Grass-covered Ground Research Articles

Fine-Scale (10 m) Dynamics of Smallholder Farming through COVID-19 in Eastern Thailand

This study aims to understand the spatiotemporal changes in patterns of tropical crop cultivation in Eastern Thailand, encompassing the periods before, during, and after the COVID-19 pandemic. Our approach involved assessing the efficacy of high-resolution (10 m) Sentinel-2 dense image time series for mapping smallholder farmlands. We integrated harmonic regression and random forest to map a diverse array of tropical crop types between summer 2017 and summer 2023, including durian, rice, rubber, eucalyptus, oil palm, pineapple, sugarcane, cassava, mangosteen, coconut, and other crops. The results revealed an overall mapping accuracy of 85.6%, with several crop types exceeding 90%. High-resolution imagery demonstrated particular effectiveness in situations involving intercropping, a popular practice of simultaneously growing two or more plant species in the same patch of land. However, we observed overestimation in the majority of the studied cash crops, primarily those located in young plantations with open tree canopies and grass-covered ground surfaces. The adverse effects of the COVID-19 pandemic were observed in specific labor-intensive crops, including rubber and durian, but were limited to the short term. No discernible impact was noted across the entirety of the study timeframe. In comparison, financial gain and climate change appeared to be more pivotal in influencing farmers’ decisions regarding crop cultivation. Traditionally dominant crops such as rice and oil palm have witnessed a discernible decline in cultivation, reflecting a decade-long trend of price drops preceding the pandemic. Conversely, Thai durian has seen a significant upswing even over the pandemic, which ironically served as a catalyst prompting Thai farmers to adopt e-commerce to meet the surging demand, particularly from China.

Energy Savings Resulting from Using a Near-Surface Earth-to-Air Heat Exchanger for Precooling in Hot Desert Climates

Given the substantial energy use for space cooling in buildings, integrating energy-efficient and sustainable cooling systems into buildings has become increasingly more important. Even though the cooling potential of a near-surface earth-to-air heat exchanger (EAHE) with grass cover was demonstrated in previous studies, the energy savings and environmental benefits resulting from using the EAHE have not yet been quantified. Therefore, in this study, we quantify the energy savings resulting from using a near-surface earth-to-air heat exchanger (EAHE) with grass-covered ground as a precooling unit in hot desert climates. The outlet air conditions of the EAHE during 9 months of the year (March to November), where space cooling is required, are predicted using a 3D transient CFD model, which is validated against field measurements. The EAHE is fabricated from a 1 mm thick aluminum tube with a diameter of 0.15 m and a length of 21.5 m, buried 0.4 m deep. The results showed that the EAHE can cool ambient air by up to 8.5 °C at an air flow rate of 607 m3/h, corresponding to a cooling capacity of 1700 W and a COP of 17. The daily average cooling capacity of the EAHE is about 560 W for an average operation period of 15.1 h per day. When used as a precooling unit for conventional cooling systems, the highest estimated monthly energy savings is 115 kWh, and the estimated annual savings is 741 kWh.

An urban ecohydrological model to quantify the effect of vegetation on urban climate and hydrology (UT&C v1.0)

Abstract. Increasing urbanization is likely to intensify the urban heat island effect, decrease outdoor thermal comfort, and enhance runoff generation in cities. Urban green spaces are often proposed as a mitigation strategy to counteract these adverse effects, and many recent developments of urban climate models focus on the inclusion of green and blue infrastructure to inform urban planning. However, many models still lack the ability to account for different plant types and oversimplify the interactions between the built environment, vegetation, and hydrology. In this study, we present an urban ecohydrological model, Urban Tethys-Chloris (UT&C), that combines principles of ecosystem modelling with an urban canopy scheme accounting for the biophysical and ecophysiological characteristics of roof vegetation, ground vegetation, and urban trees. UT&C is a fully coupled energy and water balance model that calculates 2 m air temperature, 2 m humidity, and surface temperatures based on the infinite urban canyon approach. It further calculates the urban hydrological fluxes in the absence of snow, including transpiration as a function of plant photosynthesis. Hence, UT&C accounts for the effects of different plant types on the urban climate and hydrology, as well as the effects of the urban environment on plant well-being and performance. UT&C performs well when compared against energy flux measurements of eddy-covariance towers located in three cities in different climates (Singapore, Melbourne, and Phoenix). A sensitivity analysis, performed as a proof of concept for the city of Singapore, shows a mean decrease in 2 m air temperature of 1.1 ∘C for fully grass-covered ground, 0.2 ∘C for high values of leaf area index (LAI), and 0.3 ∘C for high values of Vc,max (an expression of photosynthetic capacity). These reductions in temperature were combined with a simultaneous increase in relative humidity by 6.5 %, 2.1 %, and 1.6 %, for fully grass-covered ground, high values of LAI, and high values of Vc,max, respectively. Furthermore, the increase of pervious vegetated ground is able to significantly reduce surface runoff.

