Technical Notes for The Surface Water Record

G. Robert Brakenridge and Albert J. Kettner
Dartmouth Flood Observatory
CSDMS, INSTAAR, University of Colorado
Campus Box 450
Boulder, CO 80309-0450 USA

Observing the Earth's changing surface water in near real time


The Surface Water Record includes both map displays and the GIS data supporting the map displays. The Record maps and data provide current surface water extent and the observed history of changes (2000 to present). Surface water expansions and contractions are both observed and mapped. Mapped water expansions may be short-term, from inland flooding or storm surges. Or long-term, from post-2000 reservoir construction and increases in rice agriculture or aquaculture. Mapped contractions may also be short-term, from temporary drought conditions. Or long term: for example, now-dry water bodies within shrinking wetlands. The most recent conditions are shown on each individual map page. The accumulated areas of past flooding, also illustrated, allow the relative size of any ongoing flood events to be assessed, and can be used in evaluation of future flood hazard. The Record can serve as underlying infrastructure for more-detailed flood mapping efforts conducted to assist in disaster response; it provides wide-area coverage with frequent time steps, but spatial resolution limited to that of the two MODIS sensors bands 1 and 2 (250 m).

The map sheets are arranged in 10 degree latitude and 10 degree longitude increments, and projected in a.simple latitude and longitude coordinate system ("Plate Carre" projection; WGS 84 datum). They can be read into GIS systems by entering the corner coordinates or using the 2 degree graticule (for the.jpg versions) or directly reading in geotif files, where provided. The satellite sources for water extent mapping are mainly the MODIS sensors; where other data are displayed, the sources are noted on the maps and/or in the associated GIS file names.

In 2013, automated updating is being implemented. This uses 14-day accumulation .shp (and associated files) named "current" in the MODISlance_2wkpro directory. Date stamps showing the most recent update are provided on the maps.. Clickable black dots access DFO's River Watch 2 data at individual river or reservoir measurement sites.

Click here for access to the archived long term record for each map sheet. In 2013, all DFO GIS files are being transferred to this directory. The process is still underway as of June, 2013: many directories are empty but will be populated with the data already visualized on the maps. Choose appropriate dates to retrieve surface water changes during floods or droughts. Or retrieve the yearly .ppt slide maximum water compilation, if available, from that directory, and scroll through time.

Click here for access to the automated daily .shp file GIS record (record commences in 2011). Choose map sheet directory and appropriate dates to retrieve surface water changes of interest.

See also NRT Global MODIS Flood Mapping from NASA. With citation of the source (see above), the map displays and associated data can be further distributed and used under the Creative Commons Attribution 3.0 Unported License. Please use our names in such citation: this display and associated data are authored by Brakenridge, G.R., Kettner, A.J., Slayback, D. and Policelli, F.

Map Legend

At the time of each map's date: Large areas of purple are dry land (formerly water in February, 2000, when the reference SWBD water database was obtained). At higher latitudes, such areas may be ice-covered water. Small areas of purple are water mapped by SWBD, but are too small to be mappable by MODIS. Dark blue is current water, imaged by MODIS and by SWBD in 2000 ("permanent" water). Bright blue is flooding: expanded water areas mapped by MODIS compared to SWBD. Any post-2000 reservoir or new water body is also depicted in bright blue. Light blue-gray is all previous flooding imaged and mapped by the Flood Observatory (now dry land). Note: in mountainous areas, local shadows are commonly mis-classified as water. See also the Record Guide.

User Cautions

1) In mountainous regions, terrain shadows mimic surface water and are mis-classified as water in our current algorithm. We are working to reduce such noise.

2) Reservoirs and impoundments constructed since February 2000, the date of the SWDB reference water, appear permanently as new water (bright blue).

3) The historic observational record (light blue-gray) illustrated may not include all floods: prior to 2011 the records were obtained manually and focus was only on major flood events.

4) Because of the 3-day, 6-image compositing process used to produce each new 'daily" .shp GIS file (so actually each daily updated includes three days of data), and because of cloud cover obscuration during flooding, even major flooding may not appear on the maps immediately: lag times of several days are common, and lag times may be longer if cloud cover is sustained. The maps show all flooding and also any reduced surface water, compared to the reference water, and as visible over the 14 days ending on the map date.

Remote Sensing Data Sources

DFO water mapping using NASA's two MODIS sensors progressively evolved since Terra MODIS "first light" in late 1999. The earlier classified water results are, however, very similar to those at present. Most processing changes concern how clouds and cloud shadows are removed. Currently the Surface Water Survey uses the NASA LANCE-MODIS NRT surface reflectance products, as processed to surface water .shp file water boundaries (find appropriate 10 degree folder at Earlier, DFO used the MODIS Rapid Response Subset data, and before this, MODIS swath data while performing our own "bow tie correction" and geocoding. Use of the LANCE-MODIS 10 deg x 10 deg mosaic scenes, already in Geotiff format, improves the ability to generate new maps very quickly.

Parties Responsible

A cooperative research project funded by the NASA Earth Science Applications Program in 2010 incorporated DFO MODIS flood mapping methods into an automated processor at NASA. Fritz Policelli at NASA Goddard Spaceflight Center (GSFC) was project Principal Investigator. Project personnel included J. Sun, D. Slayback, D. Ouzounov, Science Systems and Applications, Inc. at GSFC, and, at DFO, also A. Kettner. Jeff Schmalz of the MODIS Rapid Response team was instrumental early-on in facilitating the utilization of the Rapid Response Subset data for flood mapping. NASA support of the LANCE-MODIS project now provides even more timely MODIS data (latency < 3 hours), and the source MODIS band 1 and 2 data are digitized to 12 bit rather than 8 bit formats. Cooperative work between D. Slayback and A. Kettner, in particular, has provided a near real time MODIS data transformation from satellite acquisition to GIS (.shp file) surface water polygons and automated updating of the Record maps..

Water Classification Procedures

Water classification is accomplished by a ratio approach (band2 + A)/(band1+ B), with constants A and B determined empirically. Below a numerical threshold, all pixels are classified as water and all above as land. Threshold values have ranged, as experience was gained with this algorithm, from 0.6 to 0.9. Numerical thresholds in band 1 and 7 data can also be used to remove clouds and partially shadowed clouds.

Cloud shadows on land and terrain shadows may exhibit spectral signatures in the MODIS bands used that are nearly identical to that of clear water. To remove cloud shadows, our approach evolved with experience: from a 4 scene set including two days and two scenes per day (Terra, morning, and Aqua, afternoon), to also employing a 3 day, 6 scene set in regions where such noise is a problem. In this approach, cloud shadows are removed by requiring 3 "water" classifications for each pixel to be accepted as water in the final product. Cloud shadows are largely removed, because their positions change over a three day interval. Only three, in any order, of the 6 images need show water. Intermittent cloud cover is partially removed also by this approach, which fills coverage between clouds, while at the same time removing the noise caused by cloud shadows. Terrain shadows also move from morning to afternoon and may be partially removed, but much deeply shadowed land areas in mountainous terrain remain misclassified as water.

Data Product Notes

The NRT GIS vectors, fit around each final water classification product, show in their file names single dates, but currently represent a result from six scenes, over three days.

The Surface Water Record displays, with most-current date shown, accumulate the previous 14 days of the three day .shp file product from NASA GSFC (this provides comprehensive coverage of current water, with very little obsuration from cloud cover). Older data are all coded as light blue-gray. (previous flooding). This method allows: 1) the most recent data obtained to be illustrated, and 2) the extents of flooding for each year to be accumulated and integrated into the long term archive of surface water variability.

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