In proceedings of 8th International Symposium on EcoHydraulics 2010. September 12-16, 2010 COEX, Seoul, Korea
Riparian management, phosphorus, soluble, leaching, subsurface flow, sediment
Riparian vegetation is established or restored on the basis that it physically filters and traps hillslope derived particulate nutrients in surface runoff. Whilst many studies support this conventional model of riparian function, few test this models embedded assumptions. The assumptions are that catchments are surface runoff dominated, that most surface derived nutrients are transported in particulate form, and that riparian management targets locations that will result in the greatest change in water quality. This paper reviews studies in south west Western Australia that challenge these assumptions. Plots measuring leaching and runoff of nutrients showed that 20 times more water and 2 to 3 orders of magnitude more Phosphorus (P) was transported through leaching than runoff processes. Along with soluble leachate P, most runoff samples were >75% Filterable Reactive P (FRP). A before and after riparian restoration experiment in a small catchment reduced Suspended Sediment (SS) by 90%, had no impact on Total P (TP), increased FRP by 70 % and reduced TN exports by 25%. Prior to restoration FRP leached through sandy soils and entered streams via subsurface pathways, combining with SS to give particulate P signals. Restoring riparian vegetation stabilised streambanks and exhausted SS supply and limited FRP sorption. Whilst SS transport had been stopped, FRP continued downstream. Implications arise for catchments dominated by sub-surface transport pathways through increased bioavailability of P, as well as changes in the N:P ratio of discharging waters. Hillslope experiments measuring the trapping efficiency of sediment and nutrients by grass and trees shows that trapping efficiency of 54% of surface derived TP was trapped by grass buffers compares well with other studies, however, this is discounted to 10% when considering both surface and subsurface transported nutrients. Riparian condition studies show systematic changes in condition with stream order. High order streams (<10% of the total stream length) have good condition riparian cover. Low order streams (~80% of the stream length) have poor riparian condition. Snapshot water quality programs show systematic changes in water quality with stream order, whereby low order streams have higher nutrient concentrations than high order streams. Despite the poor condition, greater representative length and poor water quality of low order streams, riparian management programs focus on high order iconic streams in good condition. These studies show riparian management is unlikely to be effective for P management in these catchments. Most P delivered to streams is soluble and travels via subsurface pathways and the restoration effort is not directed to areas that would make a significant change to water quality.
Weaver DM (2010) Riparian Conundra. In proceedings of 8th International Symposium on EcoHydraulics 2010. September 12-16, 2010 COEX, Seoul, Korea