High-resolution peat volume change in a northern peatland: Spatial variability, main drivers, and impact on ecohydrology
Nijp, Jelmer J. ; Metselaar, Klaas ; Limpens, Juul ; Bartholomeus, Harm M. ; Nilsson, Mats B. ; Berendse, Frank ; Zee, Sjoerd E.A.T.M. van der - \ 2019
Ecohydrology 12 (2019)6. - ISSN 1936-0584
compression - ecohydrology - geostatistics - groundwater - peat volume change - peatlands - photogrammetry - spatial patterns
The depth of the groundwater table below the surface and its spatiotemporal variability are major controls on all major biogeophysical processes in northern peatlands, including ecohydrology, carbon balance, and greenhouse gas exchange. In these ecosystems, water table fluctuations are buffered by compression and expansion of peat. Controls on peat volume change and its spatial variability, however, remain elusive, hampering accurate assessment of climate change impact on functioning of peatlands. We therefore (1) analysed patterning of seasonal surface elevation change at high spatial resolution (0.5 m); (2) assessed its relationship with vegetation, geohydrology, and position within the peatland; and (3) quantified the consequences for peatland surface topography and ecohydrology. Changes in surface elevation were monitored using digital close-range photogrammetry along a transect in a northern peatland from after snowmelt up to midgrowing season (May–July). Surface elevation change was substantial and varied spatially from −0.062 to +0.012 m over the measurement period. Spatial patterns of peat volume change were correlated up to 40.8 m. Spatial variation of peat volume change was mainly controlled by changes in water table, and to a lesser extent to vegetation, with peat volume change magnitude increasing from lawn < hollow < flark. Our observations suggest that patchiness and vertical variability of peatland surface topography are a function of the groundwater table. In dry conditions, the variability of surface elevation increases and more localized groundwater flows may develop. Consequently, spatially variable peat volume change may enhance peatland water retention and thereby sustain carbon uptake during drought.
An accurate retrieval of leaf water content from mid to thermal infrared spectra using continuous wavelet analysis
Ullah, S. ; Skidmore, A.K. ; Naeem, M. ; Schlerf, M. - \ 2012
Science of the Total Environment 437 (2012). - ISSN 0048-9697 - p. 145 - 152.
remote-sensing imagery - hyperspectral data - mu-m - spatial heterogeneity - reflectance data - transform - vegetation - leaves - classification - compression
Leaf water content determines plant health, vitality, photosynthetic efficiency and is an important indicator of drought assessment. The retrieval of leaf water content from the visible to shortwave infrared spectra is well known. Here for the first time, we estimated leaf water content from the mid to thermal infrared (2.5-14.0µm) spectra, based on continuous wavelet analysis. The dataset comprised 394 spectra from nine plant species, with different water contents achieved through progressive drying. To identify the spectral feature most sensitive to the variations in leaf water content, first the Directional Hemispherical Reflectance (DHR) spectra were transformed into a wavelet power scalogram, and then linear relations were established between the wavelet power scalogram and leaf water content. The six individual wavelet features identified in the mid infrared yielded high correlations with leaf water content (R 2=0.86 maximum, 0.83 minimum), as well as low RMSE (minimum 8.56%, maximum 9.27%). The combination of four wavelet features produced the most accurate model (R 2=0.88, RMSE=8.00%). The models were consistent in terms of accuracy estimation for both calibration and validation datasets, indicating that leaf water content can be accurately retrieved from the mid to thermal infrared domain of the electromagnetic radiation.
On the escape transition of a tethered Gaussian chain; exact results in two conjugate ensembles
Skvortsov, A.M. ; Klushin, L.I. ; Leermakers, F.A.M. - \ 2006
Macromolecular Symposia 237 (2006)1. - ISSN 1022-1360 - p. 73 - 80.
polymer-chain - force microscope - compression
Upon compression between two pistons an end-tethered polymer chain undergoes an abrupt transition from a confined coil state to an inhomogeneous flower-like conformation that is partially escaped from the gap. In the thermodynamic limit the system demonstrates a first-order phase transition. A rigorous analytical theory of this phenomenon for a Gaussian chain is presented in two ensembles: a) the H-ensemble, in which the distance H between pistons plays the role of the control parameter, and b) the conjugate f-ensemble in which the external compression force f is the independent parameter. A loop region for f(H) with negative compressibility exists in the H-ensemble, while in the f-ensemble H(f) is strictly monotonic. The average lateral forces taken as functions of H (or H, respectively) have distinctly different behavior in the two ensembles. This result is a clear counterexample of the main principles of statistical mechanics stating that all ensembles are equivalent in the thermodynamic limit. Another theorem states that the thermodynamic potential as a function of volume must be concave everywhere. We demonstrated that the exact free energy in the H-ensemble contradicts this statement. Inapplicability of these fundamental theorems to a macromolecule undergoing the escape transition is clearly related to the fact that phase coexistence in the present system is strictly impossible. This is a direct consequence of the tethering and the absence of global translational degrees of freedom of the polymer chain
Soil precompression stress: II a comparison of different compaction tests and stress-displacement behaviour of the soil during wheeling
Keller, T. ; Arvidsson, J. ; Dawidowski, J.B. ; Koolen, A.J. - \ 2004
Soil & Tillage Research 77 (2004)1. - ISSN 0167-1987 - p. 97 - 108.
