The study of man-made bodies of water such as dams is essential to national security. There are thousands of dams in Bulgaria. Their purposes for drinking and irrigation and fishing. Regardless of the use and size of the body of water, each dam can be both significant and dangerous.
Water has different reflectivity in different parts of the electromagnetic spectrum. And while large bodies of water, such as a reservoir, can have a high reflectivity. Ogosta, for example, can be identified by classical RGB-images (R-red G-green B-blue), smaller bodies of water cannot be well deciphered, e.g., the Chernila reservoir located in immediate vicinity of the lake Ogosta, south of the town of Montana.
Fig. 1 Left: RGB-image, middle: infrared image, right: water index, capturing the spectral reflectivity of water in different parts of the electromagnetic spectrum. Water reservoir Ogosta, nearby Montana (Bulgaria).
To classify images into objects that are water and objects that are not, we use arithmetic computations between different channels of the input image. There are several water indices (NDWI, mNDWI, etc.) by which water objects are easily identified. Some of these indices are not completely reliable under certain weather conditions, as the signal is weaker or noise laden. This is the case in the following illustration:
Fig. 2 Comparison between different indices for classifying water bodies. The example is with water reservoir Alexander Stamboliiski (June 2021). In this case, the NDWI water index is not reliable to correctly identify water bodies. The normalized vegetation index – NDVI is more reliable in this case. But this is, of course, not a rule.
Artificial water bodies are dynamic, meaning that water availability is controlled according to the season. For example, towards the end of summer, dams are released in a controlled manner to make room for snowmelt water. Consequently, in the spring the dams are relatively full, which can be both beneficial for drinking and irrigation needs and dangerous for the public and the environment.