Motueka River Research Articles

Sustainable water resource management using surface-groundwater modelling: Motueka-Riwaka Plains, New Zealand

Abstract New Zealand’s freshwater resources in coastal regions are under increasing pressure from demands for drinking, irrigation and industrial uses, and climate change effects, particularly sea level rise. The management of water resources needs to balance values associated with (but not limited to) economy, society, ecology, culture, recreation and water quality. To assist the management of water resources in the Motueka-Riwaka Plains, New Zealand, a numerical integrated surface-groundwater model was developed. Calibration of the transient integrated surface-groundwater model was achieved with a root mean square error (RMSE) of 2.6% for groundwater levels. Model verification against groundwater levels not used in the calibration also resulted in an RMSE of 2.6% for groundwater levels. Model outputs, coupled with groundwater trend analyses, were used to identify the impact of groundwater abstractions on rivers, streams and groundwater-fed springs. Trend analyses showed that groundwater levels are lowering in the central area of the plains. Model scenarios predict that additional groundwater abstraction of over 30,000 m3/day from a well field located near the central area of the plains (adjacent to the Motueka River) is unlikely to increase the risk of saltwater intrusion and will result in surface water flow depletion within acceptable limits.

Sharing stakeholder knowledge across water management boundaries and interfaces: experiences from Australian and New Zealand ‘HELP’ basins

As water management issues have grown and become more connected, the need to engage civil society and incorporate a wider range of community knowledge in decision-making is increasingly recognised. This paper discusses experiences of three river basins that are part of the UNESCO-IHP Hydrology for the Environment, Life and Policy programme. In each, water management issues cross different kinds of ‘boundaries’. At the Ord River, north-western Australia, investment in irrigation expansion and social infrastructure is driving the need for more comprehensive water planning and management incorporating a new set of economic, social and ecological values, new knowledge sources, and more collaboration with the neighbouring jurisdiction. In the lower Burdekin, north-eastern Australia, sugar cane irrigators need to reduce their impact on local groundwater, wetlands and adjacent Great Barrier Reef. And at the Motueka River, on New Zealand’s South Island, an 11-year Integrated Catchment Management programme sought solutions to the impacts of upstream land use on downstream water quality. While none of the basins physically crosses an international or national boundary, they can all be considered transboundary waters. These examples show that many of the challenges experienced in relation to international transboundary resources are replicated at other scales and in other ways: across internal borders, through institutional confines, across environmental interfaces, between economic sectors and around a range of social norms. Understanding the various boundaries can help identify a more comprehensive and inclusive suite of stakeholders, enabling their interests and knowledge to be incorporated into decision-making. Sharing knowledge across these boundaries is critical to developing the mutual understanding necessary to support better water management and more equitable benefit-sharing from available water resources.

Nitrogen, phosphorus and E. coli loads in the Sherry River, New Zealand

The Sherry River, a tributary of the Motueka River (Tasman District, upper South Island, New Zealand), has somewhat degraded water quality related to a high proportion of the catchment being used for pastoral agriculture (which comprises 30% dairying). We undertook a campaign to estimate loads of phosphorus, nitrogen and Escherichia coli for comparison with other dairy-impacted catchments and against which to measure future reductions in pollutants in the Sherry River, expected from changed management practices. Monthly water quality sampling in the Sherry River was augmented with storm chasing for one year (October 2008 to October 2009). Loads of pollutants in events increased disproportionately with event peak flow, probably because the proportion of overland flow increases systematically with increasing event size. Stormflows (occurring 9.5% of the time) transported 33% of the total flow from the Sherry River, and between 22% (for nitrate) and 92% (for E. coli) of pollutant loads.

