Crop phenology is a key parameter for precision farming and necessary in crop models for improving water and nutrients management in space and time. It has been traditionally determined at field, with observations of biophysical parameters in correspondence with different time-scales, but depending on plot size, it is not full representative of the variability inside it and of the variation between plots. Remote sensing techniques may increase the accessibility of high frequency spatialized data that in combination with meteorological information provide a tool for monitoring crops. Particularly, an important variety of sensors suited for agricultural applications on board of unmanned aerial systems, including spectral bands coherent with satellites and field radiometry, are being applied to describe within plot variability. With this purpose, an experiment along the year 2017 has been designed, to study the behavior of more than forty varieties of barley and wheat in an intensive experiment composed of 576 micro-plots of 1.4 × 10 m size. They have been monitored at field, registering the phenology in the BBCH scale and a complete set of soil and plant biophysical parameters in coincidence to sixteen multispectral very high-resolution images, using the number of days after sowing (NADS), the accumulated growing degree days (AGDD) and accumulated reference evapotranspiration (AcETo) as temporal scales, for studying the spatio-temporal distribution response of crop. The images were supplied with a parrot sequoia® multispectral camera, at a ground sample distance of 10 cm which allow to determine the variability into the micro-plots and obtaining representative results between them. The meteorological parameters were registered in a weather station close to the experimental area. The results show that the vegetation index in combination to the growing degree days scale or the accumulate reference evapotranspiration can set the emerging, flowering and maturity stages that are crucial inputs for management and crop models. AGDD and AcETo show a better-defined plateau between flag leave and early maturity. The ratio between the TNDVI along the reproductive phase (from BBCH = 55 to 89) and the growing cycle for barley show values of 0.31 ± 0.05 in NADS, 0.45 ± 0.03 in AGDD and 0.48 ± 0.03 in AcETo. For durum wheat the 0.32 ± 0.05 (NADS), 0.46 ± 0.03 (AGDD) and 0.49 ± 0.05 (AcETo). In case of bread wheat, the values are 0.27 ± 0.03 (NADS), 0.53 ± 0.06 (AGDD) and 0.54 ± 0.05 (AcETo). These results show that proximal remote sensing is very useful in intensive experiments as prospective techniques to explore new crop varieties that could be implanted in the experimental area and setting up the tools for satellite applications.
Monitoring Land Surface Temperature (LST) from satellite remote sensing plays a key role in climatic, environmental, hydrological and agricultural applications. A Single Band Atmospheric Correction (SBAC) tool was recently introduced and tested with Landsat 7/ETM+. SBAC provides pixel-by-pixel atmospheric correction parameters regardless of the pixel size using atmospheric profiles from National Centers of Environmental Prediction (NCEP) reanalysis products as inputs, accounting also for the pixel elevation through a Digital Elevation Model (DEM). This work deals now with the assessment of SBAC applied to Landsat 8/TIRS data since no operational LST product is still available. A new experiment was conducted in summer 2018, covering a variety of crops and surface conditions in the Barrax test site, Spain (39º 03’ N, 2º 06’ W) concurrent to L8/TIRS overpasses. Ground temperatures were measured using a set of hand-held infrared radiometers (IRTs) Apogee MI-210. Results show differences within ±3.5 K for all cases. Average results for SBAC show small bias (- 0.8K) and standard deviation (±1.3 K), yielding a RMSE of ±1.5 K. Finally, a comparison is established with results obtained using the NASA Atmospheric Correction Parameter Calculator tool (ACP) applied to the center of ”Las Tiesas” site coordinates. A similar standard deviation (±1.4 K) was obtained, with a larger bias, close to -1.5 K in this case, and a resulting RMSE of ±2.0 K. These results reinforce the potential of SBAC for the operational pixel-by-pixel atmospheric correction of full Landsat 8/TIRS images.
