reflected 83 % of the reference eddy-covariance flux with a good correlation. By introducing a combined eddy-covariance method (the spatial eddy covariance plus the additional flux of the temporal eddy covariance of the spatial mean values), the mean flux increases by 9 % relative to the eddy-covariance reference. Considering the typical underestimation of fluxes by the standard eddy-covariance method, this is seen as an improvement. Within the limits of the Eddy Matrix, Taylor’s hypothesis is supported by the results.

Forest ecosystems play an important role in the interaction between the land surface and the atmosphere. Measurements and modelling efforts have revealed significant uncertainties in state-of-the-art flux assessments due to spatial inhomogeneities in the airflow and land surface. Here, a field experiment is used to describe the turbulent flow across a typical Central European forest clearing. A three-dimensional model of the inhomogeneous forest stand was developed using an innovative approach based on terrestrial laser-scanner technology. The comparison of the wind statistics of two measurement campaigns (5 and 12 months long) showed the spatial and temporal representativeness of the ultrasonic anemometer measurements within the canopy. An improved method for the correction of the vertical velocity enables the distinction between the instrumental offsets and the vertical winds due to the inclination of the instrument. Despite a 13 % fraction of deciduous plants within the otherwise evergreen canopy, the effects of phenological seasons on the velocity profiles were small. The data classified according to the wind speed revealed the intermittent nature of recirculating air in the clearing. Furthermore, the development of sub-canopy wind-speed maxima is explained by considering the velocity moments and the momentum equation (including measurements of the local pressure gradient). Clearings deflect the flow downward and feed the sub-canopy flow, i.e., advective fluxes, according to wind speed and, likely, clearing size, whereas local pressure gradients play an important role in the development of sub-canopy flow. The presented dataset is freely available at the project homepage.

The eddy covariance technique was used to measure the CO2 flux over four differently grazed Leymus chinensis steppe ecosystems (ungrazed since 1979 (UG79), winter grazed (WG), continuously grazed (CG), and heavily grazed (HG) sites) during four growing seasons (May to September) from 2005 to 2008, to investigate the response of the net ecosystem exchange (NEE) over grassland ecosystems to meteorological factors and grazing intensity. At UG79, the optimal air temperature for the half-hourly NEE occurred between 17 and 20 °C, which was relatively low for semi-arid grasslands. The saturated NEE (NEEsat) and temperature sensitivity coefficient (Q 10) of ecosystem respiration (RE) exhibited clear seasonal and interannual variations, which increased with canopy development and the soil water content (SWC, at 5 cm). The total NEE values for the growing seasons from 2005 to 2008 were −32.0, −41.5, −66.1, and −89.8 g C m−2, respectively. Both the amounts and distribution of precipitation during the growing season affected the NEE. The effects of grazing on the CO2 flux increased with the grazing intensity. During the peak growth stage, heavy grazing and winter grazing decreased NEEsat and gross primary production (45 % for HG and 34 % for WG) due to leaf area removal. Both RE and Q 10 were clearly reduced by heavy grazing. Heavy grazing changed the ecosystem from a CO2 sink into a CO2 source, and winter grazing reduced the total CO2 uptake by 79 %. In the early growing season, there was no difference in the NEE between CG and UG79. In addition to the grazing intensity, the effects of grazing on the CO2 flux also varied with the vegetation growth stages and SWC.

Available climatological information of Distrito Federal does not satisfy the requirements for detailed climate diagnosis, as they do not provide the necessary spatial resolution for water resources management purposes. Annual and seasonal climatology (1971–2000) of precipitation from 6 meteorological stations and 54 rain gauges from Central Brazil were used to test eight different spatial interpolation methods. Geographical factors (i.e., altitude, longitude and latitude) explain a large portion of precipitation in the region, and therefore, multivariate models were included. The performance of estimations was assessed through independent validation using mean square error, correlation coefficient and Nash–Sutcliffe efficiency criterion. Inverse distance weighting (IDW), ordinary kriging (OK) and the multivariate regression with interpolation of residuals by IDW (MRegIDW) and OK (MRegOK) have performed the lowest errors and the highest correlation and Nash–Sutcliffe efficiency criterion. In general, interpolation methods provide similar spatial distributions of rainfall wherever observation network is dense. However, the inclusion of geographical variables to the interpolation method should improve estimates in areas where the observation network density is low. Nevertheless, the assessment of uncertainties using a geostatistical method provides supplementary and qualitative information which should be considered when interpreting the spatial distribution of rainfall.

