A framework for modeling connections between hydraulics, water surface roughness, and surface reflectance in open channel flows. Issue 9 (21st September 2017)

• Record Type: Journal Article
• Authors: Legleiter, Carl J.; Mobley, Curtis D.; Overstreet, Brandon T.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 9(2017)
• Page Start: 1715
• Page End: 1741
• Publication Date: 2017-09-21
• Subjects: remote sensing of rivers -- water surface roughness -- surface reflectance -- flow hydraulics -- Sun glint -- ray tracing; Geomorphology -- Periodicals; 551.3
• DOI: 10.1002/2017JF004323
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: This paper introduces a framework for examining connections between the flow field, the texture of the air‐water interface, and the reflectance of the water surface and thus evaluating the potential to infer hydraulic information from remotely sensed observations of surface reflectance. We used a spatial correlation model describing water surface topography to illustrate the application of our framework. Nondimensional relations between model parameters and flow intensity were established based on a prior flume study. Expressing the model in the spatial frequency domain allowed us to use an efficient Fourier transform‐based algorithm for simulating water surfaces. Realizations for both flume and field settings had water surface slope distributions positively correlated with velocity and water surface roughness. However, most surface facets were gently sloped and thus unlikely to yield strong specular reflections; the model exaggerated the extent of water surface features, leading to underestimation of facet slopes. A ray tracing algorithm indicated that reflectance was greatest when solar and view zenith angles were equal and the sensor scanned toward the Sun to capture specular reflections of the solar beam. Reflected energy was concentrated in a small portion of the sky, but rougher water surfaces reflected rays into a broader range of directions. Our framework facilitates flight planning to avoid surface‐reflected radiance while mapping other river attributes, or to maximize this component to exploit relationships between hydraulics and surface reflectance. This initial analysis also highlighted the need for improved models of water surface topography in natural rivers. Plain Language Summary: Anyone who has spent time along a river on a bright, sunny day has probably observed strong reflections of sunlight from the water surface and also might have noticed that Sun glint tends to be more pronounced in faster‐flowing areas where the water surface is more choppy. In this study, we introduce a framework for examining connections between flow speed, water surface roughness, and reflectance from the surface. To illustrate the general framework, we used data from lab experiments, as well as field measurements, to model water surfaces and trace rays of light as they interact with these surfaces. Our analysis indicates that greater flow intensity leads to a rougher water surface texture, which in turn increases the likelihood of strong reflections from the surface. The lab‐based model we used as an example might not have been appropriate for natural rivers, however. Ray tracing showed that the brightest reflections occur only when the Sun's rays approach from a certain direction and the river is viewed from a certain direction. Examining these relationships could provide a way to estimate flow speed from remote sensing images that capture reflectance from river surfaces. Key Points: Flow hydraulics influence water surface roughness, which in turn affects surface reflectance A spatial correlation model of water surface topography can be used to simulate surfaces Ray tracing shows that rougher surfaces have higher reflectance in a broader range of directions