STABILITY OF FLOW PATTERNS IN WATER REPELLENT SOILS
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Gerd Wessolek1), Heiner Stoffregen, and Karsten Taeumer
Technical University Berlin, Institute of Ecology, Dep. of Soil Protection
Salzufer 12, D-10587 Berlin, Email: gerd.wessolek@tu-berlin.de
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 (<<100%)
High recovery rate (>100%)
Irregular pattern of tracer distribution
- tracer can not penetrate the water repellent soil
- tracer has passed the sample depth
Reasons:
- high precipitation and deep drainage
- preferential flow
- runoff
- lateral water movement out of the sampled area
lateral tracer movement
Reasons:
- dry and water repellent behavior of the topsoil in
combination with
- special micro relief conditions (influx of runoff)
fingered flow, parts of the soil are excluded from
transport processes
Reasons:
water repellent spots with low water content
spring
effective cross section: a
effective cross
section: c
fall
fraction of total water content change
1
summer
effective cross section: b
a
b
0,8
c
0,6
0,4
0,2
0
0
0,2
0,4
0,6
0,8
fraction of cross-sectional area
Fig. 1: Schematic reconstruction of the changing soil water flow regime
Keywords: tracer study, water repellence, preferential flow, effective cross section, TDR
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