Explore the vast range of titles available.

The Nordic region was subjected to severe drought in 2018 with a particularly long-lasting and large soil water deficit in Denmark, Southern Sweden and Estonia. Here, we analyse the impact of the drought on carbon and water fluxes in 11 forest ecosystems of different composition: spruce, pine, mixed and deciduous. We assess the impact of drought on fluxes by estimating the difference (anomaly) between year 2018 and a reference year without drought. Unexpectedly, the evaporation was only slightly reduced during 2018 compared to the reference year at two sites while it increased or was nearly unchanged at all other sites. This occurred under a 40 to 60% reduction in mean surface conductance and the concurrent increase in evaporative demand due to the warm and dry weather. The anomaly in the net ecosystem productivity (NEP) was 93% explained by a multilinear regression with the anomaly in heterotrophic respiration and the relative precipitation deficit as independent variables. Most of the variation (77%) was explained by the heterotrophic component. Six out of 11 forests reduced their annual NEP with more than 50 g C m −2 yr −1 during 2018 as compared to the reference year. The NEP anomaly ranged between −389 and +74 g C m −2 yr −1 with a median value of −59 g C m −2 yr −1 . This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

Evaporation from the land surface, averaged over successive 8 day intervals and at 0.05° (∼5 km) spatial resolution, was calculated using the Penman‐Monteith (PM) energy balance equation, gridded meteorology, and a simple biophysical model for surface conductance. This conductance is a function of evaporation from the soil surface, leaf area index, absorbed photosynthetically active radiation, atmospheric water vapor pressure deficit, and maximum stomatal conductance (gsx). The novelty of this paper is the use of a “Budyko‐curve” hydrometeorological model to estimate mean annual evaporation rates and hence a unique value of gsx for each grid cell across the Australian continent. First, the hydrometeorological model was calibrated using long‐term water balances from 285 gauged catchments. Second, gridded meteorological data were used with the calibrated hydrometeorological model to estimate mean annual average evaporation () for each grid cell. Third, the value of gsx for each cell was adjusted to equate calculated using the PM equation with from the hydrometeorological model. This closes the annual water balance but allows the PM equation to provide a finer temporal resolution for evaporation than is possible with an annual water balance model. There was satisfactory agreement (0.49 < R2 < 0.80) between 8 day average evaporation rates obtained using remotely sensed leaf area indices, the parameterized PM equation, and observations of actual evaporation at four Australian eddy covariance flux sites for the period 2000–2008. The evaporation product can be used for hydrological model calibration to improve runoff prediction studies in ungauged catchments.

Peatlands and forests cover large areas of the boreal biome and are critical for global climate regulation. They also regulate regional climate through heat and water vapour exchange with the atmosphere. Understanding how land-atmosphere interactions in peatlands differ from forests may therefore be crucial for modelling boreal climate system dynamics and for assessing climate benefits of peatland conservation and restoration. To assess the biophysical impacts of peatlands and forests on peak growing season air temperature and humidity, we analysed surface energy fluxes and albedo from 35 peatlands and 37 evergreen needleleaf forests-the dominant boreal forest type-and simulated air temperature and vapour pressure deficit (VPD) over hypothetical homogeneous peatland and forest landscapes. We ran an evapotranspiration model using land surface parameters derived from energy flux observations and coupled an analytical solution for the surface energy balance to an atmospheric boundary layer (ABL) model. We found that peatlands, compared to forests, are characterized by higher growing season albedo, lower aerodynamic conductance, and higher surface conductance for an equivalent VPD. This combination of peatland surface properties results in a ∼20% decrease in afternoon ABL height, a cooling (from 1.7 to 2.5 °C) in afternoon air temperatures, and a decrease in afternoon VPD (from 0.4 to 0.7 kPa) for peatland landscapes compared to forest landscapes. These biophysical climate impacts of peatlands are most pronounced at lower latitudes (∼45°N) and decrease toward the northern limit of the boreal biome (∼70°N). Thus, boreal peatlands have the potential to mitigate the effect of regional climate warming during the growing season. The biophysical climate mitigation potential of peatlands needs to be accounted for when projecting the future climate of the boreal biome, when assessing the climate benefits of conserving pristine boreal peatlands, and when restoring peatlands that have experienced peatland drainage and mining.

