title: Effect of land use change on hydrology for large catchments in Central Queensland, Australia
Creators: name: Cheng, Zhuo
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Land-use change
subject: catchment hydrology

description: Land-use change and its impacts on catchment hydrology has been for a long time one of the most important environmental issues around the world. The conversion between forest to other land-use types, i.e. deforestation and afforestation, is one of the most dramatic examples of land use change. Our enhanced understanding of the interaction between water and forest is of strategic importance to decision-making in effective land-use and water-resource management, especially for dry regions where water supply is limited. While extensive research has been undertaken to determine how catchment hydrology responds to forest cover change around the world, no consistent conclusion has been drawn, especially for relatively large catchments (&gt;1,000 km2). For small catchments, experimental methodologies, such as paired catchment studies, were favoured, leading to a general conclusion that deforestation would increase the flow volume and afforestation would decrease the flow. For large catchments, however, diverse and even contradictory conclusions tended to be reached, and hydrological models were probably the most widely used tools for impact assessment compared to other methods including statistical techniques and elasticity analysis. The inconclusive results with respect to the impacts of forest cover change on flow were also obtained for a large catchment in Central Queensland, Australia. Central Queensland of Australia, having a dry sub-tropical climate, witnessed dramatic land clearing from the 1960s to the 1980s resulting from a government sponsored development scheme, known as the Brigalow Development Scheme. More than half of the region (around 79,800 km2) was cleared between 1960 and 1972, with the annual clearing rate of over 6,600 km2/yr. There have been a number of investigations into the effect of land use change on basin water balance at a range of spatial scales, while discrepant conclusions were drawn by two previous studies with respect to hydrological response to land clearing for catchments at a large scale. One of the studies suggested an increase of 40% in streamflow due to land clearing and the other attributed changes in streamflow mostly to climate variability. To determine the impact of large-scale land clearing on catchment hydrology in Central Queensland, two large catchments, namely the Comet and Upper Dawson (area&gt; 15,500 km2), were selected for investigation. For each of the two catchments, three distinct periods, i.e. pre-clearing (1920-1949), transitional (1960-1989) and post-clearing (1989-2018) periods, were identified according to land clearing rates and remanent forest cover. Overall, four research objectives were proposed: (1) to determine the direction and magnitude of the impact; (2) to quantify the impact using suited hydrological models; (3) to interpret the impacts with meaningful land-use-related parameter values and (4) to improve the modelling performance by taking into account of the spatial variability of climate, land use and vegetation cover. To address the first objective, a series of methods, i.e. the Budyko framework, Tanh function, flow duration curve, double mass curve and SimHyd, were applied to provide multiple lines of evidence for determining the impacts. It was concluded that (1) the effect of land clearing on water yield of large basins in Central Queensland are unmistakably detectable; (2) the impact of land clearing manifested itself mostly during wetter years or for wetter basins; (3) for a given basin, the greater the extent of land clearing the larger the effect on water yield. The second objective was addressed by assessing and comparing three hydrological models, i.e. AWBM, Sacramento and SimHyd, in terms of their ability in capturing rainfall-streamflow relationship for the two catchments under change, i.e. pre- and post- land clearing. It was concluded that AWBM outperformed the other two models and can be further simplified as AWBM_Ca. The model parameter, i.e. the average storage capacity (Ca), could be related to land use/cover. The Ca value was decreased by 21- 30% for post-clearing period, indicating the same degree of decrease water amount potentially available for transpiration due to deforestation. While considering the spatial variability of climate and land use/cover, performance of AWBM_Ca was improved, and different Ca values were assumed for differing land use types including cropping, grazing and forest. It was found that the Ca value for forest was higher than those for cropping and grazing by 35-53%, indicating a higher capacity for runoff generation from the catchment when forests were cleared. The distributed AWBM_Ca calibrated for the Comet catchment was validated by predicting flows for 4 nested catchments satisfactorily with the Nash-Sutcliffe coefficient of efficiency ranging from 0.69 to 0.98. For different scenarios of afforestation sequences, the relationship between forest cover change and flow was nonlinear and exhibited a “lily leaf” shaped envelope curve, which was driven by the combined effect of the spatial variability of climate and the “uncertainty” of where afforestation occurs. The “lily leaf” shaped envelope curve led to the conclusion that flow reduction was more sensitive to tree plantation in wetter areas of the catchment and that 10% of afforestation would cause on average a decrease of 5.8% in flow. Overall, this study highlights the need of a better understanding of the relationship between forest cover and water yield for large catchments, with the main findings as follows: (1) an increase in flow due to land clearing has certainly occurred for large catchments in Central Queensland supported with multiple lines of evidence; (2) hydrological models such as AWBM performed well in capturing rainfall-flow relationship to represent the hydrological response to forest cover change; (3) model parameter for AWBM_Ca, i.e. the average storage capacity (Ca), was found to be capable of representing the increased evapotranspiration from the forested area comparing the cropping and grazing areas; and (4) hydrological response to forest cover change is enhanced in relatively wetter areas, and thus the spatial variability of climate and land use need to be taken into consideration for impact assessment.