Dynamic analysis of groundwater discharge and partial-area contribution to Pukemanga Stream, New Zealand

The proportion and origin of groundwater contribution to streamflow from agricultural catchments is relevant to estimation of the effects of nitrate leached from the soil on the quality of surface waters. This study addresses the partitioning of streamflow contributions from near-surface runoff and from groundwater, each with different contributing land area, on a steep pastoral hillslope in a humid climate. The 3 ha headwater catchment of the perennial Pukemanga Stream, in the North Island of New Zealand, was instrumented for continuous observation of climatic data, streamflow and groundwater level. The dynamics of groundwater levels and groundwater contribution to streamflow were analysed by means of a one-parameter, eigenvalue-eigenfunction description of a 1-D aquifer model. Model results for seven years of daily data predict that 36–44% of the topographical catchment contributes groundwater to the stream. The remaining groundwater generated within the catchment contributes to streamflow outside the catchment. Groundwater was calculated to be 58–83% of observed annual streamflow from the topographical catchment. When the smaller groundwater catchment is taken into account, the groundwater contribution to streamflow is 78–93% on a unit area basis. Concurrent hourly data for streamflow and groundwater levels at two sites indicate the dynamic behaviour of a local groundwater system. Groundwater flow dynamics that support the perennial nature of this headwater stream are consistent with the size of the groundwater body, porosity of the subsurface material, and hydraulic conductivity derived from partitioning of streamflow contributions

Multiobjective inverse modeling for soil parameter estimation and model verification

Due to the exponential increase in computational power and increasing awareness of problems associated with vadose zone parameter estimations based on laboratory and in situ measurements during the last decades, the process of automatic model calibration against laboratory or field data is being increasingly used. This is often referred to as inverse modeling and has as a major limitation the inability to identify a single optimal parameter set. A coupled HYDRUS1D-SCE (shuffled complex evolution) simulation global optimization technique was developed and its suitability for multiobjective inverse modeling evaluated. In particular, the trade-off between goodness of fit against leachate volume and soil moisture content in unirrigated and irrigated lysimeters was evaluated. After identification of the most sensitive model parameters using a Monte Carlo based sensitivity analysis, the technique was capable of finding effective Pareto optimal parameter sets that well simulated leachate volume and soil moisture content in both unirrigated and irrigated lysimeters. The parameter variation along the Pareto fronts was large and differences existed between soil hydraulic parameter distributions along the Pareto fronts of the irrigated and unirrigated treatments. The multiobjective optimization technique was then adopted for the verification of the suitability of the conceptual model of equal parameter sets for both treatments. The technique was able to objectively reject the hypothesis of equal parameter sets for both treatments. This is probably due to (i) physical parameter changes with time due to the effect that long-term irrigation has on soil structure, and (ii) differences in acting transport mechanisms between the unirrigated and irrigated lysimeters

Accurate measurements of vadose zone fluxes using automated equlibrium tension plate lysimeters: A synopsis of results from the Spydia research facility, NZ

Automated equilibrium tension plate lysimeters (AETLs) are arguably the most accurate method to measure unsaturated water and contaminant fluxes below the root zone at the scale of up to 1 m2. The AETL technique utilizes a porous sintered stainless-steel plate to provide a comparatively large sampling area with a continuously controlled vacuum that is in “equilibrium” with the surrounding vadose zone matric pressure to ensure measured fluxes represent those under undisturbed conditions

High-frequency, in situ sampling of field woodchip bioreactors reveals sources of sampling error and hydraulic inefficiencies

Woodchip bioreactors are a practical, low-cost technology for reducing nitrate (NO₃) loads discharged from agriculture. Traditional methods of quantifying their performance in the field mostly rely on low-frequency, time-based (weekly to monthly sampling interval) or flow-weighted sample collection at the inlet and outlet, creating uncertainty in their performance and design by providing incomplete information on flow and water chemistry. To address this uncertainty, two field bioreactors were monitored in the US and New Zealand using high-frequency, multipoint sampling for in situ monitoring of NO₃–N concentrations. High-frequency monitoring (sub hourly interval) at the inlet and outlet of both bioreactors revealed significant variability in volumetric removal rates and percent reduction, with percent reduction varying by up to 25 percentage points within a single flow event. Time series of inlet and outlet NO₃ showed significant lag in peak concentrations of 1–3 days due to high hydraulic residence time, where calculations from instantaneous measurements produced erroneous estimates of performance and misleading relationships between residence time and removal. Internal porewater sampling wells showed differences in NO₃ concentration between shallow and deep zones, and “hot spot” zones where peak NO₃ removal co-occurred with dissolved oxygen depletion and dissolved organic carbon production. Tracking NO₃ movement through the profile showed preferential flow occurring with slower flow in deeper woodchips, and slower flow further from the most direct flowpath from inlet to outlet. High-frequency, in situ data on inlet and outlet time series and internal porewater solute profiles of this initial work highlight several key areas for future research

Numerical Analysis to Investigate the Effects of the Design and Installation of Equilibrium Tension Plate Lysimeters on Leachate Volume

ABSTRACT steel porous plates. Various control options for the ten-The composition and quantity of leachate as it moves down through sion applied to these porous plates have been advocated. the vadose zone is seldom measured directly because sampling in this The simplest being zero tension, where no capacity to unsaturated zone at the depths required has proven to be extremely exert a tension on the plates exists. The zero-tension difficult. A promising technique is the use of large porous plates lysimeters rely on the formation of a saturated zone known as equilibrium tension plate lysimeters (ETPLs), which have a controlled suction exerted on them that mimics the soil matric potential measured in the surrounding undisturbed soil profile. In the design phase for the installation of 15 ETPLs at five different depths (three replicates) around a central access chamber in the vadose zone in the Lake Taupo catchment of New Zealand, questions arose regarding the effects of the design and installation layout of the ETPLs above the plate before water can be collected and sam-pled. The requirement of this saturated zone for sam-pling is an artifact of the measurement technique an