Hydrologische Vernetzung und ihre Bedeutung für diffuse Nährstoffeinträge im Hotzenwald/Südschwarzwald

In order to meet the goals of the Water Framework Directive (WFD) –
namely the sustainable improvement of water quality – it is necessary to
reduce diffuse nutrient inputs. Diffuse nutrient inputs – primarily of phosphorus
(P) and nitrogen (N) – are of public interest due to their negative
ecological effects on surface water quality. Lakes, especially their trophic
status, are natural indicators reflecting anthropogenic impacts on aquatic
systems in that they integrate nutrient inputs spatially over their entire
catchment area and over time as well. The ”Bergsee” near Bad S¨ackingen
(Southern Black Forest, Germany) is a classic example of anthropogenic eutrophication
of a lake. It was the initial ”lake of contention” for this work,
which is focussing on hydrologic connectivity and its significance for diffuse
nutrient inputs in the Southern Black Forest (Germany).
An innovative combination of experimental hydrologic methods and high
resolution sampling strategies1 offered fundamental insights in the systemand
process-understanding of diffuse P-inputs from land to water. The applied
methods made it possible to
• identify the dominating preferential pathways in management-related
scales,
• check the influence of these quick flow paths on delivery mechanisms
of nutrients and
• investigate diffuse pathways on a event-orientated basis.
The application and monitoring of natural (18O), geogenic (SiO2), conservative
(2H, Uranine/Flurescine) and reactive (respectivily sorptive) tracers
(P) offered detailed insights in the process understanding of the field scale.
Close cooperation with farmers allowed conducting management-related
experiments on entire fields (”field scale”). The spatial dimension of these
experiments is therefore more in agreement with the scale of the natural phenomenon
of interest than plot- or lab-experiments. This makes the results
more valuable for the catchment scale2, but leads to more complex interpretations
of the findings due to the greater amount of processes involved. In
order to assess the variability of runoff formation, runoff concentration and
nutrient transport processes in relation to event specific boundary conditions,
the following was conducted:
• 3-year high resolution sampling of (continuously: runoff, electric conductivity,
P-fractions, silica; episodically: environmental isotopes, sediment
load, TOC, chloride, nitrate),
• monitoring of boundary conditions (land use, timing and amount of
fertilizer application, rainfall amount and intensity, air-, water- and
soil temperature, groundwater level and discharge),
• simultaneous measurement of events in various scales (from field to
catchment scale),
• determination of P-loads based on high resolution flow-proportional
sampling at two gauging stations,
• detailed experimental investigation of specific aspects like preferential
flow paths or subsurface travelling times and
• measurements at various scales in order to estimate the effects and
range of influence of integral and scale specific processes.
The dotation of liquid manure with an adequate tracer and its common
agricultural application enabled practice-related experiments for identification
and quantification of nutrient transport processes. Deuterium (2H) was
used as artifical tracer in the liquid manure experiment and delivered in combination
with high resolution measurements/determination of P-fractions in
ground and stream water valuable information on predominant flow paths
and P-retention capacity of the soil(s). The additional determination of natural
(18O) and geogenic (SiO2) tracers for storm event following directly
after the experiment offered important insights in fundamental hydrologic
processes like runoff formation and concentration with high relevance for nutrient
transfers. The data set gained thus consisting of intensive experimental
sampling and high resolution data of an extreme event is extraordinary and
especially valuable in respect to risk assessment for initial losses.
According to its land use patterns, the focus in the Bergsee catchment
was on P-loss from permanent grassland. Due to the minimal crop land
amount3, soil erosion is negligible. Even during storm runoff sediment and
wash load is comparably low. Due to the soil properties – high infiltration
capacity, hydraulic conductivity, pore volume – the extend of saturation area
is relatively low, with little variation and predominately bound to organic
soils in the riparian zone. Saturation overland flow is not contributing significantly
to runoff formation. Even the storm immediately after the liquid
manure experiment in the upper R¨uttebach valley (84mm/d on 02.06.2004)
did not produce notable overland flow in the riparian zone. Consequently,
P-rainwash4 as transport path is of minor importance.
