Today, we are pleased to feature a guest author for our blog – Nima Pahlevan, a 2011 Alexander Goetz Instrument Support Program winner. He was awarded the use of ASD’s HandHeld spectroradiometer based on his research proposal titled “The Potential of Landsat7 and LDCM Coupled with a Hydrodynamic Model for Quantitative Mapping of Water Constituents in Inland Waters”. He has written an account of his experience with the instrument and the data he acquired during his research. Nima will also be presenting his findings at the upcoming ASPRS 2012 Annual Conference. His presentation "Integrating Landsat-7 Imagery with Physics-based Models for Quantitative Mapping of Coastal Waters near River Discharges" will be on March 23 from 10 a.m. - Noon.
This study intended to evaluate the capability of a new generation of Landsat, i.e., the Landsat Data Continuity Mission, for monitoring coastal/inland waters. A physics-based approach was applied to investigate the performance of LDCM against Landsat-7, the Advanced Land Imager (ALI), and Hyperion datasets. It was demonstrated that, with its enhanced features, LDCM will dramatically improve the retrieval of water constituents, including total suspended solids, chlorophyll-a, and colored dissolved organic matter absorption.
We also proposed a coupled modeling system for dynamic monitoring of coastal waters near river discharges by integrating Landsat-7 imagery and a 3D hydrodynamic model. Under ideal atmospheric conditions, the Landsat imagery illustrates the state of the environment at one instant. A hydrodynamic model, when calibrated using the Landsat-derived products, allows a continuous monitoring of such dynamic environments. In this approach, we used Landsat-7 data as a surrogate for LDCM, which will enable an enhanced understanding of the coastal environments when launched in early 2013.
In this way, a well-accepted in-water radiative transfer model (Hydrolight) was used to model the surface reflectance associated with the profiles of suspended particles and dissolved matter predicted by the hydrodynamic modeling. An accurate modeling of the surface-leaving reflectance is subject to having appropriate knowledge of the absorption and scattering properties of water and its constituents. In this research, while the absorption properties of the water constituents were measured through laboratory measurements, their scattering properties were estimated through a curve-fitting procedure leveraging from in situ optical measurements.
During several field campaigns, the ASD HandHeld, mounted on a small boat, was used to measure the surface-leaving optical field using a 3 degree-field-of-view foreoptic. The total downwelling solar radiation and the diffuse skylight were also measured using the cosine-receiver optics to account for the atmospheric effects and sky glint (reflection off the water surface) further in the post-processing. The measurements were taken over different sites to capture the differences in the optical properties of water constituents throughout the study site. After removing the outlier measurements and accounting for the sky glint, the in situ measured reflectance spectra were compared to various modeled reflectance spectra generated using the Hydrolight code. The Hydrolight simulations are conducted by adjusting scattering spectra obtained from the literature. The best match, which is in agreement with the measured spectrum, is found through an optimization routine. The following Figure shows the best matches found for two different spectra measured for two stations (ST1 and ST2) at the Niagara River mouth, Lake Ontario, USA, in October 2010.
The in situ measurements of the scattering properties of the water constituents commonly require deployment of specific instruments, such as AC-S or AC-9, which are relatively expensive. However, with this practical technique using the ASD spectral measurements, one is able to make reasonable estimates of the scattering properties of water constituents assuming their absorption properties and their concentrations are known.