THEME: FLOODS AND DROUGHTS SATELLITE: Sentinel-2 L2A
INDEX: True
color
IMAGES:
SUBJECT ANALYTICALLY:
This optimized True color script
uses the visible light bands red, green and blue in the corresponding red,
green and blue color channels, resulting in a product with natural colours that
represents the Earth as humans would naturally see it. The visualisation uses
highlight compression and improves the contrast and color vividness through
minor contrast and saturation enhancement.
COMMENTS –
OBSERVATION:
In the left
image we can see areas depicted in dark brown, in December 2016, these are
areas with abundant drought. In contrast, in
the right image, in December 2024 respectively, the areas that had suffered
this devastating fire appear to be now overgrown.
INDEX: Moisture
Index
Normalized Difference Moisture Index
(NDMI)
IMAGES:
SUBJECT ANALYTICALLY:
The normalized difference moisture
Index (NDMI) is used to determine vegetation water content and monitor
droughts. The value range of the NDMI is -1 to 1. Negative values of NDMI
(values approaching -1) correspond to barren soil. Values around zero (-0.2 to
0.4) generally correspond to water stress. High, positive values represent high
canopy without water stress (approximately 0.4 to 1).
COMMENTS –
OBSERVATION:
In the left
image, December 2016, it is easily understood that in the burned areas, there
is intense drought, due to high temperatures and minimal rainfall. Thus, the
burned areas appear in an orange-red color.
While, in the
right image, in the same month of 2024, the same areas have returned to their
natural levels. This is understood based on the image, since the majority of
the areas are depicted in blue.
INDEX: NDWI
SUBJECT: Normalized Difference Water
Index (NDWI)
IMAGES:
SUBJECT ANALYTICALLY:
The normalized difference water
index is most appropriate for water body mapping. Values of water bodies are
larger than 0.5. Vegetation has smaller values. Built-up features have positive
values between zero and 0.2.
COMMENTS –
OBSERVATION:
In the left
image, we can see embedded features in the areas that have suffered fire. In
December 2016, these appear with an index less than 0.2 and greater than 0 and
are depicted in the images with very light shades of green.
In contrast, in
the right image, December 2024, the burned areas have returned to their natural
vegetation.
INDEX: NDMI
SUBJECT : NDMI for Moisture Stress
IMAGES:
SUBJECT ANALYTICALLY:
The Normalized Difference Moisture
Index (NDMI) for moisture stress can be used to detect irrigation. For all the
index values above 0, knowing the land use and land cover, it is possible to determine
whether irrigation has taken place. Knowing the type of crop grown (e.g. citrus
crops), it is possible to identify whether irrigation is being effective or not
during the crucial growing summer season, as well as find out if some parts of
the farm are being under or over-irrigated.
COMMENTS –
OBSERVATION:
In the left
image, we can see areas that have suffered fires. In December 2016, these
appear with an index above 0 and are depicted in white in the images, due to
the low humidity index and the lack of irrigation.
In contrast, in
the right image, December 2024, there is a uniform image on the island. More
specifically, there are not such great contrasts of blue-white colors, but the
island is depicted all in shades of blue.
INDEX: NDVI
Subject: Normalized Difference Vegetation
Index (NDVI)
IMAGES:
SUBJECT ANALYTICALLY:
The normalized difference vegetation
index is a simple, but effective index for quantifying green vegetation. It is
a measure of the state of vegetation health based on how plants reflect light
at certain wavelengths. The value range of the NDVI is -1 to 1. Negative values
of NDVI (values approaching -1) correspond to water. Values close to zero
(-0.1to 0.1) generally correspond to barren areas of rock, sand, or snow. Low,
positive values represent shrub and grassland (approximately 0.2 to 0.4), while
high values indicate temperate and tropical rainforests (values approaching 1).
COMMENTS –
OBSERVATION:
In the left
image, in December 2016, the burned areas are distinguished with a light green,
due to the minimal vegetation, due to the devastating fire a few months ago.