The Nordic region was subjected to severe drought in 2018 with a particularly long-lasting and large soil water deficit in Denmark, Southern Sweden and Estonia. Here, we analyse the impact of the drought on carbon and water fluxes in 11 forest ecosystems of different composition: spruce, pine, mixed and deciduous. We assess the impact of drought on fluxes by estimating the difference (anomaly) between year 2018 and a reference year without drought. Unexpectedly, the evaporation was only slightly reduced during 2018 compared to the reference year at two sites while it increased or was nearly unchanged at all other sites. This occurred under a 40 to 60% reduction in mean surface conductance and the concurrent increase in evaporative demand due to the warm and dry weather. The anomaly in the net ecosystem productivity (NEP) was 93% explained by a multilinear regression with the anomaly in heterotrophic respiration and the relative precipitation deficit as independent variables. Most of the variation (77%) was explained by the heterotrophic component. Six out of 11 forests reduced their annual NEP with more than 50 g C m yr during 2018 as compared to the reference year. The NEP anomaly ranged between -389 and +74 g C m yr with a median value of -59 g C m yr . This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

The aim of this work is to develop and evaluate a performance comparison for estimating satellite-based actual evapotranspiration (AET) using the Penman–Monteith (PM) and Priestley–Taylor (PT) approaches and to create a spatial AET map in an ungauged sub-humid tropical river basin. A few studies have compared the PM and PT model performance with a MODIS evapotranspiration data product (MOD16A2). Estimated AET values by the PT approach (AETPT), PM model with aerodynamic conductance (Ga) computed using the Leuning equation (AETPM), and Ga computed using the Choudhury equation (AETPMCH) were extracted for each of the 304 pixels for each day, and pixelwise comparisons were made with the corresponding MOD16A2 AET estimates for validation. As shown by the low RMSE values (0.19–0.23 mm/day), the PM model suggested in this analysis with Ga computed using the (AETPMCH) equation turned out to be the better model. Also, using the (AETPMCH) equation significantly reduced PBIAS values for all days examined. Topography, land use and land cover (LU/LC), temperature, and moisture availability conditions appear to influence AET variations across the basin. For the eight total Julian days in summer and winter for selected wet (2007) and dry (2012) years the for period 2006–2017, pixelwise maps depicting spatial variability were developed using MATLAB for the AETPMCH approach for selected available cloud-free MODIS image data.

We carried out a paired study in tallgrass prairie to evaluate the influence of vegetation on the energy exchange and evapotranspiration. Two eddy-covariance systems were installed over two adjoining sites, one of which was denuded of vegetation, with the adjacent, control site kept undisturbed. Our year-long investigation shows that, for quantifying the ground surface heat flux, the soil heat storage above the soil plates is more important than the sub-surface soil heat flux, both temporally and in magnitude. The incorporation of the soil heat storage, therefore, is indispensable for energy balance closure in areas with short vegetation. At our control site, we observed a critical threshold of 0.17 m3 m−3 in the surface (top 0.3 m) soil water content, whereby the energy partitioning is significantly affected by the presence of the photosynthetically active vegetation when the surface soil water content is higher than this critical threshold. The pattern of energy partitioning approaches that of the treated site when the surface soil water content is lower than this threshold (during drought), because of the suppression of plant physiological activities. This threshold also applies to the surface conductance for water vapour at the control site, where yearly evapotranspiration is 728 ± 3 mm (versus 547 ± 2 mm for the treated site). Thus, the soil water content and presence of active vegetation are the key determinants of energy partitioning and evapotranspiration. Any land-cover changes or vegetation-management practices that alter these two factors may change the energy and water budgets in tallgrass prairie.

