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  • Harry Vereecken  (102)
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  • 1
    Language: English
    In: Remote Sensing, 01 October 2018, Vol.10(10), p.1637
    Description: L-band radiometer measurements were performed at the Selhausen remote sensing field laboratory (Germany) over the entire growing season of a winter wheat stand. L-band microwave observations were collected over two different footprints within a homogenous winter wheat stand in order to disentangle the emissions originating from the soil and from the vegetation. Based on brightness temperature (TB) measurements performed over an area consisting of a soil surface covered by a reflector (i.e., to block the radiation from the soil surface), vegetation optical depth (τ) information was retrieved using the tau-omega (τ-ω) radiative transfer model. The retrieved τ appeared to be clearly polarization dependent, with lower values for horizontal (H) and higher values for vertical (V) polarization. Additionally, a strong dependency of τ on incidence angle for the V polarization was observed. Furthermore, τ indicated a bell-shaped temporal evolution, with lowest values during the tillering and senescence stages, and highest values during flowering of the wheat plants. The latter corresponded to the highest amounts of vegetation water content (VWC) and largest leaf area index (LAI). To show that the time, polarization, and angle dependence is also highly dependent on the observed vegetation species, white mustard was grown during a short experiment, and radiometer measurements were performed using the same experimental setup. These results showed that the mustard canopy is more isotropic compared to the wheat vegetation (i.e., the τ parameter is less dependent on incidence angle and polarization). In a next step, the relationship between τ and in situ measured vegetation properties (VWC, LAI, total of aboveground vegetation biomass, and vegetation height) was investigated, showing a strong correlation between τ over the entire growing season and the VWC as well as between τ and LAI. Finally, the soil moisture was retrieved from TB observations over a second plot without a reflector on the ground. The retrievals were significantly improved compared to in situ measurements by using the time, polarization, and angle dependent τ as a priori information. This improvement can be explained by the better representation of the vegetation layer effect on the measured TB.
    Keywords: Microwave Remote Sensing ; Vegetation Optical Depth ; Soil Moisture ; Winter Wheat ; Smos ; Smap ; Tower-Based Experiment ; Inverse Modeling ; Geography
    E-ISSN: 2072-4292
    Source: Directory of Open Access Journals (DOAJ)
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  • 2
    In: Nature, 2016, Vol.536(7617), p.E1
    Description: In their study, Evaristo et al.1 collected an extensive data set on the basis of which they statistically determined the isotopic compositions of the plant water source (δ 18Ointersect and δ 2Hintersect, called respectively δ 18Ointercept and δ 2Hintercept in their paper) as the x and y coordinates in (δ 18O, δ 2H) space of the intersection between the local meteoric water line (LMWL) and the plant xylem water 'evaporation line' (EL) for a range of climates and vegetation types.
    Keywords: Isotopes ; Groundwater ; Groundwater Recharge ; Stream Flow ; Precipitation ; Botany ; Flowers & Plants;
    ISSN: 0028-0836
    E-ISSN: 1476-4687
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  • 3
    Language: English
    In: Water, 01 September 2018, Vol.10(10), p.1290
    Description: Bedrock topography is known to affect subsurface water flow and thus the spatial distribution of pore water pressure, which is a key factor for determining slope stability. Therefore, the aim of this study is to investigate the effect of bedrock topography on the timing and location of landslide initiation using 2D and 3D simulations with a hydromechanical model and the Local Factor of Safety (LFS) method. A set of synthetic modeling experiments was performed where water flow and slope stability were simulated for 2D and 3D slopes with layers of variable thickness and hydraulic parameters. In particular, the spatial and temporal development of water content, pore water pressure, and the resulting LFS were analyzed. The results showed that the consideration of variable bedrock topography can have a significant effect on slope stability and that this effect is highly dependent on the intensity of the event rainfall. In addition, it was found that the consideration of 3D water flow may either increase or decrease the predicted stability depending on how bedrock topography affected the redistribution of infiltrated water.
