Coherence between sunlight quantum and soil water content in the shallow Quaternary gravel layer: Evidence of the spectral analysis

Abstract

Due to the heterogeneity and nonlinear process in the unsaturated soil zone, it is challenging to represent the analytical mechanism and actual in-situ water content in the soil layer. With this in mind, this study provides a novel approach to discern the water content in the shallow Quaternary gravel layer at various depths using temporal sunlight and air temperature observation. It demonstrates significant components in spectral density to evaluate the crucial importance concerning the time-dependent soil water content, sunlight quantum, and air temperature. It shows that the soil water content at 10 cm depth can distinguish the fluctuated manner of light quantum at the time lag of about 6.6 and 5.1 h for diurnal and semidiurnal processes, respectively. The time progress of soil water contents at 30 cm depth is less relevant to 10 cm depth, but the phase at 60 and 120 cm depth is nearly in phase. Due to heterogeneity in the shallow layer, the temporal changes of soil water content below 10 cm depth have varying coherence degrees. Thus, using spectral analysis, practically estimating the coherence and time lag between light quantum, air temperature, and soil water content becomes feasible and achievable.