Hydrological indicators of flow in headwaters for assessing farm management impacts: Streamside forestry management case study

Hydrological indicators have been used for interpreting stream flow patterns in large catchments. It would be useful to evaluate such indicators for use also in headwater catchments close to the source of on-farm management impacts. Reforestation of cleared farmland is encouraged internationally for water quality and biodiversity improvements. One practice option is to establish fast-growing plantations in streamside management zones (SMZs), but impacts on stream flows are not well understood. A eucalypt plantation was established in a steep headwater catchment in temperate Australia and stream flows compared 2 years pre- and 4 years post-establishment with an adjacent unplanted catchment. The flow ratio of SMZ to control was never significantly different from one (no SMZ effect) despite a significant decreasing trend during the six-years of monitoring. The decrease in flow ratio was attributable mainly to a reduction in some higher flows resulting from increased surface roughness and interception of overland flow. Nonparametric analyses also indicated a reduction of some low flows due to plantation establishment. Overall, changes in flow patterns appeared minor, but the value placed on such changes will depend on social and environmental contexts. Because of the rarity of such data and the importance of understanding water flows as affected by plantations established on farmland, similar studies should be conducted elsewhere. The range of flow indicators tested in this case were useful and should be considered for other similar applications. In addition, a simulation capability needs to be developed to enable the effects of SMZ plantations on stream flows to be predicted for a range of climate, landscape and management contexts and to contribute further to policy and practice discussions.