Responses of streamflow to vegetation and climate change in southwestern Australia

Southwestern Australia has experienced recent climate change, with an increase in air temperature of 0.6°C and a reduction in mean annual precipitation of -15% since 1970. Along with the warming and drying trends, dramatic declines of streamflow have occurred across the region. However, both forest mortality and an increase in leaf area index have been observed in the southwestern forest, suggesting varied responses of vegetation to climate change. In this study, 30 catchments were analyzed using the Mann-Kendall trend test, Pettitt’s change point test and the theoretical framework of the Budyko curve to study the rainfall-runoff relationship change, and effects of climate and land cover change on streamflow. A declining trend and relatively consistent change point (2000) of streamflow were found in most catchments, with 14 catchments showing significant declines (p < 0.05, -68.1% to -35.6%) over 1970-2000 and 2001-2015. Most of the catchments have been shifting towards a more water-limited climate condition since 2000. For the period of 1970 to 2015, the dynamic of vegetation attributes (land cover/use change and growth of vegetation) dominated the decrease of streamflow in about half the study catchments. In general, a coequal role of climate and vegetation on the decline in streamflow was found in the study, suggesting the importance of vegetation management on future water management and production. Precipitation is predicted to decline in the future; therefore, some forest management intervention is required to maintain forest growth and water supply in the southwest of Australia

The relation between runoff generation and temporal stability of soil macropores in a fine sandy loam

During rainfall events, macropores are generally considered to play a dominant role in infiltration after matrix ponding has occurred. Once ponding has been initiated on the soil matrix, surface runoff may be generated at rainfall intensities less than the saturated hydraulic conductivity of the soil. The amount of runoff will depend on detention storage and how efficiently the surface flow is captured by soil macropores. The efficiency of surface water removal by macropores is diminished if surface vents become clogged sealed by washed-in sediment during the runoff event. Post-event opening of surface vents by the animals that created them can remove evidence of the sealing process and so it is particularly important to examine the temporal stability of the soil surface during rainfall events. In this paper evidence of macropore clogging and post-event clearing of the surface vents is presented. A fine sandy loam passed through a 2 mm diameter sieve was packed into two boxes, each with a surface area of 0.5 m2. The boxes were irrigated at 28 mm h-1 using a low energy rainfall sprinkler. This application rate exceeded the saturated hydraulic conductivity of the soil matrix. After measuring runoff and infiltration from the boxes, one box was held as a control and the second was inoculated with earthworms. After four weeks the inoculated box had a burrow density at the soil surface of 380 m-2, with an average diameter of 5 mm. Macropore sealing occurred immediately after ponding and runoff from the macroporous soil was only 10.7% less than a control with no macropores. Within 24 h after cessation of simulated rainfall the earthworms had cleared washed in material from over 95% of burrow vents. Time to matrix ponding was well predicted using hydraulic parameters characteristic of the soil matrix, indicating that matrix sealing was not significant under the experimental conditions

Analysis of water flow from cylindrical macropores

Water flow from cylindrical macropores of finite length is predicted using an approximate analysis derived previously for flow from a steady constant-head uncased borehole permeameter in homogenous unsaturated soil. This analysis is appropriate in cases where the macropores fill rapidly and remain completely water filled. Under these conditions predicted macropore flow rates compared well to those obtained from measurements of flow into both 'artificial' and natural macropores. The study emphasises the effect of pore continuity, with pore length exerting a dominant influence on the magnitude of macropore volume fluxes over the range of macropore diameters commonly encountered in agricultural soils

Soil water movement in relation to soil structure and input conditions

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