Ground Deformations around the Toktogul Reservoir, Kyrgyzstan, from Envisat ASAR and Sentinel-1 Data—A Case Study about the Impact of Atmospheric Corrections on InSAR Time Series

Julia Neelmeijer,Volker Klemann,Mahdi Motagh,Tilo Schöne,Robert Dill

doi:10.3390/rs10030462

Abstract

We present ground deformations in response to water level variations at the Toktogul Reservoir, located in Kyrgyzstan, Central Asia. Ground deformations were measured by Envisat Advanced Synthetic Aperture Radar (ASAR) and Sentinel-1 Differential Interferometric Synthetic Aperture Radar (DInSAR) imagery covering the time periods 2004–2009 and 2014–2016, respectively. The net reservoir water level, as measured by satellite radar altimetry, decreased approximately 60 m (∼13.5 km3) from 2004–2009, whereas, for 2014–2016, the net water level increased by approximately 51 m (∼11.2 km3). The individual Small BAseline Subset (SBAS) interferograms were heavily influenced by atmospheric effects that needed to be minimized prior to the time-series analysis. We tested several approaches including corrections based on global numerical weather model data, such as the European Centre for Medium-RangeWeather Forecasts (ECMWF) operational forecast data, the ERA-5 reanalysis, and the ERA-Interim reanalysis, as well as phase-based methods, such as calculating a simple linear dependency on the elevation or the more sophisticated power-law approach. Our findings suggest that, for the high-mountain Toktogul area, the power-law correction performs the best. Envisat descending time series for the period of water recession reveal mean line-of-sight (LOS) uplift rates of 7.8 mm/yr on the northern shore of the Toktogul Reservoir close to the Toktogul city area. For the same area, Sentinel-1 ascending and descending time series consistently show a subsidence behaviour due to the replenishing of the water reservoir, which includes intra-annual LOS variations on the order of 30mm. A decomposition of the LOS deformation rates of both Sentinel-1 orbits revealed mean vertical subsidence rates of 25 mm/yr for the common time period of March 2015–November 2016, which is in very good agreement with the results derived from elastic modelling based on the TEA12 Earth model.

Highlights

The water levels of large artificial water reservoirs constructed for hydroelectric power generation and irrigation are prone to significant changes over the course of a year
A first general result of our study is that Sentinel-1 products are superior to Envisat results in the following aspects: (1) the wide swath of the Sentinel-1 sensor allows more freedom in cutting the scenes to the desired area of interest; we capture the western region of the reservoir better with the Sentinel-1 time series than with the Envisat ones; (2) due to the higher temporal acquisition sampling, Sentinel-1 interferograms are affected less by decorrelation, which leads to a higher point density than in the Envisat time series
In the Toktogul case, small- to medium-scale bands generally perform better than longer ones (Table 3), but the results are not consistent among the three different SAR time series, which may again be explained by the different SAR frame extents

Summary

Introduction

The water levels of large artificial water reservoirs constructed for hydroelectric power generation and irrigation are prone to significant changes over the course of a year This periodic loading of the crust causes ground deformations of the surrounding area, alters pore pressure and changes stress on underlying faults and fractures, which may induce seismicity [1]. Our aim in this study is to measure ground deformations induced by water level changes in the Toktogul Reservoir, which is located at N 41.8° E 72.9° in the northwest of Kyrgyzstan, Central Asia (Figure 1). This reservoir is fed by the Naryn River, which originates from glacial melt water of the Tien Shan mountain range. These activities lead to a trans-boundary water policy conflict, which resulted in an exaggerated use of water in some years that could not be compensated by the incoming amount of water until the beginning of the following winter season (Figure 2)

Methods

Results

Discussion

Conclusion