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There are numerous studies reported in literature which investigate the impacts of land use/land cover change and climate change on catchment water availability and there is sufficient evidence that afforestation can reduce streamflow substantially. The most commonly used methods for estimating impacts of plantations on water availability are catchment experiment method, statistical analysis method and the hydrological modelling method. Most of the studies reported in literature normally use either one of the sensitivity-based approach or a hydrological model with few actually comparing the impact results from these different approaches. This paper investigates the impacts of increase or decrease in plantations and climate variability on streamflow using two approaches: the sensitivity-based approach (seven methods) and the hydrological modelling approach (two models) for three medium sized catchments in Australia. The results from the different methods show that both plantation expansion/reduction and climatic differences can have major effects on catchment streamflow. There is a small variability in the reduction or increase in streamflow estimated by the nine methods. The results from hydrological modelling are compared to those from the sensitivity-based methods. For all the three catchments, when compared to the hydrological modelling results, the Budyko based approaches overestimate the reductions in streamflow due to increase in plantations and underestimate the streamflow reductions due to drier climate. The results from the non-parametric approach are similar to those from hydrological modelling for the Crawford River and Darlot Creek catchments but the non-parametric approach underestimates the streamflow reductions due to increase in plantations and overestimates the streamflow reductions due to drier climate for the Tinana Creek catchment. When comparing the results for reduction in plantations for the Crawford River catchment, the Xinanjiang hydrological model underestimates the increase in streamflow due to reduction in plantations and overestimates the streamflow increase due to wetter climate when compared to the results from the other eight approaches. For the Darlot Creek catchment, the results from all the nine methods are similar and for the Tinana Creek catchment, the non-parametric approach underestimates the increase in streamflow due to reduction in plantations and overestimates the streamflow increase due to wetter climate when compared to the results from the other eight methods. The results from this study show that the estimates of plantation impacts from the dynamic hydrological models are similar to those from the commonly used sensitivity-based approaches. The sensitivity-based approaches are only applicable where long term data sets are available and they only provide results at a mean annual time scale. The hydrological models simulate plantation impacted streamflow time series and so they can be used to estimate the relative contributions of land cover changes and climate change/variability at a daily, monthly or annual time-step. The outputs from the hydrological models can also be used to investigate the impacts of plantation expansion or reduction and climate change/variability on different runoff characteristics.

Comparison of land-cover change and climate variability impacts on runoff using hydrological modelling and sensitivity-based approaches