32 research outputs found
Extrapolating continuous vegetation water content to understand sub-daily backscatter variations
No full textMicrowave observations are sensitive to vegetation water content (VWC). Consequently, the increasing temporal and spatial resolution of spaceborne microwave observations creates a unique opportunity to study vegetation water dynamics and its role in the diurnal water cycle. However, we currently have a limited understanding of sub-daily variations in the VWC and how they affect microwave observations. This is partly due to the challenges associated with measuring internal VWC for validation, particularly non-destructively, and at timescales of less than a day. In this study, we aimed to (1) use field sensors to reconstruct diurnal and continuous records of internal VWC of corn and (2) use these records to interpret the sub-daily behaviour of a 10 d time series of polarimetric L-band backscatter with high temporal resolution. Sub-daily variations in internal VWC were calculated based on the cumulative difference between estimated transpiration and sap flow rates at the base of the stems. Destructive samples were used to constrain the estimates and for validation. The inclusion of continuous surface canopy water estimates (dew or interception) and surface soil moisture allowed us to attribute hour-to-hour backscatter dynamics either to internal VWC, surface canopy water, or soil moisture variations. Our results showed that internal VWC varied by 10 %–20 % during the day in non-stressed conditions, and the effect on backscatter was significant. Diurnal variations in internal VWC and nocturnal dew formation affected vertically polarized backscatter most. Moreover, multiple linear regression suggested that the diurnal cycle of VWC on a typical dry day leads to a 2 (HH, horizontally, and cross-polarized) to almost 4 (VV, vertically, polarized) times higher diurnal backscatter variation than the soil moisture drydown does. These results demonstrate that radar observations have the potential to provide unprecedented insight into the role of vegetation water dynamics in land–atmosphere interactions at sub-daily timescales.Water ResourcesMathematical Geodesy and Positionin
Monitoring vegetation dynamics using vegetation optical depth retrieved from L-band single-incidence angle backscatter observations: A field-based study over corn
No full textVegetation water content (VWC) is an important parameter for sustainable land and water management. In agriculture, VWC can be used to monitor drought and assess crop productivity. Being able to monitor VWC is important to reduce agricultural vulnerability and scientifically manage agricultural water use. Due to the dense in-situ networks are expensive and have difficulties in capturing the large spatial variability of VWC. Therefore, remote sensing has great potential in VWC monitoring. With the application of remote sensing, worldwide data with coarse or fine spatial and temporal resolution can be extracted with relatively low cost. Vegetation Optical Depth (VOD), extracted from the radar remote sensing observations, is a dimensionless parameter that highly related to VWC. Thus, VOD can be used as an indicator for VWC. The present research aims to thoroughly analyze the relation between VOD and VWC. High temporal backscatter data and detailed field experiment data of soil moisture and vegetation water content during a full growing season of corn (between 18 April 2018 to 13 June 2018) were used. Correlations between VOD and VWC were analyzed. The result shows that VOD is highly related to . However, the linear relation between VWC and VOD is only valid before the heading stage at both co- and cross-polarization. Then, random forest machine learning was conducted to determine the sensitivity of VOD to the water content of different parts of the plant. This sensitivity analysis contains two parts: a) the sensitivity of VOD to the water content of different vegetation components and b) The sensitivity of VOD to stem and leaf water content at different heights. The results of a) show that VOD is more sensitive to stem and leaf water content in the vegetative stage whereas more sensitive to ear water content during the reproductive stage. Besides, stem, leaf and ear water content can better capture the VOD variation during the vegetative stage. The result from b) suggests that VOD can provide information about the vertical distribution of moisture inside the canopy. Finally, a cross-comparison was conducted between VOD and other commonly used vegetation indicators, which includes NDVI and Cross-Ratio. VOD is available regardless of cloud conditions and is, therefore, more reliable than NDVI. Compared with cross-ratio, VOD is better related to vegetation moisture dynamics
Impact of diurnal variation in vegetation water content on radar backscatter from maize during water stress
