Dynamic groundwater-river interaction model for planning water allocation in a narrow valley aquifer system of the Upper Motueka catchment

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1 Dynamic groundwater-river interaction model for planning water allocation in a narrow valley aquifer system of the Upper Motueka catchment Timothy Hong t.hong@gns.cri.nz Gilles Minni g.minni@gns.cri.nz GNS Science, New Zealand Tim Davie Tim.davie@ecan.govt.nz ECAN, New Zealand Joseph Thomas Joseph.thomas@tdc.govt.nz TDC, New Zealand ABSTRACT: This example shows a 3D transient groundwater river interaction model developed to assets the effect of groundwater abstraction on river stages. INTRODUCTION The Upper Motueka catchment is a 887km2 area of fertile river terraces suitable for irrigated agriculture (Figure 1). There has been an increasing demand for irrigation water, especially groundwater, in this catchment since the 1990s. The Upper Motueka catchment includes a shallow unconfined alluvial aquifer located on a Quaternary river terrace. Similar to many regions in New Zealand, the Upper Motueka aquifer is strongly connected to the associated surface water system. New Zealand Regional authorities in charge of water allocation are concerned that an increase in abstraction may affect stream baseflow during summer. GNS Science has developed a FEFLOW 3-D finite-element groundwater-river interaction model (Figure 2). The purpose of this model is to guide utilization of groundwater resources while ensuring acceptable amounts of stream flow during low flow periods. PROBLEM DEFENITION The Upper Motueka (UM) catchment is located on the Northwest part of the South Island in New Zealand (Figure1). The catchment is dominated by mountains and hill country, (about 67% of the catchment has slopes greater than 15 O ). The Upper Motueka Catchment is composed of three main river valleys: Motupiko River (344 km 2 ), Tadmor River (124 km 2 ) and Motueka River (419 km 2 ). Mean annual rainfall for the catchment is estimated to be 1600 mm with strong spatial variation, related to topography. Groundwater in the catchment is abstracted from a shallow Quaternary alluvial aquifer corresponding to five gravel formations that have been identified within the study area (Stewart, et al, GNS Science report 2003/32). Assumptions made consider the aquifer to be 9-10 meters thick within the northern part and of about meters in the southern part of the Upper Motueka catchment. MODEL DEVELOPPEMENT The model grid covers the entire Upper Motueka aquifer and is descritized 6-node triangular prisms with nodes and finite-element meshes. During mesh generation we refined the mesh to be finer along the river network and take into account the location of wells as additional points. Mesh refinement along the rivers allows better resolution as our study is focused on interactions between groundwater and surface water (Figure 2). Incorporating well locations during mesh generation ensured nodal location at well coordinates. Ground elevation was generated using a 20 meter resolution DTM, also used to approximate the bottom of the model aquifer according to geological data (Stewart, et al, GNS Science report 2003/32). Hydraulic conductivity values were estimated from slug test and constant rate pump test analysis provided by Tasman District Council these are summarized in Table 1. At some localities where tests were not undertaken, the hydraulic conductivity was assumed to be the same as at the closest hydraulic test site, or an average of several nearby sites. Rainfall recharge was calculated using a daily soil water balance model (Davie, 2007). Three different recharge zones (P1, P2, and P3) were identified based on soil types and then were divided