Preferential flowpaths in soils are potentially an important diffuse transport
paths for agricultural P-losses to surface waters (P-leaching/wash out
and initial losses). Runoff concentration in the Bergsee catchment – and therby
as well lateral transport of water bound soluble substances to the water
course – takes place predominantly in the soil. The primary storm runoff formation
process is fast lateral subsurface flow (subsurface stormflow, SSF).
Vertical macropores and lateral pipe networks in the soil are connecting in
the soil forming preferential flowpaths and results in hydrologic connectivity
with increased infiltration and rapid drainage of the soil. Depending on the
groundwater level, soil moisture content, precipitation intensity and precipitation
amount, either event water or pre-event water comprises the bulk
of the discharge. In case of high groundwater level the major part of the
soil drainage pipe network is situated mainly in the phreatic zone already.
In this case the preferential flow paths are for the most part draining old,
pre-event water stored in the soil, which is mobilised by the rainfall impulse.
If the pipe systems is located mainly in the vadose zone, new event water
infiltrates in macropores and quickly brigdes the vadose zone by bypassing
the soil matrix. Above the groundwater level a temporarily saturated zone
composed of pre-event water and event water is formed and drains quickly
in the stream channel through the lateral pipe system.
The fast transport of the tracers in the experiments indicates the good
hydrologic connectivity. Remarkable is the high event water fraction of about
60% for the storm on 2 June 2004, which highlights the predominance of
preferential pathways with respect to runoff formation and concentration.
The field experiments, the analysis of geogenic tracers and the isotope tracer
inverstigations in the upper R¨uttebachtal are indicative agruments for the
following hypothesis:
"An extended unsaturated zone – due to low ground water
levels – in combination with high rainfall intensities results in
high event water fractions and prevailingly preferentially formed
discharge, because under these conditions hydrologic connectivity
takes place in the temporary saturated area (small cross-section
with high flux rates/high hydraulic conductivity). This temporarily
saturated zone increases in areas with high lateral pipe density. In case of high groundwater level even intensive rainfall
results in low event water fraction, due to dominating hydraulic
connectivity – predominatly old pre-event water from the midterm
saturated zone is hydraulically mobilsied (big/large crosssection
with low flux rates/low hydraulic conductivity)."
Crucial Points for P-retention in soils are on one hand the way how
preferential infiltration in vertical macropores and lateral drainage in pipes
are connecting and how connectivity beetween different pipe networks (interpipe
system connectivity) is created on the other hand. In both cases a
direct link (direct connectivity) is possible. But a non-direct macropore-pipeconnection
with matrix passage seams to be more plausible for the Bergsee
catchment, as this would explain the high P-retention in the soil for the
liqiud manure experiment. If a contact time of less than 30 min is sufficant
for the (high) P-retention potential of macropore and soil pipe walls allone
is questionable, as the exchange with the soil matrix for such short residence
times is (perhaps) strongly limited.
The R¨uttebach is the primary source for the high P-concentrations and
loads in the headwater catchment(s) of the Bergsee. The strong increase of
P-concentrations and loads in stormflow conditions in the straitend, strongly
deep eroded lower part of the R¨uttebach results from mobilisation of
P-enriched river sediments. The significant enriched P-concentrations of the
river sediments of the R¨uttebach is a result of the point source sewage treatment
plant R¨uttehof. The high P-event loads in the Bergsee catchment are
in the majority of cases not due to diffuse P-losses from agriculture, but a
caused mainly from instream sources. Hydrologic connectivity is consequently
a necessary, but not sufficient condition for water bound diffuse P-losses
from agricultural land. High event water fractions are not defenitely leading
to high P-losses – even in cases of fertilisation of riparian grassland during
or shortly after intensive rainfall.