Surface Water Watch and Record

Web Portal and Map, 100W040N

By G. R. Brakenridge and A. J. Kettner

 

Display 1: Flood

Blue: Reference water (February, 2000); Red: Currently observed flooding; also, reservoirs built since February, 2000; Light gray: Satellite record of flooding, 1999-2016.

Click on River Watch (colored dots) to retrieve daily discharge values from River Watch. Click on reservoir, wetland, and lake areas to retrieve current surface water extents from the Flood Observatory's Web Map Server. A large, simplified Geotif of this map is also available.

 

Display 2: Drought

Yellow: New dry Land*; Blue: Current water (past 14 days); Light gray: Satellite record of flooding, 1999-2016.

Click on River Watch (colored dots) to retrieve daily discharge values from River Watch. Click on reservoir, wetland, and lake areas to retrieve current surface water extents from the Flood Observatory's Web Map Server. *Small streams and rivers are not observed in the Current Water layer and will appear yellow even though not dry.

Explanation

Purpose:

Since late 1999, satellite data have been obtained by the Dartmouth Flood Observatory, processed to detect water/land boundaries, and analyzed to produce flood inundation limits in vector GIS format. Surface Water Watch and Record compiles this information to produce a comprehensive record of the Earth's surface water variability.

A geotif file provides a large scale, permanent record of land areas that were flooded during this time interval.These lands are subject to signficant flood hazard. This site is also a portal to examine surface water changes occurring today. The top display shows current water expansions (floods, red). The bottom display shows water contractions (hydrologic drought, yellow).

Many areas of both maps include hyperlinks to more detailed map or river flow status information: hover with the mouse to locate active links.

Map Symbology:

The map sheet number gives geographic coordinates of the upper left corner of map. Most maps measure 10 degrees longitude and latitude and are in plate-carree (equirectangular) projection. Sheets above 60 degrees north are in UTM projections and measure 20 degrees longitude.

The Shuttle Water Boundary Data (SWBD, obtained in February, 2000) are at 90 m spatial resolution and provide a useful reference water between latitudes 60 degrees south and 56 degrees north. It is the uppermost (blue) layer in the flood display. In the top display, red areas thus illustrate current MODIS surface water where it extends beyond the reference water. In the lower display, the SWBD layer is yellow, and is beneath the current MODIS water layer (blue). Thus, yellow areas indicate dry land that was water in February 2000, and the blue areas show currently imaged water. A date of last update is also shown on each map.

Use:

Flood Response: In the Flood display, red areas are either flooding (compared with the winter, 2000, water extent), reservoirs constructed since then, or reservoir/lake areas larger than in 2000. Red areas in mountainous terrain, or during winter in the higher latitudes may be "false positive" water detections. Also visit the Web Map Server (WMS) for more detailed examination.

Flood Hazard Assessment: In either display, the light gray area indicates remote sensing of surface water during the period of record. Download the full resolution geotif file to retrieve this important information about flood hazard. Also, where River Watch sites monitor the floodplain, recurrence intervals can be calculated for the mapped flooding; these are shown for local reaches.

Reservoir and Lake Monitoring: In the Drought display, reservoirs with yellow fringes are low compared to February 2000. The maximum reservoir extents recorded are also shown, in light gray. For additional information, click on many of the reservoirs and lakes to access the Web Map Server (WMS). Allow time for the WMS to refresh, and select only appropriate water layers for display. For example, to compare current surface water with the maximum in the year to date, display only two layers: "January till current" and "Two week water extent".

Notes on Data For These Maps:

Twice-daily (NASA Terra and Aqua) MODIS data are first processed by NASA's LANCE system. The daily surface water outcome is then transmitted to the Flood Observatory and further processed and then archived. The "current" water layer is an accumulation of such data over 14 days to remove cloud obscuration. The map date refers to a two week period ending on that date; the curent file is updated each day and replaces the previous one..

Floods in mountainous regions are difficult remote sensing targets and not always observable at MODIS spatial resolution of 250 m/pixel. Also, cloud cover sometimes restrict the ability to capture peak inundation, even using the rolling-forward 14 day accumulation. The maps do not illustrate all areas of possible flood hazard, but do provide an objective record of where flooding has been observed. Finally, terrain shadows in upland areas may produce abundant "false positive" water detections; we are in the process (in 2017) of addressing this issue.

GIS Data Access:

Click here for access to the automated daily MODIS-derived .shp file GIS record (record commences in 2011; processor is now located at NASA LANCE). Choose appropriate 10 deg x 10 deg map sheet directory and appropriate dates.

Data from the Global Surface Water Explorer is included in both displays as part of the light gray satellite-mapped water extent. These data are based on Landsat data at a spatial resolution of 30 m (Jean-Francois Pekel, Andrew Cottam, Noel Gorelick, Alan S. Belward, High-resolution mapping of global surface water and its long-term changes. Nature 540, 418-422, 2016). The NASA Shuttle Water Boundary Data (SWBD) surface water extent (90 m resolution) was derived from NASA's 11-day February, 2000, SRTM mission; this "permanent" water layer was also corrected using Landsat data.

NASA NRT Global Flood Mapping annual maximum water extent for the years 2013-2015, at 250 m spatial resolution, also are included in the light gray satellite-mapped water extent and are also provided separately via the Web Map Server. DFO creates these annual water extent layers from data provided by the NASA project, by accumulating into one annual file all of the daily .shp files for each year.

Finally, since late 1999, DFO produced flood extent files through mapping of individual floods; these are also included in the maximum water extent layer.

River Watch Measurements (colored dot symbols above):

At selected locations, a time series of satellite microwave-based daily river discharge measurements are available from the Flood Observatory. See River Watch Global Display for more information. A sample from Site 445 in this map sheet is shown below. For current conditions: White triangles: ice covered. Yellow dots: low flow (<5th percentile discharge for this date, 1998-2012). Blue dots: normal flow. Purple dots: moderate flooding (>1.5 year recurrence interval). Red dots, major flooding (> 5 year recurrence interval).

Funding, Data, and Institutional Support:

The NASA Earth Sciences Program, the Latin American Development Bank, the World Bank, the European Commission (Global Disaster Alert and Coordination System, GDACS), the Google Earth Engine research awards program, the Committee on Earth Observation Satellites (CEOS) Disasters Working Group-Flood Pilot, the Japanese Space Agency, and the European Space Agency all have supported this work via research grants and contracts or by provision of free satellite-derived data.

The Dartmouth Flood Observatory was founded in 1993 at Dartmouth College, Hanover, NH USA and moved to the University of Colorado, INSTAAR, CSDMS in 2010. The institutional support of both universities is gratefully acknowledged.

Creative Commons License
This work, web site, and associated data are by G. R. Brakenridge and A. J. Kettner and are provided for use under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

 

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