Comparative evaluation of 24 reference evapotranspiration equations applied on an evergreen-broadleaved forest

Abstract Reference evapotranspiration (ET0) is a major component of the hydrological cycle. Its use is essential both for the hydrological rainfall–runoff assessment models and determination of water requirements in agricultural and forest ecosystems. This study investigates the performance of 24 different methods, which produce ET0 or potential evapotranspiration estimates above a grass-covered ground in a Mediterranean forest environment in Greece and compares the derived results with those of the presumed most accurate and scientifically acceptable Penman–Monteith method (ETP-M). Their performance was evaluated on a daily basis for a period of 17 years, using 17 different statistical parameters of goodness of fit. The results showed that some empirical methods could serve as suitable alternatives. More specifically, Copais (ETCOP), Hargreaves original (ETHAR), and Valiantzas2 (ETVA2) methods, exhibited very good values of the model efficiency index, EF (0.934, 0.932, and 0.917, respectively) and the index of agreement, d (0.984, 0.982, and 0.977, respectively). Additionally, the differences of the estimated mean daily value against the respective ETP-M value (rt index) for all methods had a range of −27.8% (Penman – ETPEN) to +59.5% (Romanenko – ETROM), while Copais (ETCOP), Hargreaves–Samani modified1 (ETHS1), and STU (ETSTU) yielded the best values (−0.06%, +0.06%, and 0.22%, respectively).

Frontal extents in virtual environments are not immune to underperception.

Distance is commonly underperceived by up to 50 % in virtual environments (VEs), in contrast to relatively accurate real world judgments. Experiments reported by Geuss, Stefanucci, Creem-Regehr, and Thompson (Journal of Experimental Psychology: Human Perception and Performance, 38, 1242-1253, 2012) indicate that the exocentric distance separating two objects in a VE is underperceived when the objects are oriented in the sagittal plane (depth extents), but veridically perceived when oriented in a frontoparallel plane (frontal extents). The authors conclude that "distance underestimation in the [VE] generalizes to intervals in the depth plane, but not to intervals in the frontal plane." The current experiment evaluated an alternative hypothesis that the accurate judgments of frontal extents reported by Geuss et al. were due to a fortunate balance of underperception caused by the VE and overperception of frontal relative to depth extents. Participants judged frontal and depth extents in the classroom VE used by Geuss et al. and in a sparser VE containing only a grass-covered ground plane. Judgments in the classroom VE replicated findings by Geuss et al., but judgments in the grass VE show underperception of both depth and frontal extents, indicating that frontal extents are not immune to underperception in VEs.

Impedance tube experiments as a tool to teach acoustics of porous media

As a practical matter, sound propagation in many everyday situations includes interactions with porous materials. Such materials include textiles, foams, gravel, grass-covered ground, asphalt, and trees. Modeling sound propagation in complex environments, such as passenger cabins or urban spaces, with porous materials like these is essential to understanding realistic acoustics problems. The theory behind the sound absorption in porous material is not typically included in introductory acoustics courses. This is, at least in part, attributable to the mathematical complexity of the topic. Despite this barrier, acoustic properties of porous material can be easily introduced to typical undergraduate students using simple experiments to demonstrate the acoustic effects of different materials. This presentation describes acoustic property measurements of rigid-frame porous material with straight cylindrical pores as well as foam and soil included in an “Introduction to Acoustics” class at the Catholic University of America. The apparatus is composed of a LabVIEW measurement interface, a pair of microphones, and a simple aluminum impedance tube. The modular design allows students to adapt the software or hardware for other experiments. These hands-on experiences are intended to impart a conceptual understanding of the theory of sound absorption and the acoustic performance of sound absorbing (porous) media.

