Control of runoff and retention of diffuse P-pollutants by sink landscape structures of agricultural watershed

Experiments were conducted in Taohuasi agricultural watershed in Northern China to study the interactions between landscapes and diffuse P-pollutants during 2001 to 2003. Watershed investigation, rainfall-runoff events monitoring, sampling activities and simulation experiments were carried out all along. Agricultural landscape in North of China was different from the landscape in humid area in South of China. The contrast of the heterogeneity landscape was obvious, and the dependence of water source was high. Stability of watershed ecosystem was low and sensitive to natural or human interference. Heterogeneity of landscape in agricultural watershed was high. Channels, valleys, plains and ephemeral streams were intermixed. Artificial or natural sink landscape structures for buffer and detention were distributed broadly. It was found that a series of artificial or natural sink landscape structures as stone dams, grassed ditch, grassed filter strip, dry ponds and buffer zone distributed along the ephemeral stream in the watershed could effectively control the flow velocity and volume of surface runoff with their detention and buffering function. Hydrological control by these structures could not only decrease the kinetic energy of the runoff, but also provide a good opportunity for sand sediment as well as P-pollutants maintenance and transform due to prolong the detention time. The retaining efficiency of sediment and P was high during both continuous and discontinuous runoff events. During the continuous runoff event on June 22nd, 2003, surface flow velocity could be reduced from 37.2cm·s~(-1) to 11.1cm·s~(-1) after passing through these structures. And the volume of the polluted runoff could be cut down to 50.2%. The sink structures greatly influenced the concentration of the pollutants along the transporting pathway. Usually, high concentration appeared in the middle part of the watershed where the village and orchard were located, and decreased gradually after passing through these structures. During the continuous runoff event on June 22nd, 2003, the concentration of TSS,TP,TDP,DRP in surface runoff from village area were 3.68g·L~(-1), 3.56mg·L~(-1), 0.48mg·L~(-1), 0.33mg·L~(-1), and the concentration decreased to 0.39g·L~(-1), 0.26mg·L~(-1), 0.08mg·L~(-1), 0.06mg·L~(-1) in the outflow of the watershed after it had passed through a series of sink structures. The retention rates of TSS,TP,TDP,DRP were 72.7%、69.3%、59.8% and 57.9%, respectively. During the discontinuous runoff event, the reduction of the flow velocity and volume was obvious, and the retention efficiency of pollutants was higher due to the relative small runoff volume and the intermitted transportation of the polluted runoff in the stream channel. The control mechanisms of the sink landscape structures included many factors in which hydrological control was significant. The adsorption by landscape matrix could retain and adsorb pollutants, especial for phosphorus, during they transported in the flow pathway. Furthermore, the vegetation growing in these sink landscape areas could also adsorb large quantity of nutrients. The statistical analysis of 21 rainfall-runoff events indicated that dry pond and grassed filter strip were reliable structures for controlling the effluent of the pollutants from this watershed. The sink landscape fragments could be connected, repaired and conformed to control the production and transportation of the non-point source pollutants in agricultural watershed.