RIVERBED DIGITAL ELEVATION MODELS AS A TOOL FOR HOLISTIC RIVER MANAGEMENT: MOTUEKA RIVER, NELSON, NEW ZEALAND

ABSTRACTRiver management in New Zealand's laterally active gravelly rivers has permitted floodplain development and protection of agricultural resources and infrastructure. Management of these dynamic systems has been hailed as a success for the approaches adopted, namely straightening and confining the river using bank protection and managing riverbed levels by gravel extraction. However, this activity also impacts river morphological/habitat diversity and potential gravel resource, by replacing broad riparian corridors with narrower channels and reducing lateral connectivity with the floodplain. This paper quantifies river behaviour in three laterally confined reaches in the upper Motueka River over a 7‐year period, using annual high‐resolution ground surveys to address the nature of morphological change and associated sediment flux in these reaches with a view to informing management of the gravel resource. Surveys between 2004 and 2010 acquired data to construct digital elevation models (DEMs) of the active riverbed in three ~1‐km‐long reaches. Morphological budgeting based on differencing between successive DEM surfaces reveals complex spatial and temporal patterns of erosion and deposition, demonstrating complex reach dynamics. Overall, volumetric changes suggest these narrowed reaches have been net exporters of sediment, associated with continued channel degradation. This has left bar features, traditionally the focus of gravel extraction in the reaches, relatively isolated from all but extreme flows, limiting replenishment of the gravel resource. The paper demonstrates the utility of riverbed DEMs as a potential tool to frame river character and behaviour at the reach scale in gravel‐bed rivers, thereby providing an important contribution to holistic river management in these systems. Copyright © 2012 John Wiley & Sons, Ltd.

Spatial delineation of the depositional footprint of the Motueka River outwelling plume in Tasman Bay, New Zealand

Progress towards development of a river plume ecosystem basis for management of coastal resources requires a thorough understanding of the nature and spatial extent of terrestrial influences on receiving water and seabed characteristics. As a case study, we estimate suspended sediment (SS) loading rates from the Motueka River catchment into Tasman Bay, New Zealand and describe benthic characteristics along a series of transects encompassing the coastal river plume. A mineral-rich geological formation in the headwaters of the river was identified as a storm-generated source of highly elevated concentrations of nickel and chromium in river margin sediments, and coastal sediments extending >5 km offshore. A major storm in 2005, focused in the upper catchment, resulted in an estimated SS discharge of 161,000 tonnes into the Bay. Spatial gradients of a suite of sediment trace metal signatures, organic content and infauna community structure were used to define a river plume depositional footprint of c. 180 km2. We suggest that the boundaries of the Motueka catchment be redefined to include this area in order to facilitate engagement of marine stakeholders in management decisions that may affect coastal biological resources.

Motueka River plume facilitates transport of ruminant faecal contaminants into shellfish growing waters, Tasman Bay, New Zealand

Hydrographic and water quality surveys of the Motueka River and its river plume were conducted during a moderate flood event (peak flow of 420 m3/s) to assess the source and fate of faecal contaminants transported into Tasman Bay. Escherichia coli (E. coli) and enterococci concentrations in the river were up to 10000 and 7300 Most Probable Number (MPN)/100 ml during peak flow, respectively. A coastal survey revealed a shallow low-salinity plume that extended at least 6 km into Tasman Bay and the region's largest aquaculture management areas (AMAs). Mussels within the influence of the river plume, including those collected within an AMA, had E. coli and enterococci concentrations as high as 1300 and 2200 MPN/100 g tissue, respectively. Application of microbial source tracking markers using end-point PCR assays identified the presence of faecal contamination from ruminant animals (cows, sheep) in water and mussel samples. The detection of ruminant faecal contamination within shellfish located 6 km offshore highlights the close connection between land use and the quality of New Zealand's highly valued coastal resources.

Variation in nutrient loading from the Motueka River into Tasman Bay, New Zealand, 2005–2009: implications for the river plume ecosystem

The monthly mass transport of nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), total nitrogen (TN), dissolved reactive phosphorus (DRP), total phosphorus (TP) and dissolved reactive silica (DRSi) from the Motueka River catchment into Tasman Bay (South Island, New Zealand) was estimated for the period 2005–2009 based on observed historical relationships between recorded river flows and the associated measured concentrations of each nutrient. The resulting average annual loading estimates for TN, TP and DRSi ranged from 397–829, 25–72 and 4053–6576 tonnes, respectively, over the 5-year period. Large seasonal variations in nutrient loadings were also estimated with the highest rates generally, but not always, occurring during winter. The observed patterns suggest that phytoplankton biomass within the river outwelling plume may also vary in response to seasonal and inter-annual variations in catchment nutrient loading rates with potential follow-on food web effects including the growth of cultured GreenshellTM mussels. The estimated average annual TN loading rate (2005–2009) from the Motueka River catchment (613 tonnes) is only c. 20% of the annual loss of nitrogen based on extrapolated literature values for denitrification rates in Tasman Bay. We therefore suggest that, under the present land use scenario in the Motueka catchment, there is little potential for resulting dysfunctional river plume ecosystem enrichment effects to occur.

