Catchment Changes Effect On Future River Flow

Land-use changes on a catchment scale can lead to variation in catchment water yield and event response. Reduction or change in vegetation and management techniques disturbs the water balance within a catchment by changing parameters such as soil and water storage, which can have adverse effects on the productivity of farmland. Within New Zealand economic pressures to increase farming productivity will consequently increase farming intensity. Along with future land-use modification New Zealand’s water resources will face added pressures from projected climate change. Projected changes of 0-30% in precipitation rates and +0.6 oC to +2.2 oC in temperature, will alter hydrologic responses around the country. The aim of this research is to use a semi-distributed watershed model to determine the impact farming intensity will have on river flow in the Kakanui River in North Otago, particularly in future climates. Fourteen years of climate and river flow data from the Otago Regional Council were used to develop a semi-distributed watershed model for the Kakanui using SWAT. Due to the flexibility of SWAT it was chosen as a suitable tool to investigate the effects of land use change on the Kakanui catchment, specifically on water yields and low flows. Three land-use scenarios were used, current state: no modification to land-use, intensive farming: doubling of farming practises, and severe intensive farming: quadruple of farming practises. Through the use of data from 1997-2011 the SWAT model showed 25% decrease in monthly flow within the intensive farming scenario, and a further 56% decrease in monthly flow for the severe-intensive farming scenario. Nine CMIP3 climate models were used to modify Kakanui’s climate data from 1997-2011 to project future changes in 2085-2099. Without any change to land-use in the Kakanui catchment the effects of climate change will decrease flow by 27%. If farming practises are intensified over this time there will be a 50% decrease in monthly flow, and if the catchment and river resource is pushed to its farming intensification limit the decrease in monthly flow will be 70%. The Kakanui River is a valuable resource for irrigation and household use, this reduction in hydrologic response will in turn affect the productivity of farming within the area. Adaptive strategies have been put in place and there are schemes to move water into the Kakanui catchment for irrigation; though the introduction of an intergraded catchment scheme, such as Motueka, would help provide targeted adaptive strategies for the Kakanui and insure the future of this valuable resource