    Keywords: Slope Stability ; Local Factor of Safety ; Bedrock Topography ; Soil Depth ; Soil Layering ; Rainfall Induced Landslide ; 2d and 3d Simulation ; Landslide Initiation ; Subsurface Flow ; Rainfall Intensity ; Engineering
    E-ISSN: 2073-4441
    Source: Directory of Open Access Journals (DOAJ)
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  • 4
    Language: English
    In: Remote Sensing, 01 September 2015, Vol.7(9), pp.12041-12056
    Description: We tested an off-ground ground-penetrating radar (GPR) system at a fixed location over a bare agricultural field to monitor the soil freeze-thaw cycles over a snow-covered surface. The GPR system consisted of a monostatic horn antenna combined with a vector network analyzer, providing an ultra-wideband stepped-frequency continuous-wave radar. An antenna calibration experiment was performed to filter antenna and back scattered effects from the raw GPR data. Near the GPR setup, sensors were installed in the soil to monitor the dynamics of soil temperature and dielectric permittivity at different depths. The soil permittivity was retrieved via inversion of time domain GPR data focused on the surface reflection. Significant effects of soil dynamics were observed in the time-lapse GPR, temperature and dielectric permittivity measurements. In particular, five freeze and thaw events were clearly detectable, indicating that the GPR signals respond to the contrast between the dielectric permittivity of frozen and thawed soil. The GPR-derived permittivity was in good agreement with sensor observations. Overall, the off-ground nature of the GPR system permits non-invasive time-lapse observation of the soil freeze-thaw dynamics without disturbing the structure of the snow cover. The proposed method shows promise for the real-time mapping and monitoring of the shallow frozen layer at the field scale.
    Keywords: Ground-Penetrating Radar ; Freeze-Thaw Cycles ; Seasonal Snow Cover ; Inversion ; Geography
    E-ISSN: 2072-4292
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  • 5
    Language: English
    In: Remote Sensing, 01 March 2015, Vol.7(3), pp.2808-2831
    Description: Leaf Area Index (LAI) is an important variable for numerous processes in various disciplines of bio- and geosciences. In situ measurements are the most accurate source of LAI among the LAI measuring methods, but the in situ measurements have the limitation of being labor intensive and site specific. For spatial-explicit applications (from regional to continental scales), satellite remote sensing is a promising source for obtaining LAI with different spatial resolutions. However, satellite-derived LAI measurements using empirical models require calibration and validation with the in situ measurements. In this study, we attempted to validate a direct LAI retrieval method from remotely sensed images (RapidEye) with in situ LAI (LAIdestr). Remote sensing LAI (LAIrapideye) were derived using different vegetation indices, namely SAVI (Soil Adjusted Vegetation Index) and NDVI (Normalized Difference Vegetation Index). Additionally, applicability of the newly available red-edge band (RE) was also analyzed through Normalized Difference Red-Edge index (NDRE) and Soil Adjusted Red-Edge index (SARE). The LAIrapideye obtained from vegetation indices with red-edge band showed better correlation with LAIdestr (r = 0.88 and Root Mean Square Devation, RMSD = 1.01 & 0.92). This study also investigated the need to apply radiometric/atmospheric correction methods to the time-series of RapidEye Level 3A data prior to LAI estimation. Analysis of the the RapidEye Level 3A data set showed that application of the radiometric/atmospheric correction did not improve correlation of the estimated LAI with in situ LAI.