Modelling storm-event E. coli pulses from the Motueka and Sherry Rivers in the South Island, New Zealand

Storm-induced Escherichia coli pulses in the Motueka River (2074 km2) and the Sherry River (78.4 km2) are modelled. The model focuses on the catchment outlets, representing key processes, including E. coli transfer to and from the river bed, with account taken of the hysteresis in, and non-linear, non-stationary, response of E. coli concentrations to river stormflows. The model fits the Motueka River observations well, but less well in the Sherry River. A greatly simplified description of headwater and riparian inputs is satisfactory at the larger catchment scale where near-field, in-channel processes dominate the response. Spatial heterogeneity in rainfall-run-off and faecal sources probably contribute to the poorer fit in the smaller catchment. Despite using a relatively small number of driving variables and parameters, the model has the potential to predict real-time E. coli input to Tasman Bay in river plumes causing shellfish and bathing beach contamination.

Sediment yield response to large storm events and forest harvesting, Motueka River, New Zealand

Suspended sediment yields (SY) were measured at seven sites in the Motueka catchment to characterise major influences on sediment yield variation and estimate sediment delivery to Tasman Bay. Relationships between storm event SY and peak flow were used with long-term flow records to calculate SY. At large catchment scale, there were clear differences in SY related to geology and rainfall, but the major influence on SY was the impact of a >50-year return period storm that caused an increase in event SY ranging from 10–20 times in the headwaters to 2–3 times at the coast. Forest harvesting was an important influence at small catchment scale, producing a five-fold increase in event SY. The magnitude of the SY increase, the time to recover and the total area affected were all smaller than the impact of large storms. Annual SY at the coast was extremely variable, ranging from 49,000 t to 1.7 Mt.

Integrated catchment management—a decade of research in the Motueka River catchment

The need for more holistic approaches to land and water management has long been recognised (Cullen 1990). These approaches have a common focus on research and management, which recognises that lan...

Establishing native plants in a weedy riparian environment

Riparian zones are important for influencing stream habitat and water quality. Efforts to populate these areas with native species are frequently hindered by fast-growing weed species. A trial was installed to examine riparian establishment alongside the Sherry River in the Motueka river catchment with seven native species and four weed-control treatments. After 2 years, herbicide use significantly improved seedling survival compared with the other weed-control treatments, but weed mats promoted greater seedling height and crown diameter growth. Seedling survival and growth varied significantly with species. After comparisons of costs and practical issues, weed mats were concluded to provide the most suitable weed control. Cordyline australis had the highest survival rate, developed the most crown area and performed well in terms of height increment; Pittosporum tenuifolium and Plagianthus regius also performed well. All three native species are recommended for use in further plantings at this and other similar locations.

Influences of geology and land use on macroinvertebrate communities across the Motueka River catchment, New Zealand

The influences of geology and land use on macroinvertebrate communities were investigated in the Motueka River catchment, New Zealand. Comparisons of functional feeding groups, multivariate community composition and biotic indices (MCI, QMCI, %EPT) were made between native forest stream reaches that differed in subcatchment geology (ultramafic, hard sedimentary, granite, gravel, karst) and between stream reaches surrounded by different land uses (native forest, exotic forest and pasture) within two of these geologies (granite, gravel). Differences in invertebrate community density and taxon richness were greater across the three land uses in the same geology than among the five geologies. Macroinvertebrate communities in streams flowing through native forest had greater numbers of shredders, and higher MCI scores than pastoral streams. Exotic forest macroinvertebrate communities were similar to the native forest communities in granite geology, but distinct from both native and pastoral communities in sites with underlying gravel geology. The results demonstrate that there is potential for underlying geology to influence the magnitude of land use impacts on invertebrate communities. The geologies within a catchment should be accounted for when considering the consequences of land development on river and stream communities.

Recruitment sources of brown trout identified by otolith trace element signatures

This study examined whether element:Ca ratios within the otoliths of juvenile brown trout could provide accurate trace element signatures for specific natal tributaries, and attempted to match these to trace element natal signatures found within the otoliths of adult trout caught in the main stem rivers of the same catchment. The trace element signatures of juvenile trout otoliths were analysed from a sample of eight tributaries representing the main sub-catchments of the Motueka River catchment, New Zealand. Trace element signatures were determined using laser ablation inductively coupled plasma mass spectrometry, and differentiated using linear discriminant function analysis with an overall cross-validated classification success of 96.8%. Temporal stability in element:Ca ratios was investigated by repeat collections of juvenile fish over two years. Natal signatures from 11 of 23 adult trout sampled from the catchment main stems were matched to one of the eight tributary signatures showing recruitment sources to be spread relatively evenly throughout the catchment. This study demonstrates the potential of using otolith trace element analysis to determine the natal origins of freshwater fish within a catchment.

