Wavelet analysis of soil moisture measurements for hillslope hydrological processes

Abstract

Spatial and temporal distributions of soil moisture are important for explaining hydrological processes at the hillslope scale. The fate of various hillslope flow paths can vary seasonally and their relationship with rainfall events can be nonstationary. We collected high resolution in situ soil moisture datasets at 30 points (10 locations in three depths) between March and November in a steep mountainous hillslope. The wavelet coherence analysis of eco-hydrometeorological factors and soil moistures indicated distinctive controls of rainfall and evapotranspiration over soil moisture at time scales of under a day. The vertical flow in upslope areas can be explained by significant wavelet coherence in soil moisture profiles. The appearance of lateral flow with vertical flow in the downslope is associated with even more significant wavelet coherence between identical depths. The wavelet coherence analysis between two soil moisture time series both along hillslope layers and depth profiles provided good spatial and temporal representations of hillslope hydrological processes in terms of their timing and scale. The distributions of soil moisture coherences showed distinctive characteristics for three different seasons. The seasonally distinctive role of vegetation to infiltration was apparent for the upslope area, but the combined impact of vegetation and topography controlled more complicated hydrological processes in the downslope area. The phase angle analysis was introduced to show the profile difference of hydrological processes on various time scales.