No full textMicrowave backscatter from vegetated surfaces is influenced by vegetation structure and vegetation water content (VWC), which varies with meteorological conditions and moisture in the root zone. Radar backscatter observations are used for many vegetation and soil moisture monitoring applications under the assumption that VWC is constant on short timescales. This research aims to understand how backscatter over agricultural canopies changes in response to diurnal differences in VWC due to water stress. A standard water-cloud model and a two-layer water-cloud model for maize were used to simulate the influence of the observed variations in bulk/leaf/stalk VWC and soil moisture on the various contributions to total backscatter at a range of frequencies, polarizations, and incidence angles. The bulk VWC and leaf VWC were found to change up to 30% and 40%, respectively, on a diurnal basis during water stress and may have a significant effect on radar backscatter. Total backscatter time series are presented to illustrate the simulated diurnal difference in backscatter for different radar frequencies, polarizations, and incidence angles. Results show that backscatter is very sensitive to variations in VWC during water stress, particularly at large incidence angles and higher frequencies. The diurnal variation in total backscatter was dominated by variations in leaf water content, with simulated diurnal differences of up to 4 dB in X- through Ku-bands (8.6–35 GHz). This study highlights a potential source of error in current vegetation and soil monitoring applications and provides insights into the potential use for radar to detect variations in VWC due to water stress.Water ManagementCivil Engineering and Geoscience
Impact of Soil Moisture and Vegetation Water Content on Backscatter Simulated by WCM at Different Radar Parameters in Maize Field
No full textThis paper analysis the effects of soil moisture and vegetation water content (VWC) on total backscattering (σ_tot^0) simulated by water cloud model (WCM) throughout a growth cycle of maize at different frequencies, polarization modes, and incidence angles. Firstly, the bare soil backscatter (σ_soil^0) was simulated by Integral Equation Method (IEM) surface scattering model [1] or Dubois empirical backscattering approach [2]. Then, to analysis the effect of vegetation cover, a standard and a double layer WCM based on parameter sets of three published studies [3]-[5] are applied in this study area to simulate the two components of σ_tot^0, direct backscatter from vegetated surface (σ_veg^0) and attenuated soil backscatter (〖γ^2 σ〗_soil^0). The input parameters of IEM, Dubois and WCM are supported by a series of ground measurements performed in Florida during the entire growth season, which includes soil moisture, surface roughness and vegetation biomass measurement. According to the analysis at different frequency and incident angle, the increase of bulk VWC can lead to either an increase or decrease in σ_(tot ). The different impact is determined by either σ_veg^0 or 〖γ^2 σ〗_soil^0 is the main contributor to σ_tot^0. At higher frequencies and larger incident angles, where the dominant part is from σ_veg^0, σ_tot^0 will increase with increasing bulk VWC. While, when 〖γ^2 σ〗_soil^0 becomes the main contributor to σ_tot^0, increasing bulk VWC leads to a denser canopy and thus more incoming microwave is attenuated. Therefore, increasing bulk VWC results in a decrease in σ_tot^0. Besides, according to the results obtained at C-band, different incident angle, HH-polarized microwaves are more sensitive to changes in bulk VWC, especially at larger incident angle. VV-polarization is less affected by vegetation cover, σ_tot^0is sensitive to soil moisture even at peak biomass and large incidence angles, which is attributed to scattering along the soil-vegetation pathway
The influence of surface canopy water on the relationship between L-band backscatter and biophysical variables in agricultural monitoring
No full textThe presence of surface water on the canopy affects radar backscatter. However, its influence on the relationship between radar backscatter and crop biophysical parameters has not been investigated. The aim of this study was to quantify the influence of surface canopy water (SCW) on the relationship between L-band radar backscatter and biophysical variables of interest in agricultural monitoring. In this study, we investigated the effect of SCW on the relationship between co- and cross-polarized radar backscatter, cross ratios (VH/VV and HV/HH), and radar vegetation index (RVI) and dry biomass, vegetation water content (VWC), plant height and leaf area index (LAI). In addition, the effect of SCW on estimated vegetation optical depth (VOD) and its relationship with internal VWC was investigated. The analysis was based on data collected during a field experiment in Florida, USA in 2018. A corn field was scanned with a