The International Water Research Alliance Saxony (IWAS) is addressing the global challenges concerning water quality in the areas of drinking water and sanitation, agricultural irrigation and the quality of surface and ground waters, as well as developing specific ecosystem-relevant services to be implemented on an exemplary basis in selected model regions. Locations (model regions) have been selected in Eastern Europe (R1), Central and Southeast Asia (R2 and R3), the Middle East (R4) and Latin America (R5) that are representative international regions with respect to climate, land use and demographic change (Ibisch et al., Helmholtz Centre for Environmental Research—UFZ, Department of Aquatic Ecosystem analysis and management, 2013). The causes of water problems and the relevant boundary conditions vary from region to region (Borchardt and Ibisch, Integrated water resources management in a changing world : lessons learnt and innovative perspectives, pp 225, 2013). Mongolia and Vietnam were selected model regions in the first IWAS phase; the research was transferred and continued in one of the cross-cutting projects in IWAS II (Vietnam → capacity development) or in the frame of related project activities in Central Asia (R2 Mongolia, Karthe et al., Environ Earth Sci, doi: 10.1007/s12665-014-3789-1 , 2014). The IWAS consortium exists on scientific institutions like the Technische Universitat Dresden and the Helmholtz Centre for Environmental Research—UFZ, Leipzig, as well as partners from industry like the Stadtentwasserung Dresden GmbH (SE-DD), DREBERIS—Dresden consulting for international strategies, and itwh—institute for technical-scientific Hydrology, Hanover. This thematic issue compiles the most important scientific results of the second phase of the IWAS project. The project itself and findings of the first phase were already introduced in a previous special issue by Kalbus et al. (Environ Earth Sci 65:1363–1366, 2012). Main results: The IWAS project is structured by the model regions (R) as well as by cross-cutting activities scenario analysis (Q1), technology development (Q2), governance (Q3) and capacity development (Q4).

The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately $$328\,\mathrm {m}$$ length and $$172\,\mathrm {m}$$ width at the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a highly resolved, three-dimensional plant area distribution representing the actual canopy. Hence, the vegetation maintains a rich horizontal and vertical structure including the three-dimensional clearing. The scanned plant area density is embedded in a larger domain, which is filled with a heterogeneous forest generated by the virtual canopy generator of Bohrer et al. (Tellus B 59:566–576, 2007). Based on forest inventory maps and airborne laser scanning, the characteristics of the actual canopy are preserved. Furthermore, the topography is extracted from a digital terrain model with some modifications to accommodate for periodic boundary conditions. A large-eddy simulation is performed for neutral atmospheric conditions and compared to simulations of a two-dimensional plant area density and an one-year-long field experiment conducted at the corresponding field site. The results reveal a considerable influence of the plant heterogeneity on the mean velocity field as well as on the turbulent quantities. The three-dimensional environment, e.g., the oblique edges combined with horizontal and vertical variations in plant area density and the topography create a sustained vertical and cross-flow velocity. Downstream of the windward forest edge an enhanced gust zone develops, whose intensity and relative position are influenced by the local canopy density and, therefore, is not constant along the edge. These results lead us to the conclusion that the usage of a three-dimensional plant area distribution is essential for capturing the flow features inside the canopy and within the mixing layer above.

