Period analysis and trend forecast for soil temperature in the Qinghai-Xizang Highway by wavelet transformation

Abstract

Soil temperature is a key factor in engineering design, construction and operation involved in permafrost engineering, and is closely associated with engineering stability. Here, the study shows that the wavelet transformation analysis method provides a better understanding of soil temperature in the time–frequency domain. Semimonthly soil temperature time series for two monitoring sites along the Qinghai-Xizang Highway were analyzed using wavelet analysis from 1996 to 2012. Periodic characteristics and trends in soil temperature were determined. The results show that soil temperatures under natural ground and embankment settings have similar periodic change at both sites. Six periods of 18, 36, 78, 130, 220 and 342 half-months were observed. Cold/warm cycles in soil temperature were qualitatively appraised using Morlet wavelets for soil temperatures at these two sites over a cold period in 2012, as well as for soil temperature below embankment at site no. 5 over a warm period. The changes in soil temperature were quantitatively appraised by decomposing and reconstructing soil temperature using dbN wavelets; soil temperature at both sites showed an increasing trend over the past 17 years, with a rise in soil temperature under the natural ground and the embankment corresponding to 0.022 and 0.072 °C a−1 in warm permafrost and 0.048 and 0.007 °C a−1 in cold permafrost.