Multiannual measurements of ozone (O3) fluxes were performed from 2012 to 2020 in a broadleaf deciduous forest of the Po Valley, Italy. Fluxes were measured with the eddy covariance technique on a 41-m high tower, 15 m above the forest canopy. Different partition methodologies, based on concomitant water and carbon dioxide measurements, were compared for the calculation of the stomatal and non-stomatal components of the O3 fluxes. Total O3 fluxes revealed a marked interannual variability that was mainly driven by the stomatal activity in summer. Therefore, those factors that influence stomatal conductance were responsible for the flux variability, with soil water content being the main physiological driver. Despite the variability of the total O3 fluxes, the annual mean of the stomatal fraction was similar in the different years, around 42% on a 24-h basis, with an average summer value of 52% and a maximum around 60% during the summer daylight hours. The non-stomatal deposition was mainly driven by air humidity, surface wetness, and chemical sinks such as reaction of O3 with nitric oxide. Wind speed, turbulence intensity, and surface temperature showed a negative relationship with the non-stomatal fraction, but this was probably the result of a temporal misalignment between the daily cycles of non-stomatal conductance and those of temperature, turbulence, and wind speed. During the 7 years of measurements, the forest experienced a phytotoxic O3 dose of 10.55 mmolO3 m−2, as annual average, with an estimated reduction of the forest growth rate around 3% yr−1 according to the dose–effect relationships of the United Nations Economic Commission for Europe for broadleaf deciduous forests. Besides their implication for the O3 risk assessment for vegetation, these long-term measurements could be useful to test the deposition models used to correctly assess the O3 budget in troposphere on a multiannual time span.

Energy and water exchange between the surface and the atmosphere are important drivers to Earth’s climate from local to global scale. In this study, the energy dynamic and the biophysical mechanisms that control the energy partitioning over a natural grassland pasture over the Brazilian Pampa biome are investigated using two micrometeorological sites located 300 km apart, in Southern Brazil. The latent heat flux, LE, was the main component of the energy balance in both autumn-winter (AW) and spring-summer (SS) periods. Annually, approximately 60% of the available energy is used for evapotranspiration (ET). However, the Bowen ratio presents seasonal variability greater in AW than SS. Global radiation, Rg, is the atmospheric variable controlling LE and sensible heat flux, H. Hysteresis curves in the daily cycle were observed for ET and surface conductance, Cs, regarding the environmental variables, net radiation, vapor pressure deficit, and air temperature. Among the variables analyzed in the Pampa biome, surface conductance and evapotranspiration respond more strongly to the vapor pressure deficit. The hysteresis cycles formed by ET and conductance show a substantial biophysical control in the ET process. The results obtained here allowed a comprehension of the biophysical mechanisms involved in the energy partition process in natural grassland. Therefore, this study can be used as a base for research on land-use changes in this unique ecosystem of the Pampa biome.

This paper presents a transient model of streaming potential generated when fluid flows through a porous transducer, which is sintered by glass microspheres and embedded in the circular tube of a liquid circular angular accelerometer (LCAA). The streaming potential coupling coefficient (SPC) is used to characterize this proposed transient model by combining a capillary bundle model of a porous transducer with a modified Packard’s model. The modified Packard’s model is developed with the consideration of surface conductance. The frequency-dependent streaming potential is investigated to analyze the effect of structure parameters of porous media and the properties of the fluid, including particle size distribution, zeta potential, surface conductance, pH, and solution conductivity. The results show that the diameter of microspheres not only affects bandwidth and transient response, but also influences the low-frequency gain. In addition, the properties of the fluid can influence the low-frequency gain. Experiments are actualized to measure the steady-state value of permeability and SPC for seven types of porous transducers. Experimental results possess high consistency, which verify that the proposed model can be utilized to optimize the transient and steady-state performance of the system effectively.

Electrical conductance of Si(111)6×6-Tl and Si(111)β√3×√3-Bi surfaces has been monitored in the course of fullerenes deposition. It has been found that dependence of surface conductivity on the adsorbed C60 dose can be understood in terms of charge transfer induced by interaction of fullerene molecules and substrate which can be explained by the acceptor-like behavior of fullerenes. For the Si(111)6×6-Tl surface decreasing of conductance is connected with depletion of metallic bands of the Tl double layer. For the Si(111)β√3×√3-Bi surface the conductivity is almost unchanged with C60 dose, but due to the fact that C60 layer on this surface form close-packed hexagonal arrays in the layer-by-layer mode, it can be used as a template for alkali-induced ultra-thin fulleride formation.