Effect of urban development on water balance in the Southern River catchment

[摘要] EXECUTIVE SUMMARYThe Southern River catchment is one of the fastest developing areas of Perth. Local urban development is challenged by shallow groundwater and multiple wetlands, some of which are of high conservation value. It is expected that the future land use alteration may significantly affect the current hydrological cycle. As inundation and shallow watertables are incompatible with much urban infrastructure, it is expected that the urban development will include implementation of watertable control measures.A project funded by the Western Australia Water Foundation aims to evaluate the likely impact of the changes in land use on water fluxes and water quality within the Southern River catchment. This report examines the effects of urbanisation on the hydrological cycle. A coupled surface water-groundwater model, MODHMS, was used to simulate hydrological and hydrogeological processes on a catchment scale. The MODHMS model, initially developed for the entire Southern River catchment (Barr and Barron, 2009), was applied to a sub-area incorporating the Wungong Urban Waters (WUW) area, i.e. the newly proposed urban development.The main objectives of the undertaken research were to define: •changes in surface outflow from the WUW area after the development is completed •sustainable groundwater abstraction rates for a non-potable water supply in the development.The WUW sub-model was used to examine the effect of land and water management variables and their combinations including:•shallow groundwater table control measures using only subsurface drains or a combination of subsurface drains and surface fill•density of urban development, including high (R35), medium (R25) and low (R17.5) urban density •effect of groundwater abstraction for non-potable water use in the new development for irrigation of public open space (POS) and domestic gardens and for indoor use for toilet flushing and hot water use in laundry The WUW area is located in close proximity to the Darling Scarp. Wungong Brook and four tributaries flow through the development area. The modelling results demonstrated that in pre-development conditions the water balance in the area is dominated by rainfall and evaporative losses. The volumetric runoff from the WUW area in pre-development conditions is extremely low (4% annual rainfall) and occurs within the winter months. The WUW area contribution to Wungong Brook is low, composing 4–7% of the annual brook discharge.These results suggest that the current discharges from the WUW area are unlikely to be accurately estimated. No gauging station exists immediately downstream from this area. The available flow data comes from occasional in-stream flow measurements. However, the low estimated contribution from the WUW area falls below the accuracy of the in-situ flow measurements in the stream channels, which according to the Western Australian Department of Water is greater than 15% of recorded instantaneous discharge rate. It was demonstrated that when urban development takes place, the magnitude of the urbanisation impact on the catchment fluxes is most strongly influenced by urban density, the extent of local water resources used for non-potable water supply and climate variability. Overall, urbanisation of a catchment, where groundwater table is shallow and occurs within 1–3 m of the ground surface, can significantly influence volumetric discharge from the area. This is related to both high flow seasons (winter) and low flow seasons (summer baseflow). However, typically summer baseflow does not occur in pre-development conditions within the Southern River catchment on the Swan Coastal Plain except where the channel is deeply incised close to the outlet to the Canning River.The changes in fluxes are linked with the effect of urbanisation on subsurface evaporative losses. Subsurface evaporation is a dominant water balance component inpre-development conditions but is reduced from 73% of infiltration to 13% of infiltration after urbanisation takes place.Up to 90% of pre-development evapotranspiration losses were losses from the shallow groundwater table, which was most significant during the period when the groundwater table was the shallowest (August to October). The requirement to control shallow watertables resulted in a significant reduction in this water balance component as a result of urban development.Urbanisation also leads to an increase in infiltration rates mainly due to roof runoff being directed to the subsurface. This, alongside a reduction in evaporative losses, causes a significant increase in net groundwater recharge. This effect is most significant for high density urban development.Consequently, the increase in net recharge triggers a greater drainage discharge, but it also results in an increase in available groundwater resources for local non-potable water supply. High rates of groundwater abstraction allow reduced drainage discharge from the urbanised area, and lead to groundwater table drawdown during the summer months. This drawdown generates an additional storage capacity in the local aquifer for increased recharge during wet seasons. The two simulated shallow watertable control measures, namely subsurface drains only and a combination of subsurface drains and surface fill, had similar effects on drainage discharge. The subsurface drains only scenarios show slightly greater annual average volumetric runoff (5%). This difference mainly occurred in the summer months when monthly volumetric runoff for the subsurface drains only scenario was on average double the discharge associated with the fill and subsurface drains combination. During the high flow period in winter the monthly discharge from the WUW area was similar for both scenarios. The modelling presented in this report reflects the future changes in the catchment and as such cannot be directly verified at this stage, therefore in order to validate the model outcomes, the modelled discharge from the WUW area was compared with the observed data in similar catchments within the Perth metropolitan area. The modelled discharge is similar to the discharge in the Bayswater and Mills Street drains at between 180 and 380 mm/annum/m2.The modelling examined the effect of WUW urbanisation on surface water discharge in the catchment. It was shown that•Wungong Brook annual average volumetric runoff at the outflow from the WUW area is likely to increase by between 50% to nearly 200% •Southern River annual average volumetric runoff at the outflow from the catchment is likely to increase by between 5% to 95%Such an increase in river discharge may be used as a basis for the estimation of water quality targets for the new urban development. In order to achieve a zero net increase in nutrient load from the catchment, the nutrient concentrations should be reduced proportionally to the increase in the river flow. The sustainable yield within the WUW under the selected bore field design (64 bores) varies across the area. The abstraction rate in an individual bore is greatest (6 L/s) in the areas where the superficial aquifer is characterised by a higher hydraulic transmissivity in the west and north-west, and is smallest (2 L/s) in the south and south-east. The sustainable yield increases with wetter climate and higher urban densities.

[发布机构] CSIRO

[学科分类] 地球科学(综合)