Suchergebnisse

Abstract
Leaf area index (LAI) can be measured either directly, using destructive methods, or indirectly using optical methods that are based on the tight relationship between LAI and canopy light transmittance. Third, innovative approach for LAI measuring is usage of remote sensing data, especially airborne laser scanning (ALS) data shows itself as a advisable source for purposes of LAI modelling in large areas. Until now there has been very little research to compare LAI estimated by the two different approaches. Indirect measurements of LAI using hemispherical photography are based on the transmission of solar radiation through the vegetation. It can thus be assumed that the same is true for the penetration of LiDAR laser beams through the vegetation canopy. In this study we use ALS based LiDAR penetration index (LPI) and ground based measurement of LAI obtained from hemispherical photographs as a reference in-situ method. Several regression models describing the corellation LAI and LPI were developed with various coefficients of determination ranging up to 0,81. All models were validated and based on the tests performed, no errors were drawn that would affect their credibility.

28 Pags., 1 Tabl., 3 Figs. The definitive version is available at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291755-0238 ; Background and Aims: Most optical devices for indirect measurement of leaf area index (LAI) from canopy-transmitted light are tailored for homogeneous canopies, thus limiting their application to discontinuous canopies such as vertically trained vineyards. This study evaluates the influence of sun position on the reliability of LAI estimates provided by a ceptometer and proposes a measurement protocol for use of such instruments on vineyards under direct illumination. Methods and Results: Ceptometer readings at several sun elevation and azimuth angles were recorded in two fields. LAI estimated at different sun positions using several measurement protocols were then compared against destructive leaf area measurements. The best results when the sun position departs from the zenith (sun elevation < 40°) were achieved by reading the transmitted, photosynthetically active radiation in all the inter-row spaces, whereas measurements below the vines were suitable only when the sun illuminates close to the zenith. Conclusions: The homogeneity of the canopy fraction measured along the ceptometer at each individual reading is a major requirement in order to obtain non-biased LAI estimates. Therefore, the protocol followed to measure the transmitted photosynthetically active radiation at every sun position proved critical for the accuracy of LAI determination. Significance of Study This study provides guidelines for reducing LAI uncertainties associated with vineyard canopy structure in LAI estimation with linear array optical devices such as ceptometers. ; This research was funded by the Government of Aragon, Spain, as part of a demonstration project, and by the Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) under research project RTA 2005-00230-00-00, and a grant for author Raúl López-Lozano. ; Peer reviewed

© 2021 by the authors. ; We proposed a direct approach to validate hectometric and kilometric resolution leaf area index (LAI) products that involved the scaling up of field-measured LAI via the validation and recalibration of the decametric Sentinel-2 LAI product. We applied it over a test study area of maize crops in northern China using continuous field measurements of LAINet along the year 2019. Sentinel-2 LAI showed an overall accuracy of 0.67 in terms of Root Mean Square Error (RMSE) and it was used, after recalibration, as a benchmark to validate six coarse resolution LAI products: MODIS, Copernicus Global Land Service 1 km Version 2 (called GEOV2) and 300 m (GEOV3), Satellite Application Facility EUMETSAT Polar System (SAF EPS) 1.1 km, Global LAnd Surface Satellite (GLASS) 500 m and Copernicus Climate Change Service (C3S) 1 km V2. GEOV2, GEOV3 and MODIS showed a good agreement with reference LAI in terms of magnitude (RMSE ≤ 0.29) and phenology. SAF EPS (RMSE = 0.68) and C3S V2 (RMSE = 0.41), on the opposite, systematically underestimated high LAI values and showed systematic differences for phenological metrics: a delay of 6 days (d), 20 d and 24 d for the start, peak and the end of growing season, respectively, for SAF EPS and an advance of −4 d, −6 d and −6 d for C3S. ; This research was funded by the National Natural Science Foundation of China under Grant (41971282; 42001303), the Sichuan Science and Technology Program (2021JDJQ0007; 2020JDTD0003), the Copernicus Global Land Service (CGLOPS-1, 199494-JRC), and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant 835541.

Double-skin green facades using deciduous climbing plants are easy-to-implement construction systems stated to be effective energy-saving tools for buildings during cooling periods. Although the leaf area index (LAI) has been identified as a key parameter for characterizing foliar density and, consequently, the green facade's potential as a passive tool for energy savings, a lack of knowledge still remains on this index's values and measurement methods. The present paper aims to characterize the annual LAI evolution of a Boston ivy double-screen green facade under Mediterranean continental climate (Csa), by using an original non-destructive methodology during two consecutive years. Moreover, the influence of the green facade's foliage density, characterized by LAI, on the external building wall temperatures and the energy consumption by season and orientation was addressed. From the results it can be noticed that LAI changed seasonally over the course of five periods with a related differentiated energy performance: early summer (LAI of 4.8; 54% savings for cooling), late summer (LAI of 4.4; 30% savings for cooling), autumn (LAI of 1.7; 5.4% increase for heating), winter (LAI of 0.9; 5.4% increase for heating), and spring (LAI of 3.6; 11.9% increase for heating). The increase of energy consumption during leaf-off stage was directly linked to woody material and remaining leaves. Two crucial effects were identified and characterized: firstly, the influence of facade orientation and, secondly, a slight 'insulation effect' at night, with the green screen acting as a thermal barrier. ; The authors at GREiA research group would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.

Abstract. In the present work, the role played by vegetation parameters, necessary to the hydrological distributed modeling, is investigated focusing on the correct use of remote sensing products for the evaluation of hydrological losses in the soil water balance. The research was carried out over a medium-sized river basin in Southern Italy, where the vegetation status is characterised through a data-set of multi-temporal NDVI images. The model adopted uses one layer of vegetation whose status is defined by the Leaf Area Index (LAI), which is often obtained from NDVI images. The inherent problem is that the vegetation heterogeneity – including soil disturbances – has a large influence on the spectral bands and so the relation between LAI and NDVI is not unambiguous. We present a rationale for the basin scale calibration of a non-linear NDVI-LAI regression, based on the comparison between NDVI values and literature LAI estimations of the vegetation cover in recognized landscape elements of the study catchment. Adopting a process-based model (DREAM) with a distributed parameterisation, the influence of different NDVI-LAI regression models on main features of water balance predictions is investigated. The results show a significant sensitivity of the hydrological losses and soil water regime to the alternative LAI estimations. These crucially affects the model performances especially in low-flows simulation and in the identification of the intermittent regime.