How Could Unmanned Aerial Systems (UAS) Be Used for Ecohydrological and Ecosystem Research? Experiences of First Operations with UAS in River Flood Plains of Northern Mongolia

Abstract

This paper proposes the use of unmanned aerial systems (UAS) as a method for monitoring biotic resources and ecohydrological systems in river floodplains. Small scale mapping based on LANDSAT and SRTM or ASTER data is of limited applicability since a spatial resolution of 30 to 90 m is not sufficient to meet the demands of habitat mapping and large scale 3D -modelling. Newer satellites like WorldView2 and SENTINEL (space mission from European Space Agency within the Copernicus Programme) could be an option to gain a 0.5 m resolution, but the availability of image data is limited. UAS allow the collection of very high spatial and temporal resolution image data and the generation of digital elevation models (DEM). A spatial resolution of less than 10 cm and multispectral or hyperspectral image data, which can be provided by UAS sensors, is needed for mapping of habitats and riparian vegetation. Indicators for water quality such as chlorophyll (a) and suspended matter concentration can be efficiently derived from multispectral image data. Thermal image data, which can also be recorded by UAS-borne sensors, provides information on thermal heterogeneity of water temperature and the interaction of river and groundwater discharge from the river floodplain. In addition, cloud cover rarely affects UAS-generated aerial images because flying altitudes are usually low and flight missions can be timed very flexibly. UAS are also much more cost-effective to operate than manned aircraft. In a first field survey in September 2012, several field plots were investigated in northern Mongolia in different watersheds of the Selenge River Basin (SRB) with varying types of land use and environmental impacts. The regional focus was on the Kharaa River Basin (KRB), which is a paradigm for transformation from nearly natural conditions to an increasingly altered state by economic activities. Within the BMBF funded project “Integrated Water Resources Management in Central Asia: model region Mongolia (MoMo)” the actual situation of water quality, quantity and ecological impacts in this area has been investigated since 2006. A first analysis of nutrient and ecological gradients of the Kharaa Rver Basin indicates a ‘good’ chemical and ecological status for the headwaters and some parts of the middle reaches. Evidence for initial processes of ecosystem degradation and biodiversity loss were detected in the middle and increasingly in the lower reaches. Despite many efforts, several questions remained unsolved. Among them, the impact of erosion and particle transport on ecosystem degradation is a key issue. Fine sediment intrusion caused by erosion predominantly from the river banks but also from upland areas seems to be the most likely cause. However, based on the experiences of our existing monitoring scheme with a combination of intense fieldwork and continuous measuring with data loggers, the need of more spatial information (e.g. riparian vegetation structure, hydromorphology) with a high resolution became evident to confirm this hypothesis. Therefore, an unmanned aerial vehicle (UAV) equipped with a calibrated RGB camera was used to record image data for photogrammetric processing. DEM and orthophotos as well as spherical panoramic views were derived. Furthermore, thermal image data were terrestrially collected using an Infratec Variocam hr. Integration of thermal, multi- or hyperspectral sensors on various UAS (e.g. Archaeocopter), as well as analysis algorithms are the next steps for future work. The applicability of remote sensing approaches is discussed to better foster the development of ground truthing for a sustainable river basin management plan. The application of UAS offers a sound scientific base to assess especially the riparian zones in areas with difficult access