Investigating the relationship between air and ground temperature variations in shallow depths in northern Greece

In this paper the relationship between ground temperature and ambient air temperature is investigated in Northern Greece. To this end, 10 years records of daily average values of ambient air, bare and grass-covered ground surface, and ground under bare surface temperatures at depths of 0.02, 0.05, 0.10, 0.20, 0.50, 1.00 and 1.50 m measured by the Meteorological Station of Aristotle University of Thessaloniki are used.Seasonal variation of surface and ground temperatures, time lag regarding ambient air temperature variations, as well as the vertical distribution of ground temperature up to 1.50 m, are examined and analyzed.The ground-to-air temperature relation is examined by correlation of average daily, as well as monthly values, and best trend lines equations are defined in order to be used in technical applications. The defined analytical equations are evaluated by comparison of calculated and measured ground temperature values and a very good fitness is found.

Acoustical characterization of grass-covered ground

An investigation of acoustical properties of soil covered with live grass is presented. Measurements of normal surface impedance of such samples have been performed over a 200–2000 Hz frequency band using a vertical impedance tube. A phenomenological model is used to predict acoustic properties of the samples. In this work, the samples are considered as a three-component system: soil, grass roots, and foliage. Acoustic impedance of this composite material has components resulting from the different constituent elements. In order to differentiate the acoustic absorption contribution of each element, grass that was controlled for both water content and grass blade height was grown. The acoustic contribution of the soil was determined by performing measurements on unseeded ground with an equivalent watering protocol. Contributions of the roots and foliage were determined by making impedance measurements before and after shearing the mature foliage near the soil surface. The effect of water content in the soil was estimated by making measurements of the samples before and after oven desiccation. We show the effects of roots and foliage on acoustic absorption of grass-covered ground and the acoustic parameters of these complex media estimated using an equivalent fluid model.

Experimental investigation of induced suction distribution in a grass-covered soil

Evapotranspiration from a grass-covered ground is known to induce suction by soil evaporation and grass transpiration. However, grass-induced suction in the ground when it is subjected to wetting and drying are not yet well understood. In this study, a laboratory test program was conducted to investigate the magnitude and distribution of suction induced by Bermuda grass growing in silty sand. In total, four test boxes compacted with silty sand were prepared, three of which covered with Bermuda grass while one test box was left bare as control. All the four test boxes were subjected to wetting and drying in a plant room with temperature and humidity controlled. Under identical atmospheric conditions and initial soil density and water content, peak suction induced within the root zone in grassed soil was 1.5 times higher than that in bare soil after 20 days of drying. A vertical suction influence zone was identified to be up to four times the root depth while the lateral suction influence zone was one diameter of ring collar away from the centre of the plot. Upon wetting, suction retained at depth right below the root zone in grassed soil was found to be 40% higher than that in bare soil. For three grass replicates that were germinated under identical atmospheric conditions, they produced different shoot lengths and induced different magnitudes of suction. No direct correlation between grass shoot length and grass-induced suction could be found.

The effects of vegetation on reducing traffic noise from a city ring road

This study was designed to investigate the reduction of road noise by vegetation along the ring road of Thessaloniki. Road noise was measured at two sites, one through a belt of trees and the other on a grass-covered ground. A total of 245 measurements were taken from each site over a two-month period and expressed as differences in LAeq. Also measurements of the number of vehicles per minute, the type of vehicles and analytic description of vegetation were taken at both sites. The results showed higher noise reductions through the belt of trees than over the grass-covered ground. According to the results of this research the largest reduction, 6 dB, was seen in the Pinus brutia belt of trees, 60 m away from the road. This noise reduction is considered satisfactory due to the tree belts extending on both sides of the ring road.

Wind turbine noise propagation: Comparison of measurements and predictions by a method based on geometrical ray theory

The objective of the present work has been to investigate the accuracy of the Nord2000 propagation model when used to predict noise from high sources like wind turbines. The model is representative to a new generation of �engineering� models developed on basis of analytical and heuristic solutions. The accuracy is investigated by comparison with measurements. Three experiments have been carried out for this purpose: 1) downwind and upwind propagation over flat grass-covered ground from an elevated loudspeaker, 2) downwind and upwind propagation over non-flat ground from a loudspeaker placed on top of a wind turbine nacelle, and 3) downwind propagation over non-flat ground from a single wind turbine. Experiment 1) showed for downwind propagation a very fine agreement between measured and predicted A-weighted noise levels in the considered range of propagation distances (1500 m). The bias error was 0.1 dB with a standard uncertainty of 0.7 dB. The agreement for upwind propagation was not as good. In experiment 2) the accuracy was slightly reduced compared to the downwind results of experiment 1) with a bias error of 0.5 dB and a standard uncertainty of 1.9 dB. In experiment 3) where a wind turbine was used as source the agreement was less than seen in the loudspeaker experiments.

