Assessment of Climate Change Impact on Water Balance of Forested and Farmed Catchments

Water balance modeling is commonly used to quantify the impacts of climate change on water availability over a region or a catchment. Under climate change, significant variability in precipitation and evapotranspiration would dramatically affect the catchment water balance. Changes in soil and vegetation also have large impacts on water resources. However, current water balance modeling is mainly dependent on precipitation, while evapotranspiration is a fixed proportion of precipitation. Also, the interaction of the various phases of rainfall-runoff transformation within the soil is not fully computed. This paper for the first time investigates the combined effects of precipitation and evapotranspiration on the water balance of three typical forested and farmed catchments in the Waikato basin of New Zealand. A conceptual lumped water-soil model is employed to simulate the land phase of the hydrological cycle including soil moisture and ground water recharge from rainfall and evapotranspiration at catchment scale for both historical and future time slices. Observation data from 1971 to 2000 are used for model calibration. Future data up to year 2090 is obtained from a model. Future simulations are projected accordingly. The results show that changes in precipitation and especially potential evapotranspiration have a large impact on daily streamflow even though they do not much affect runoff volume. Streamflow is projected to dramatically decrease in 2030, 2060, and 2090 in the grassed catchments, while an inconsiderable reduction is found in the forested catchment. Even though bias correction is used to improve the accuracy of the potential evapotranspiration and the resulting catchment runoff, other errors are addressed but not yet resolved. They originate from regional climate model (RCM) outputs, scenarios, data observation, and interpretation as well as model performance.