The IWAS project Ukraine deals among others with the water balance in the headwaters of the river Western Bug. During investigations in the sub-catchment Sasiv, a gap in the water balance was detected by Pluntke et al. (Environ Earth Sci, this issue, 2014). The runoff of this sub-catchment is significantly higher than that of any other catchment in the region. Unexpectedly, in 2011 the runoff was observed to be higher than precipitation. We hypothesize additional subsurface flow in the Sasiv catchment from adjacent aquifers. The hydrological model BROOK90 was used to model water flows. In addition to publicly available meteorological input data from the NOAA and ECA&C databases, own precipitation measurements in Sasiv were used. The mean seasonal leaf area index (LAI)—cycle of forest, grass, and agricultural land was determined using satellite data. To improve the output quality, the model was adapted for the superior catchment Kamianka-Buzka/Western Bug (2,500 km2). It was calibrated for the period 2001–2006 and validated for 2007–2011. Infiltration and drainage parameters were unknown, and were therefore calibrated applying Monte Carlo simulations. It was not possible to calibrate the sub-catchment Sasiv (107 km2) due to high deviations of the natural water balance. Thus, the model was calibrated for the superior catchment and afterwards applied to the Sasiv catchment. The monthly estimated inflow was calculated based on, first, a constant term, which is a fixed monthly inflow with its minimum in October (22 mm) and its maximum in May (41 mm). Secondly, a dynamic term was added. Here, different dependencies were tested, e.g., precipitation, soil moisture, groundwater level, and modelled runoff. The best coefficient of determination was achieved using a monthly modelled runoff from the 4 months before the current time. The model showed a good performance in the validation period (R2 = 0.88). On average, the runoff observed in Sasiv consists of 60 percent groundwater inflow. At the gauge Kamianka-Buzka, this fraction is still about 5 %. The determined LAI cycles and the usage of an additional precipitation station near Sasiv significantly improved the quality of the water balance modelling in the Sasiv sub-catchment. The presented hydrological model approach was extended by hydrogeological simulations to enhance the understanding of processes in groundwater and surface water interactions. Therefore, it was a special challenge to model with scarce data the heterogeneous hydrogeological character of the catchment. The multi-level calibration enables a comprehensible estimation of annual groundwater recharge rates (30–700 mm) of the shallow quaternary aquifer, which is drained by the river Bug system.

A key challenge for climate projection science is to serve the growing needs of impact assessments in an environment with substantial differences in the projections of climate models and an increasing number of relevant climate model results. In order to assist the assessment of water resources impacts under future climate change, this work provides a synthesis of the simulations of General Circulation Models (GCMs) for the region of Distrito Federal, Brazil. The work analyzes projections of mean surface air temperature and precipitation of 22 GCMs, as well as seven extreme indices of 10 GCMs. Trends of the multi-model ensemble median, as well as their significance, were calculated. The consistency in the sign of change was assessed through the percentage of agreement of simulations with the median. Finally, the probability density function of the multi-model ensemble provides valuable information about the uncertainties of projections. Investigations were performed for annual and seasonal temporal scales for the period 2011–2050. The main results here identified are: (a) a consensus of the multi-model ensemble and median to increasing temperature; (b) a slightly, but less consistent, decrease of precipitation in the dry season; and (c) increase of heat waves and droughts events, although changes in precipitation extremes are much less coherent than for temperature. The approach used gives a comprehensive assessment of the possible future climate until the middle of the twenty-first century, as well as the uncertainties involved in the multi-model ensemble projections.

One subproject within the project IWAS aims at the identification of effective and sustainable measures to improve the water quality of the river basin Western Bug in the Ukraine in preparation of changes of climate, land use and socio-economy. An important part of the system analysis is the quantification of water balance components. Rare and uncertain data impede hydrological modelling in the region. Approaches to reduce the modelling uncertainty are needed. In this work, we focus on the reduction of uncertainty that results from precipitation observations and parameter estimation during the calibration process. The main aim is to set up a well performing model for water balance simulations. The semi-physically based model Soil and Water Assessment Tool (SWAT) was applied. A calibration and uncertainty reduction strategy was set up that consists of time- and spatial-scale dependencies as well as alternative precipitation inputs. The single models that were set up and calibrated with alternative precipitation inputs are treated as an ensemble and were averaged with different methods. Nine parameters were chosen from the completed sensitivity analysis for all calibration approaches. The calibration strategy revealed the benefit of applying a complex bottom-up calibration that starts with sub-basins and ends with the entire basin. Not only performance improved significantly, but also water balance gaps were identified and henceforth reduced. The usage of daily runoff data in the calibration procedure did not enhance simulations in comparison to monthly data. Simulations improved and parameter uncertainty was reduced applying the SWAT model variants, where precipitation was included in calibration and by the ensemble averaging of the different models. Nevertheless, chosen uncertainty measures are not optimal and indicate that parameter uncertainty is still high. This is attributed to the low density of precipitation stations in the region, their low representativity, and scarce data situation in general. Differences between modelled and observed runoff are large in some occasions and can not be balanced completely by the applied methods. Although this approach reached its limit, it was demonstrated that simulations can be improved and modelling uncertainty can be reduced with an appropriate calibration strategy, with extended precipitation information and the application of an ensemble approach.

