Surface Water Tool
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Welcome to the SERVIR-Mekong Surface Water Tool.
This application allows you to carry out live calculations using a sophisticated water detection algorithm for the SERVIR-Mekong region. It is based on the algorithm described in this paper.

You can change the time period over which results are calculated and download the results for a region of your choosing. If you are confident that you understand the implications, you can also check out the expert settings which give you greater control over the outcome of the algorithm.

Temporary water can, in certain cases, be seen as in indication of a flood. However, both permanent and temporary water are relative to the selected time period. For example, if a long time period is chosen (e.g. 10 years), temporary water does not necessarily indicate a flood has occurred there, but it does show that water was detected there for some time within the selected period. To detect flooded water, it is advised to use a time period of 1 year or shorter.

Please note that, because everything is calculated on-the-fly, it might take some time for results to load.

This is the Surface Water Tool Demo!

Sadly, the demo is not implemented yet. Coming soon!
We will first go over some of the basics of the application, before we dive in a little deeper.

All info panels are toggled on/off with their 'i' button.
Turn the general information panel off so that we can see a bit more of the map.
Then, click 'Next' to go to the next step of the demo.
The map shown is a standard Google Maps instance with most of its basic functionality.
You can zoom in/out with the buttons in the bottom right corner, or with the scroll wheel of your mouse.
You can toggle different Google Maps displays with the buttons in the top left corner.

The data generated by the algorithm is visualized on top of this map.
It displays permanent and temporary water in different shades of blue,
as shown in the legend.
As a user of this tool, you can specify yourself for which time period you want to visualize the data.
This will change the input of the algorithm, and make it carry out a new run in Earth Engine.
Once finished, the new results will be displayed on the map. Because of this on-the-fly calculation, it might take some time for the results to load.

Try it out yourself by changing the time period.
Congratulations! You've reached the end of the demo!

Enter or select a time period to perform a new calculation.
Check out the documentation to learn more about this tool.

Legend:
Study area
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Permanent water
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Temporary water

Controls:

Period:
-
Region:
Filename:
Exporting results is enabled once you have selected a region (or completed drawing a polygon) on the map. This is done by first choosing a region selection method above. This will either load a new map layer from which you can select one ore more areas, or it will allow you to draw a polygon on the map yourself. Since this accesses the same database as the other map layers it is advised to wait for those to load before making a region selection.

You can select multiple areas (up to a maximum of four) by holding down the Ctrl key. Pressing the Esc key will reset the region selection.

Optionally, a filename for the to-be-downloaded data can be entered. If omitted, a default filename will be constructed using the selected time period.

Clicking the Export button reveals a panel on the top right of the screen. Download links will be shown there once they are ready for use. Map (water) layers are downloaded as zipped GeoTIFF files (0=no water, 1=temporary water, 2=permanent water). Metadata with input parameters can be downloaded directly as a CSV file.

NOTE: Due to limitations with the current implementation of the download option, there is a constraint on the size of the area that can be downloaded. You will see a warning if the area you have selected or drawn is too big. Near the limit there is a possibility that you receive a corrupted zip file. In that case, simply delete it and retry the download. We are working on a better implementation of the download option, which will remove all area constraints and other issues.
Expert controls

WARNING! These settings should only be adjusted by expert users! Please check the documentation for more details.

Show months:
Instead of a single map, this will show results for each individual month. This makes it possible to detect monthly trends. Since this substantially reduces the total amount of images within each map, this does require a longer time period. Instead of the usual 90 days minimum, the minimum time period when climatology is activated is 3 years. Even so, it is still possible that a certain month will not have sufficient satellite coverage to show accurate information over the entire area. Since 12 maps are calculated instead of 1, this can also slow down the application a lot. Please consider these things when activating this option.

When activated, a slider for the different months will appear on the lower right of the screen.
Defringe images:
Some Landsat satellite images contain noise due to sensor failures. See the following page for background information: http://landsat.usgs.gov/. The defringe algorithm attemps to mask out the pixels where sensor failures are an issue, reducing noise in the final result. However, due to the masking involved, this does result in less water being detected (especially temporary water), as it reduces the total amount of information. It is therefore important to consider what is most relevant; reducing the noise (apply defringing) or maintaining as much information from the data as possible (do not apply defringing). Since defringing requires additional steps in the calculation, it delays the visualization of results slightly.
Percentiles:
The percentile value controls the how the complete stack of satellite images is reduced to a single image (see GEE documentation for more info on this) . This parameter can take a value between 0 and 100, but in the lower ranges the effect of shadows will deteriorate results, while clouds will do the same in the higher ranges. The value on the left is used to calculate permanent water, the value on the right for temporary water. The value for permanent water should always exceed the one for temporary water. For permanent water, it is recommended to stay within the range of 15 - 60. For temporary water, it is recommended to stay within the range of 5 - 15.
Water threshold:
The water threshold value controls when something is classified as water from the Modified Normalized Difference Water Index (MNDWI). For more information on MNDWI, check out the relevant paper by Xu (2006). The parameter can take a value between -1 and 1, with lower values resulting in more areas identified as being water. However, it is recommended to stay within the range of 0.0 - 0.4.
Vegetation threshold:
The vegetation threshold value controls when something is classified as vegetation, which is used to mask out false-positives from the water detection algorithm. The latter especially occurs in areas with dark vegetation. This uses the Normalized Difference Vegetation Index (NDVI), which is often used to study vegetation.
A little background information on NDVI can be found, amongst others, at Wikipedia. The parameter can take a value between -1 and 1, but it is recommended to stay within the range of 0.2 - 0.6.
HAND threshold:
HAND (Height Above Nearest Drainage) is a method to normalize land elevation values relative to streams. For more information about HAND see the relevant papers by Rennó et al. (2008) and Nobre et al. (2011). This makes it possible to use a single value to mask out false-positives related to elevation, such as hill shades (these can appear similar to water in the processed satellite images). The parameter can take a value between 0 and 9999 meter. It is recommended to choose a value of at least 10 meter.
Clouds threshold:
The cloud threshold value controls when something is classified as a cloud, which will subsequently be removed from the analysis. This is done to reduce the impact of clouds on the algorithm. The parameter can take a value between 0 and 100, or be turned off completely, by setting it to -1 (default). This parameter an effect on the percentile calculation, as cloud masking is carried out before that step. As such, changes in this parameter could permit the use of a different percentile value, and vice versa.
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Click here to download water layer Click here to download water layer (2) Click here to download water layer (3) Click here to download water layer (4) Click here to download metadata