| The Greenland ice sheet (GrIS) is the second largest ice body on Earth. In the last two decades, the GrIS has undergone an accelerated loss of mass. This alarming mass loss can lead to considerable global sea level rise in the near future. In addition, a moderate or high rate of GrIS melting can weaken the global thermohaline ocean circulation. Eventually, all these processes will have substantial societal and economic impacts on human survival and development. Surface meltwater runoff is believed to be the main cause of the alarming GrIS mass loss. Each melt season, large volume of meltwater is produced on the ice sheet and forms a complex supraglacial hydrologic system mainly under the control of topography. This hydrologic system includes supraglacial lakes, streams, crevasses, and moulins. Supraglacial lakes can store meltwater. Supraglacial streams are in charge of meltwater transport. Moulins play as the terminations of meltwater on the ice sheet surface. By far, the mass loss caused by the surface runoff is commonly derived from the Surface Mass Balance (SMB) modeling. However, this modeling approach contains large uncertainties due to the knowledge gaps in certain fundamental hydrologic processes. Therefore, the SMB modeling may lack the ability to represent the storage, transport, and release processes of the surface meltwater appropriately. It is crucial to analyze the supraglacial hydrologic system in order to better understand the impact of the surface meltwater runoff on the GrIS mass balance.Unfortunately, very few attempts have been conducted to study the GrIS supraglacial hydrologic system. A prime reason is that this drainage pattern is extraordinarily complex, consisting of a largely unstudied ephemeral patchwork of supraglacial lakes, streams, crevasses, and moulins. In addition, the relative importance of different pathways for meltwater on the ice sheet surface, i.e., sinking into the ice sheet, running off over the surface via organized supraglacial stream drainage patterns, or remaining ponded at the surface, is undetermined. Consequently, our understanding of its patterns and dynamics is fairly limited at present. This dissertation studied the GrIS supraglacial hydrology by combining multi-source, multi-resolution satellite images with field observations. Prime attention was paid to address the fundamental problems in the GrIS supraglacial hydrology, such as feature extraction, feature analysis, and key hydrologic parameter calculation. The supraglacial hydrologic features were automatically extracted from remotely sensed imagery and their characteristics were studied. The storage, transport, and release processes of surface meltwater on the ice sheet were analyzed.The contents of this dissertation are as follows:(1) Delineation of supraglacial lakes and streams in satellite imagery. The first step to study supraglacial hydrologic system is to delineate supraglacial lakes and streams in satellite imagery with good accuracy. However, remote sensing of these two hydrologic features is challenging owing to their proximity to other features having similar visible/near-infrared reflectance or shape. This dissertation presented three automated procedures for delineating supraglacial lakes and streams in satellite imagery. First, a modified normalized difference water index adapted for ice enhanced the spectral contrast between open water and drier snow/ice surfaces. Second, a global and local thresholding method was proposed to delineate the supraglacial lakes. More specifically, a global threshold was applied to detect the initial boundaries of the lakes, and then these initial boundaries were updated using adaptive thresholds based on local spectral variations. Third, an automated procedure was presented to exploit both spectral and shape information to delineate supraglacial streams on the GrIS ablation zone using high-resolution visible/NIR satellite imagery. Its overall strategy was to use both spectral and shape information to discern actively flowing streams from other features with similar visible/NIR reflectance and/or shape. Next, another automated multi-scale procedure was proposed, focusing on small river identification, enhancement and delineation. This method segmented the large and small rivers separately and combined the two segmented results to generate the final delineated stream networks. Finally, a novel stream ordering procedure was presented to order the supraglacial stream networks with numerous bifurcations and confluences. The experimental results showed that the proposed methods delineated supraglacial hydrologic features in remotely sensed imagery with good success.(2) Study of meltwater storage on the ice sheet. Topographic depressions are important meltwater storage on the ice sheet. This dissertation analyzed the maximum storage capability of these depressions and demonstrated its potential hydrologic effects by combining multi-source, multi-resolution digital elevation models (DEMs) with remotely sensed imagery. First, the volume-area relationship for each depression was determined by using DEM data. The depression volume was calculated and estimated. High-resolution DEM data were then generated by using WorldView-1/2 stereo images and employed as validation data. Second, time-series moderate ASTER satellite images were used to delineate maximum spatial coverage of the supraglacial lakes. Third, combining the delineated supraglacial lakes and the volume of the topographic depressions, the largest meltwater storage capability in the study area were calculated and analyzed. The result showed that the largest meltwater volume that can be host by all the supraglacial lakes is 2.34 km3 in the southwest Greenland ice sheet during 2000-2011. This meltwater volume accounted for 4.8% of the average annual runoff (48.43 km3) in this area. This percentage was an important supraglacial hydrologic parameter, which revealed that the majority of the meltwater produced on the ice sheet was not stored in the supraglacial lakes.(3) Study of meltwater transport and release on the ice sheet. Numerous supraglacial streams form on the ice sheet and transport large volume of meltwater each melt season. This dissertation analyzed the meltwater transport and release processes on the ice sheet using high-resolution satellite images and field measured hydraulic data (depth, velocity, discharge). First, supraglacial stream networks were delineated and mapped by using high-resolution WorldView-2 images. Second, moulins were visually interpreted as the outlets of the supraglacial stream networks, which depicted the locations where meltwater was released into the ice sheet; in addition, the internal drainage basins were extracted and analyzed, which exhibited moulins as their outlets. Third, various network morphometric parameters (e.g., stream length, Strahler order, bifurcation ratio, braided index, relief ratio, drainage density, and slope) were studied. Next, by combining high-resolution satellite imagery and field measured data, the hydraulic geometry of the supraglacial stream networks was analyzed. Finally, Manning’s equation was used to calculate the discharge of the supraglacial stream networks, revealing the capability of meltwater transport and release on the ice sheet. The result showed that a total of 523 supraglacial stream networks formed in the Kangerlussuaq area, southwest Greenland during July 18-23, 2012. All the meltwater produced in this area was released into the ice sheet by moulins. The well-developed (high order) and discrete (low order) supraglacial stream networks both form on the ice sheet, draining meltwater into moulins. The bifurcation ratio values of these networks were comparable to other terrestrial river network values. Higher order streams were more braided. Supraglacial drainage densities were found to be negatively correlated with the mean elevation of the stream networks, and there were quantitative relationships between the stream networks delineated from the multi-spectral images and the ones delineated from the panchromatic image. The total discharge of these supraglacial stream networks was 1336 m3/s. If this discharge was assumed to be constant in July 2012, the meltwater that would be transported and released by those networks was 3.58 km3, which accounted for 62.3% of the total meltwater runoff that could be produced in the same area during the same time. Most of the meltwater produced in this area was transported by the supraglacial stream networks and released into the ice sheet, without storing in the supraglacial lakes or leaving the ice sheet. |
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