Today, integrated water resources management (IWRM) is an important approach for sustainable management and protection of catchment areas. One of the core challenges for a successful IWRM program is the assessment of climate change impacts on the quantity and quality of water resources as well as related socioeconomic sectors. In this context, the climate impact on the hydrology of the catchment “Inflow Reservoir Dobrotvir” situated in Western Ukraine was investigated. The results of the regional climate model CCLM (COSMO—Climate Limited-area Modeling) were used to evaluate the climate conditions for two 30-year future periods in the framework of the future emissions scenarios A2 and B1 as laid out by the IPCC. Based on the projected climatic conditions, a hydrologic impact study was conducted using the Soil Water Assessment Tool (SWAT). Signals of possible future climate and future water budgets were analyzed having the period 1961–1990 as a reference for current climatic conditions. Climatic and hydrologic indices were calculated to assess possible risks and opportunities for the water management sector. In a more generic manner, the implications of climatic changes for the sectors of agriculture, forestry, ecology, energy business and human health were examined with respective literature. Increasing temperatures, declining summer rainfalls and a decreasing climatic water balance were the primary simulated results for the period 2071–2100. These meteorological conditions lead to decreasing soil water content as well as decreasing runoff and groundwater recharge through nearly all seasons. Reduced water yields may affect the energy sector, water supply and water quality negatively. Water stress, especially in summer, might cause declining yields in agriculture and forestry. By contrast, rising temperatures will lead to an extended growing season, which represents an opportunity for higher agricultural and silvicultural yields. However, rising temperatures may also cause indirect effects such as higher risks of pest infestation and germs, which can have a negative impact on a variety of the evaluated socioeconomic sectors. In this work, the impact of possible future scenarios on climate and hydrology as well as resulting risks and opportunities have been identified to serve as a basis for further investigations.

This study presents an assessment of global climate model (GCM) performances for the regions Arabian Peninsula, Brazil and Ukraine, all facing problems related to water availability as well as quality and so are the focus regions for investigations within the Integrated Water Resources Management Project ‘International Water Research Alliance Saxony’. Results from the model performance assessment have been applied to derive estimates of the future climate in the regions by means of ensemble averaging. The study uses GCM data from the third Climate Model Intercomparison Project. Model performances were assessed by the Model Performance Index (MPI) as well as Taylor diagrams applying different reference data sets like reanalysis data as well as observational data sets. The model performances were analysed for different domain sizes. Climate variables of different tropospheric levels were classified by applying projections of the overall model ensemble as well as projections of a subset of models (selected according to the MPI) in variables with different levels of certainty of change. The MPI as well as the reliability ensemble averaging method were employed to calculate ensemble averages for variables with high certainties of change. Results are presented for mean values within the time periods 2040–2060 an 2080–2100. Applying different data sets as reference data shows that model performance crucially depends on the reference data set. The size of the model domain is of minor importance. Model rankings vary between regions. Certain changes are found for temperatures and geopotential heights in all levels but, e.g., not for precipitation.

