DOI: 10.5066/p1q3calh
title: Image-based measurements and gage records used to test a method for inferring river discharge from remotely sensed data based on critical flow theory

description: This data release makes available the data used to test a method for remote sensing of river discharge based on critical flow theory. In rivers where flow conditions are near critical and well-defined standing waves are present, simple measurements of the wavelengths of the standing waves and the width of the channel made using readily available image data can be used to infer river discharge. This approach could provide an efficient, non-contact method of estimating streamflow in certain rivers.
 

Sites near established USGS gaging stations with standing waves (also known as undular hydraulic jumps) were identified by examining images within Google Earth Pro (https://www.google.com/earth/about/versions/#earth-pro). The time slider within that software was then used to select specific image dates on which waves were clearly expressed. For each site on each date, Google Earth Pro was used to manually digitize four types of features: 1) a shadow to infer the image acquisition time using a sundial method; 2) one initial wavelength measurement made between the first and second crests in a standing wave train; 3) a second wavelength measurement made between the second and third crests in a standing wave train and thus located adjacent to and immediately downstream of the first wavelength measurement; and 4) the channel width, measured perpendicular to the two waves at the midpoint between them. These features are saved in the file CriticalFlowMeasurements.kmz, which consists of a separate folder for each site with subfolders for each date. In cases where more than one site was associated with a given gaging station, the description field in the parent folder name was used to provide a unique name for each rapid.
 

In addition to the CriticalFlowMeasurements.kmz file created in Google Earth Pro, the file CriticalFlowResultCompilationWithMeasurements.csv also contains the latitude and longitude coordinates of each of the digitized features. This tabular data file also contains the measurements of wavelength and width derived from these features, the image acquisition time inferred from shadow orientation using the sundial method, the discharge recorded at the gaging station at that time, the results of the critical flow calculations, and an accuracy assessment based on a comparison of the inferred discharge to that recorded at the gaging station. Please refer to the entity and attribute section of the metadata for further detail regarding this file.
 

Associated primary publications provide further detail on the sundial method and the underlying critical flow theory, but the relevant calculations can be summarized briefly as follows. Measuring the orientation of a shadow provides a means of inferring the acquisition time of an image by calculating the position of the sun at various times throughout the day and identifying when the sun would have been positioned so as to cast the observed shadow. Note that the latitude, longitude, and image acquisition date must be known to perform this analysis. Under critical flow conditions, relationships between the wavelengths observed in a standing wave train and the depth and velocity of the flow can be used to estimate river discharge. Critical flow occurs when the Froude number (Fr) is equal to 1 and the velocity v is equal to the square root of the product of depth d and acceleration due to gravity g: Fr = 1 --> v = sqrt(gd). Velocity is also related to wavelength L as v = sqrt((L*g)/(2*pi)), which can in turn be used to calculate depth via the definition of the Froude number: d = v^2/g. Finally, the discharge Q is calculated by multiplying the estimated depth by the measured width to obtain the cross-sectional area and then multiplying this result by the estimated velocity: Q = wdv.
 

Users are advised to thoroughly read the metadata file associated with this data release to understand the appropriate use and limitations of the data and code provided herein.
 

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