truck-mounted, fully polarimetric, L-band radar along with continuous monitoring of SCW (dew, interception) and soil moisture every 15 min for 58 days. In addition, pre-dawn destructive sampling was conducted to measure internal vegetation water content and dry biomass. Results showed that the presence of SCW can increase the radar backscatter up to 2 dB and this effect was lower for cross ratios (CRs) and RVI. The Spearman's rank correlations between radar observables and biophysical parameters were, on average, 0.2 higher for dry vegetation compared to wet vegetation. The estimated VOD from wet vegetation was generally higher than those from dry vegetation, which led to different fitting parameter (so-called b) values in the linear fit between VOD and VWC. The results presented here underscore the importance of considering the influence of SCW on the retrieval of biophysical variables of interest in agricultural monitoring. In particular, they highlight the importance of overpass time, and the impact that daily patterns in dew and interception can have on the retrieval of biophysical variables of interest.Mathematical Geodesy and PositioningWater Resource
Detecting forest response to droughts with global observations of vegetation water content
No full textDroughts in a warming climate have become more common and more extreme, making understanding forest responses to water stress increasingly pressing. Analysis of water stress in trees has long focused on water potential in xylem and leaves, which influences stomatal closure and water flow through the soil-plant-atmosphere continuum. At the same time, changes of vegetation water content (VWC) are linked to a range of tree responses, including fluxes of water and carbon, mortality, flammability, and more. Unlike water potential, which requires demanding in situ measurements, VWC can be retrieved from remote sensing measurements, particularly at microwave frequencies using radar and radiometry. Here, we highlight key frontiers through which VWC has the potential to significantly increase our understanding of forest responses to water stress. To validate remote sensing observations of VWC at landscape scale and to better relate them to data assimilation model parameters, we introduce an ecosystem-scale analog of the pressure-volume curve, the non-linear relationship between average leaf or branch water potential and water content commonly used in plant hydraulics. The sources of variability in these ecosystem-scale pressure-volume curves and their relationship to forest response to water stress are discussed. We further show to what extent diel, seasonal, and decadal dynamics of VWC reflect variations in different processes relating the tree response to water stress. VWC can also be used for inferring belowground conditions-which are difficult to impossible to observe directly. Lastly, we discuss how a dedicated geostationary spaceborne observational system for VWC, when combined with existing datasets, can capture diel and seasonal water dynamics to advance the science and applications of global forest vulnerability to future droughts
Extreme rainfall impacts on soil CO2 efflux in an urban forest ecosystem in Beijing, China
No full textExtreme rainfall events are infrequent disturbances that affect urban environments and soil respiration (Rs). Using data measured in an urban forest ecosystem in Beijing, China, we examined the link between gross primary production (GPP) and soil respiration on a diurnal scale during an extreme rainfall event (i.e., the ‘21 July 2012 event’), and examined diel and seasonal environmental controls on Rs. Over the seasonal cycle, Rs increased exponentially with soil temperature (Ts). In addition, Rs was hyperbolically related to soil volumetric water content (VWC), increasing with VWC below a threshold of 0.17 m3 m-3, and then decreasing with further increases in VWC. Following the extreme rainfall event (177 mm), Rs showed an abrupt decrease and then maintained a low value of ~0.3 μmol m-2 s-1 for about 8 h as soil VWC reached the field capacity (0.34 m3 m-3). Rs became decoupled from Ts and increased very slowly, while GPP showed a greater increase. A bivariate Q10-hyperbolical model, which incorporates both Ts and VWC effects, better fit Rs than the Q10 model in summer, but not for whole year.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Parkinsonian signs are associated with subtle functional deterioration in community-dwelling Chinese older persons
