Effect of physiographic conditions on the spatial variation of seasonal topsoil moisture in Mediterranean soils

Abstract

This paper studies the spatial variation of topsoil moisture within different soil types in a medium-size catchment at the Spanish Pre-Pyrenees (Estaña catchment, 2.5 km2) using seasonal data at high spatial resolution. Topsoil moisture was measured with a Delta-T Theta Probe ML2x device to 80 mm soil depth and at seasonal scale during 2005 and 2006. Measured values were related to the soil water content at field capacity to calculate the relative topsoil moisture (θR, %). The highest values of θR were obtained in autumn (53.2%) and the lowest in summer (32.1%), showing a positive correlation with seasonal precipitation and an inverse relationship with seasonal solar radiation (SR). Steep northern slopes presented the highest values of θR in spring, summer, and winter, and topsoil moisture progressively decreased from steep northern slopes to gentle slopes and from gentle slopes to steep southern slopes, defining a topographical trend. Any topographical trend was observed in autumn when values of θR were very high within the whole catchment. No significant correlation was found between values of θR and those of topographic wetness index (TWI) except in summer. Relative topsoil moisture varied greatly for the different soil types, and the highest values were also measured in steep northern slopes in spring, summer, and winter. Correlation between θR and SR was good for the different soil types in winter when SR varied the most for the different physiographic units and the range of values of θR was the highest. No significant correlation was found in autumn for any soil type due to the very low values of solar radiation. Results of this work draw attention to the importance of considering the slope steepness and aspect conditions for a more precise characterisation of moisture-sensitive areas in Mediterranean environments and soils at seasonal scale.