Within the IWRM research project IWAS, we generated regional climate projections via dynamic downscaling for the catchment area of the Western Bug river in the border area of Poland, Belarus, and Ukraine. Data sources used are WMO meteorological station data and global climate ERA40 reanalysis data for an evaluation run, as well as global simulations from the ECHAM5 model for a twentieth-century control run and for the IPCC emission scenarios A2 and B1. We evaluated the performance of the regional climate model CCLM on the basis of simulated 2m temperature and precipitation. Furthermore, we analyzed the hydro-climatic conditions of the past and their projected future changes in the catchment based on 2m temperature, precipitation, potential evaporation and climatic water balance. The latter is discussed as an indicator for potential water availability in the region. Our evaluation, like many other studies, attests that the CCLM performs well for 2m temperature. Precipitation is not modelled adequately for the Western Bug basin. Remarkably, the precipitation bias is five times higher in the ECHAM5-driven control run than in the ERA40-driven evaluation run. Despite all the uncertainties, the significance of the modelled changes clearly suggests robust model results for the last three decades of this century. Up to the end of the century, both scenarios A2 and B1 lead to highly significant warming for each month in the long-term mean, with highest warming rates in winter. Instead, precipitation does not change significantly in the long-term yearly mean, but the intra-annual distribution of monthly precipitation sums shifts, with an increase in winter and a strong decrease in summer. Combined, this leads to a changed climatic water balance with a stronger deficit in summer and a higher gain in winter. Particular in the south-eastern part, the summer deficit cannot be compensated within the annual cycle. This might have serious implications for many socio-economic sectors.

In the framework of the IWAS/Agua-DF project, this study focuses on changes in mean surface air temperature and accumulated precipitation in Central Brazil over the past 40 years. It has two main objectives: (1) comparison between two climatological periods (2001–2010 and 1971–2000) and (2) trend analysis of climate variables. Time series of meteorological and rain gauge stations from Central Brazil have been organized in a databank, which contains tools for homogeneity tests. From that, 4 temperature and 55 precipitation time series were sufficient homogeneous, while 1 temperature and 5 precipitation time series were identified as inhomogeneous. Reliable spatial distribution was produced using proper interpolation method. Trends and significance levels were calculated by Rapp’s estimator of slope and Mann–Kendall test, respectively. The most important results of the comparisons and trend analysis in the last four decades are: (1) marked increase in annual and seasonal mean surface air temperature, (2) evident decreases of accumulated rainfall in winter and autumn, and (3) apparent increase of precipitation amounts in the rainy season.

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing.

Based on the carbon fluxes measured over the grassland ecosystems in Inner Mongolia (UG79 site), Loess Plateau (SACOL site), and Tongyu, Jilin Province (TY site) in the semiarid areas from 2007 to 2008 with the eddy covariance method, we have investigated the carbon exchange processes over semiarid grassland ecosystem and its main affecting environmental variables. The precipitations at UG79 and TY sites in 2007 were below the historical average, especially for TY site, which was 50% below the historical average annual precipitation. The precipitation in SACOL site was close to average in 2007 but below average in 2008. The variation of monthly diurnal average NEE showed that the diurnal mean NEE decreased in the order of TY site, UG79 site, and SACOL site. However, a longer net carbon uptake period was observed at SACOL site. The diurnal course of NEE at UG79 site was similar between 2007 and 2008. The diurnal average NEE remained large during July and August in growing season (May to September) at UG79 site, with maximum values approaching 0.08 mg C m−2 s−1 in August of 2008. The diurnal average NEE of 2007 was larger than 2008 at SACOL site, with maximum values of 0.07 mg C m−2 s−1 in September of 2007. A shorter carbon uptake period was recorded in 2007 at TY site, lasting from July to August. A larger diurnal average NEE occurred in 2008 at TY site, with maximum values of 0.12 mg C m−2 s−1. The ecosystem respirations of three sites were controlled by both soil temperature and soil volumetric water content (at a depth of 5 cm below the land surface). Both UG79 site and SACOL site acted as a carbon sink during the growing periods of 2007 and 2008. Annual NEE in the growing seasons of 2007 and 2008 ranged from −68 to −50 g C m−2 at UG79 site and from −109 to −55 g C m−2 at SACOL site. Alternation between carbon source and carbon sink was found at TY site, with respective values of annual NEE in the growing seasons of 0.32 g C m−2 and −73 g C m−2 in 2007 and 2008. The magnitude and duration of carbon uptake depended mainly on the amount and timing of precipitation and the timing of the first effective rainfall during the growing season in semiarid grassland ecosystems.

