『Abstract
Phosphorus (P) and suspended sediment (SS) can impair surface
water quality. Recent work purports that the majority of loss
comes from a small part of the catchment where areas of high potential
for supply (source) and transport (e.g., surface runoff) overlap.
However, these areas, termed critical source areas (CSAs) may
also be small enough to enable the targeting of mitigation practices
so that the approach is cheaper and more efficient than managing
whole catchments. We aimed to determine CSAs in two headwater
catchments. Firstly, we measured P fractions and SS in baseflow
and stormflow in sub-catchments on the premise that CSAs were
most active during stormflow. Using stormflow data we then aimed
to determine the contribution to streamflow P and SS load from
saturation- and infiltration-excess areas and the utility of five
hydrological models to isolate transport areas which would also
help define CSAs. While concentrations were greater in summer-autumn,
loads were greater in winter-spring. Stormflow loads accounted
for on average >60% of P and SS loss, with the largest storms
accounting for >75% of stormflow load. Data collected from samples
located in infiltration-excess areas indicated that stream P and
SS load during small events were dominated by infiltration-excess
surface runoff, whereas larger events included more saturation-excess
surface runoff. Utilising estimates of flow from five hydrological
approaches for modelled storms and empirical equations for P loss
from various sources (e.g., surface runoff from soil and dung),
the topographic index combined with infiltration-excess areas
gave the best estimate of P loads in the stream. While large events
accounted for most P loss, we may not be able to manage for them
since they involve most of the catchment. Furthermore, since they
occurred in winter, they may have little effect on surface water
quality parameters such as periphyton growth in summer. Since
during small storms most saturation-excess surface runoff occurred
within a short distance either side of the stream channel, the
simplest approach to mitigating P and SS losses would be to target
mitigation strategies to infiltration-excess areas like gateways
and water troughs and fence-off animals from near-stream areas.
Keywords: Hydrology; Infiltration-excess; Saturation-excess; Tracks;
Lanes』
Introduction
Materials and methods
Catchment descriptions and management
Flow and water sampling
Flow and water quality analysis
Estimation of critical source areas (CSAs)
Results and discussion
Phosphorus and sediment losses in baseflow and stormflow
Isolation of critical source areas
Preliminary modelling of CSAs of P loss to headwater streams
The merit of CSA management
Conclusions
Acknowledgements
References