    Keywords: Leaf Area Index ; Red-Edge Band ; Rapideye ; Atmospheric Correction ; Validation ; Time-Series ; Geography
    E-ISSN: 2072-4292
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  • 6
    Language: English
    In: Vadose Zone Journal, 01 February 2018, Vol.17(1)
    Description: The spatiotemporal distribution of root water uptake (RWU) depends on the dynamics of the root distribution and compensatory uptake from wetter regions in the root zone. This work aimed to parameterize three RWU models with different representations of compensation: the Feddes–Jarvis model that uses an empirical function, the Feddes model without compensation, and the Couvreur model that is based on a physical description of water flow in the soil–root system. These models were implemented in HYDRUS-1D, and soil hydraulic parameters were optimized by inverse modeling using soil water content and potential measurements and observations of root distributions of winter wheat ( L.) in horizontally installed rhizotubes. Soil moisture was equally well predicted by the three models, and the soil hydraulic parameters optimized by the models with compensation were comparable. The obtained RWU parameters of the Feddes–Jarvis model were consistent with data reported in the literature, although the pressure heads and for lower and higher transpirations rates, respectively, could not be uniquely identified. Response surfaces of the objective function showed that the root-related parameters of the Couvreur model could be identified using inverse modeling. Furthermore, these parameters were consistent with combined root architectural and hydraulic observations from the literature. The Feddes–Jarvis and Couvreur models simulated similar root-system-scale stress functions that link total RWU to the effective root zone water potential, suggesting that parameters may be transferable between the two models. Simulated RWU profiles differed due to different water redistribution by the root system, but the measurements were not sufficiently precise to observe this redistribution.
    Keywords: Agriculture
    ISSN: 1539-1663
    E-ISSN: 1539-1663
    Source: Directory of Open Access Journals (DOAJ)
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  • 7
    Language: English
    In: Vadose Zone Journal, 01 March 2018, Vol.17(1)
    Description: Soil greenhouse gas (GHG) emissions contribute to global warming. To support mitigation measures against global warming, it is important to understand the controlling processes of GHG emissions. Previous studies focusing mainly on paddy rice fields or wetlands showed a strong relationship between soil redox potential and GHG emission (e.g., NO). However, the interpretation of redox potentials for the understanding of the controlling factors of GHG emission is limited due to the low number of continuous redox measurements in most ecosystems. Recent sensor developments open the possibility for the long-term monitoring of field-scale soil redox potential changes. We performed laboratory lysimeter experiments to investigate how changes in the redox potential, induced by changes in the water level, affect GHG emissions from agricultural soil. Under our experimental conditions, we found that NO emissions followed closely the changes in redox potential. The dynamics of redox potential were induced by changing the water-table depth in a laboratory lysimeter. Before fertilization during saturated conditions, we found a clear negative correlation between redox potentials and NO emission rates. After switching from saturated to unsaturated conditions, NO emission quickly decreased, indicating denitrification as the main source of NO. In contrast, the emissions of CO increased with increasing soil redox potentials. After fertilization, NO emission peaked at high redox potential, suggesting nitrification as the main production pathway, which was confirmed by isotope analysis of NO. We propose that redox potential measurements are a viable method for better understanding of the controlling factors of GHG emissions, for the differentiation between different source processes, and for the improvement of process-based GHG models.
    Keywords: Agriculture
    ISSN: 1539-1663
    E-ISSN: 1539-1663
    Source: Directory of Open Access Journals (DOAJ)
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  • 8
    Language: English
    In: Journal of Hydrology: Regional Studies, September 2015, Vol.4, pp.294-312
    Description: The Federal State of North Rhine-Westphalia (NRW), Germany. On behalf of the Federal State Agency for Nature, Environment and Consumer Protection of North Rhine-Westphalia, Germany (LANUV) the mGROWA model is applied, in order to simulate the water balance components actual evapotranspiration, total runoff, direct runoff and groundwater recharge at Federal State level. mGROWA-simulations were performed in daily time steps for the hydrological reference period 1971–2000 and in a spatial resolution of 100 by 100 m. mGROWA results for groundwater recharge and State-wide recorded groundwater withdrawal rates have been used to assess the extent of groundwater exploitation in NRW. Simulated groundwater recharge levels are presented as long-term annual averages and as long-term monthly values in order to indicate the seasonal fluctuation of groundwater recharge rates. Quantitative statistics indicate that mGROWA enables the simulation of total runoff and groundwater recharge without significant tendency of over- or underestimation. Against this background mGROWA simulation results are used by LANUV to support regional water resources management, e.g. for determining the status of groundwater exploitation in NRW. The majority of groundwater bodies in NRW are at present not at risk of unsustainable groundwater usage. A small number of groundwater bodies along the river Rhine, however, have been assessed as being close to unsustainable groundwater exploitation.