2 between irrigated and non-irrigated areas. The rainfall recharge model considers a total of six different rainfall recharges rates over the catchment (Table 2 and Figure 3). Groundwater abstraction was implemented using FEFLOW s 4 th type flow boundary for the 39 wells identified within the catchment (Figure 4). Daily groundwater abstraction at each well was estimated by Landcare Research (Davie, 2007) considering land use pattern over the simulation period. The river system in the model domain is composed of three rivers: Motueka River, Motupiko River and Tadmor River. Six river stations were used to extrapolate daily river stages over the catchment. Rivers stages are implemented in the model domain using a head-dependent, third-type Cauchy s boundary condition within the model area. Four small streams coming from the hills are defined as a single point river and are implemented using a head-dependent, third-type Cauchy s boundary. Landcare identified eleven hill slope zone where seepages infiltrate and directly recharge into the aquifer. The eleven hills slopes are implemented as a flux boundary condition. MODEL CALIBRATION The model has been calibrated with a daily time step based on 5 groundwater monitoring wells for the one year period of 1 July 2001 to 30 June The model was then evaluated for the period of 1 July 2002 to 30 June The model prediction shows that drawdowns at five groundwater monitoring wells during the irrigation season as well as winter recovery of groundwater levels due to rainfall recharge closely simulate actual data for the one year evaluation period. Figure 5 illustrates an example of calibration site results. MODEL S AIM The Upper Motueka groundwater model is capable of simulating complex groundwater-surface water interactions occurring in the catchment. River networks within the catchment were identified as main recharge source for the Upper Motueka aquifer. A series of river gaining-losing sections were observed to determine the relative sensitivity of the aquifer to abstraction. Management scenarios are currently being carried out to determine the impact of changes in water allocation. Modifications of the current water allocation will affect river flow differently over the study area. Climate change can also be simulated and studied trough the model recharge package or river stages. Our study shows the importance and usefulness of a well-calibrated regional-scale groundwater-river interaction model for water management, when used for assessing water availability and vulnerability to groundwater abstraction. Further development of a dynamic river interaction model incorporating a river flow model developed in Mike11 is currently being undertaken to simulate the dynamic exchanges of water between the shallow aquifer and stream, and to evaluate the effects of different hydrological conditions and changes land use pattern. SUMMARY The Upper Motueka aquifer strongly interacts with the associated surface water stream. New Zealand Regional authorities in charge of water allocation are concerned that an increase in groundwater abstraction may affect stream baseflow during summer. A FEFLOW 3D transient groundwater-river interaction model was developed to asset the effect of groundwater abstraction on river stage. The model was calibrated in a daily time step based on 5 groundwater monitoring wells over a one year period. Further developments currently undertaken are being made to couple the existing FEFLOW model to a 1D river flow model using MIKE11. ACKNOWLEDGEMENTS This study was conducted in a cooperative project between GNS Science, Landcare Research and thetasman District Council (TDC) as a component of the Integrated Catchment Management (ICM) project of Landcare.

3 REFERENCES Davie, T Personal communication, Landcare Research, Christchurch, New Zealand. Hong, T., Thomas, J. and Davie, T River-aquifer interaction modelling in the Upper Motueka River catchment: three-dimensional finite-element groundwater flow model, Motueka Integrated Catchment Management (Motueka ICM) Programme Report Series, Landcare ICM Report No /02. Stewart, M., Hong, T., Cameron, S., Daughney, C., Tait, T. and Thomas, T Investigation of groundwater in the Upper Motueka River Catchment, Institute of Geological and Nuclear Sciences Report 2003/32. Wells ID X Y GWL RL (m) Surveyed Ground Level (m) Well Depth (m) Hydraulic Conductivity (m/d) Table 1 Hydraulic conductivities at well locations Recharge (mm/day) P1 P2 P3 Irrigated Non-irrigated Annual recharge rates (% of rainfall) P1 P2 P3 Irrigated Non-irrigated Total rainfall for year P1 P2 P Table 2 Rainfall recharge used within the model

4 Figure 1: Location of the Upper Motueka catchment (New Zealand) and model boundary Figure 2: Finite element structure looking at Motupiko and Motueka rivers confluences (Black points represent 3 of the 5 monitoring sites used to calibrate the model; grey points represent 2 of the 5 gauging stations used)

5 Figure 3: Delineation of six rainfall recharge areas within the catchment Figure 4: Location of 39 abstraction wells modeled (black points). Grey triangles represent the 5 monitoring sites used to calibrate the model

6 Figure 5: Calibration results found at Hyatts well