1,721,063 research outputs found
Estimation of Surface Characteristics Using GNSS LH-Reflected Signals: Land Versus Water
Full text linkEstimating the characteristics of soil surface represents a significant area in applications such as hydrology, climatology and agriculture. Signals transmitted from Global Navigation Satellite Systems (GNSSs) can be used for soil monitoring after reflection from the Earth’s surface.
In this paper, the feasibility of obtaining surface characteristics from the power ratio of left-hand (LH) reflected signal-to-noise ratio (SNR) over direct right-hand (RH) is investigated. The analysis was done regardless of the surface roughness and the incoherent components of the reflected power. First, the analysis was carried out on data collected during several in situ measurements in controlled environments with known characteristics. Then, further data were collected by a GNSS receiver prototype installed on a small aircraft and analyzed. This system was calibrated on the basis of signals reflected from water. The reflectivity and the estimated permittivity showed good correlation with the types of underlying terrain
Soil moisture retrieval based on LHCP and RHCP GNSS-R signals
No full textSoil moisture can be retrieved using radar techniques or L-band microwave radiometry. Recently, Global Navigation Satellite System Reflectometry (GNSS-R) techniques have been demonstrated to be useful for many applications as sea-state retrieval and soil moisture retrieval. In this paper, GNSS-R signals reflected from the ground are acquired by means of a commercial receiver. These signals are raw sampled and digitalized, before being processed into Delay Doppler Maps (DDMs) and Delay Waveforms (DWs) with an open loop scheme. In this open loop approach, data are continuously acquired, independently on the tracking status of the receiver. An antenna with two channels, left and right circularly polarized and a low level of cross-polarization is used for the acquisition. The value of the permittivity is computed from the SNR with a standard algorithm based on LH polarized reflected signals, and an improved algorithm based on LH and RH reflected signals in order to keep into account the roughness of the surface. Various in situ measurement campaigns on a terrain with known characteristics are described. The values of the permittivity evaluated from the GNSS-R signals are compared with the results obtained by local measurements based on Time Domain Reflectometry technique (TDR). The measurements were performed with a three-rod sensor and a Tektronix Metallic Cable Tester 1502 instrumen
Evaluation of Turbulent Fluxes on a mountain slope
Full text linkStudies about turbulent exchanges, momentum and mass transfer and energy balance on mountain slopes allow a better comprehension of the interactions between soil and atmosphere in complex orography. In addition, if long periods of observations are considered, the evolution of energy and mass fluxes can be derived. This is useful
for model delicate ecosystems such as in the highlands. Furthermore, the study on carbon dioxide fluxes can be related to the increase of greenhouse gas. The eddy-covariance technique has some critical points: one of the most important is related to the relative uncertainty in the fluxes estimation when there are bad weather or low-wind and nocturnal conditions. Our aim is divided into two parts: in the first one, the meandering was explored. In the second part, we compared two approaches, the planar fit and the double rotation techniques for the computation of turbulent fluxes. Because of the high number of low-wind speed conditions (LWS), we investigated the “meandering”: in LWS conditions, wind speed components and scalars such as temperature can show oscillations visible in the
auto-correlation function of the signals. In these cases, turbulent fluxes estimation may be difficult. We analysed 11 months of data collected at 10 Hz, considering a 1-hour time scale, with the identification of surface-layer parameters. Meandering phenomenon was explored following the works of Mortarini et al. (2013, 2015). We
evaluated also the impact of clear-sky conditions on our data. We observed the validity of the formula for spectral analysis proposed in the aforementioned papers in most part of the analysed hours. Meandering conditions occur in 305 hours over more than 8000, especially during winter and night, although there are diurnal episodes. Meteorological conditions seem to play some role on the local phenomena because, although no certain relationship between stability and meandering parameters was found, the sky was cloudy in most part of meandering hours. In the second part, 30-minutes turbulent fluxes (sensible heat flux, latent heat flux and mass fluxes of water vapour and carbon dioxide) were determined using planar fit and double rotation techniques and the eddy-covariance technique use was tested for our site having a slope of about 26. Then, computation of the energy balance was done. We made comparisons between estimated and measured data and considerations on sensible and latent heat fluxes, then energy and mass fluxes and net radiation were computed also at the daily scale. We found that anemometer rotations improve robustness of computation and the difference between planar fit and double rotation is not so high in fluxes computations. Planar fit seems to give more reliable values. Considering the ground heat flux, G, we obtained a better approximation of energy balance. In particular, the computation of the energy balance ratio (EBR) showed that in general the balance is better during the daytime, while the seasons in which the energy balance is nearer to closure are summer and autumn