    Keywords: Water Balance Components ; Soil Water Balance ; Runoff ; Runoff Components ; Groundwater Recharge ; Groundwater Management ; Geography
    ISSN: 2214-5818
    E-ISSN: 2214-5818
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  • 9
    Language: English
    In: Remote Sensing, 01 March 2018, Vol.10(3), p.427
    Description: Several studies currently strive to improve the spatial resolution of coarse scale high temporal resolution global soil moisture products of SMOS, SMAP, and ASCAT. Soil texture heterogeneity is known to be one of the main sources of soil moisture spatial variability. With the recent development of high resolution maps of basic soil properties such as soil texture and bulk density, relevant information to estimate soil moisture variability within a satellite product grid cell is available. We use this information for the prediction of the sub-grid soil moisture variability for each SMOS, SMAP, and ASCAT grid cell. The approach is based on a method that predicts the soil moisture standard deviation as a function of the mean soil moisture based on soil texture information. It is a closed-form expression using stochastic analysis of 1D unsaturated gravitational flow in an infinitely long vertical profile based on the Mualem-van Genuchten model and first-order Taylor expansions. We provide a look-up table that indicates the soil moisture standard deviation for any given soil moisture mean, available at https://doi.org/10.1594/PANGAEA.878889. The resulting data set helps identify adequate regions to validate coarse scale soil moisture products by providing a measure of representativeness of small-scale measurements for the coarse grid cell. Moreover, it contains important information for downscaling coarse soil moisture observations of the SMOS, SMAP, and ASCAT missions. In this study, we present a simple application of the estimated sub-grid soil moisture heterogeneity scaling down SMAP soil moisture to 1 km resolution. Validation results in the TERENO and REMEDHUS soil moisture monitoring networks in Germany and Spain, respectively, indicate a similar or slightly improved accuracy for downscaled and original SMAP soil moisture in the time domain for the year 2016, but with a much higher spatial resolution.
    Keywords: Smos ; Smap ; Ascat ; Soilgrids ; Soil Moisture Variability ; Scaling ; Soil Texture ; Geography
    E-ISSN: 2072-4292
    Source: Directory of Open Access Journals (DOAJ)
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  • 10
    Language: English
    In: Sensors, 01 January 2017, Vol.17(1), p.208
    Description: Soil water content is a key variable for understanding and modelling ecohydrological processes. Low-cost electromagnetic sensors are increasingly being used to characterize the spatio-temporal dynamics of soil water content, despite the reduced accuracy of such sensors as compared to reference electromagnetic soil water content sensing methods such as time domain reflectometry. Here, we present an effective calibration method to improve the measurement accuracy of low-cost soil water content sensors taking the recently developed SMT100 sensor (Truebner GmbH, Neustadt, Germany) as an example. We calibrated the sensor output of more than 700 SMT100 sensors to permittivity using a standard procedure based on five reference media with a known apparent dielectric permittivity (1 〈 Ka 〈 34.8). Our results showed that a sensor-specific calibration improved the accuracy of the calibration compared to single “universal” calibration. The associated additional effort in calibrating each sensor individually is relaxed by a dedicated calibration setup that enables the calibration of large numbers of sensors in limited time while minimizing errors in the calibration process.
    Keywords: Soil Water Content ; Permittivity ; Calibration ; Sensor-to-Sensor Variability ; Smt100 Sensor ; Engineering
    E-ISSN: 1424-8220
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