Aircraft-based System Research Articles

HYPERSPECTRAL UAS IMAGERY FOR GRASS SWARDS BIOMASS AND NITROGEN ESTIMATION

Abstract. Monitoring agricultural grass fields is particularly important for meat and milk production in Northern Europe, where three harvests occur during a growing season to maximize yields. Reliable data on forage, including biomass and nitrogen concentration, are essential for making informed decisions regarding seed mixtures, fertilizer rates, and harvest timing. Miniaturized hyperspectral cameras mounted on unmanned aerial systems (UAS) have become increasingly accessible and efficient. These cameras, operating in the visible to near-infrared (VNIR) range, have shown potential in estimating grass sward quantity and feeding quality. Additional advancements in hyperspectral technology have emerged the short-wave infrared (SWIR) range for UAS applications, previously utilized mainly in laboratory and aircraft-based systems. This study aims to explore the potential of VNIR and SWIR hyperspectral UAS-based remote sensing in biomass and nitrogen estimation during primary and re-growth stages. Grass fresh yield and nitrogen concentration prediction models were built after selecting the most significant features from the cameras to cope with the high dimensionality of the data. Using best features and machine learning, both fresh yield and nitrogen concentration were estimated with normalized root mean square error better than 10%. This work contributes to the development of accurate remote sensing techniques, supporting sustainable agricultural practices and climate change studies.

Multiscale estimates of N 2O emissions from agricultural lands

Nitrous oxide (N 2O) emissions are a large proportion of the agriculture sector's contribution to the greenhouse gas inventory of most developed countries. The spatial and temporal variability of N 2O emissions from agricultural soils has long been considered the main factor limiting our ability to estimate N 2O emissions, particularly the emissions associated with the spring snowmelt period. Tower and aircraft-based flux measurement systems and a process-based model were used to quantify N 2O emissions for four years (2000, 2001, 2003 and 2004) in an agricultural area of eastern Canada, near Ottawa, where a corn-soybean crop rotation dominates. A tower-based system, which relies on the flux gradient technique, provided diurnal N 2O emissions at a field scale. An aircraft-based system, which relies on the relaxed eddy accumulation technique, provided N 2O emissions for two similar agricultural regions and the DeNitrification and DeComposition (DNDC) model was used to estimate daily N 2O emissions at a regional scale. In most cases, aircraft-based N 2O emissions measurements were comparable for the two agricultural regions. Corresponding tower-based measurements which were collected over a field in the Ottawa area showed similar emission patterns to the aircraft-based measurements but in some cases the tower-based emissions were larger, as expected. This is because the footprint of aircraft-based measurements always incorporated a significant amount of crops such as soybean and other types of vegetation which do not receive additional nitrogen fertilization as well as waterlogged areas that do not emit N 2O. While in three of the four years, the tower-based measurements were made over a tile drained field where nitrogen fertilizer had been applied the previous year. The N 2O emissions patterns after planting were also similar for both aircraft and tower-based systems, but again they were slightly larger for the tower-based system. Aircraft-based N 2O flux measurements are also compared to the N 2O emissions obtained using the most recent version of the process-based model DNDC. Tests showed that DNDC gave comparable N 2O emissions estimates for the measurement period as a whole, but was not always able to correctly predict the timing of peak emissions.

Atmospheric Response to a Partial Solar Eclipse over a Cotton Field in Central California

Abstract The partial solar eclipse on 11 July 1991 in central California, with 58.3% maximum coverage, provided an exceptional opportunity to study the temporal response of processes in the atmospheric boundary layer to an abrupt change in solar radiation. Almost laboratory-like conditions were met over a cotton field, since no clouds disturbed the course of the eclipse. Tower-based and complementing aircraft-based systems monitored the micrometeorological conditions over the site. Temperature profile measurements indicated neutral stratification during the maximum eclipse in contrast to the unstable conditions before and after the eclipse. Accordingly, the sensible heat exchange completely stopped, as a wavelet analysis of the tower measurements and airborne eddy-covariance measurements showed. Turbulent fluxes of water vapor, carbon dioxide, and ozone were reduced by approximately ⅔ at the peak of the eclipse. Wavelet analysis further indicated that the same eddies contributed to the turbulent transport of water vapor and carbon dioxide, whereas sensible heat was transported by different ones. An analysis of the decay of turbulent kinetic energy followed a power law of time with an exponent of −1.25. The response of the sensible heat flux was 8–13 min delayed relative to the solar forcing, whereas no significant time lag could be detected for the turbulent fluxes of air constituents.

Effects of Land Use and Meteorological Conditions on Local and Regional Momentum Transport and Roughness for Midwestern Cropping Systems

