Pseudo-transient groundwater-stream interaction model for determination of the effect of groundwater abstraction on spring-fed stream flow in the Poukawa basin, Hawke's Bay

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Cameron, S.G.; Gusyev, M.A.; Meilhac, C.; Minni, G.; Zemansky, G.M. 2011 Pseudo-transient groundwater-stream interaction model for determination of the effect of groundwater abstraction on spring-fed stream flow in the Poukawa basin, Hawke's Bay. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/07 76 p.

Abstract: A steady-state groundwater flow model of the Poukawa basin (Poukawa Stream catchment) has been developed using FEFLOW software. The model was primarily developed to simulate the cumulative effects of groundwater pumping from wells in the south-eastern area of the basin on groundwater levels, spring-fed stream flow and groundwater discharge into Lake Poukawa, under mean monthly low-flow conditions. A pseudo-transient model was constructed from the calibrated steady-state model. In the pseudo-transient model all boundary conditions are steady-state with the exception of groundwater pumping. Inclusion of transient groundwater pumping allowed for the assessment of a more realistic worst-case groundwater pumping duration (90 days) on the catchment water budget, compared to steady-state pumping. The model was calibrated to observed groundwater levels, sub-catchment flow rates in the southern area of the basin, flow in the Poukawa Stream at Stock Rd and mean monthly minimum low-flow in the Poukawa Stream at Douglas Rd (45 L/s or 3,888 m3/d). The model was also calibrated to observed drawdown in Brownrigg Agriculture (BA) irrigation wells and to observed stream depletion effects due to the pumping (1–2 days) of these wells. The pseudo-transient model indicated that worst-case, simultaneous, 90-day pumping from four wells (12,879 m3/d) in the south-eastern area of the model will cause negligible (5 m3/d) reduction in groundwater discharge to Lake Poukawa, but will cause surface water flow into the lake to decrease by about 13% (1055 m3/d) under low flow conditions. While the model was not calibrated to flow in tributaries of the Poukawa Stream downstream of Douglas Rd, modelling results indicate that worst-case scenario pumping will reduce Poukawa Stream flow at Stock Rd by about 10% (1215 m3/day) under low flow conditions. The pseudo-transient model has been significantly revised from an earlier steady-state model of the Poukawa basin reported by Minni et al. (2010). This latest model has also been used to assess the effect of all consented groundwater pumping in the basin at the request of Hawke’s Bay Regional Council. The results of these model simulations are reported separately in Cameron and Gusyev (2011). (auth)