The influence of natural variation in discharge on juvenile brown trout population dynamics in a nursery tributary of the Motueka River, New Zealand

The effects of natural flow variation on juvenile brown trout population dynamics were investigated by biannual sampling over 5.5 years in the Rainy River, a tributary of the Motueka River. A large flood in late March (50-year return period) substantially reduced the density (by 66%) and biomass (by 73%) of 0+ trout over autumn–spring, but the cohort responded with compensatory survival to achieve similar density and biomass by spring as in other years. A low-flow event in February–April (return period >8.4 years), when 7-day low flows fell to 56% of the 7-day mean annual low flow (MALF) and were less than the MALF for 46 days, had no adverse affect on the population. We found no evidence for density-dependent growth. However, there was strong evidence for a two-phase self-thinning response in density, with no self-thinning occurring over summer (i.e. the 0+ population remained below carrying capacity) until a threshold mass of 22.08 g (length=123.7 mm) was attained in autumn after which severe self-thinning took place over autumn to spring. The results indicate that over spring–autumn the population is insensitive to flow reduction and that over autumn–spring the effects of high (and probably low) flow events on local abundance and biomass are offset by compensatory (density-dependent) survival. However, effects on the contribution of migrants to the downstream population remain unknown. The study identified ecological redundancy, which could be exploited for flow allocation. Significantly, it has shown that minimum flows equivalent to the MALF (often advocated by New Zealand conservation and fisheries management organisations) are not always necessary for sustaining juvenile trout populations.

Integrated catchment management research: lessons for interdisciplinary science from the Motueka Catchment, New Zealand

Integrative research projects are becoming more common and inherently face challenges that single-discipline or multi-disciplinary projects seldom do. It is difficult to learn what makes a successful integrative research project as many of these challenges and solutions often go unreported. Using the New Zealand Integrated Catchment Management (ICM) for the Motueka River research program, we reflect on the demands confronting research programs attempting to operate in an integrative interdisciplinary manner. We highlight seven key lessons that may help others learn of the benefits and difficulties that confront scientists and stakeholders involved in undertaking similar research. These are (1) clarify the goal and work with key people; (2) manage expectations; (3) agree on integrative concepts and face the challenge of epistemology; (4) leadership; (5) communication in an atmosphere of mutual trust and respect; (6) acknowledge that different modes of learning mean that a wide range of knowledge products are needed; and (7) measure and celebrate success. The recognition that many environmental problems can only be solved through the creation of new knowledge and through social processes that engage the research and management domains has been a major benefit of the research program.

Stormflow dynamics and loads of Escherichia coli in a large mixed land use catchment

AbstractStorm events are major transporters of faecal microbial contaminants, but few studies have reported storm loads or concentration dynamics in relation to discharge or other pollutants, notably fine sediment. Episodically, high loads of faecal contamination during storm flows impact downstream uses of water bodies, particularly contact recreation and shellfish harvesting. We examined the storm dynamics of Escherichia coli, turbidity and discharge in the mixed land use Motueka catchment (2047 km2; 60% forest and 19% pasture) to gain insights into E. coli sources and transport. We also explored different approaches for calculating E. coli loads. Discharge and field turbidity were recorded continuously, and E. coli concentrations were sampled during events, over a 13‐month period near the mouth of the Motueka River. E. coli loads were estimated by interpolation, averaging estimators and by using linear regression with smearing correction of the log‐transformed variables: discharge, turbidity, and both turbidity and discharge. The annual E. coli load was dominated (∼98%) by export during events. Comparison of monthly monitoring with the intensive storm monitoring campaign suggests that simple stratification of the sampling into storm and baseflow would greatly improve export estimates. E. coli peak concentrations always preceded discharge and turbidity peaks (which had similar timing). Turbidity can be a useful surrogate for faecal microbes in smaller catchments, but in the Motueka turbidity was no better for predicting E. coli concentration than discharge. Runoff from grazed pasture and direct deposition from livestock are probably the ultimate E. coli sources in the Motueka catchment. However, in‐channel stores seem to dominate E. coli dynamics during events and account for the typical feature of bacterial concentrations peaking ahead of discharge and turbidity. This study demonstrates the importance of storm events to faecal microbial loads and shows that E. coli concentration dynamics may contrast with those of turbidity. Copyright © 2009 John Wiley & Sons, Ltd.