In contrast, in
the right image, in December 2024, an intense green color is observed almost
throughout Thassos. Thus, we understand that there is reforestation in the
burned areas.
INDEX: SWIR
Subject: Short wave infrared composite (SWIR)
IMAGES:
SUBJECT ANALYTICALLY:
Short wave infrared (SWIR)
measurements can help scientists estimate how much water is present in plants
and soil, as water absorbs SWIR wavelengths. Short wave infrared bands (a band
is a region of the electromagnetic spectrum; a satellite sensor can image Earth
in different bands) are also useful for distinguishing between cloud types
(water clouds versus ice clouds), snow and ice, all of which appear white in
visible light. In this composite vegetation appears in shades of green, soils
and built-up areas are in various shades of brown, and water appears black.
Newly burned land reflects strongly in SWIR bands, making them valuable for
mapping fire damages. Each rock type reflects shortwave infrared light
differently, making it possible to map out geology by comparing reflected SWIR
light.
COMMENTS –
OBSERVATION:
In the left
image, December 2016, the lack of water in the burned areas is depicted in dark
brown, while the overgrown areas are depicted in green. In contrast, in
the right image, December 2024, all areas are now depicted in green.
INDEX: False Color
Urban
Subject: False Color Urban composite
IMAGES :
SUBJECT ANALYTICALLY:
This composite is used to visualise
urbanized areas more clearly. Vegetation is visible in shades of green, while
urbanized areas are represented by white, grey, or purple. Soils, sand, and
minerals are shown in a variety of colors. Snow and ice appear as dark blue,
and water as black or blue. Flooded areas are very dark blue and almost black.
The composite is useful for detecting wildfires and calderas of volcanoes, as
they are displayed in shades of red and yellow.
COMMENTS –
OBSERVATION:
In the left
image, December 16, the areas with intense vegetation are depicted in shades of
green. While areas with less vegetation are depicted in lighter green. More
specifically, areas that have suffered fires are depicted in yellow.
In contrast, in
the right image, December 2024, there is a uniform image on the island. More
specifically, there are not such large deviations in dark-light green colors,
but the island is depicted all in shades of intense green.
INDEX:
Highlight Optimized Natural Color
IMAGES:
COMMENTS –
OBSERVATION:
In the left
image we can see areas depicted in dark brown, in December 2016, these are
areas with abundant drought. In contrast, in
the right image, in December 2024 respectively, the areas that had suffered
this devastating fire appear to be now overgrown.
GROUND DATA:
A dry storm
occurs when there is too much drought. Thus, on the island of Thassos, after
the great drought of the summer, a dry storm broke out, resulting in the fire
on the island in 2016.
The island of
Thassos, after the devastating fires of 2016, also suffered catastrophic floods
because its mountains were stripped bare due to the fact that their trees
burned and could not retain the water of the rainfall.
There were 2
catastrophic floods:
• November 2019
In Thassos, the
biggest problems are found in the southern part of the island, where six areas
were declared in a state of emergency.
More
specifically, the heavy rainfall mainly affected the southern part of Thassos,
with many roads and houses flooded. Roads turned into torrential torrents,
sweeping away cars and anything else in their path, streams and irrigation
canals overflowed, many parts of the road network are inaccessible, while the
assistance of the Fire Department was needed to free residents in settlements
where water literally entered their homes and made it impossible for them to
move.
The situation
has gotten out of control and in Potamia, the central stream had overflowed,
resulting in water "running" uncontrollably in any direction.
• November 2024
Thassos
recorded an impressive rainfall record. The Meteokav weather station in
Limenas, Thassos recorded a daily rainfall of 153mm, surpassing the previous
record of 152.4mm recorded in November 2019. The rainfall significantly
affected the areas of Prinos and Kalivia Limenaria, causing flooding.
The bad weather
severely affected the road network and infrastructure in Maries to Kalivia,
Prinos, where the road connecting it to Skala Prinos, Kalivia Limenaria was
flooded, where they were affected by the flooding phenomena. The main road
flooded again, as had happened in 2019, causing serious problems in traffic and
the movement of residents.
