4. The need for an integrated catchment-wide perspective on riparian management
4.1 Hydrologic pathways
An understanding of hydrological pathways is critical to determining buffer strip effectiveness. For example, the key nitrate pathway upon porous pumice soils in the central North Island of New Zealand is vertical, down to groundwater, which may take several decades to emerge into surface waters. In this case, the nitrate predominantly bypasses riparian vegetation and is difficult to mitigate using conventional riparian management (Howard-Williams and Pickmere 1999). Determining overall buffer effectiveness, therefore, not only requires an understanding of the attenuation efficiency with respect to nutrients washed into the buffer, but also quantification of the nutrient load that bypasses the buffer. Both pieces of information are required across the catchment to fully evaluate buffer effectiveness.
A further example of the importance of hydrological pathways is found on flat or gently sloping dairy land. Typically, these areas are underlain by artificial drains that feed a network of open drains discharging directly to streams. The subsurface drains reduce surface ponding and runoff by aiding infiltration. Studies within the Toenepi catchment, Waikato have shown that the N-load leaving the catchment outlet can be accounted for by the sum of all drainage inputs to the stream network (R. Wilcock-NIWA personal communication). In other words, the presence of riparian buffers at Toenepi would be ineffectual (other than preventing direct access to streams) at attenuating Nitrogen delivery to waterways.
Williamson et al. (1996) predicted that total phosphorus loads of the Lake Rotorua catchment had been reduced by 20% after implementation of the Upper Kaituna Catchment Control Scheme, in one of the few studies that has extrapolated specific yield estimations of nutrient and sediment reductions from retired riparian margins to a whole catchment estimation of whether riparian management was effective in improving water quality. The measures included tree plantings on erosion-prone hillslopes, preservation of wetlands and lake margins, and retirement and planting of stream riparian zones. The study took data from the Ngongotaha stream catchment , which showed reductions of 85% for sediment and 26-40% for nutrients and scaled up the findings to the whole catchment. The reduction in phosphorous loads was expected to reduce the chlorophyll a concentration by enough to shift the lake’s trophic status from eutrophic to mesotrophic. The reduction in phosphorus was achieved by the improved land management, despite hydrological characteristics of the Ngongotaha catchment where springs from deep ground water contain naturally high levels of soluble P. However, dissolved N concentrations were higher after control measures were implemented, partly due to an increase in nitrate concentrations in the deep groundwater bypassing riparian zones.
4.2 Changing functions with stream size
In a catchment context, it has been suggested that maximum water quality benefits will occur if buffer strips are located along headwater reaches, partly because most of the water in a catchment originates in the headwaters (Fennessy & Cronk 1997). Many small wetlands are distributed throughout the upper reaches of catchments, providing denitrification and uptake of soluble pollutants. Small streams are intimately linked to their riparian zones and riparian buffers in these systems will achieve many of the benefits of shading and nutrient filtering. As streams get larger, e.g., rivers, their main interaction with the riparian zone are when flood waters over top the banks. Riparian vegetation in buffers is then more useful for slowing flood flows, rather than shading or filtering functions. The spatial pattern of riparian planting has, therefore, a clear influence upon overall buffer effectiveness.
4.3 Shade impacts on nutrient removal and sediment
Streams convert inorganic nutrients (nitrogen and phosphorus) to instream plant biomass under stable flow conditions. Given the same nutrient inputs, a shaded stream can be expected to retain less nutrient as plant biomass than an unshaded stream. Thus, as noted by Rutherford et al. (1999), restoration of shade through riparian planting can change the way streams transform and process nutrients, and lead to increased transport of inorganic nutrients downstream. In streams dominated by macrophytes (i.e., where their biomass is much greater than that of algae) it would be reasonable to infer that they will have a much greater influence on nutrient removal than algae (B. Wilcock, NIWA, pers. comm.). Typically, plant uptake of N varies with stream size; headwater streams are better processes of N than larger channels. Consequently, the impact of shading will vary with stream size, supporting the need for a catchment wide holistic approach to riparian management.
