STABILITY OF FLOW PATTERNS IN WATER REPELLENT SOILS 1)
Gerd Wessolek1), Heiner Stoffregen, and Karsten Taeumer Technical University Berlin, Institute of Ecology, Dep. of Soil Protection Salzufer 12, D-10587 Berlin, Email:
[email protected]
A time-delayed double tracer experiment was conducted to study the characteristics and stability of flow paths on a water repellent sandy soil. The first tracer, bromide, was applied in spring, while the second one, chloride, was applied in autumn. After a travel time of 328 days for bromide respectively 87 days for chloride, soil samples were taken from a three-meter long and one meter deep soil profile. The chloride tracer showed a typical distribution for preferential flow: no chloride in water repellent areas and high transport of chloride through wettable soil parts. The drastically changes of extremely high and relatively low recovery rates indicate the lateral flow in the profile. During summer the soil dries out successively and becomes water repellent. Consequently, water infiltrates the soil along flow fingers. If the dry periods in the summer are prolonged, the complete topsoil becomes water repellent. According to our measurements, the established flow paths i.e. the position of the dry spots and flow paths remain stable. In contrast, the bromide tracer was highly concentrated in dry, water repellent areas. This distribution is a result of the water flow during the spring time, where a complete different flow regime was predominant. The varying distribution of the bromide and chloride tracer make clear that different flow regimes predominated at the time of the tracer transport. Thus, a more or less consistent transport of bromide took place during spring, whereas the continuing desiccation and the negative water balance led to water repellence and preferential flow paths in the topsoil which stopped the bromide transport. Seasonal changes of the effective cross section have a great influence on transport events in the soil. During the winter months until spring (January – April) a relatively homogeneous even distribution of moisture and percolation rate prevailed, only few parts remain water repellent, while fingering is dominated during the summer months and autumn. Fig. 1 illustrates the changing flow regime and effective cross section during the year. However, the double tracer experiment does not allow any conclusions about the temporal stability of the flow paths for consecutive precipitation events within one season. Tab. 1 shows the reasons exist for low or high recovery rates of the tracers within the same soil profile. Our experiences lead to the conclusion, that only high temporal TDR-measurements on an adjacent plot give the information of flow path changes at distinct positions in the soil profile. The changes of the soil water contents after sequential precipitation events show that the same flow paths are still active. However, comparing long time intervals, e.g. from autumn to autumn, a spatial alteration of several flow fingers becomes typical. A complete flow path change occurs at least only after the soil was dried out completely and starts to rewet. In this situation the flow path memory was reset and soil water transport can take in new pore systems. Tab.1: Criteria for interpreting tracer distribution and recovery rates
Low tracer recovery rate (