physical-properties - compression - pressure - strength
Precompression stress is often used as a criterion for soil susceptibility to compaction. The objective of this study was to compare precompression stresses derived from different compression test methods and relate these values to measured stress and displacement during wheeling in the field. Precompression stress was measured at three depths at two sites on a Eutric Cambisol in Sweden using in situ plate sinkage test, and by compressing soil cores in the laboratory with sequential loading (oedometer) and at constant displacement speed, respectively. At the same sites, wheeling experiments were carried out where vertical soil displacement and vertical soil stress were measured simultaneously at three depths. No clear relationship between precompression stress and compression method was found. Precompression stress values derived from the oedometer and the in situ plate sinkage test did generally not differ from each other. Precompression stress derived from the constant speed test was either higher (site A, silty clay loam) or lower (site B, silty clay) compared to the other methods. Precompression stress did not work as a threshold value between reversible and irreversible deformation when precompression stress values derived from the different methods were compared with stress and displacement measured in the field during the wheeling experiment. Even when the observed stress was lower than the precompression stress, a residual displacement was observed. The study demonstrates that the precompression stress is not a sharp value but depends on the compression test method and its analysis, and that compression tests might not represent the soil behaviour in the field sufficiently. There is an urgent need to study soil behaviour in the field and link these experiments to soil mechanical (laboratory) tests.
Partition function, metastability, and kinetics of the escape transition for an ideal chain
Klushin, L.I. ; Skvortsov, A.M. ; Leermakers, F.A.M. - \ 2004
Physical Review. E, Statistical nonlinear, and soft matter physics 69 (2004)6,1. - ISSN 1539-3755 - p. 61101 - 1-61101-15.
atomic-force microscopy - polymer-chain - surface interactions - rolling transition - gaussian chain - afm tip - spectroscopy - compression - mushrooms - laws
The exact partition of the gaussian chain squeezed between two cylinders for a phase transition in a single macromolecule is analyzed. The polymer chain is squeezed between two pistons which results in abrupt transition from a confined coil state to an inhomogeneous conformation. The landau free energy is used in a one dimensional fokker-plank equation to predict the life-time of the metastable states. The analysis shows that the mean first passage time is estimated on the basis of the fokker-planck formalism.
Exactly solvable model with stable and metalstable states for a polymer chain near an adsorbing surface
Klushin, L.I. ; Skvortsov, A.M. ; Leermakers, F.A.M. - \ 2002
Physical Review. E, Statistical nonlinear, and soft matter physics 66 (2002). - ISSN 1539-3755 - p. 036114/1 - 036114/16.
statistical-mechanical analysis - atomic-force microscope - escape transition - rolling transition - gaussian chain - ising-model - elasticity - compression - macromolecule - adsorption
We report on the conformational properties and transitions of an ideal polymer chain near a solid surface. The chain is tethered with one of its ends at distance z0 from an adsorbing surface. The surface is characterized by an adsorption parameter c. The exact expression for the partition function is available. We obtained the distribution of complex zeros of this function. The comparison with the Yang-Lee theory allows the characterization of the phase transitions. A first-order conformational transition from a coil to a (adsorbed) flower conformation occurs at c*=6z0/N. The flower is composed of a strongly stretched stem and a pancake that collects the remaining adsorbed segments. The degree of stretching of the coil or of the stem serves as an order parameter which parametrizes the analytical expressions of the Landau free energy. The phase diagram with one binodal and two spinodal lines is presented. The height of the barriers between metastable and stable states is obtained and the lifetime of metastable states is estimated. A two-state ansatz is used to develop scaling arguments to account for the effects of excluded volume.
|Snoeiafval verdichten voor transport en opslag
Evers, E. - \ 1998
Tuin en Park Techniek 5 (1998)6. - ISSN 1380-3212 - p. 30 - 31.
wood products - pruning - plant residues - willows - straw - compression - houtproducten - snoeien - plantenresten - wilgen - stro - compressie
Onderzocht werd hoe de biomassa van zowel gehakseld als ongehakseld stro en wilgenstengels het beste te verdichten is om de kosten voor transport en opslag te verlagen
Comparison of various methods to evaluate fracture phenomena in food materials
Luyten, H. ; Vliet, T. van; Walstra, P. - \ 1992
Journal of Texture Studies 23 (1992)3. - ISSN 0022-4901 - p. 245 - 266.
gouda cheese - compression - gels
The usage, including eating, of foods and intermediate products normally involves large deformations. Fracture and/or yielding then become the salient features. Several methods to study the behaviour food materials at large deformations are discussed. It is shown that it is possible to measure real material properties, independent of the method and the size and shape of the test-piece used. Results for Gouda cheese and potato starch gels are given