Advancing IUWM through an understanding of the urban water balance

Integrated urban water management (IUWM) requires the management of the urban water cycle in sympathy with the hydrological water cycle. Urban systems significantly alter the hydrological water cycle. Under natural conditions the water inputs at any point in the landscape are precipitation and overland flows; while the outputs are via surface flows, evapo-transpiration and groundwater recharge. The large volumes of piped water introduced with the change to an urban setting and the introduction of vast impervious areas strongly impact on the water balance, increasing in-flows and dramatically altering the out- flow components. IUWM seeks to change the impact of urban development on the natural water cycle. One approach is to establish an inner, urban, water cycle loop through the implementation of reuse strategies. Developing this urban water cycle loop requires an understanding both of the natural, pre-development, water balance and the post-development water balance. Accounting for flows in the pre- and post-development systems is an important step toward limiting urban impacts on the natural water cycle. This paper investigates urban water balancing and preliminary identification of reuse strategies for the case study of the proposed Tamala Park development in Perth, as the first steps toward an integrated water reuse strategy. The hydrology and hydrogeology of the site are examined. Overland flows are not a feature of the site with rainfall leaving the site as either evapotranspiration or recharge to groundwater. The unconfined Superficial aquifer is identified as suited to aquifer storage and recovery. Changes to the site water balance due to urbanisation include the significant inflow and outflow of piped water. It is projected that the small allotment sizes proposed would have a beneficial impact on garden water use and hence piped inflow. Water accounting for the developed case showed a decrease in evaporation and an increase in recharge. Reuse strategies should therefore aim to exploit the excess recharge - through groundwater abstraction and roof runoff capture - as a component of an inner urban water cycle loop. Volumetrically, this excess could supply projected in-house uses. Further, if wastewater was reused for irrigation demands the need for mains supply could be eliminated and wastewater discharges halved.