Predicting the effect of forest cover changes on flow duration curves

Paired catchment studies have been widely used as a means of determining the magnitude of water yield changes resulting from changes in vegetation. This thesis reviews the use of paired catchment studies for determining the changes in water yield at various time scales resulting from permanent changes in vegetation. Comparisons are made between paired catchment results and a mean annual water balance model and show good agreement between the two methodologies. Four types of paired catchment studies are considered in the thesis and the results highlight the potential underestimation of streamflow changes if regrowth experiments are used to predict the impact of permanent alterations to forest cover. The literature review summarises seasonal changes in water yield, highlights the proportionally larger impact on low flows, and demonstrates the potential for using FDCs to gain a greater understanding of the impact of vegetation on the distribution of daily flows.

The literature review highlighted that different methods used to assess the impact of forest cover make general conclusions difficult to draw. A consistent analysis of data from 46 paired catchment studies assessed annual streamflow and FDCs. The results indicate that it takes between 8 and 25 years for a catchment to reach a new equilibrium following a permanent change in forest cover. Analysis of the FDCs in the afforestation and deforestation experiments showed that the responses could be grouped into three categories. These are: (1) catchments that a change from perennial to ephemeral, or ephemeral catchments have a change in the number of zero flow days, (2) perennial catchments that have a proportionally larger reduction in low flows compared to high flow, and, (3) perennial catchments have a uniform reduction in all flow percentiles.

Forests use more water than grass. This statement has implications for how we manage land-use change and water resources. This thesis uses paired catchment studies to understand how alterations in forest cover affect streamflow. This understanding is used to develop a model for predicting how a catchment’s flow duration curve (FDC) will change following a change in forest cover. The model has been tested on catchments undergoing a permanent change in forest cover and has proven to be robust.

The analysis of paired catchment studies demonstrated that FDCs could alter in different ways following a change in forest cover. To develop a predictive methodology for adjusting a catchment’s FDC for forest cover change a five-parameter model was developed to describe the shape of any FDC. The model proved to provide good predictions for all flow percentiles. To adjust the shape of the FDC for change in forest cover, the parameters of the FDC model for current vegetation conditions are adjusted based on an estimated change in mean annual streamflow. The linkage between the estimated change in mean annual streamflow and the parameters of the FDC model comes from the knowledge that the area under the FDC must be equal to the mean annual streamflow. The FDC adjustment methodology was tested on 17 experimental catchments that have undergone large percentage changes in forest cover and has proved to be a robust procedure. To demonstrate the applicability of this methodology to real world catchments, the method was applied to sub-catchments of the Murrumbidgee River Basin, Australia for two potential forest-cover change scenarios. The adjusted FDCs were used to developed new input time series to the daily river planning model. This allows an assessment of the impacts of forest cover change on water uses at both a local and regional scale.