Climate change is one of the key factors influencing the quantity and quality of water resources in hydrologically sensitive regions. In order to downscale global climate simulations from horizontal resolutions of about 125–200 km to about 7 km, a double nesting strategy was chosen. The modelling approach was implemented with the Regional Climate Model CCLM (COSMO-Climate Local Model) with a first nesting covering a central part of Europe and with a second nesting covering parts of Poland, Belarus, and the Ukraine. A control run—driven by global reanalysis data—was evaluated by comparing the model results with corresponding reference data. Long-term yearly and monthly mean differences of temperature and precipitation were statistically assessed. As reference data for the first nesting, the gridded CRU data set with a horizontal resolution of about 55 km was used. Station data of the NOAA and ECA databases were interpolated to provide an appropriate reference data set for the second nesting. Both nestings overestimated long-term yearly precipitation means. Seasonal evaluation of the first nesting showed positive precipitation biases for spring and winter months and negative biases in summer. Furthermore, differences in the spatial precipitation patterns occured, especially in the high mountain area of the Carpathians. The second nesting overestimated precipitation in spring and summer with smaller biases than in the first nesting. Long-term area means of temperature were properly reproduced. The first nesting showed an overestimation for all months with maximal deviations in summer and spring. In contrast, the second nesting was slightly too warm for summer and autumn and too cold for winter and spring.

Applications of flow models to tall plant canopies are limited, amongst other factors, by the lack of detailed information on vegetation structure. A method is presented to record 3D vegetation structure and make this information applicable to the derivation of turbulence parameters suitable for flow models. The relationship between wind speed, drag coefficient (CD) and plant area density (PAD) was experimentally investigated in a mixed conifer forest in the lower part of the Eastern Ore Mountains. Essential information was gathered by collecting multi-level high-frequency wind velocity measurements and a dense 3D representation of the forest was obtained from terrestrial laser scanner data. Wind speed dependence or streamlining was observed for most of the wind directions. Edge effects, i.e. the influence of the here not regarded pressure gradient and the advective terms of the momentum equation, are assumed to cause this heterogeneity. Contrary to the hypothetic shelter effect, which would reduce the drag on sheltered plant parts, the calculated profiles of drag coefficients revealed an increasing CD with PAD (i.e. a dependence on canopy and plant structure).

Conditions, distribution and development of vegetation in semi arid regions are highly variable. In this study we detected the temporal and spatial variability of vegetation in the Xilin river catchment from 2000 to 2008 by analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) data products of that period. The study is based on LAI (Leaf area index) and supported by NDVI (Normalized difference vegetation index) and LST (Land surface temperature) data with a spatial resolution of 1 km. The mean LAI of the catchment from 2000 to 2008 is 0.59. Precipitation data of the study period governs the conditions and distribution of vegetation in the catchment. In dry years, e.g. 2001 and 2005, LAI was clearly lower (0.52) compared to 2003 (LAI = 0.72) which was a wet year. As precipitation generally decreases from south-east to north-west, LAI values decrease according to this gradient. The influence of heavy grazing in the vicinity of the Xilin river is obvious as LAI is low (0.4) in these areas. The high temporal variability of the LAI is displayed by its high mean CV (coefficient of variation) which is 48% for the observed years. The analysis of sample areas illustrates temporal and spatial differences in vegetation development within the catchment and shows a generally delayed growth start in the north-west of the catchment.

Detecting the influences of land management on seasonal dynamics and interannual variabilities of grassland surface reflectance is of scientific and practical importance as it can help to monitor grazing effects on the grassland ecosystem. We conducted spectral reflectance measurements at five differently grazed sites in Inner Mongolia, China, during the growing seasons of 2005 and 2006 using a portable, highly resolving spectrometer. Through analyses of the measured surface reflectance spectra and the derived visible albedo and NDVI, we found that grazing influences the reflectance spectrum shape and affects the visible albedo and NDVI seasonal pattern; visible albedo and NDVI of grazed sites are more sensitive to precipitation than that of ungrazed sites. In addition, we observed a well-defined linear relationship between total shortwave and visible (PAR) albedo, with R = 0.83 and R = 0.94 for 2005 and 2006, respectively, and between NDVI and total shortwave albedo with R = −0.92. Thus, NDVI spectrometer measurements and total shortwave albedo pyranometer measurements are interchangeable when addressing different grazing intensities.

