Delta-X: In Situ Spectral Reflectance of Water Surface at MRD at LA, USA, Spring 2021

This dataset includes above water measurements of remote-sensing reflectance measured in situ at field sampling stations during the Delta-X 2021 Spring campaign. Samples were collected in the Atchafalaya River and Terrebonne Basins on the southern coast of Louisiana from 2021-03-25 to 2021-04-22 using a handheld Portable SpectroRadiometer (PSR-1100f). Reflectance is reported in the dataset as remote sensing reflectance at 1 nm intervals in the range 360 nm-900nm, with uncertainty reported for each wavelength. During the field campaign, reflectance was measured near-simultaneously with collection of field samples and in-water sediment parameters in multiple channels of varying width (from a few meters to >100m), near Delta-X intensive study sites, and in open bays and lakes and at few locations in the nearshore Gulf of Mexico. PSR-1100f, manufactured by Spectral Evolution, was used to measure radiance from: (1) a highly reflective (> 99% reflectance) Lambertian Spectralon panel, ( 2) from the sky, measured at 40 degrees from the solar zenith and at 135 degrees from the sun azimuthal plane, and ( 3) from the water, measured at 40 degrees from nadir and at 135 degrees from the sun azimuthal plane. These measurements were used to calculate remote-sensing reflectance, the water-leaving radiance relative to downwelling irradiance, including a correction for the influence of reflected skylight. At each station, measurement spectra of a single target (water, panel, or sky) were combined through the use of a median filter to create a single representative radiance measurement for the target from multiple spectra. This dataset is used to calibrate and validate Delta-X's algorithms used for the retrieval of total suspended solids concentration from AVIRIS-NG imagery and to inform and validate Delta-X's sediment transport models.

This dataset includes one file in comma-separated (*.csv) format. 

This dataset includes above water measurements of remote-sensing reflectance measured in situ at field sampling stations during the Delta-X 2021 Spring campaign. Samples were collected in the Atchafalaya River and Terrebonne Basins on the southern coast of Louisiana from 2021-03-25 to 2021-04-22 using a handheld Portable SpectroRadiometer (PSR-1100f). Reflectance is reported in the dataset as remote sensing reflectance at 1 nm intervals in the range 360 nm-900nm, with uncertainty reported for each wavelength. During the field campaign, reflectance was measured near-simultaneously with collection of field samples and in-water sediment parameters in multiple channels of varying width (from a few meters to >100m), near Delta-X intensive study sites, and in open bays and lakes and at few locations in the nearshore Gulf of Mexico. PSR-1100f, manufactured by Spectral Evolution, was used to measure radiance from: (1) a highly reflective (> 99% reflectance) Lambertian Spectralon panel, ( 2) from the sky, measured at 40 degrees from the solar zenith and at 135 degrees from the sun azimuthal plane, and ( 3) from the water, measured at 40 degrees from nadir and at 135 degrees from the sun azimuthal plane. These measurements were used to calculate remote-sensing reflectance, the water-leaving radiance relative to downwelling irradiance, including a correction for the influence of reflected skylight. At each station, measurement spectra of a single target (water, panel, or sky) were combined through the use of a median filter to create a single representative radiance measurement for the target from multiple spectra. This dataset is used to calibrate and validate Delta-X’s algorithms used for the retrieval of total suspended solids concentration from AVIRIS-NG imagery and to inform and validate Delta-X’s sediment transport models. 

Project: Delta-X

The Delta-X mission is a 5-year NASA Earth Venture Suborbital-3 mission to study the Mississippi River Delta in the United States, which is growing and sinking in different areas. River deltas and their wetlands are drowning as a result of sea level rise and reduced sediment inputs. The Delta-X mission will determine which parts will survive and continue to grow, and which parts will be lost. Delta-X begins with airborne and in situ data acquisition and carries through data analysis, model integration, and validation to predict the extent and spatial patterns of future deltaic land loss or gain.

Related Dataset:

Jensen, D.J., T.M. Pavelsky, and C. Lion. 2020. Pre-Delta-X: Spectral Reflectance of Water Surface, Atchafalaya Basin, LA, USA, 2016. ORNL DAAC, Oak Ridge, Tennessee, USA. https://doi.org/10.3334/ORNLDAAC/1804.

• Remote-sensing reflectance measurements made using a hand-held spectrometer from the Atchafalaya River Basin during the Pre-Delta-X Fall 2016 campaign.

Acknowledgement

This study was funded by the NASA Earth Venture Suborbital-3 Program, grant number NNH17ZDA001N-EVS3.

