Modelling the potential of integrated vegetation bands (IVB) to retain stormwater runoff on steep hillslopes of Southeast Queensland, Australia

Rainfall intensity is predicted to increase under a changing climate, leading to increased risks of hillslope erosion, downstream sedimentation and flooding. For many catchments used for grazing and agricultural land uses, it will become increasingly important to maintain ecohydrological functioning despite climatic extremes. One means to achieve this is through strategic reforestation using locally endemic species, in spatial configurations that effectively intercept, retain or and redistribute overland flows. This paper adopts a modelling approach for investigating the potential of one such design termed “integrated vegetation bands” (IVB), to increase the retention of runoff across steep hillslopes, particularly in the sub-tropics where rainstorms are becoming increasingly intense. A spatially distributed simulation model (MIKE-SHE) was applied to a steep, grazed catchment (Maronghi Creek catchment, Southeast Queensland, Australia) to compare stormwater runoff characteristics between: (1) the existing pasture land cover; and (2) a series of hypothetical IVB added across this pasture land. The IVB were approximately 20 m wide, and configured at 5% gradient towards ridgelines. Results for estimates of overland flow depth and infiltration (spatial), and accumulative water balance (temporal), confirm that the area of hillslope retaining > 10 mm/day more runoff increased by 22% under IVB compared to the pasture land use. Excluding the IVB themselves, the area of hillslope where runoff retention increased was 11%. During the most intense rainfall, IVB held up to 25% greater water depth and had 10% greater infiltration at the hillslope scale. At the sub-catchment scale, discharge decreased by 7% and infiltration increased by 23%. The findings for sub-tropical landscapes presented here are consistent with studies conducted in temperate regions. Based on the results of this preliminary modelling work, the IVB concept has been established as a paired-catchment field trial in a high rainfall catchment in Southeast Queensland, Australia

Model design for the hydrology of tree belt plantations on hillslopes

When selecting or developing a model to use for research it is important that the model structure and complexity meet the objectives of the research while avoiding problems from overparameterisation. In this paper a procedure is outlined for the selection or development of a model to be used to assist in locating and designing tree belt plantations on hillslopes. Sensitivity analysis and field data interpretation are used to define the important hillslope properties and processes occurring at a field site in southern New South Wales. These are combined with the research objectives to identify the model requirements for further study on tree belt plantations. A brief review of potentially suitable models available reveals that no single model meets all of the requirements. It is concluded that field data should be used to develop a simple cascading bucket model for hillslope hydrology using a top-down approach

Assessing the value of trees in sustainable grazing systems

The retention of trees in strips provides an option for managing non-remnant woody vegetation in native and sown pastures in northern Australia. However, the impact of tree strips on pasture production has not been previously researched in detail in southern Queensland. The influence of existing tree strips on pasture production in southern Queensland was measured at three grazing properties during 2004 and 2005. Soil and pasture attributes were sampled along transects 80 to 300 metres in length positioned perpendicular to tree strips. The tree strips ranged from 15 to 75 metres wide and were 120 to 500 metres apart. The effects of tree strips along the pasture transect were quantified in terms of pasture microclimate (e.g. temperature, humidity and, at one location, wind), pasture growth in grazed and exclosed situations, soil water, soil nutrients and condition, and nutrient availability. An experimental approach using exclosed pasture transects provided a useful ‘bioassay’ potentially integrating beneficial and competitive effects of tree strips on pasture growth as well as other factors (e.g. soil variability). Averaged across two locations and two years, the competitive effects of the tree strip were compensated to some extent by enhanced pasture growth at distances of 1-6 x tree height from the tree strip edge. However, the observed effects on pasture growth along the transect were likely to be due to different causes: pasture microclimate at one site, soil texture and microtopography at a second site and pasture establishment history at a third site. Thus, the trial highlighted the difficulty of attributing effects in real-world situations, given the number of possible causes including the tree strip effects on pasture microclimate and nutrient availability, soil surface disturbance, and systematic variation on soil and water redistribution due to soil micro-topography and felled timber. Despite these many sources of variation, general effects were derived from the field data consistent with other studies on tree strips and wind breaks across Australia. To extrapolate the project results to other locations, tree strip configurations and climates, a new version of the soil waterpasture growth simulation model GRASP was developed allowing simulation of tree and pasture effects and processes for various distances along a pasture transect perpendicular from the tree strip