The eddy-covariance method is the state-of-the-art tool to measure mass and energy fluxes, though many measuring systems (particularly closed-path systems) show strong spectral attenuation that causes significant underestimates of actual mass fluxes. The standard way to correct these underestimates is to use theoretically derived transfer functions. Practical implementation has shown that the attenuation, especially of closed-path systems, is underestimated by this method, even when the additional tube damping is considered. This paper introduces and tests three alternative site-specific and hardware specific correction methods based on spectral analysis, which typically enhance—additionally to the classical correction—the calculated mass fluxes based on closed-path analysers, as their inevitable tubing and the filters used cause additional flow attenuation. Two of the three methods are based on a site-specific and hardware specific transfer functions, the third uses direct comparison of cospectra. Primarily the methods based on transfer functions proved to be easy-to-handle once established for the specific set-up and measurement site. They represent practical and robust methods to correct for spectral attenuation.

A roving tower concept was used to compare a semi-arid grassland site in Inner Mongolia (China), which was fenced in 1979 and ungrazed thereafter (UG79) with differently grazed semi-arid steppe ecosystems. The study was conducted during three consecutive years characterised by contrasting precipitation. The different grazing intensities included continuously and moderately grazed (CG), winter grazed (WG), and heavily grazed (HG). Here, we compare the energy fluxes and surface parameters that characterise the differently managed plots. The main focus is on sensible heat flux (H), available energy (AE), surface temperature (T ( s )), and surface albedo (alpha). Systematic errors were excluded by a side-to-side intercomparison of the instruments, and systematic climatic differences were minimised by the close distance between the fixed and the roving eddy covariance tower. Statistically, AE and T ( s ) were always significantly different between two simultaneously measured grazing intensities. Whereas AE was higher at UG79 in all years (mean difference of about 19Wm(-2)), T ( s ) was typically lower at UG79 (mean differences of 0.4 degrees C to about 2 degrees C). The exception was the end of the vegetation period in 2004 when T ( s ) was 0.6 degrees C higher at UG79 compared to CG. At UG79 alpha was typically significantly lower, and H was typically significantly higher. Consequently, latent heat fluxes (both as energy balance residual and directly measured) do not differ much between the different grazing intensities. It is concluded, that (1) the roving tower concept is able to detect differences due to grazing, (2) differences between the sites can be attributed to real surface differences, and (3) differences due to grazing intensities are small compared to interannual differences in surface fluxes.

The Kyoto-protocol permits the accounting of changes in forest carbon stocks due to forestry. Therefore, forest owners are interested in a reproducible quantification of carbon stocks at the level of forest management units and the impact of management to these stocks or their changes. We calculated the carbon stocks in tree biomass and the organic layer including their uncertainties for several forest management units (Tharandt forest, Eastern Germany, 5,500 ha) spatially explicit at the scale of individual stands by using standard forest data sources. Additionally, soil carbon stocks along a catena were quantified. Finally, carbon stocks of spruce and beech dominated stands were compared and effects of thinning intensity and site conditions were assessed. We combined forest inventory and data of site conditions by using the spatial unions of the shapes (i.e., polygons) in the stand map and the site map. Area weighted means of carbon (C) stocks reached 10.0 kg/m² in tree biomass, 3.0 kg/m² in the organic layer and 7.3 kg/m² in mineral soil. Spatially explicit error propagation yielded a precision of the relative error of carbon stocks at the total studied area of 1% for tree biomass, 45% for the organic layer, and 20% for mineral soil. Mature beech dominated stands at the Tharandt forest had higher tree biomass carbon stocks (13.4 kg/m²) and lower organic layer carbon stocks (1.8 kg/m²) compared to stands dominated by spruce (11.6, 3.0 kg/m²). The difference of tree biomass stocks was mainly due to differences in thinning intensity. The additional effect of site conditions on tree carbon stocks was very small. We conclude that the spatially explicit combination of stand scale inventory data with data on site conditions is suited to quantify carbon stocks in tree biomass and organic layer at operational scale.