Spatial Coverage: Atchafalaya and Terrebonne Basins, Mississippi River Delta (MRD) floodplain, southern coast of Louisiana, USA

Spatial Resolution: Points

Temporal Coverage: 2021-03-25 to 2021-04-22

Temporal Resolution: One-time measurement

Site Boundaries: Latitude and longitude are given in decimal degrees.

Site	Westernmost Longitude	Easternmost Longitude	Northernmost Latitude	Southernmost Latitude
Atchafalaya and Terrebonne Basins	-91.47	-90.57	29.75	28.79

Data File Information

This dataset contains one file in .csv format: DeltaX_WaterReflectance_PSR_Spring2021.csv. The file contains measurements of remote-sensing reflectance measured in situ at field sampling stations on the southern coast of Louisiana. Reflectance is reported in the dataset as remote sensing reflectance at 1 nm intervals in the range 360 nm-900nm, with uncertainty reported for each wavelength. Missing values are indicated by -9999.

Table 1. Data dictionary for DeltaX_WaterReflectance_PSR_Spring2021.csv

Variable	Units	Description	Collected during Pre-Delta-X campaign?
basin		Atchafalaya or Terrebonne	Yes
site_id		Name of site. See Section 5: Data Acquisition, Materials, and Methods for site_id naming convention.	Yes
campaign		e.g. Spring_2021	Yes
latitude	Decimal degrees	Latitude of measurement station	Yes
longitude	Decimal degrees	Longitude of measurement station	Yes
date	YYYY-MM-DD	Date of sampling and measurement	Yes
time	HH:MM	UTC time of measurement	Yes
rrs###, where ### is wavelength	sr-1	Remote sensing reflectance at a given wavelength per steradian (sr-1) measured above the surface of the water at 1 nm intervals	Yes. Note radiometric resolution of this dataset is 1nm from 360-900nm and radiometric resolution from Pre- Delta-X Fall 2016 measurements is 5nm from 446.5-1002.5 nm.
uncertainty_rrs###, where ### is wavelength	sr-1	Uncertainty in the remote sensing reflectance at a given wavelength representing the propagated error from variability in measured DN values as well as uncertainty in viewing geometry, and wind speed	No

During the Delta-X Campaigns, above-water measurements of remote-sensing reflectance (R_rs) were collected at a number of sites across the Atchafalaya and Terrebonne basins. These sampling sites spanned large and small channels at locations chosen to cover a representative range of suspended solids concentration from a variety of hydrodynamic and physical settings typically encountered across the Atchafalaya and Terrebonne basins. This dataset is used to calibrate and validate Delta-X’s algorithms used for the retrieval of total suspended solids concentration from AVIRIS-NG imagery and to inform and validate Delta-X’s sediment transport models. These in situ measurements of reflectance offer direct validation of the reflectance measured by AVIRIS-NG to assess the performance of the instrument for retrieving surface reflectance. In situ reflectance measurements are paired with concurrent measurements of TSS concentrations and other in situ water-quality indicators (water temperature, salinity, turbidity, chlorophyll-a fluorescence) and in situ measurements of beam attenuation coefficient at 670 nm, average suspended particle size, and suspended particle size distributions (all provided as separate data sets). These location- and time-specific measurements can be used to calibrate local empirical algorithms relating remote-sensing reflectance to sediment content allowing for the remote estimation of suspended sediment across airborne images. Remote-sensing-derived maps of suspended sediment distribution will be compared to numerical models to calibrate and validate its parameters. The models quantify the mesoscale (i.e., on the order of 1 ha) patterns of soil accretion that control land loss and gain and predict the resilience of deltaic floodplains under projected relative sea-level rise. Understanding and mitigating the impact of the relative sea-level rise on coastal deltas is urgent. If ignored, relative sea-level rise will very soon have devastating consequences on the livelihood of the half-billion people that live in these low-lying coastal regions.

Uncertainty in the remote sensing reflectance at a given wavelength representing the propagated error from variability in measured DN values as well as uncertainty in viewing geometry, and wind speed is provided.

Study Sites

The Spring 2021 season collected R_rs data at different locations across the Atchafalaya and Terrebonne Basins (Figure 2). Each sampling site was named using a combination of abbreviations provided in Table 2 as follows: BBB_MMDD_TN where BBB is the abbreviation for the basin (WLD or TB), MMDD is the month and day of 2021 (e.g., 0327 for March 27, 2021), T is the abbreviation for the station type (F for Full station, D for Dry station, and B for Basic station), and N is the sequential number of the station of each type sampled that day (B1 for first basic station, S1 for first full station, S2 for second full station, etc.). Full stations included the collection of in-water parameters, radiometry, and sampling for laboratory total suspended sediment analysis, dry stations did not include sample collection, and basic stations only included the collection of in-water water quality parameters and sediment size distribution (YSI multi-parameter probe and LISST-200X size transmissometer). Reflectance measurements were attempted at all full and dry stations.

