The Response of Soil Moisture to Long-Term Variability of Precipitation

Abstract

Soil hydrology is a widely recognized low-pass filter for the interaction between land and atmosphere. However, the lack of adequate long-term measured data on soil moisture profiles has precluded examination of how soil wetness responds to long-term precipitation variations. Such a response can be characterized by its amplitude damping, phase shifting, and increasing persistence with soil depth. These should be correlated with the climate spectra through the interactions between the land and the atmosphere. The major objective of this study is to investigate how precipitation signals are manifested in vertical soil moisture profiles in the context of timescales. Thus, the natural variability of soil moisture profiles is documented using 16 yr of field observational data of soil moisture measured at 11 levels of various depths down to 2.0 m at 17 locations over Illinois. Detailed statistic analyses are made of the temporal variations of soil moisture profiles and concurrently measured precipitation over the 16-yr period of 1981?96. Cross-spectral analysis is performed to obtain the coherency pattern and phase correlation of surface and profile soil moisture time series to determine phase shift and amplitude damping. A composite of the drought events during this time period is analyzed and compared with the 16-yr climatology. The major findings are that 1) the amplitude decreases with soil depth, with the dryness signal penetrating more deeply than the wetness signal; 2) the phase shift with soil depth is correlated with the timescales of the variation, such that it is deeper with longer timescales; and 3) the seasonal variation of soil moisture is amplified in the drought-year composite, with an increased phase shift from soil surface to bottom. Hence, the observations provide a description of the soil moisture profile variability as a function of soil depth. Whether or not climate models can reproduce this variability should be a good test of their land process representations in the treatment of soil hydrology.