Stream baseflow preservation with optimal aquifer management

Abstract

The public water supply in the Gosford-Wyong area of New South Wales, Australia, is reliant on streams that originate in elevated sandstone country. About half of the stream flow is believed to be baseflow from the sandstone aquifer system in the Kulnura-Mangrove area. At the same time as the population is growing steadily on the coast, there is increased demand for groundwater for horticultural. agricultural and industrial purposes along the sandstone ridges. Hence, good groundwater management is critical, to ensure that stream baseflow is not jeopardised. The study area consists of nine catchments and is located north of Sydney and inland from Gosford with an area of about 1,400 km2. Baseflow has been estimated for seven flow gauges located at the creeks by applying a digital filtering algorithm to separate the baseflow from the total stream flow. The groundwater hydrographs for 20 monitoring bores show strong correlation with residual rainfall mass, which suggests that rainfall recharge provides the major control on aquifer behaviour. Hydrographs at the same location show that, under natural conditions, there is a huge vertical head difference between layers of alternating sandstone (as much as 30 metres). A management model that couples a simulation model (MODFLOW -SLRFACT) with an optimisation model (OPTl MAQ) has been developed to preserve stream baseflow, MODFLOI,V -SURFACT was selected 10 simulate the complex multi-layer Kulnura-Mangrove aquifer system. The model has an area of 40 km x 59 km with approximately 400 m difference in elevation. The model was divided into 30 flat layers that reflect the alternation between sheet and massive facies in the Hawkesbury Sandstone Formation. A uniform cell size 500 m x 500 m results in a grid mesh of 118 rows and 80 columns. The model was calibrated for both steady state and transient conditions. The results of steady-state calibration revealed that the model perfonms very well in representing the values and the patterns of the composite groundwater level contours map. Also. the results showed a good agreement between the observed and computed target values across all the model layers with a coefficient of determination of 0.994. The transient model started in January 1985 and ended in October 2003 with a monthly stress period, The results of the transient calibration illustrated that the model matched very well with all observed hydrographs, even in the areas that have high vertical head difference, Also, the results showed a good agreement between the estimated baseflow and that simulated by MODFLOW -SURFACT for all the flow gauges. OPTIMAQ software, based on generic optimisation software (GAMS). solves the management problem with linear or nonlinear objectives by using the response matrix approach. OPTIMAQ was linked successfully with MODFLOW -SURFACT to compose the management model for the multi-layer aquifer system in the Kulnura- Mangrove area. The main target for the management model is to preserve baseflow in the creeks by determining the optimal limits on groundwater extraction from the existing or planned bores. The objective function of the management model is to maximise the pumping rates of the bores, subject to groundwater level constraints imposed along the creeks. In effect, this approach determines a sustainable yield for the aqui fer system that is compliant with surface water constraints.