Spatial Gauging on Canterbury Plains Using RADARSAT
Study area
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The Waimakariri river, a broad braided, gravel bed river crossing the Canterbury Plains is the subject of this investigation.
Data
Discharge data collected routinely by the Regional Council is compared to surface water areas observed on RADARSAT imagery supplied by the Canadian Space Agency. RADARSAT has an active microwave sensor which "sees through" the cloudy atmosphere and so it is a more reliable source of information for a study of this sort than satellites which use optical wavelengths. The sensor can collect coarse spatial resolution imagery from large areas up to c.300km across, or more detailed images from small areas (c. 50km across). The more detailed Fine Beam data with spatial resolution of about 8m was used in this study.
The results presented here refer to this 7km long test reach.
Method
RADARSAT's Synthetic Aperture Radar (SAR) looks obliquely at the planet's surface. The satellite illuminates the surface by transmitting a radar beam, some of which is backscattered to the satellite where it is recorded. Gravel tends to backscatter much more of the beam than does water, which tends to forward reflect the beam in much the same way as a mirror reflects a beam of light. Consequently water in the test reach will appear darker on the radar image.
Unfortunately the recorded image has some uncertainty associated with it which is manifest as speckle. All SAR imagery suffers from this. It means that some pixels in the image are brighter than they should be, and others are darker. A raw SAR image has a fuzzy appearance which is very evident in the image of the test reach above. Imagery must be processed to remove the speckling before it can be properly interpreted.
Processing steps are illustrated here for a small portion of the reach.
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Roll your mouse over the titles to view the image as:
Results
RADARSAT imagery was acquired on 16 occasions, mainly during 1997 and 1998. For each occasion the mean water width at the test reach (= Water Surface Area divided by Reach Length) was calculated and plotted against the River's discharge recorded from the Regional Council's gauging station.
The correlation coefficient is 0.88 for this relationship, good enough to suggest that this crude measure of hydraulic geometry may be of use, albeit in rather limited circumstances.
References
- Hockey, J.B. Spatial river gauging on the Canterbury Plains, New Zealand. Proceedings of the RADARSAT ADRO symposium, Montreal, 13-15 October 1998, ADRO Technical volume: Hydrology Applications. Canadian Space Agency, 1999:1-11
- Kuittinen R. (1992) Remote Sensing for Hydrology, Progress and Prospects. WMO Operational Hydrology Report No 36 (62pp).
- Smith L.C. (1997) Satellite remote sensing of river inundation area, stage, and discharge - a review. Hydrological Processes. 11(10) pp1427-1439
- Smith L.C., Isacks B.L., Forster R.R., Bloom, A.L. & Preuss, I. (1995) Estimation of discharge from braided glacial rivers using ERS 1 synthetic aperture radar: First results. Water Resources Research, v31 pp1325-1329