The Runoff And Sediment Regulation Of Grass Covers In Loess Slopes

The coexistence of drought and soil and water loss is the main ecological problem of the hilly region of the Chinese Loess Plateau, where the ecosystem is fragile. The grass cover is an efficient water and soil conservation practice. It promotes the harvesting of rainfall water in the storm season(from June to September) in the Loess Plateau. In the present thesis, the water and sediment regulation benefits in the slope covered by four kinds of perennial pastures(cock’ foot, Dactylis glomerata L.; white clover, Trifolium repens L.; crown vetch, Coronilla varia L.; bird′s foot trefoil, Lotus corniculatus L.) was studied in a patch scale. The roles of roots and canopies of the vegetation covers in various growth stages(early May, early June, middle July and late August) in the water and sediment regulation were observed and compared. The mechanism of water and sediment reduction of the grass slopes was also discussed to provide the pursuance for the optimization of grass plantation and mowing in the hilly region of the Chinese Loess Plateau. The key conclusions of the thesis included:(1) The water and sediment reduction of all the grasses were apparent. When compared with the bare slope, their runoff and sediment reduction ratios were 31%~73% and 84%~99%, respectively, indicating the better sediment reduction effects. For the cock’ foot, the average runoff reduction and sediment reduction were 60.5% and 97.8% respectively, which were the highest for all the growth stages, followed by the white clover, The lowed averaged runoff reduction and sediment reduction were observed in the bird′s foot trefoil treatment. The runoff and sediment reduction reached the top points for the cock’ foot and the white clover treatments in middle July(runoff reduction: 76.0% and 63.9%, sediment reduction: 99.1% and 98.4%). The crown vetch and the cock’ foot treatments had the highest runoff and sediment reduction in the early June and middle July respectively(runoff reduction: 58.8% and 50.0%, sediment reduction: 98.0% and 90.2%). The temporal dynamics of soil and water conservation of the cock’ foot and the white clover matched the storm season in the study regain well, especial the cock’ foot treatment.The root and canopy had different contribution in the soil and water conservation process. The root was more important in the erosion prevention while the canopy was more critical in trapping the runoff. Besides the cock’ foot treatment, the contribution of canopy in runoff reduction were 53%~79%, and the contribution of root in sediment reduction were 58%~96%. The top average canopy runoff reduction and root sediment reduction were observed in the bird′s foot trefoil treatment(40.75% and 86.8%) among the four grasses. The regression analysis indicated that the relationship between leaf area index and grass or canopy reduction as well as sediment rate were power function or exponential function.(2) The initial and stable infiltration rate of grass slopes were significantly higher than bare slopes(P<0.05), which was tribute to the altering of soil physical property of grass cover. Among the four grasses, the bird′s foot trefoil had the highest soil infiltration(0.79~1.00 mm min-1), followed by the white clover. The depth of infiltration and soil water increase of grass slopes were also different with the bare slopes. The depth of infiltration of grass slopes were 40~60 cm while this value was only 30 cm in the bare slopes. In the same growth stage, the averaged soil water increase showed the same trend with the soil infiltration among the treatments. For the first two growth stages, the order of soil water increase after rainfall was: cock’ foot>white clover>bird′s foot trefoil>crown vetch>bare land, while in the late two stages, this order changed to: cock’ foot>white clover>crown vetch>bird′s foot trefoil>bare land.(3) The modification of grass cover on the flow velocity was apparent. The bare land slopes had far greater velocity than grass slopes. The bird′s foot trefoil showed greater velocity than bird′s foot trefoil, white clover and crown vetch treatments(P<0.05). After mowing the canopy, the velocity increased by 23.5% ~61.1%. With the development of grass covers, velocity first decreased then increased. The minimal velocity were observed in middle July for cock’ foot, white clover and crown vetch treatments were 1.9, 2.7, and 3.3 cm s-1respectively. While for the bird′s foot trefoil treatment, the minimal velocity observed in early June was 5.8 cm s-1. The slowing effects and contribution rate of root in slowing flows were larger than that of canopy, especially for the cock’ foot. Slowing effects and contribution rate were 72.0%~83.3% and 89.4%~93.5%, respectively. The flow depth of different treatments followed the order: cock’ foot>white clover>crown vetch>bird′s foot trefoil>bare land. After mowing the cannopy, the flow depth increase lightly.The Reynolds number were 10.30~28.29, indicating the laminar flow occurred. After mowing the crown, the Re had no significant changes. The Froude number of the flow in bare land was larger than 1, indicating supercritical flow. The Froude number of the flow in cock’ foot, white clover and crown vetch were smaller than 1 both with and without cannopies, indicating subcritical flow. After mowing, the Froude number increased lightly, but the status of the flow was remained.The resistant coefficient and Manning coefficient reflected the resistant status of the flow and show the identical order in this study: cock’ foot>white clover>crown vetch>bird′s foot trefoil>bare land. The two coefficients increased lightly after mowing. The sediment rate showed good power function relationships with velocity, Reynolds number, Froude number, resistant coefficient and Manning coefficient, especially velocity, but mean flow depth.