Figure 2.  Measurement stations sites for the Spring 2021 Delta-X campaign.

Table 2. Site names and descriptions.

Abbreviated Site name	Full Site name
WLD	Atchafalaya Basin (including Wax Lake Delta)
TB	Terrebonne Basin
S	Full station (Water samples for TSS was collected along with water reflectance, measurements from the LISST, and water-quality indicators from ProDSS probe)
D	Dry stations (No Water samples for TSS was collected, but measurements of water reflectance, from the LISST, and water-quality indicators from ProDSS probe were collected)
B	Basic station (only measurements of LISST, and water-quality indicators from ProDSS probe were collected)

Measurement Procedure

For each in situ collection, a handheld Portable SpectroRadiometer (PSR-1100f), manufactured by Spectral Evolution, a radiance was used to measure radiance from:

1. from a highly reflective (> 99% reflectance) Lambertian Spectralon® panel
2. from the sky, measured at 40^o from the solar zenith and at 135^o from the sun azimuthal plane
3. from the water, measured at 40^o from nadir and at 135^o from the sun azimuthal plane.

Measurement of each target was repeated 5-12 times. These radiance measurements were used to calculate the water-leaving reflectance corrected for specular reflection of skylight at the air-water interface. Care was taken during measurement collection to maintain viewing geometry (135 ^o away from the sun direction, and 40 ^o off-nadir/azimuth) and to minimize the influence of shading or light reflected from the boat. Measurement of solar zenith angle (calculated from latitude, date and time of day) and wind speed were also collected coincidently in order to estimate the fraction of skylight reflected at the air-sea interface from the tables compiled by Mobley et al.  (2015). Care was also taken to avoid measurements during variable illumination conditions (e.g., moving clouds) so that all measurements were made during short windows of time (1 to 2 minutes) when illumination conditions were stable.

In situ, hyperspectral, Rrs(360−900 nm), spectra were estimated from the above-water measurements as follows:   

Rrs(λ) = (DN_water+sky – DN_sky*ρ)/(π*DN_panel/R),

where DN_water+sky is the measured radiance signal from the water and includes both the water-leaving signal and the signal of skylight reflected at the air-water interface. DN_sky is the measured signal from the sky, DN_panel is the measured signal from the white Spectralon panel, and R is the reflectivity of the plaque (approximately 99%; actual measured spectral values used in the calculation). Here, the factor π allows for the integration of the signal reflected off the Spectralon® panel (Lambertian diffuser) to convert it from a radiance to an equivalent of an irradiance. The measured sky signal DN_sky is also multiplied by ρ, a reflectance factor relating the relative signal measured when the detector views the sky to the reflected sky signal actually measured when the detector views the sea surface (the fraction of skylight reflected at the water’s surface). This factor ρ varies substantially with wind speed, solar zenith angle, and viewing geometry, and was estimated for each station from lookup tables compiled in Mobley et al. (2015) using field measurements of wind speed and calculated solar zenith angle.

At each station, multiple measurement spectra of a single target (water, reflectance panel, or sky) were combined through the use of a median filter to create a single representative radiance measurement for the target from multiple spectra. Outlier spectra, defined as spectra differing from the median spectrum by more than 3 median absolute deviations at more than one third of wavelengths, were also removed from the analysis in this step. Median spectra were then used to calculate remote sensing reflectance for each station following the approach (Mobley 2015) described above.

Finally, an empirical correction was applied to remove any residual sun glint from reflectance measurements. This correction utilized an empirical relationship demonstrated between a shoulder in the absorption spectrum of pure water between 780 nm and 840 nm and the magnitude of remote sensing reflectance at 810 nm (Jiang et al., 2020). The empirical relationship between water spectral shape and the magnitude of glint-free reflectance at 810 nm allowed for the calculation of a scalar correction factor that was subtracted from all wavelengths to remove the influence of glint.

Jiang, D., B. Matsushita, and W. Yang. 2020. A simple and effective method for removing residual reflected skylight in above-water remote sensing reflectance measurements. ISPRS Journal of Photogrammetry and Remote Sensing 165: 16–27. https://doi.org/10.1016/j.isprsjprs.2020.05.003.

Mobley, C.D. 2015. Polarized reflectance and transmittance properties of windblown sea surfaces. Applied Optics 54: 4828. https://doi.org/10.1364/ao.54.004828.