## 1 Introduction
### Study Objectives Associated with Environmental Issues of Lake Sevan when Changing Its Water Level  
Lake Sevan, as Armenia's largest freshwater resource, is of vital importance not only nationally but throughout the South Caucas region. Being of great significance to irrigation, fisheries, hydropower, recreation, and tourism, as well as to sustaining valuable freshwater, associated populations, and endemic species, Lake Sevan has been subjected to a complex set of environmental issues since the early 1930s, when the water level was significantly dropped due to extensive water withdrawal for irrigation (Figure 1). In the 2000s, the Republic of Armenia enacted the \"Law on Lake Sevan\" and \"Law on Measures for Restoration, Maintenance, Reproduction, and Use of Lake Sevan Ecosystem\" (2001) [34] and a spatial planning project for the Lake Sevan drainage basin (2003) [35]. These laws refer to plans for raising the water level by 6 m up to the optimal altitude of 1903.5 m by the year 2030. Taking into account the current inflow and withdrawal, this means a raise of the water level by 21.6 cm annually [34]. However, the rapidly raised water level has led to serious issues of shore submergence and flooding of the built-up coastal zone. In 2011, it was officially approved to limit the increase in the lake water level [36]. Such long-term and short-term changes in the water level of Lake Sevan have caused drastic transformations in coastal and aquatic ecosystems as well as the immunological and hydro-chemical properties of lake waters [37, 38, 39]. The first algal bloom in Lake Sevan was observed in 1964, and, since 2018, repeated summer outbreaks of _Cyanophyta_, have encompassed the whole lake area and are clearly classified as toxic [40]. The coastal zone and the entire Lake Sevan's basin area are now subject to anthropogenic impacts that are large in extent and diverse in type. This, combined with ineffective environmental management and regulation measures, exacerbates the state of coastal and water environments, which are also vulnerable to climatic changes and seismic events [41]. Nevertheless, in spatial and temporal terms, there is a little knowledge about factors and drivers of negative impacts and coastal ecosystem responses to them. Enacted measures of water, agricultural, and environmental policies are not well balanced in Sevan Basin [36, 42]. There is a low public awareness on all the aspects of environmental responsibility, resource use, and ecosystem services provided by Lake Sevan. Under such conditions, there is evidently a need to devise and implement a remote sensing system for regular monitoring of the coastal environments and rapid response to their changes based on such a flexible and operative tool as UAV surveys. Taking into account the very complex set of environmental issues and their intricate spatio-temporal patterns, the present study  
Figure 1: (**a**) Annual changes in Lake Sevan water level from 1890 to 2020. (**b**) Monthly changes in water level during unmanned aerial vehicle (UAV) surveys, April 2018-July 2019. Red lines and figures mark the following events: 1–the start of artificial drop of the water level, 2–the decrease of water withdrawal on irrigation purposes, 3–Arpa-Sevan tunnel is fully operational (inflow of 200 mln. m\({}^{3}\) of water annually), 4–Vorot-Arpa tunnel is operational (additional water inflow of 165 mln. m\({}^{3}\)). Credits: data from 1890 to 1920 [30], from 1927 to 1998 [31], from 2002 to 2016 [32], and then according to data from the Ministry of Emergency Situations, Republic of Armenia [33]. examines the possibilities, specific features, and applications of UAV-based environmental monitoring of Lake Sevan's coastal zone as an initial and important elaboration stage of multi-platform, multi-scale, and multi-temporal remote sensing scheme for monitoring this highly dynamic and vulnerable area. In this paper, we focus on several topical thematic applications based on optical and thermal UAV imagery.