Pasture streams in New Zealand typically display marked seasonal changes in dissolved nutrient concentrations that reflect seasonal growth of the streambank and aquatic vegetation (Howard-Williams et al. 1986). A long-term study of Whangamata Stream draining into Lake Taupo has clearly demonstrated how dissolved nutrient levels can fluctuate in response to changes in instream plant biomass as riparian plantings grow (Howard-Williams & Pickmere 1994, 1999). This study recognised 3 phases in changes to water quality over 24 years following riparian planting:
Years 1-5 – an initial moderate decline in dissolved nutrients (30-50% for NO3-N and 10-60% for DRP) for 1-2 months in summer as channel vegetation increased (mainly watercress).
Years 5-13 – very high dissolved nutrient removal (up to 100% for NO3-N and DRP) for 4-5 months of the year due to the proliferation of plants that do not die back in winter (mainly monkey musk).
Years 13-24 – decreasing nutrient removal capacity as increased levels of shade limited the biomass of light-requiring plants.
Davies-Colley (1997) found that 2nd order Waikato streams were wider in native forest than in pasture catchments, and that the streams formerly in pasture catchments that were now covered in mature pine plantations had actively eroding streambanks (Fig. 5). This observation raised the concern that riparian planting along pasture streams could lead to the mobilisation of stored sediment if stabilising streamside grasses are shaded out, and that if this occurs there could be a period of increased water turbidity, streambed sedimentation and sediment export until the channels reach a new equilibrium. However, it is expected that the ultimate (forest) channel width will be much more stable than under pasture where appreciable bank erosion occurs during floods.
The findings of Davies-Colley & Quinn (1998), who compared stream widths and light climates for a range of streams throughout the northern North Island, provide wider-scale qualified corroboration of the phenomenon of stream narrowing in pasture catchments. Davies-Colley (1997) reviewed several studies from overseas which suggest that the phenomenon of channel narrowing in pasture is a general feature of formerly forested stream channels.
Collier et al. (2001) estimated that the total mass of sediment stored in streambanks in the 250 ha Mangaotama catchment near Hamilton is about 13,000 tonnes, equivalent to around 21 years of current annual sediment yield (assumed to be from hillslope sources). Forecasts of the mass of sediment exported following riparian planting in this catchment (assuming eventual doubling in channel cross-sectional areas consistent with Davies-Colley 1997) suggest that, over a 25-year timescale, there would be an increase in sediment yield compared to the status quo as stream channels widened in response to shaded conditions. Over the longer term, however, once this stored bank sediment has been exported, banks are expected to stabilise as channels reach new steady-state ‘forest’ morphology and sediment yield will eventually decline to a lower level than currently experienced.
Figure 5: Change in stream channel width from native forest (A) to pasture (B), where pasture grasses trap sediment resulting in narrow and incised channels. (see Davies-Colley 1997).
The significance of these studies for riparian management are that if a whole catchment perspective is not considered when implementing forested buffers, then shade may cause problems downstream if there are estuaries or lakes sensitive to sediment and nutrients.
In a conceptual modelling exercise, Parkyn et al. (unpubl. data) investigated concerns that riparian management could lead to increased yields of nutrients and sediments. A simple model of the trade-off between interception of nutrients in runoff by forest buffers versus reduction of in-stream uptake due to shade, predicted that a buffer strip alongside a small headwater stream would reduce nutrient export, while a buffer strip instigated as an isolated patch alongside a larger stream (> c. 2.5 km2 upstream catchment size) would increase nutrient export, as the relative amount of nutrients trapped by the buffer decreases as the nutrient load present in the stream water increases. However, in these larger streams with width exceeding approx. 6 m, sufficient light may reach the streambed for plant and algal growth, which in turn would promote instream nutrient processing. At the peak of streambank erosion after planting, predicted total sediment yield (hillslope plus bank sources) was appreciably higher than the hillslope pasture yield, but sediment yield stabilised c. 35-40 years after planting. When planting was extended over 40 years in the model, the sediment yield never exceeded that in pasture before planting. This conceptual modelling exercise highlighted the problems associated with implementing riparian tree planting programmes in a piecemeal fashion and concluded that planting should commence in the headwaters and progress downstream to avoid nutrient yield increases. To avoid or reduce peak losses of sediment downstream, riparian planting would need to be implemented slowly or some riparian management options such as grass filter strips or spaced plantings may need to be investigated if downstream environments are sensitive to sedimentation.
Contact for Enquiries
MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
Contact this person