Drought effects investigation of a forested site at different spatial scales with eddy covariance technique, cosmic ray sensors, electrical resistivity tomography and 2 meters deep soil moisture and matric potential profile
Full text linkThe forested area is growing in Italy. The eco-hydrological monitoring of such an ecosystem is not trivial, because of canopy height, deep root system and soil heterogeneity. Hence, it is important to merge multiple measurement approaches to quantify the ecohydrological dynamics at the sites. In addition, it is also important to consider multiple temporal and spatial scales from point measurements to areal measurements of the soil-atmosphere interactions. At the Bussoleno - Grangia dell’Alpe forest site (Piedmont, Northwest Italy), we monitored two years, and in particular, two growing seasons (2021 and 2022, with a severe drought in Italy) with areal measures in the atmosphere of actual evapotranspiration (ETa) estimated via eddy covariance technique overcanopy (25 m mast) and areal estimates of soil water content measured continuously with cosmic ray sensors. Moreover, the soil resistivity was measured at the plot scale with Electrical Resistivity Tomography (ERT) technique with several campaigns in which two measurement transects were explored. The point scale with continuous measurements was monitored via soil water content and matric potential probes installed at several depths between 0.1 m and 2 m. In addition, during the ERT campaigns, the soil water content of the first 30 cm profile was also measured via TDR probes in different locations of the experimental site. All this effort allows the reconstruction of a forest volume from about 3 m of soil depth to 23 m of height (height of the eddy covariance setup), including the whole canopy effect. Results highlight the consistency of the soil water content estimation with different approaches (cosmic ray sensors, ERT technique, TDR and capacitive probes). Moreover, using different soil moisture measurements, the ETa regimes can be correctly and well identified. Furthermore, the drought effects are explored also using eddy covariance technique, highlighting that, despite a very low water content above 2 m of soil depth, the vegetation is not severely stressed, likely because of its resilience (the site is characterized by low precipitation, usually below 600 mm/year)
Long-Range Low-Power Electronic System for Drip Irrigation in Precision Agriculture
Full text linkNowadays, food security is threatened by population growth, wars, climate change, and desertification due to human activities. Precision agriculture is a novel concept to minimize the usage of natural resources in the agriculture field, mitigating the anthropological effects. This is possible by adopting electronic systems to measure plants' requirements and to make optimal decisions on the crop, avoiding wastefulness. One critical aspect of agriculture is the handling of potable water: an essential resource for all living entities. This paper proposes a long-range, low-power electronic system for drip irrigation in orchards, especially Actinidia and apple trees, to control the distributed water to the cultivars. In this way, the irrigation cycle depends on the plants' needs, saving water and energy resources. The node communicates using LoRa radiofrequency protocol, and it can be used in rural areas where no internet connection is present
Evapotranspiration of an Abandoned Grassland in the Italian Alps: Modeling the impact of shrub encroachment
Full text linkShrub encroachment of grasslands in the Alps is still a poorly studied phenomenon. Therefore, this study analyses the possible effect of shrub encroachment on actual evapotranspiration (ETa) at an abandoned grassland in the Northwestern Italian Alps, colonised by Elaeagnus Rhamnoides shrubs. This is done by means of micrometeorological and eddy covariance data collected during four growing seasons. Additionally, the Hydrus 1D hydrological model modified to account for a soil column with two vegetation types is used. This modified model is run with a variable percentage of shrubs on evapotranspiration, ranging from 0 to 80% and it is validated by using the measured eddy covariance-derived ETa. The Hydrus 1D model is also applied in its usual set-up, having only one vegetation type, to estimate the ETa from both grassland and shrubs separately.
The performance of the modified model with two vegetation types is acceptable, although it is very variable between different growing seasons and in dry condition it could be further improved (R between 0.50 in 2016 and 0.73 in 2014 considering the probable actual percentage of ETa affected by shrubs. The percentage varies between 20% in 2016 and 60% in 2014). Besides, the model captures the inter-annual variability of ETa. The agreement of cumulative simulated and observed ETa is good, since the deviation between observed and modelled cumulative ETa is always lower, in the four analysed growing seasons, than 50 mm.