Importance des interactions entre les eaux de surface et les eaux souterraines pour les modèles thermiques au niveau de biefs et pour le potentiel de croissance des truites: exemple de la rivière de Motueka (Nouvelle Zélande)

To assess whether reaches of the Motueka River (New Zealand) that gain water from groundwater were likely to represent significant cold-water refugia for brown trout during periods of high water temperatures, water temperature was monitored for more than 18 months in two gaining reaches of the Motueka River and three reaches that were predicted to be losing water to groundwater. These data were used to predict brown trout (Salmo trutta) growth in gaining and losing reaches. Groundwater inputs had a small effect on water temperature at the reach-scale and modelling suggests that the differences observed were unlikely to result in appreciable differences in trout growth. Several coldwater patches were identified within the study reach that were up to 3.5°C cooler than the mainstem, but these were generally shallow and were unlikely to provide refuge for adult trout. The exception was Hinetai Spring, which had a mean water temperature of close to 16°C during the period January–March, when temperatures in the mainstem regularly exceeded 19°C. Trout were observed within the cold-water plume at the mouth of Hinetai Stream, which would allow them to thermoregulate when mainstem temperatures are unfavourable while still being able to capitalise on food resources available in the mainstem.

Modelling Impacts of Land Cover Change on Critical Water Resources in the Motueka River Catchment, New Zealand

After the SWAT (Soil and Water Assessment Tool) model was calibrated and validated to historic flow records for the current land use conditions, two additional land cover scenarios (a prehistoric land cover and a potential maximum plantation pine cover) were used to evaluate the impacts of land cover change on total water yields, groundwater flow, and quick flow in the Motueka River catchment, New Zealand. Low-flow characteristics and their potential impacts on availability for water abstraction and for support of in-stream habitat values were focused on. The results showed that the annual total water yields, quick flow and baseflow decreased moderately in the two scenarios when compared with the current actual land use. The annual water balance for the pine potential land cover scenario did not differ substantially from the prehistoric scenario for the catchment as whole. However, there were more notable differences among individual tributary catchments, which could be attributed to the relative area of land cover altered and location of those catchments. Simulated low flows for the prehistoric and potential pine land cover scenarios were both significantly lower than the low flows for the current land use. In summary, under the current land use conditions, both annual water yield and low flow are higher than was the case before human intervention in the area or in a maximum commercial reforestation scenario.

Comment on Cao W, Bowden BW, Davie T, Fenemor A. 2006. ‘Multi‐variable and multi‐site calibration and validation of SWAT in a large mountainous catchment with high spatial variability’. Hydrological Processes 20(5): 1057‐1073

Recently, a manuscript was published with the objective to complete a multivariable and multisite calibration and validation for the Soil and Water Assessment Tool (SWAT) model on the Motueka River basin by Cao et al. (2006). Physically based distributed parameter models, such as SWAT, have become a preferred tool to predict watershed response not only at the watershed outlet, but also at intermediate points within the watershed. Calibrating and validating such models before they can be applied to make watershed decisions is always a challenge. A methodology to perform multisite and multiparameter calibration should be of interest to a wide rage of stakeholders working in the area of watershed management. However, upon reviewing this paper, we felt two topics warranted additional discussion.

Tidal circulation in Tasman and Golden Bays: Implications for river plume behaviour

An investigation into the tidal residual circulation in the Tasman/Golden Bays system was performed. This investigation involved collecting hydrographic and current meter data, and configuring a numerical circulation model. The model was able to reproduce the gross features of the major tidal flows within the system revealed by the data. The simulated tidal residual circulation conformed to that previously suggested, except along the western side of Tasman Bay where this investigation suggests that residual flows move northwards. The validated model was then used to produce simulations of the fate of the plumes from the major rivers. Salinity field simulations describing different river flow and wind direction/velocity scenarios suggest that the Motueka River plume can cover considerable proportions of western side Tasman Bay, extending into Golden Bay during flood conditions. The embedded sediment transport model was also used to investigate the fate of fine sediments entering the bays. Simulated distribution patterns of fine sediments entering the bays from the four major tributaries were found to be consistent with existing bathymetric and seabed substrate characteristics.