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Bishopdale, New Zealand

Fraser C.E.,Aqualinc Research Ltd | Mcintyre N.,Imperial College London | Mcintyre N.,University of Queensland | Jackson B.M.,Victoria University of Wellington | And 2 more authors.
Water Resources Research | Year: 2013

Metamodeling uses computationally efficient models to emulate the outputs of complex models, trading off computational time against prediction accuracy and/or precision. Although potentially powerful, there is limited understanding of the uncertainty introduced by the metamodeling procedure. In particular, the errors associated with transformations of the predictions, such as aggregations during upscaling or differences between results used for impacts analysis, have not been explored in the metamodeling literature. We present an application of metamodeling that upscales physics-based model predictions to make catchment scale predictions of land management change impacts on peak flows. Two parallel sets of simulations are conducted, one with the original physics-based models and the other with metamodels. Despite good performance in emulating the local scale physics-based model simulations, once incorporated into a catchment scale model and especially once impacts of change are calculated, errors associated with the metamodeling procedure alone become significant, accounting for almost half of the prediction uncertainty in peak flows. The additional (metamodel-contributed) uncertainty is introduced both through biases in peak flows and through increases in peak flow variance. In the context of land management impacts, the results demonstrate the importance of tracking propagation of errors during upscaling, and of evaluating a model's ability to predict change, as well as independent observations. Despite these errors, the predictions of land management impacts from both physics-based models and metamodels are broadly consistent between each other, and in accordance with expectations from the literature. Key Points Metamodelling used to predict impacts of land use change Metamodelling significantly reduces computational time Metamodel errors increase when transformations on the predictions are performed ©2013. American Geophysical Union. All Rights Reserved. Source

Booker D.J.,NIWA - National Institute of Water and Atmospheric Research | Snelder T.H.,Aqualinc Research Ltd | Greenwood M.J.,NIWA - National Institute of Water and Atmospheric Research | Crow S.K.,NIWA - National Institute of Water and Atmospheric Research
Ecohydrology | Year: 2015

Robust relationships between biological characteristics and hydrological indices are required to provide a quantitative basis for environmental flows. Data from 1075 river sites distributed across New Zealand were used to investigate relationships between invertebrate communities and flow regimes, whilst also including the influence of additional environmental factors. Variance decomposition analysis was used to investigate the proportion of variance explained by hydrological, geomorphological, land cover, and catchment characteristics for the community matrix and each of three biotic indices representing taxon richness, Macroinvertebrate Community Index, and percentage of species in the Ephemeroptera, Plecoptera, and Trichoptera orders. Results showed that hydrological regime contributes a unique component to the explainable variation in the community, but this contribution is overestimated if other explanatory factors are not considered. A gradient forest model comprised of 93 random forest models, each predicting the probability of occurrence of a taxon, indicated the importance of high flows and also other dimensions of hydrological variation for predicting invertebrate taxa and biotic indices. Although many freshwater invertebrates in New Zealand are well adapted to a range of flow conditions through resistance traits and/or rapid colonization, this study suggests that several aspects of the flow regime influence invertebrate communities. These results suggest that environmental flows may be designed to sustain or even optimize specific ecological attributes or taxa, but changes along several dimensions of hydrological variability are likely to disadvantage other taxa and change invertebrate community composition. © 2014 John Wiley & Sons, Ltd. Source

Burbery L.F.,Institute of Environmental Science and Research | Burbery L.F.,Y Ventures | Flintoft M.J.,Institute of Environmental Science and Research | Flintoft M.J.,Aqualinc Research Ltd | Close M.E.,Institute of Environmental Science and Research
Journal of Contaminant Hydrology | Year: 2013