The traditional limitation in the lower spatial resolution of Thermal Infrared (TIR) versus Visible Near Infrared (VNIR) satellite data can be faced by applying recent disaggregation techniques. These techniques are based on the VNIR-TIR variable regressions at coarse spatial resolution, and the assumption that the relationship between spectral bands is independent of the spatial resolution. A comprehensive analysis of different disaggregation methods in the literature using MODIS and Landsat images was already addressed by [1] in a previous publication. The aim of this work is to evaluate the performance of the downscaling method that showed the best results, when applied now to the MODIS/Sentinel-2 tandem for the estimation of daily land surface temperature (LST) at 10 m spatial resolution. An experiment was carried out in an agricultural area located in the Barrax test site, Spain (39º 03’ 35’’ N, 2º 06’ W), for the summer of 2018. Ground measurements of LST transects centered in the MODIS overpasses, and covering a variety of crops and surface conditions, were used for a robust local validation of the disaggregation approach. An additional set of Landsat-7/ETM+ images were also used for a more extended assessment of the LST product generated. Data from 6 different dates were available for this study, covering 10 different crop fields. Despite the large range of temperatures registered (300-325 K), differences within ±4.0K are obtained, with an average estimation error of ±2.2K and a systematic deviation of 0.6K for the full dataset. A similar error was obtained for the extended assessment of the high resolution LST products, based on the pixel-to-pixel comparison between Landsat and disaggregated Sentinel-2 LST products.
A two-source energy balance model that separates surface fluxes of the soil and canopy was applied to a drip-irrigated
vineyard in central Spain, using a series of nine Landsat-5 images acquired during the summer of 2007. The model
partitions the available energy, using surface radiometric temperatures to constrain the sensible heat flux, and computing ET as a residual of the energy balance. Flux estimations from the model are compared with half-hourly and daily values obtained by an eddy covariance flux tower installed on the site during the experiment. The performance of the twosource model to estimate ET under the low vegetation cover and semiarid conditions of the experiment, with RMSD between observed and model data equal to 49 W m-2 for half-hourly estimations and RMSD=0.5 mm day-1 at daily scale, is regarded as acceptable for irrigation management purposes. Model results in the separation of the beneficial (transpiration) and non-beneficial (evaporation from the soil) fractions, which is key information for the quest to improve water productivity, are also reported. However, the lack of measures of these components makes it difficult to draw conclusions about the final use of the water.
A linear relationship between NDVI and basal crop coefficient (Kcb) allows to compute the spectral crop coefficient (Krcb). Due to the influence of soil variations varying surface humidity on NDVI, five soil optimized indices have been used to obtain a linear relationship normalized for soil background effect (SAVI, OSAVI, TSAVI, MSAVI and
GESAVI). Data used on this work have been obtained from a field campaign for corn in the area of Barrax (Spain), describing crop growth stages with green fraction cover (GFC), and leaf area index (LAI). SAVI with optimized factor L set to 0.5 is a good estimator of Krcb from sparse to dense vegetation, nevertheless the soil line based index ( GESAVI) due to a wider range of variation are more sensitive to leaf variations at high levels of vegetation amount. Spectral crop coefficients obtained from SAVI and soil line based GESAVI are sensitive to crop hazards by weather anomalies and
estimates in real time the basal crop coefficients to estimate the amount of water removed by the crop from the active root zone.
KEYWORDS: Vegetation, Reflectivity, Near infrared, Soil science, Data modeling, Remote sensing, Optical properties, Signal to noise ratio, Multiple scattering, Information operations
Operational monitoring of vegetative cover by remote sensing currently involves the utilization of vegetation indices (VIs), most of them being functions of the reflectance in red (R) and near-infrared (NIR) spectral bands. A generalized soil-adjusted vegetation index (GESAVI), theoretically based on a simple vegetation canopy model, is introduced. It is defined in terms of the soil line parameters (A and B) as: GESAVI equals (NIR-BR-A)/(R + Z), where Z is related to the red reflectance at the cross point between the soil line and vegetation isolines. Z can be considered as a soil adjustment coefficient which let this new index be considered as belonging to the SAVI family. In order to analyze the GESAVI sensitivity to soil brightness and soil color, both high resolution reflectance data from two laboratory experiments and data obtained by applying a radiosity model to simulate heterogeneous vegetation canopy scenes were used. VIs (including GESAVI, NDVI, PVI and SAVI family VIs) were computed and their correlation with LAI for the different soil backgrounds was analyzed. Results confirmed the lower sensitivity of GESAVI to soil background in most of the cases, thus becoming the most efficient index. This good index performance results from the fact that the isolines in the NIR-R plane are neither parallel to the soil line (as required by the PVI) nor convergent at the origin (as required by the NDVI) but they converge somewhere between the origin and infinity in the region of negative values of both NIR and R. This convergence point is not necessarily situated on the bisectrix, as required by other SAVI family indices.
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