Abstract Eddy covariance measurements of wind speed u and shear velocity u* from tower- and aircraft-based systems collected over rapidly developing corn- (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields were used in determining the local and regional (effective) surface roughness length zo and 〈zo〉, respectively. For corn, canopy height increased from ∼1 to 2 m and the leaf area index changed from ∼1 to 4 during the study period, while for soybean, canopy height increased from ∼0.1 to 0.5 m and the leaf area index increased from ∼0.5 to 2. A procedure for the aggregation of local roughness values from the different land cover types based on blending-height concepts yielded effective surface roughness values that were from ∼1/2 to 1/4 of the magnitude estimated with the aircraft data. This indicated additional kinematic stress caused by form drag from isolated obstacles (i.e., trees, houses, and farm buildings), and the interaction of adjacent corn- and soybean fields were probably important factors influencing the effective surface roughness length for this landscape. The comparison of u* measurements from the towers versus the aircraft indicated that u* from aircraft was 20%–30% higher, on average, and that u* over corn was 10%–30% higher than over soybean, depending on stability. These results provide further evidence for the likely sources of additional kinematic stress. Although there was an increase in zo and 〈zo〉 over time as the crops rapidly developed, particularly for corn, there was a more significant trend of increasing roughness length with decreasing wind speed at wind speed thresholds of around 5 m s−1 for the aircraft and 3 m s−1 for the tower measurements. Other studies have recently reported such a trend. The impact on computed sensible heat flux H using 〈zo〉 derived from the aggregation of zo from the different land cover types, using the blending-height scheme, and that estimated from the aircraft observations, was evaluated using a calibrated single-source/bulk resistance approach with surface–air temperature differences from the aircraft observations. An underestimate of 〈zo〉 by 50% and 75% resulted in a bias in the H estimates of approximately 10% and 15%, respectively. This is a relatively minor error when considering that the root-mean-square error (rmse) value between single-source estimates and the aircraft observations of H was 15 W m−2 using the aircraft-derived 〈zo〉, and only increased to approximately 20 and 25 W m−2 using the 1/2 and 1/4 〈zo〉 values, as estimated from the blending-height scheme. The magnitude of the excess resistance relative to the aerodynamic resistance to heat transfer was a major contributing factor in minimizing the error in heat flux calculations resulting from these underestimations of 〈zo〉.

VESSEL-BASED DISPERSANT APPLICATION: NEW APPROACHES, EQUIPMENT, AND LOGISTICS

ABSTRACT The paper consolidates the findings of a number of projects recently completed at S.L. Ross Environmental Research Ltd. that provide new information on dispersant use, particularly in the area of vessel-based application systems. Included are discussions of: (1) various advantages that vessel-based systems have over aircraft-based systems in certain spill situations; (2) comparison of application platforms over a range of spill scenarios; (3) recent research on single-nozzle dispersant application systems for use on vessels; and (4) a recent study showing the potential inefficiencies associated with the application of popular modern dispersants in diluted form.

Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m -2 s -1, with a mean of 0.59±0.34 μg m -2 s -1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m -2 s -1, with a mean of 0.14±0.09 μg m -2 s -1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m -2 s -1, with a mean of 0.92±0.33 μg m -2 s -1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m -2 s -1, with a mean of 0.36±0.21 μg m -2 s -1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO 2 and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO 2 assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.

Mapping surface soil moisture using an aircraft-based passive microwave instrument: algorithm and example

Microwave remote sensing at L-band (21 cm wavelength) can provide a direct measurement of the surface soil moisture for a range of cover conditions and within reasonable error bounds. Surface soil moisture observations are rare and, therefore, the use of these data in hydrology and other disciplines has not been fully explored or developed. Without satellite-based observing systems, the only way to collect these data in large-scale studies is with an aircraft platform. Recently, aircraft systems such as the push broom microwave radiometer (PBMR) and the electronically scanned thinned array radiometer (ESTAR) have been developed to facilitate such investigations. In addition, field experiments have attempted to collect the passive microwave data as part of an integrated set of hydrologic data. One of the most ambitious of these investigations was the Washita'92 experiment. Preliminary analysis of these data has shown that the microwave observations are indicative of deterministic spatial and temporal variations in the surface soil moisture. Users of these data should be aware of a number of issues related to using aircraft-based systems and practical approaches to applying soil moisture estimation algorithms to large data sets. This paper outlines the process of mapping surface soil moisture from an aircraft-based passive microwave radiometer system for the Washita'92 experiment.

Flux estimates of latent and sensible heat, carbon dioxide, and ozone using an aircraft-tower combination

Several recent large-scale experiments in land-surface climatology have used the combination of aircraft- and tower-based flux measurements. Within these experiments, much effort has been made to assess the potential of aircraft to serve as extended observation platforms for the scaling up from local (tower-based) to regional estimates of surface-atmosphere exchange. Data collected as part of the California Ozone Deposition Experiment (CODE), during a one-month period of consistent day-to-day wind and radiation conditions, were particularly conducive to such a study. Aircraft-based flux estimates obtained at 30 and 60 m above two vineyard sites of approximately 9 km2 (site A, 6 km x 1.5 km and site B, 4.5 km x 2 krn) were compared to tower-based measurements made at 9.4 m above a section of site A. This paper shows that over the measuring period the sum of sensible and latent heat flux at 30 m, as measured with the aircraft-based system, was about 11 % less than that measured with a tower-based system at 9.4 m. It also shows that fluxes at 30 m were on the average about 10% larger than those at 60 m. It documents the flux variability of CO2, sensible and latent heat and ozone observed over relatively homogeneous surfaces. Although aircraft-tower comparisons must be interpreted with caution, due to the dissimilar surface areas sampled by the stationary and moving sensors located at different elevations, the consistent relationship obtained between these two sampling systems demonstrates the complementary nature of such measurements. It confirms that very repeatable measurements can be obtained with aircraft-based systems over distances as short as a few kilometers. It also demonstrates that compositing data for several runs and for several days !From aircraft- and tower-based measurements can provide diurnal patterns of fluxes of sensible and latent heat as well as CO2 and O3 on a regional scale.