Who is in Control? Competing Influences of Geology, Land use and Topography on Soil Water Storage and Soil Temperature Dynamics

Abstract:

Can we identify distinct signatures of landscape elements in the responses of soil moisture and soil temperature? Moisture and temperature dynamics in soils are largely controlled by the climatic boundary conditions of rainfall, evapotranspiration and radiation. However, certain landscape features also leave characteristic finger prints on soil moisture and soil temperature time series. The extent of these influences and their time variable relative importance are relevant in a number of contexts, such as landscape scale prediction of soil moisture patterns, vegetation stress patterns or runoff generation, process predictions in ungauged basins or the improvement of hydrological model structures for the mesoscale.

The competing influences of geology, land use and topography on temperature and moisture characteristics in the vadose zone are explored at the CAOS hydrological observatory in Luxembourg (http://www.caos-project.de/) with a unique experimental setup of 45 sensor clusters. These sensor clusters cover three different geologies (schist, sandstone, marls), two land use classes (forest and grassland), five different landscape positions (plateau, top-, mid- and lower hillslope as well as near stream/floodplain locations), and contrasting expositions. At each of these sensor clusters three soil moisture profiles with sensors at depths from 10 to 70 cm, four soil temperature profiles as well as air temperature, relative humidity, global radiation, rainfall/throughfall, sapflow and shallow groundwater and stream water levels were measured continuously. Time series of ca. 2 years for the schist region and up to 6 months for the complete set of sites allow for a first intercomparison of characteristic response behavior.

First results of our ongoing work show that mean soil water storage dynamics varied strongly from site to site. Nevertheless distinct differences between the geologies could be identified and especially the marls region showed both higher overall values and a more dampened soil water storage response compared to the other geologies (Figure 1A). Differences between the mean dynamics of soil temperature and soil water storage within a certain geological area and individual sites were examined, for example by rank stability plots (Figure 1B, C and D). Exemplary results include the distinctly higher soil temperatures at the grassland sites of the schist region (Figure 1C) and the strong differentiation in soil water storage of slope and non-slope locations within the sandstone area (Figure 1D). This difference was most pronounced in the sandstone area, while in the schist area a difference between north and south facing slopes could also be identified (data not shown). Analyzing the differences between “geological mean dynamics” and individual sites using color matrix plots (Figure 1E) allows for the identification of sites and time periods where local dynamics deviate from the mean dynamics and a subsequent further examination of the local controls underlying these “deviations”.

We can conclude that geology, land use, slope position and aspect are all found to exert a certain control on the dynamics of soil water storage and soil temperature, however, the relative importance of these factors shifts based on geology and also system state. The interrelationships between the different controls and their temporally variable importance will be examined more closely in the coming months and results will be reported.

Acknowledgements: We thank Britta Kattenstroth, Jeff Iffly and all helpers in the field.