No full textObjective: To report the prevalence of mild parkinsonian signs and their association with functional impairment in a population-based study of clinically non-demented Chinese persons. Participants and Methods: A random sample of 765 Chinese older persons from a thematic household survey was recruited. There were 389 normal elderly controls (Clinical Dementia Rating = 0), 291 with mild cognitive impairment, and 85 with very mild dementia. The prevalence of mild parkinsonian signs and its association with everyday functional performance were investigated. Results: Mild parkinsonian signs were defined as a score of 2 or more in the Unified Parkinson's Disease Rating Scale-motor section. The prevalence of mild parkinsonian signs was 16.5%, 33.0% and 49.4% in the normal controls, those with mild cognitive impairment and very mild dementia, respectively. In each group, subjects with mild parkinsonian signs had lower functional scores than those without such signs, even after adjusting for the effect of age, sex, and education. Abnormality in axial function, bradykinesia, and rigidity were associated with lower scores for Instrumental Activities of Daily Living, and rigidity was associated with lower Basic Activities of Daily Living scores. Conclusion: The prevalence of mild parkinsonian signs increased with the severity of cognitive impairment in clinically non-demented older persons. Such signs were associated with functional impairment in older persons with mild cognitive impairment and very mild dementia. © 2008 Hong Kong College of Psychiatrists.link_to_subscribed_fulltex
Using the Recovery Phase in Wheelchair Racing to Estimate the Resistance Forces
No full textInsight into in-field mechanical power estimation in wheelchair racing is useful for athletes and coaches. A non-invasive method to estimate mechanical power is by using inertial measurement units (IMUs) to estimate the power lost to resistive forces during wheelchair propulsion. During the recovery phase, no propulsive force acts on the athlete/wheelchair combination and therefore, the deceleration of the athlete/wheelchair combination during this phase is caused by the power lost to resistive forces. The aim of this study was to investigate whether using deceleration in the recovery phase for estimation of resistance using IMUs is applicable in wheelchair racing. To approach the instantaneous velocity of the athlete/wheelchair combination and therefore the deceleration during the recovery phase, the kinematics of the wheelchair and the upper body were measured and used for three different methods. The simplest method is to use the velocity of the wheelchair (vwc), the second method takes the influence of the trunk movement into account (vcom,2seg) and the last method takes the influence of all upper body segments into account (vcom,tot). The results of this study indicate that using vwc, vcom,2seg or vcom,tot for estimating the deceleration during the recovery phase is not yet suitable to provide accurate estimation of the total resistance compared to drag test based results. This indicates that application of this method of estimating resistance forces in the recovery phase is not as straightforward as initially anticipated. Despite the potential benefits of this approach, the results suggest that refinement of this method or development of a new method is necessary to obtain an accurate estimation of the total resistance.Project WheelPowerBiomedical Engineering | Neuromusculoskeletal Biomechanic
Tracking soil temperature and moisture in a multi-factor climate experiment in temperate grassland: Do climate manipulation methods produce their intended effects?
No full textPassive open-top chambers (OTCs) and rainout shelters (RSs) have been used for over two decades to manipulate temperature and water availability
in experiments on plant communities. These types of manipulations have been independently evaluated; however, as experiments become more complex and multiple factors are evaluated the potential for unknown or undesirable treatment effects increases. We present the effects of temperature manipulations (with OTCs), water manipulations (with RSs and water additions), and a clipping treatment, implemented in a fully factorial
design, on soil moisture and temperature over 2 years in a temperate grassland. Temperature was increased 0.2C by OTCs. Soil volumetric water
content was reduced 3% by RSs and increased 2% by watering. However, clipping vegetation, treatment interactions, and weather conditions also affected soil temperature and moisture. For example, in OTCs RSs increased the temperature an additional 0.4C, watering lowered it 0.4C, and clipping
raised temperature 2C. Similarly, changes in soil moisture due to the RSs decreased VWC by 3% and increased 1% by clipping whereas soil moisture
due to watering was reduced 1% by the OTCs and clipping. We also found that OTCs are more effective at raising temperatures on cooler days when
soil temperatures are below 16.3C. Our results suggest that all treatment types generally affect soil variables in predicable ways, but use of such devices should be adopted with caution, as they do not act independently, or exclusively, on the target variables.
KeyPeer reviewedopen-top chamberrainout shelterclimate changesoil volumetric water contentclippingdisturbanc
