Abstract
Abstract. Recent melt events on the Greenland ice sheet (GrIS) accentuate the need to constrain estimates of sea level rise through improved characterization of meltwater pathways. This effort will require more precise estimates of the volume of water stored on the surface of the GrIS. We assessed the potential to obtain such information by mapping the bathymetry of supraglacial lakes and streams from WorldView2 (WV2) satellite images. Simultaneous in situ observations of depth and reflectance from two streams and a lake with measured depths up to 10.45 m were used to test a spectrally based depth retrieval algorithm. We performed optimal band ratio analysis (OBRA) of continuous field spectra and spectra convolved to the bands of the WV2, Landsat 7 (ETM+), MODIS, and ASTER sensors. The field spectra yielded a strong relationship with depth (R2 = 0.94), and OBRA R2 values were nearly as high (0.87–0.92) for convolved spectra, suggesting that these sensors' broader bands would be sufficient for depth retrieval. Our field measurements thus indicated that remote sensing of supraglacial bathymetry is not only feasible but potentially highly accurate. OBRA of spectra from 2 m-pixel WV2 images acquired within 3–72 h of our field observations produced an optimal R2 value of 0.92 and unbiased, precise depth estimates, with mean and root mean square errors < 1% and 10–25% of the mean depth. Bathymetric maps produced by applying OBRA relations revealed subtle features of lake and channel morphology. In addition to providing refined storage volume estimates for lakes of various sizes, this approach can help provide estimates of the transient flux of meltwater through streams.
Highlights
Recent accelerated melting of the Greenland ice sheet (GrIS) (Tedesco et al, 2013), including a record melt season in 2012 (Nghiem et al, 2012; Bennartz et al, 2013), underscores the need to improve our understanding of the ice sheet’s hydrology and constrain estimates of the current and future contributions of the GrIS to sea level rise
This study focused on WV2 images, conducting optimal band ratio analysis (OBRA) for the other, more widely used sensors provided a means of assessing whether strong depth–reflectance relations could be obtained from coarser spectral resolution data
Because the OBRA R2 values for all three sensors were similar, the primary advantage of WV2 would be enhanced spatial resolution, but our results indicate that robust depth–reflectance relations can be derived for Landsat 7, Moderate Resolution Imaging Spectroradiometer (MODIS), and ASTER data as well
Summary
Recent accelerated melting of the Greenland ice sheet (GrIS) (Tedesco et al, 2013), including a record melt season in 2012 (Nghiem et al, 2012; Bennartz et al, 2013), underscores the need to improve our understanding of the ice sheet’s hydrology and constrain estimates of the current and future contributions of the GrIS to sea level rise. The limited information available for supraglacial water bodies hinders efforts to determine the volume of meltwater present on the surface of the GrIS and to develop estimates of meltwater storage and flux Incorporating this type of information into a hydrologic budgeting framework would help to quantify the proportion of melt that is retained within and/or beneath the ice sheet and that which propagates to proglacial rivers. Legleiter et al.: Remote sensing of supraglacial bathymetry and the ocean (Rennermalm et al, 2013) Motivated by these challenges, this study used simultaneous field measurements and high spatial resolution commercial satellite image data to evaluate the feasibility of measuring the bathymetry of Greenland’s supraglacial lakes and streams via spectrally based remote sensing
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