The simulated ETa approximates the eddy covariance-derived ETa, however the modelled soil water content is very sensitive to precipitation events, more than the measured soil water content. Both models, with the modified and the usual setup, tend to overestimate the vegetation stress during dry periods. Nevertheless, the single vegetation model results allow us to conclude that the shrubs likely are responsible for an enhancement of ETa and an alteration of the hydrological cycle accordingly. Finally, we explore how some micro-meteorological drivers of ETa (vapour pressure deficit – VPD, net radiation, wind speed, air temperature and ground heat flux - G0) affect the difference between modelled and simulated ETa, and between simulated ETa from shrubs and from grass. Frequently, higher deviations from zero are found especially with high VPD and G0
Wappfruit - an automatic system for drip irrigation in orchards based on real-time soil matric potential data
Full text linkWater is a not-so-renewable resource. Agriculture is impacting for more than 70% of fresh water use worldwide. Considering the increase of population it is fundamental to act in order to reduce water usage. The WAPPFRUIT project aims to design an automatic irrigation system, based on data of water availability in the soil gathered directly in the orchards. Matric potential data are used to determine the exact water demand of the trees, thanks to specific thresholds adapted to the actual soil and crop type. Furthermore, an electronic system based on simple, small, and ultra-low-power devices works together an automatic algorithm to manage the watering events. We tested this approach in three orchards in north-west Italy, comparing our approach to the one used by the farmers. The results show an average water saving of nearly 50% keeping the fruit production comparable to the reference solution. This approach is a clear example of how electronics and technology can really impact agriculture and food production
The contribution of W-band radar monitoring for understanding of runoff and soil erosion response at field scale
Full text linkVegetation cover has a great influence on hydrological response at field scale, and, consequently, on runoff and soil erosion processes. The maintenance of bare soil in vineyard inter-rows with tillage, as well as the tractor traffic, are known to expose the soil to compaction, reduction of soil water holding capacity and increase of runoff and erosion. The use of grass cover is one of the most common and effective practices in order to reduce such threats. Rain-driven runoff (RO) and soil loss (SL) at sites with different cover have been investigated over last decades. It has been found that RO and SL often correlate with rain properties. This correlation, however, is highly variable among different sites and also for different time periods. In many studies rain is represented only by a few parameters such as e.g. maximum intensity and total precipitation. Size of rain drops is rarely analysed, although it is important for an accurate estimation of kinetic energy of rain. Polarimetric millimetre-wavelength radars are one of the instruments capable of drop size measurements. In contrast to in-situ rain sensors, such radars have much larger sampling area and can estimate range profiles of drop size distributions with high spatial and temporal resolution. The objective of this work is to relate runoff and soil erosion to rain properties based on traditional monitoring techniques complemented by observations from a radar. With this aim, a site in the Alto Monferrato vine-growing area (Piedmont, NW Italy) was equipped with a 94-GHz radar in June 2023. The site has two vineyard-field-scale plots with inter-rows managed with conventional tillage (CT) and grass cover (GC), respectively. The radar is located about 100 m from the plots. The radar elevation was set to 30° so that the radar samples rain above the plots. During the summer and autumn seasons of 2023, 26 rain and 13 runoff events were observed. The preliminary results of the conventional analysis show that in this period runoff is directly related to erosivity index (EI30) both in CT and GC plots, and, only in GC treatment to maximum rainfall intensity over 10 minutes and antecedent rainfall in previous 7 days. Maximum rainfall intensity over 30 and 60 minutes, on the contrary, has a negative direct proportion with runoff. Soil erosion for both treatments was also directly related also with maximum rainfall intensity over 10 minutes and antecedent rainfall in previous 7 days and, in addition has a negative proportion with rainfall energy. It should be noted the relevant role played by rainfall intensity over short time interval and the antecedent rainfall, resulting in increased soil moisture. Relationships are different from those obtained in the same site in a previous study, reflecting the peculiarity of summer 2023, characterized by few rainfall events occurred on very dry soil. Information obtained from W-Band radar monitoring allows to investigate relationships in a deeper way among rainfall characteristics and generation of runoff and soil erosion
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