Anemometer and CO2 concentration data from temporary campaigns performed atsix CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulentfluxes in nighttime conditions. While storage was observed to be significant only duringperiods of both low turbulence and low advection, the advective fluxes strongly influence thenocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. Onthe basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’,which takes net radiation and local topography into account, was introduced as a criterion tocharacterise the conditions of storage enrichment/depletion. Comparative analyses of the sixsites showed several common features of the advective fluxes but also some substantial dif-ferences. In particular, all sites where advection occurs show the onset of a boundary layercharacterised by a downslope flow, negative vertical velocities and negative vertical CO2concentration gradients during nighttime. As a consequence, vertical advection was observedto be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. Themain differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which isprobably determined by the source/sink distribution. As a consequence, both positive andnegative horizontal advective fluxes (corresponding respectively to CO 2 removal from theecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates.

International audience; Measuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO2 between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO2 between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid. In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO2-flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.

Modelling of water relations at the landscape and regionalscale has largely suffered from simplifications hardly val-idated or examined in their consequences for interpretingand quantifying interrelationships between system ecologyand hydrology. On such grounds, the overall aim of thestudies gathered under the special theme of this issue of‘‘European Journal for Forest Research’’ was to foster theprocess-related ecological and hydrological basis of eco-system and landscape modelling with particular attention tothe spatio-temporal variation in water fluxes. Researchobjects were mountainous forests with emphasis on theNorthern Alps of Central Europe, including comparisonswith colline and subalpine scenarios (e.g. Matyssek et al.2009) and with the specificities of high-altitude forests inthe tropics (Engelhardt et al. 2009).Regarding the Northern Alps, knowledge on the waterdemand of spruce/beech forests, in particular, with respectto mature stands, was rather limited, being in distinctcontrast with the ample evidence available on low-altitudeforests (e.g. Ko¨stner et al. 1998;Ko¨stner 2001). Such latterevidence can hardly be extrapolated, however, to moun-tainous scenarios, given the specific constraints at highaltitude imposed by climate and topography, their effectson stand structure and canopy exposure and the ultimatelyresulting consequences for system hydrology. Researchdemand is also given in more general terms in that thor-ough process understanding requires the analysis ofextremes in ecological gradients. In fact, mountainousforests represent such extremes—the more so as underlyingmechanisms of high-altitude hydrology still requireclarification.Emphasis on mountainous forests is also timely in viewof their increasing sensitivity to environmental impact asapproaching the timberline ecotone (Wieser and Tausz2007;Ko¨rner 2003). This tendency has become increas-ingly relevant in relation to climate change with non-clarified but probable effects on high-altitude ecosystems,and to efforts in forestry of re-establishing mixed stands(Dertz 1996), including at high elevation sites. In the caseof the Northern Alps, re-structuring at least comprisesbroadleaf European beech (Fagus sylvatica) and conifer-ous Norway spruce (Picea abies), although uncertaintyprevails about the water balance of such systems (Gesleret al. 2007). Conversely, mountainous forests are typicallyassociated with moist climatic and edaphic site conditions.In humid regions, vegetation becomes a crucial hydrolog-ical link, however, between soils and atmosphere, so thathigh-altitude ecology becomes a key determinant in achanging environment of evapotranspiration and run-off,and as a consequence, of hazardous risks by erosion,landslides and flooding, altogether arising from the scopeof site water balances. An exacerbating challenge forclarifying the linkage between water balance components

A single layer (Penman-Monteith) and a two layer (modified Shuttleworth-Wallace) evapotranspiration (ET) model are used alternatively to derive conductances related to the dominant fluxes of water vapor from a semi-closed Scots pine plantation. The derivations are based on micrometeorological measurements of above canopy energy flux densities and a simple resistance network. For a period of consecutive fine weather days, below canopy net radiation and below canopy ET were about 20 percent of the corresponding above canopy values. Resulting conductances for latent heat flux agreed well with porometric measurements of pines and understory scaled to canopy level. The shift from single to two layer modelling reduced the canopy conductance to pine conductance by the fraction of understory ET.