Characterizing soil water dynamics on steep hillslopes from long-term lysimeter data

- Long-term observation of soil moisture covers large variance in the seasonality.
- Spatio-temporal moisture differences show migration of a moisture front.
- Sub-surficial discharge led to the quantification of non-vertical moisture movement.
- Qualification of moisture dynamic through cross correlation depth-step-wise.
- With the discharge/moisture behavior we could quantify hysteresis effects.

SummaryUnderstanding soil water dynamics on hillslopes is of crucial importance to the prediction of floods and other hydrological events in mountainous catchments, to the identification of natural vegetation patterns, and to the optimization of agricultural land use. In principle, such information can be obtained from lysimeters, but most experimental lysimeter facilities have been installed on flat terrain. This study presents a long-term and high-resolution investigation of soil moisture, surface and subsurface flow using three large-scale lysimeters on a slope with 23.5° inclination on a landfill site in Karlsruhe, Germany. Data from a 10-year observation period were evaluated for this study, including weekly soil moisture data obtained by neutron probes, continuous discharge data from the land surface and several layers within the soil zone, and hydrometeorological data from a climate station. The results reveal (i) clear temporal and spatial patterns of soil moisture variations down to a depth of 250 cm, (ii) substantially higher discharge and faster percolation rates in the lower part of the lysimeter field, indicating significant downhill flow at various depths within the soil profile, (iii) characteristic threshold values for flow processes in the soil, associated with a hysteresis effect between soil moisture and flow processes. These results can be used as a basis of improved numerical models for the simulation of floods, soil moisture distributions, and vegetation patterns.

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