Influence of short-term meteorological variations over flat grass-covered ground on noise level fluctuation and application of mean meteorological data to noise prediction

For environmental noise prediction, it is practicable to use meteorological data available from local meteorological observatories. However, these observations have limitations induced by the methods of measuring and data processing. Usually only mean meteorological values averaged over one 10 min period every hour are calculated. To apply these mean meteorological variables to noise propagation appropriately, we need to investigate the characteristics of both acoustic and meteorological parameters within the 10 min period. We made simultaneous measurements of both parameters over flat grass-covered ground and estimated effective sound speed profiles by similarity theory, using the meteorological data measured under conditions similar to those at local observatories. The changes in sound pressure level in periods around sunrise and sunset were similar and were smaller than those around culmination, in which fluctuations of approximately 20 dB were measured at higher frequencies at a distance of 100 m. Noise predictions by the parabolic equation method and sound speed profiles determined from instantaneous meteorological variations generally agreed with the measurements except in the time period around culmination. When we used 10 min mean meteorological values in combination with the parabolic equation method, we obtained reasonable agreement with the measurements at middle frequencies in time periods around sunrise and sunset.

A linearized Eulerian sound propagation model for studies of complex meteorological effects.

Outdoor sound propagation is significantly affected by the topography (including ground characteristics) and the state of the atmosphere. The atmosphere on its part is also influenced by the topography. A sound propagation model and a flow model based on a numerical integration of the linearized Euler equations have been developed to take these interactions into account. The output of the flow model enables the calculation of the sound propagation in a three-dimensionally inhomogeneous atmosphere. Rigid, partly reflective, or fully absorptive ground can be considered. The linearized Eulerian (LE) sound propagation model has been validated by means of four different scenarios. Calculations of sound fields above rigid and grass-covered ground including a homogeneous atmosphere deviate from analytic solutions by < or = 1 dB in most parts of the computed domain. Calculations of sound propagation including wind and temperature gradients above rigid ground agree well with measured scale model data. Calculations of sound propagation over a screen including ground of finite impedance show little deviations to measured scale model data which are probably caused by an insufficient representation of the complex ground impedance. Further calculations included the effect of wind on shading by a screen. The results agree well with the measured scale model data.

The insertion loss of screens under the influence of wind

Point source propagation over a screen located on a finite impedance surface representative of grass-covered ground is investigated under upwind and downwind conditions. The theoretical part of the investigation involves extended use of parabolic equation methods (PE) allowing for the changes in the vertical wind speed profile when the wind field passes the screen. The influence of turbulence is also implemented. The experimental part of the investigation relies on a scale model technique based upon a 1:25 scaling ratio and a triggered spark source. The main results relate to the size of the insertion loss of a screen under windy conditions and to the acoustic importance of the redirection of the flow before and after the screen.

Ground effect for A-weighted noise in the presence of turbulence and refraction

Closed-form approximations, of ground effect for continuous A-weighted noise from a point source near to finite impedance ground, obtained by Li et al. [J. Acoust. Soc. Am. 88, 1170–1172 (1990)] and Makarewicz [J. Acoust. Soc. Am. 82, 1706–1711 (1987); 88, 1172–1175 (1990)], are extended to take account of more general models for ground impedance including revised variable-porosity, thin-layer, and rough porous ground where the roughness is small compared with incident wavelengths. The influences of both turbulence and weak atmospheric refraction within the illuminated zone are included. Approximate relationships between optimum ground effect for A-weighted noise, ground parameters, source spectrum, source height, and receiver height are predicted and explored. For low source heights and noise sources with A-weighted spectra with low peak frequencies, it is possible to indicate ground surface parameters that will give significantly higher excess attenuation than those typical of grass-covered ground. It is predicted that, in the illuminated zone, the optimum ground effect parameters are fairly robust to weak refraction effects.

Climate, Grazing and Disturbance, and the Population Dynamics of Leucochrysum albicans at Ross, Tasmania

A population of an endangered daisy, Leucochrysum albicans (syn. Helipterum albicans), was monitored monthly in grazed and ungrazed plots for several years in a paddock near Ross, Tasmania. The population declined dramatically during the extremely dry summer of 1987, and also experienced high mortality in the dry summer of 1988. Adult plants and germinates were more abundant in grazed than in ungrazed plots for most of the period after this decline. The development of axillary branches on adult plants was much more prominent outside than inside the exclosures. Seedling establishment preferentially occurred on ground dominated by herbs, with establishment being low and extremely brief on grass-covered ground. The frequently disturbed margins of the exclosures were the most favourable sites for establishment of new individuals. The future of this unpalatable rare species seems to be dependent upon management that maintains open and, preferably, disturbed ground.