Five re-circulating tracer well tests (RCTWTs) have been conducted in a variety of aquifer settings, at four sites across New Zealand. The tests constitute the first practical assessment of the two-well RCTWT methodology described by Burbery and Wang (Journal of Hydrology, 2010; 382:163-173) and were aimed at evaluating nitrate reaction rates in situ. The performance of the RCTWTs differed significantly at the different sites. The RCTWT method performed well when it was applied to determine potential nitrate reaction rates in anoxic, electro-chemically reductive, nitrate-free aquifers of volcanic lithology, on the North Island, New Zealand. Regional groundwater flow was not fast-flowing in this setting. An effective first-order nitrate reaction rate in the region of 0.09 d- 1 to 0.26 d- 1 was determined from two RCTWTs applied at one site where a reaction rate of 0.37 d- 1 had previously been estimated from a push-pull test. The RCTWT method performed poorly, however, in a fast-flowing, nitrate-impacted fluvio-glacial gravel aquifer that was examined on the South Island, New Zealand. This setting was more akin to the hypothetical physiochemical problem described by Burbery and Wang (2010). Although aerobic conditions were identified as the primary reason for failure to measure any nitrate reaction in the gravel aquifer, failure to establish significant interflow in the re-circulation cell due to the heterogeneous nature of the aquifer structure, and natural variability exhibited in nitrate contaminant levels of the ambient groundwater further contributed to the poor performance of the test. Our findings suggest that in practice, environmental conditions are more complex than assumed by the RCTWT methodology, which compromises the practicability of the method as one for determining attenuation rates in groundwater based on tracing ambient contaminant levels. Although limited, there appears to be a scope for RCTWTs to provide useful information on potential attenuation rates when reactants are supplemented to the aquifer system under examination. © 2012 Elsevier B.V. All rights reserved. Source

Penas F.J.,University of Cantabria | Barquin J.,University of Cantabria | Snelder T.H.,Aqualinc Research Ltd | Booker D.J.,NIWA - National Institute of Water and Atmospheric Research | Alvarez C.,University of Cantabria
Hydrology and Earth System Sciences | Year: 2014

Hydrological classification has emerged as a suitable procedure to disentangle the inherent hydrological complexity of river networks. This practice has contributed to determining key biophysical relations in fluvial ecosystems and the effects of flow modification. Thus, a plethora of classification approaches, which agreed in general concepts and methods but differed largely in specific procedures, have emerged in the last decades. However, few studies have compared the implication of applying contrasting approaches and specifications over the same hydrological data. In this work, using cluster analysis and modelling approaches, we classify the entire river network covering the northern third of the Iberian Peninsula. Specifically, we developed classifications of increasing level of detail, ranging from 2 to 20 class levels, either based on raw and normalized daily flow series and using two contrasting approaches to determine class membership: classify-then-predict (ClasF) and predict-then-classify (PredF). Classifications were compared in terms of their statistical strength, the hydrological interpretation, the ability to reduce the bias associated with underrepresented parts of the hydrological space and their spatial correspondnece. The results highlighted that both the data processing and the classification strategy largely influenced the classification outcomes and properties, although differences among procedures were not always statistically significant. The normalization of flow data removed the influence of flow magnitude and generated more complex classifications in which a wider range of hydrologic characteristics were considered. The application of the PredF strategy produced, in most of the cases, classifications with higher discrimination ability and presented greater ability to deal with the presence of distinctive gauges in the data set than using the ClasF strategy. © Author(s) 2014. Source

Barkle G.F.,Aqualinc Research Ltd | Wohling T.,Lincoln Ventures Ltd | Stenger R.,Lincoln Ventures Ltd | Mertens J.,Laborelec | And 3 more authors.
Vadose Zone Journal | Year: 2011

In this technical note we present the design, installation, and evaluation of a field monitoring system to directly measure water fluxes through a vadose zone. The system is based on use of relatively new measurement technology-automated equilibrium tension lysimeters (AETLs). An AETL uses a porous sintered stainless-steel plate to provide a comparatively large sampling area (0.20 m 2) with a continuously controlled vacuum applied under the plate. This vacuum is in "equilibrium" with the surrounding vadose zone tension to ensure measured fluxes represent those under undisturbed conditions. Fifteen of these AETLs have been installed at five depths through a layered volcanic vadose zone to study the impact of land use changes on water quality in Lake Taupo, New Zealand. We describe the development and testing of the AETLs, the methods used for installing these devices, a condensed data set of the measured physical properties of the vadose zone, and the initial results from the in situ operation of the AETLs, including the preliminary results from a bromide tracer test. For an AETL installed at the 0.4-m depth, where soil pressure heads are most dynamic, the average deviation between the target reference pressure head, as measured in the undisturbed vadose zone and the pressure head measured above the sampling plate was only 5.4 hPa over a 180-d period. The bromide recovered in an AETL at the same depth was equivalent to 96% of the bromide pulse applied onto the surface area directly above the AETL. We conclude that this measurement technique provides an accurate and robust method of measuring vadose zone fluxes. These measurements can ultimately contribute to bett er understanding of the water transport and contaminant transformation processes through vadose zones. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. Source

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