Determining geometric links between glaciers and lakes on the Tibetan plateau

Abstract

The Tibetan plateau and surrounding mountain ranges contain the largest amount of ice outside the polar region. The Tibetan plateau also contains more than one thousand lakes and is the origin of a large part of the water resources of South and East Asia, the most densely populated regions on earth. Recent studies concluded that the glacial area on the Tibetan plateau and surroundings has decreased significantly in the last decades. To directly assess changes in glacial mass balance from remote sensing data is challenging in this area because of the high relief and the relatively small size of the thousands of different glaciers. Recently however, trends for glacier elevation change in the Himalaya Mountain Range could be estimated based on terrain elevation profiles captured by ICESat. In addition to glacier changes, in recent research lake level changes on the Tibetan plateau were observed as well. Water level changes of several big Tibetan lakes have been monitored for the last decade by in-situ measurements and/or satellite radar / laser altimetry data. As an interesting result, about 150 lake level trends on the Tibetan plateau between 2003 and 2009 were obtained using ICESat laser altimetry. In fact, water level changes of a Tibetan lake are caused by direct precipitation, snow melt, glacial melt, moisture conditions, evaporation and rainwater runoff. Thus, clearly interpreting these lake level trends on a large region as Tibet is still a challenge in future. In this study, we exploit different remote sensing products to explicitly create links between Tibetan glaciers, lakes and rivers. Main data sources consist of the so-called CAREERI glacier mask, a lake mask based on the MODIS MOD44W water product and the HydroSHEDS river network product derived from SRTM elevation data. Based on a drainage network analysis, we first differentiate between lakes with and without outlet and then all drainage links between glaciers and lakes are determined all over the Tibetan plateau. The results show that 25.3% of the total glacier area directly drains into one of 244 Tibetan lakes. Consequently, dependency of a Tibetan lake on glacial runoff is defined as the ratio between the total area of glaciers draining into a lake and the area of the catchment of the lake. These dependencies are determined for all ~900 Tibetan lakes over one kilometer square. This result indicates which lakes are expected to be strongly affected by the predicted further shrinkage of the glaciers on the Tibetan plateau. Geometric links between Tibetan glaciers and lakes in this analysis are expected to strongly assist in understanding the often very local differences in lake level change patterns and in total this indirect methodology could turn out in a new way to better constrain local glacial mass balance variations in this climate-sensitive region. This paper gives an overview of methodology and results as presented in HESSD [18]. Here however, we do give more details on the implementation of the workflow in a GIS environment, by specifying how exactly the different input data products are combined in terms of GIS layers in order to obtain the required output data.