800 MHz mobile radio propagation prediction using Kalman filtering techniques

A method is presented to derive a propagation model to predict the received median signal voltage for an 800-MHz mobile radio system. Results are given of an investigation into the performance of a particular model that was derived using this method. The method uses a Kalman filter that uses propagation measurements to derive a best fit for a particular propagation model to the measured data. The model that was derived is based on propagation measurements made in the State of Florida. The new propagation model uses a plane-Earth propagation model that is corrected with an environmental propagation loss term that takes into account the various environmental effects near the mobile unit. The parameters in the environmental propagation loss model were determined using a Kalman filter that computes minimum mean square estimates of the parameters using measured propagation data. The environmental propagation loss term accounts for the effects of Earth diffraction, hills, valleys, urban and suburban areas, bare and grass-covered ground, bushes, trees, swamps, and propagation over fresh and salt water. It was found that, with the 800-MHz propagation model derived with this method, the prediction error for the received median signal voltage had a standard deviation of 5.08 dB.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Impedance of grass-covered ground at low frequencies measured using a phase difference technique

Although the impedance of natural outdoor ground surfaces has been measured extensively at frequencies above about 200 Hz, measurements at lower frequencies are scarce and have been inaccurate. To overcome some of the difficulties at low frequencies, a two-microphone, phase difference technique to obtain the ground impedance has been adapted. A point source is suspended above the ground and the sound field is measured with two microphones along the vertical line below the source. The magnitude and phase of the reflection coefficient, and hence the impedance, is inferred from the variation of the phase difference between microphones as a function of height above the ground surface. Measurements were made in the range 25–300 Hz on flat, grass-covered ground, free of reflecting objects. The results obtained are consistent with recent seismic/acoustic coupling experiments and theoretical calculations based on layered poroelastic frame models for the ground. In particular, the measured results show evidence for fine structure expected from the layered nature of the ground.

Outdoor sound propagation under the influence of wind and temperature gradients

The situation investigated is sound propagation from a monopole point source located over an impedance surface. The sound propagation is assumed to be influenced by wind and temperature gradients. A very accurate calculation method for taking into account the effect of wind and temperature gradients on sound propagation outdoors is presented and used for verification of a new approximate calculation model. This comparison shows that the approximate model is accurate. A series of loudspeaker measurements has been carried out over a grass-covered ground for distances up to 80 m. The measurements were carried out for a wind speed fo 2–2·5 m/s measured 10 m above the ground. The measured data agree very well with the calculated results. Furthermore the results from the approximate calculation model agree with results from previous investigations [1,2]. Hence, the main conclusion is that a simple and powerful approximate model for sound propagation under the influence of wind and temperature gradients has been developed. However, the influence of turbulence is not taken into account in this paper, and the wind and temperature gradients are assumed to be constant as functions of height.

Acoustical impedance models for outdoor ground surfaces

A theory which predicts the acoustical characteristics of rigid porous materials in terms of four physical parameters is used to provide impedance versus frequency models for various types of ground surface. It is found possible to obtain tolerable agreement with measurements of surface normal impedance for grass-covered ground, bare ground and layered forest floor when measured values of flow resistivity and porosity and estimated values of the other two parameters are used. The agreement with measured acoustical data is shown to be superior to that obtainable with the single-parameter, semi-empirical model that is widely used to predict ground effect even when the single parameter is adjusted for best-fit. A low frequency/high flow resistivity approximation is derived in a form that requires the adjustment of only a single parameter when fitting impedance versus frequency data. This model predicts that the real part of the surface normal impedance, or resistance, and imaginary part, or reactance, are equal and decrease as the inverse square root of frequency. The low frequency/high flow resistivity approximation makes possible the derivation of a two-parameter impedance versus frequency model for a medium with a porosity that decreases exponentially with depth. If the two parameters are considered to be adjustable for best-fit, then some measurements of surface impedance are found to be explicable by means of such a model. The low frequency/high flow resistivity approximation is used, together with the assumption of layer thickness small compared with internal wavelength, to derive another simple two-parameter model for grounds that behave acoustically as thin hard-backed porous layers. Examples of the validity of this model are provided. Both of the two-parameter models predict values of reactance which are greater than those of resistance over the audio-frequency range. Methods of adjusting the two parameters of each model in fitting short-range excess-attenuation data are suggested as alternatives to the single-parameter method, and as the basis for predicting ground effect at long range and low frequencies.