Quantifying Freshwater Mass Balance in the Central Tibetan Plateau by Integrating Satellite Remote Sensing, Altimetry, and Gravimetry

Kuo-Hsin Tseng, Chung-Pai Chang, C. Shum, Chung-Yen Kuo, Kuan-Ting Liu, Kun Shang, Yuanyuan Jia, Jian Sun
2016 Remote Sensing  
The Tibetan Plateau (TP) has been observed by satellite optical remote sensing, altimetry, and gravimetry for a variety of geophysical parameters, including water storage change. However, each of these sensors has its respective limitation in the parameters observed, accuracy and spatial-temporal resolution. Here, we utilized an integrated approach to combine remote sensing imagery, digital elevation model, and satellite radar and laser altimetry data, to quantify freshwater storage change in a
more » ... storage change in a twin lake system named Chibuzhang Co and Dorsoidong Co in the central TP, and compared that with independent observations including mass changes from the Gravity Recovery and Climate Experiment (GRACE) data. Our results show that this twin lake, located within the Tanggula glacier system, remained almost steady during 1973-2000. However, Dorsoidong Co has experienced a significant lake level rise since 2000, especially during 2000-2005, that resulted in the plausible connection between the two lakes. The contemporary increasing lake level signal at a rate of 0.89˘0.05 cm¨yr´1, in a 2˝by 2˝grid equivalent water height since 2002, is higher than the GRACE observed trend at 0.41˘0.17 cm¨yr´1 during the same time span. Finally, a down-turning trend or inter-annual variability shown in the GRACE signal is observed after 2012, while the lake level is still rising at a consistent rate. Remote Sens. 2016, 8, 441 2 of 18 atmosphere, cryosphere, hydrosphere, and lithosphere. The reason it is so engaged in geoscience is not only because of its unique geographical formation, with an average altitude over 4000 m formed by geological processes, but also its sensitivity to the anthropogenic activities and climate change [2] . The freshwater storage in glaciers apparently experienced an accelerated loss arguably due to the contemporary climate-warming episode in recent years. From remote sensing and in situ records, the rising rate of temperature over the entire TP is estimated at 0.01-0.05˝C¨yr´1 [3] [4] [5] during the past 2-3 decades. Rapid glacier depletion, along with permafrost degradation also caused by temperature changes, indicates that the water in a frozen state changes tremendously over the TP. This warming procedure has caused many glacier-fed lakes to increase, especially over the last two decades. Thus, monitoring of lake dynamics is important and can be used as a proxy to study the amount of melting ice from thinning glaciers and/or due to changing or recently increased patterns of precipitation [6]. However, most of studies [7] conducted so far either focused on lake extent change, or short-term water level change using limited sensors. These limitations, either spatial or temporal, have made the exact quantification of water storage changes difficult. Hence, other supplemental observation is desirable to study long-term lake volume changes. The observation of freshwater storage change has been realized by satellite radar altimetry, laser altimetry, optical remote sensing, and gravimetry approaches. However, each spaceborne sensor has its own advantages and limitations in spatial-temporal resolution. For example, radar altimetry satellites, such as Jason-2 (Envisat), has a 10 (35) day repeat cycle and weather-free capability to measure surface elevation, but its large footprint (3-5 km in radius), customized for open ocean study, hinders the retrieval of surface heights within steep and rugged terrain. The radar waveform, contaminated by a mixture of surface types, is still a challenge for current waveform retracking technologies. Another major limitation in radar altimetry are the large spatial gaps between ground tracks. The interval of parallel ground tracks is about 60 km for Envisat and 250 km for Jason-2 at the latitude of the TP. Hence, A large number of lakes within the TP cannot be monitored. The advantage of laser altimetry missions, such as the Ice, Cloud, and land Elevation Satellite (ICESat), a mission dedicated to cryosphere studies, is the usage of laser photons to detect the height within small footprints (~70 m in radius). Unfortunately, the mission had just been operated in campaign mode with a short time span in 2003-2009. The next opportunity would be the ICESat-2 mission scheduled for launch in 2018. Finally, The Gravity Recovery and Climate Experiment (GRACE) [8] provides measurements of time-variable gravity that can be converted to mass redistribution. However, GRACE has a coarse resolution of a few hundred kilometers that makes lake studies difficult [9]. A potential solution to fill spatial-temporal gaps left by the instruments mentioned above is an integration of multiple sensors and products, such as combining altimetry and synthetic aperture radar (SAR) images [10], or relating water level with extent/volume by coincident altimetry and high-resolution satellites images [11] . Optical satellite remote sensing can effectively assist the monitoring of water storage changes. For a relative large lake such as Chibuzhang Co and Dorsoidong Co, mid-resolution remote sensing images may by sufficient to detect lake surface variations. For example, since the 1970s, Landsat series provide 16-day revisit cycle with 30 m medium resolution raster file with multispectral information. It could further compose false-color images to analyze land use/land cover (LULC). The Thematic Imagery-Altimetry System (TIAS) proposed in [12] intended to estimate water level change using historical Landsat imageries in Lake Mead, NV, USA. They exploited historical Landsat imageries to extend the time series of water level towards multi-decadal coverage, longer than a connection of operated and existing altimetry satellites. They applied the concept of hypsometry (reconstruction of contour from instant outline of a waterbody) to compute the elevation of the water level by overlapping water outline with a digital elevation model. The application of TIAS at Lake Mead, with relatively steep terrain, has accuracy at 0.85˘0.63 m as compared with gauge data. Based on their simulation of error budget, the accuracy at a terrain slope of <20˝and a number of shoreline pixels around 7000, similar to the geographical setup in Chibuzhang-Dorsoidong Co, the
doi:10.3390/rs8060441 fatcat:qnx2ovdyajfwdpmrv3j2x6g7p4