| Separating streamflow into its components is valuable for understanding the sources and
flowpaths of water and solutes in catchments, in particular nutrient flowpaths. Tracers give an
objective basis for hydrograph separations, but such tracer data is usually quite limited in
time even if available for a catchment. A new separation method (the bump and rise
method or BRM, Stewart 2014) gives a filter that can be calibrated by fitting to
tracer separations and then applied to the whole streamflow record. Or if no tracer
data is available, can be calibrated more approximately by fitting to the recession
hydrograph.
The value of the procedure is illustrated by applying it to Toenepi Stream, which drains a
lowland dairy farming catchment of 15.1 km2 in Waikato, New Zealand. Tracer (chemical
and tritium) measurements show that streamflow is made up of three major end-members or
components in varying proportions: high-nitrate quickflow, young nitrate-bearing fast
groundwater from a shallow aquifer, and old nitrate-free slow groundwater from a deeper
aquifer. Hydrographs of these three components were determined by applying the BRM filter
twice, once to the streamflow and then again to the baseflow. The results show that
(1) quickflow responds rapidly to rainfall but contributes only a minor part of the
stream peak, (2) fast groundwater also responds rapidly and contributes most of
the stream peak, and (3) slow groundwater shows little immediate response but
begins a very gradual rise in contribution after rainfall. By assuming constant nitrate
concentrations for the three components, the continuous variation of nitrate in the streamflow
was calculated and showed good agreement with spot streamflow measurements.
Nitrate concentrations reached very low levels during very low flows when the
stream was dominated by the slow groundwater, and increased with flow as the
proportions of quickflow and fast groundwater increased. The BRM method was
flexible enough to enable separation of the streamflow into three components, and
the procedure gave results comparable to the modelling study of Woodward et al.
(2013).
References
Stewart, M.K. 2014: New baseflow separation and recession analysis approaches for streamflow.
Hydrol. Earth System Sci. Disc. 11, 7089-7131. doi:10.5194/hessd-11-7089-2014
Woodward, S.J.R., Stenger, R., Bidwell, V.J. 2013: Dynamic analysis of stream flow and
water chemistry to infer subsurface water and nitrate fluxes in a lowland dairying catchment.
J. Hydrol. 505, 299-311. |