Effect of barometric pumping on relative humidity in the loessal soil of the loess Plateau

Abstract

Monitoring experiments of soil air in column show that relative humidity (RH) of air in the soil of the Loess Plateau fluctuates with atmospheric pressure (AP), which is barometric pumping resulting in the vertical movement of soil air and the change of its RH. When AP increases, soil air is compressed and atmospheric air enters the soil, causing RH to decrease. When AP decreases, soil air expands and rises, causing the flow of moist air outwards from the soil and increasing RH. Therefore, RH fluctuates reversely with AP (the correlation coefficient between RH and AP can reach –0.74). This paper studies the effect of barometric pumping on RH in the loessal soil of the Loess Plateau, and reveals the mechanism responsible for the vertical movement of soil air. On a yearly timescale, the volume of soil air expands with a general decrease in AP from January to July, then compresses with an increase in AP from July to December. On a daily timescale, the air in soil shows bimodal fluctuations. Barometric pumping is the driving force underpinning the soil air movement, and therefore dominating its fundamental characteristics. From a preliminary establishment of model calculation, the volume of soil air movement occurring in the Loess Plateau is proportional to the amplitude of AP fluctuations, aerated porosity, and thickness of the local loess layer. The amplitude of air fluctuations in the soil is independent from the aerated porosity.