Three in one: GPS-IR measurements of ground surface elevation changes, soil moisture, and snow depth at a permafrost site in the northeastern Qinghai–Tibet Plateau

Jiahua Zhang,Tao Che,Lin Liu,Lei Su

doi:10.5194/tc-15-3021-2021

Abstract

Abstract. Ground surface elevation changes, soil moisture, and snow depth are all essential variables for studying the dynamics of the active layer and permafrost. GPS interferometric reflectometry (GPS-IR) has been used to measure surface elevation changes and snow depth in permafrost areas. However, its applicability to estimating soil moisture in permafrost regions has not been assessed. Moreover, these variables were usually measured separately at different sites. Integrating their estimates at one site facilitates the comprehensive utilization of GPS-IR in permafrost studies. In this study, we run simulations to elucidate that the commonly used GPS-IR algorithm for estimating soil moisture content cannot be directly used in permafrost areas, because it does not consider the bias introduced by the seasonal surface elevation changes due to active layer thawing. We propose a solution to improve this default method by introducing modeled surface elevation changes. We validate this modified method using the GPS data and in situ observations at a permafrost site in the northeastern Qinghai–Tibet Plateau (QTP). The root-mean-square error and correlation coefficient between the GPS-IR estimates of soil moisture content and the in situ ones improve from 1.85 % to 1.51 % and 0.71 to 0.82, respectively. We also propose a framework to integrate the GPS-IR estimates of these three variables at one site and illustrate it using the same site in the QTP as an example. This study highlights the improvement to the default algorithm, which makes the GPS-IR valid in estimating soil moisture content in permafrost areas. The three-in-one framework is able to fully utilize the GPS-IR in permafrost areas and can be extended to other sites such as those in the Arctic. This study is also the first to use GPS-IR to estimate environmental variables in the QTP, which fills a spatial gap and provides complementary measurements to ground temperature and active layer thickness.

Highlights

Permafrost refers to the ground where the temperature remains at or below 0 ◦C for at least 2 consecutive years
Λ where A(e) is the oscillation amplitude varying with satellite elevation angle e; f is the oscillation frequency of the signal-to-noise ratio (SNR) interferogram; H is the vertical distance between the antenna and the reflecting surface, conventionally called the reflector height; λ is the carrier wavelength of GPS signals; and φ (e) is the phase varying with satellite elevation angle as well
The parameters ds and d0 are −1.7 ± 0.8 and 1.2 ± 0.6 cm, respectively. We use these parameters and the normalized thermal indices to simulate the ground surface elevation changes, which are presented in Fig. 6a as a curve superimposed on the GPS interferometric reflectometry (GPS-IR) measurements

Summary

Introduction

Permafrost refers to the ground where the temperature remains at or below 0 ◦C for at least 2 consecutive years. In the Qinghai–Tibet Plateau (QTP), permafrost occupies around 40 % of its area (Zou et al, 2017) and has been warming and degrading over the last several decades (Zhao et al, 2010, 2020). Based on the records at 10 sites, the average thickening rate was 19.5 cm per decade from 1981 to 2018 (Zhao et al, 20120). The dynamics of the active layer and permafrost (collectively called frozen ground alternatively) has a crucial impact on geomorphological, hydrological, ecological processes, and infrastructures (Wu et al, 2002; Cheng and Wu, 2007; Yang et al, 2010; Gao et al, 2018)

Methods

Results

Discussion

Conclusion