Using Width-Based Rating Curves from Spatially Discontinuous Satellite Imagery to Monitor River Discharge

Abstract:

One of the significant technical hurdles to remote estimation of river discharge using river widths is the acquisition of suitable imagery. Most high-resolution, radar-based sensors acquire imagery infrequently (e.g. once or twice per month), while optical sensors are subject to the effects of clouds. Many more images that are partially cloud-free or cover only part of a study area are often available, but their use is problematic because the spatial extent of observable subreaches differs from image to image. Here, a method is presented that allows spatially discontinuous remote observations of river width to be successfully used to estimate discharge despite partial cloudiness and/or inconsistent spatial coverage. The method is demonstrated over 62 km of the Tanana River downstream of Fairbanks, AK. This reach is divided into 1 km subreaches and a separate discharge-width rating curve is produced for each subreach. For any given image, final discharge is estimated as the median discharge calculated from all visible and cloud-free reaches. Application of the method to 28 RapidEye images at 5 m spatial resolution and useable coverage ranging from <20% to 100% suggests that it produces highly accurate estimates of discharge (Root Mean Squared Error (RMSE=6.7%, r²=0.92) when compared with in situ observations. Similar but slightly less accurate results are evident from eleven Landsat TM and ETM+ images (RMSE=12.6%, r²=0.86). These results suggest that the method used here produces estimates of discharge comparable in error characteristics to ground-based measurements, at least when high resolution imagery is used over a river reach with substantial temporal width variability.