Detection of Lake Extent Changes

//VERSION=3
/*
DETECTION OF LAKE EXTENT CHANGES

Detect changes of water body extent between two Landsat images. The scenes can be from the Landsat 4-5 TM and Landsat 8-9 Level-2 data sets which together range from 1984 to 2022. The water body detection is simply based on a MNDWI threshold.

Author: Jan Landwehrs (https://www.linkedin.com/in/jan-landwehrs-a37009130)
*/

// Set date1 and date2 to the dates of the two Landsat scenes that shall be compared. 
// (date1 should be the older one)

// EXAMPLES:
// // POYANG LAKE (https://apps.sentinel-hub.com/eo-browser/?zoom=10&lat=29.14511&lng=116.18321)
var date1="1988-08-13", date2="2022-08-19"
// // ARAL SEA (https://apps.sentinel-hub.com/eo-browser/?zoom=9&lat=45.61116&lng=59.55688)
// var date1="1989-08-16", date2="2022-08-27" 
// // DEAD SEA (https://apps.sentinel-hub.com/eo-browser/?zoom=10&lat=31.38588&lng=35.47623)
// var date1="1985-08-24", date2="2022-08-22"

var date1_date = new Date(date1);
var date2_date = new Date(date2);
var year1 = date1_date.getFullYear();
var year2 = date2_date.getFullYear();
// The last  first dates for which the Landsat 4-5 TM / Landsat 8-9 data sets are available
var Landsat45TM_end_date = new Date("2012-05-01T00:00:00Z");
var Landsat89_start_date = new Date("2013-02-01T00:00:00Z");

// Landsat 4-5 TM and Landsat 8-9 differ in their band configuration. For the water body detection, the red, green, blue and SWIR1 values have to be taken from the corresponding bands. 
if (date1_date > Landsat89_start_date) {
    var ds2bands=["B02","B03","B04","B06"];
    var ds1bands=["B02","B03","B04","B06"];}
else if (date1_date < Landsat45TM_end_date && date2_date > Landsat89_start_date) {
    var ds2bands=["B02","B03","B04","B06"];
    var ds1bands=["B01","B02","B03","B04"];}
else if (date2_date < Landsat45TM_end_date) {
    var ds2bands=["B01","B02","B03","B04"];
    var ds1bands=["B01","B02","B03","B04"];}
    
function setup() { return { input: [
    {datasource: "ds2", bands: ds2bands, mosaicking: "ORBIT"},
    {datasource: "ds1", bands: ds1bands, mosaicking: "ORBIT"}],
    output: [ { id: "default", bands: 3, sampleType: SampleType.AUTO } ] }; } 

let minVal = 0.0;
let maxVal = 0.4;
let viz = new DefaultVisualizer(minVal, maxVal);

function evaluatePixel(p) {
 
    let s1 = p.ds1[0] 
    let s2 = p.ds2[0] 
        
    var b_1=s1[ds1bands[0]],g_1=s1[ds1bands[1]],r_1=s1[ds1bands[2]],swir1_1=s1[ds1bands[3]];
    var b_2=s2[ds2bands[0]],g_2=s2[ds2bands[1]],r_2=s2[ds2bands[2]],swir1_2=s2[ds2bands[3]];
    
    // The MNDWI is used as a simple way to detect open water bodies
    var water1=0, water2=0;
    var MNDWI_threshold=0.1;
    mndwi_1=(g_1-swir1_1)/(g_1+swir1_1)
    mndwi_2=(g_2-swir1_2)/(g_2+swir1_2)
    if (mndwi_1>MNDWI_threshold) {water1=1;}
    if (mndwi_2>MNDWI_threshold) {water2=1;}
    
    // Compute the difference between the two water masks. 
    water_diff=water1 - water2;
    
    // True color for surrounding land
    let RGB=[r_2,g_2,b_2].map(a=>2.25*a);
    
    // Visualize the detected water body changes with surrounding land as true color from image 2.
    // Red color: Water detected in image 1, but not in image 2 (-> receded water body)
    // Dark blue color: Water detected at image 2, but not at image 1 (-> expanded water body)
    // Light blue color: Water detected in both images
    if (water_diff==1) return [1,0,0]; if (water_diff==-1) return [0,0,1]; else if (water1==1) return [0.44,0.54,1]; else return RGB;
}

Evaluate and visualize

EO Browser - Poyang Lake

Data sources used

Landsat 8-9 Level 2
Landsat 4-5 Level 2

General description of the script

Climate change contributes to more frequent or more severe droughts and floods in many regions of the world, among other things. Satellite Earth observation is extremely useful to document these changes, for example by monitoring water bodies. The Landsat program provides particularly useful data for demonstrating variations in the extent of lakes over the last decades, for example, as it provides the longest-running record of satellite observations since the 1970s.

This script is a custom script for the Sentinel Hub EO Browser. It maps the extent of water bodies in two Landsat images defined by the user and then visualizes the changes between both scenes. The script is able to compare images both from the Landsat 8-9 Level-2 (available since 02-2013) and Landsat 4-5 TM Level-2 data sets, which together range from 07-1984 to 05-2012.

Limitations

The script currently only compares two single Landsat images which have to be selected manually for the area of interest. A multi-temporal approach which automatically composes water body masks from multiple images is not implemented yet. Also, the water body detection is simply performed using a MNDWI (Modified Normalized Difference Water Index) threshold [2]. This does not feature masking of clouds, snow or buildings, or other more advanced methods for a more accurate water body detection. Some of these have been previously implemented in a script by Mohor Gartner.

How to use the script

Find two dates with suitable Landsat 4-5 TM or Landsat 8-9 records that you would like to compare.
Load the scene of the later date in the EO Browser.
Tick “Use additional datasets”. Set the primary Datasource alias to “ds2” and the additonal Datasource alias to “ds1”.
For the additional Datasource tick “Customize timespan” and set both fields to the date of the older scene.
In the script, set date1 and date2 to match the dates of the scenes that shall be compared.
Run the script. (“Refresh Evalscript”)

Description of representative images

Example 1: Lake Poyang

As an example, the pictures below show the Poyang Lake in August 1988 and 2022 as well as the lake extent changes detected by the presented script. The Poyang Lake is China’s largest freshwater lake and experiences significant lake level variations between the dry and the wet monsoon seasons. However, it experienced an extreme shrinkage in 2022 associated with a severe drought and heat wave in Southern China [1]. Red and dark blue colors indicate retraction or expansion of the detected water bodies from the older to the more recent image, respectively.

Landsat images of Lake Poyang on 1988-08-13 (left, Landsat 4-5-TM Level-2 True Color Image) and on 2022-08-19 (middle, Landsat 8-9 Level-2 True Color Image). The rightmost panel shows changes in the water body extent between both scenes detected by the presented script.(Red / Dark Blue: detected water body receded / expanded. Light Blue: water detected in both scenes.)

Lake Poyang