Modelling of temperature depression in rivers downstream from storages

Management of the water released from dams, particularly the temperature of this release, will have direct impacts on the ecological health and biodiversity of regulated rivers downstream of the dams. Water released from the hypolimnion of the storage can decrease maximum summer temperatures and diurnal temperature ranges which may adversely affect the aquatic ecology. These impacts can be reduced using multilevel storage offtakes; however, these structures can be costly, and their efficacy needs to be assessed. The NSW Department of Land and Water Conservation (DLWC) has developed a model that can investigate a river system?s thermal response to variable inputs. The DLWC?s Integrated Quantity Quality Model (IQQM) can be applied to any river system, and operates on a time step of one day or less to simulate river system behaviour. IQQM has been implemented to manage water quantity and salinity issues in river systems in NSW and Queensland, and has now been upgraded to simulate daily water temperature. Thermal loading in IQQM is calculated as the sum of heat exchange from river inflows, river outflows, short and long wave radiation, evaporation and surface conduction/convection. Assumptions in the model are that transport processes are significant only in the main flow direction; mixing is complete and instantaneous within each computational reach, and advection can be accounted for using hydrological routing and lagging. This paper reports on how temperature modelling was implemented in IQQM, a process that included a review of other water quality models with temperature capability. Calibration, testing and validation of the model were undertaken using temperature data downstream of Windamere Dam in the Macquarie River System. Information from this study will be used to develop decision support tools for operation of multilevel offtakes and to investigate the effect of storage and stream management policies on the stream thermal regime.