Modelling the hydrological impact of afforestation in hill country catchments in New Zealand

Abstract

Study regionThree hill-country catchments in Waikato region, New Zealand Study focusLand use change (LUC), particularly the conversion of pastoral grazing land to plantation forestry, can significantly impact stream hydrology. Measuring the hydrological impact of LUC is time consuming and expensive. Modelling approaches provide a cost-effective means to determine and understand the mechanisms behind the impact of LUC on stream hydrology. In this study, we assessed the hydrological impact of LUC, mainly in the form of conversion of grazed pasture to plantation forest by using the Soil and Water Assessment Tool (SWAT). The effect of LUC on hydrology was evaluated by comparing model results under two scenarios: with and without LUC. New hydrological insights for the regionSWAT predicted the magnitude, occurrence, and variation of streamflow well at both daily and monthly time steps, before and after LUC periods. SWAT reported a significant reduction in streamflow due to converting pasture to pine plantation, which was mainly caused by increased evapotranspiration and reduced surface runoff. Predicted groundwater flow was affected by LUC but did not have a clear and consistent change. Our results are consistent with previous studies assessing the hydrological impact of afforestation, including one observation-based study from the same study site. This study illustrates the usefulness of dynamic hydrological modelling to evaluate LUC impact on hydrology, which is rarely available in New Zealand.