## 1 Introduction  
Mountain glaciers on the Tibetan Plateau are an important hydrological buffer in sustaining the seasonal availability of water for most densely populated downstream areas, and, thus, predicting the fate of these glaciers and consequent water supplies are at the forefront of public attention [1; 2; 3; 4; 5; 6; 7]. According to whether there is continuous debris cover in the ablation zone, mountain glaciers can be divided into clean-surface and debris-covered glaciers [8]. For clean-surface glaciers, the englacial debris is entrained by ice flow and exposed at the glacier margins. For debris-covered glaciers, supraglacial debris will affect ablation substantially depending on the debris characteristics, particularly for its thickness [9; 10; 11; 12; 13; 14]. Theoretically, these two types of glacier states might be mutually transformed for the same glacier as it expands and shrinks with varying ice influx [15; 16; 17]. For large valley glaciers with abundant debris material sources, [PERSON] and [PERSON] [18] reported that their debris load has evolved from sparse to heavy since the Last Glacial Maximum. Increasing evidence exists that present-day climate warming has resulted in glacier debris cover expanding at higher elevations for typical debris-covered glaciers worldwide, including the Tibetan Plateau [19; 20; 21; 22]. In comparison, the transformation of smaller glaciers with a clean-surface to a debris-covered surface will take longer, which hinders research into these glaciers using the available data with a limited record length. A case that clearly occurred is the d'Estelette Glacier in the Italian Alps, where the glacier surface had shifted into typical supraglacial debris cover in the 1990s from the clean-surface type, as shown in photographs from the 1920s [23]. However, despite the fact that the debris changes in smaller glaciers on the western Himalaya has been investigated using the Landsat data [24], the coarse-scale satellite imagery and the paucity of field observation still make it unclear whether such transitions are taking place on the glaciers of the Tibetan Plateau. In recent years, the rapid development of commercial unmanned aerial vehicle (UAV, or more popularly known as \"drone\") technologies has made the UAV-based remote sensing data an important complement to satellite images with limited spatial and temporal resolutions [25], particularly for studies of environmental change or surface process over mountain glaciers with a hostile climate and poor approachability. For instance, by using the small UAV (<25 kg) at a relatively low cost, the cm-scale digital orthophotograph map (DOM) and the digital elevation model (DEM) datasets can be obtained to provide a detailed and on-demand map of such rugged mountainous areas [26; 27; 28]. In addition to the UAV data, the newly available commercial satellite data of meter/sub-meter resolution (e.g., IKONOS, GeoEye, Orbview, QuickBird, WorldView, etc.) and the declassified high-resolution satellite images from the American reconnaissance program Hexagon (KH-9, 1971-1984) also offer opportunities to fill the gap in observations. Combining all of these data sources, we are enabled to investigate the long-term surface process (from the 1970s) of smaller mountain glaciers (e.g., the Qiyi Glacier) at meter/sub-meter resolutions. The Qiyi Glacier on the northern Tibetan Plateau (Figure 1d) was the first glacier in the region investigated by Chinese scientists in 1958 [29]. It is a cirque-valley glacier covering an area of 2.6 km\({}^{2}\) with an elevation of 4300-5150 m, and it has long been categorized as a clean-surface glacier [30]. The slopes of the surrounding valley walls are gentle, and the material source of the englacial debris is primarily from sub-glacial erosion, which is transported to the lower part of the glacier by internal ice flow. However, in 2005, a small pile of supraglacial debris was found on the central part of the glacier tongue along the mainstream line for the first time, which has since expanded (Figure 1a-c).