Runoff Generating Process From Slope Land With Biological Soil Crusts And Vegetation In The Hilly Loess Plateau Region

Biological soil crusts(biocrusts) are non-ignorable influence factors of soil and water loss in the Hilly Loess Plateau region. However the impact of biocrusts on runoff generating process from slope land is poorly understood so far. The impact of biocrusts and vegetation on runoff generating characteristics were studied through simulated overland flow experiment and simulated rainfall experiments. The runoff generating characteristics include runoff time、water depth、runoff velocity and runoff coefficient etc., exploring runoff generating process from slope land of biocrusts and vegetation, the study was aimed at understanding hydrological effect of biocrusts and providing a scientific basis for soil and water conservation. The main results are as follows.1、Biocrusts were important influence factors on runoff generating process, which may affect the runoff generated time、runoff velocity and water distribution across the slopes, and the influence extent was related to the succession stages and the community composition of biocrusts.1)The time of runoff initiation were significantly reduced by both light, dark cyanobacterial crusts and mixed crusts compared with bare land, the time of runoff initiation of light cyanobacterial, dark cyanobacterial and mixed crust were reduced by 89.0%、96.2% and 96.0% compared with bare land; While the time of runoff initiation was markedly increased by the moss dominated crust. The time of runoff recession duration was markedly increased by light cyanobacterial crust and mixed crust compared with that of bare plots, while that of dark cyanobacterial crust had no significant differ from that of bare land plot.2)The runoff velocity was reduced by 29.1% by the dark cyanobacterial crust and 67.3% by the moss dominated crust compared with bare land; The runoff velocity of moss dominated crust was markedly slower than that of the other biocrust plots. The runoff depth of light cyanobacterial crust was significantly deeper than that of dark cyanobacterial crust.3)The runoff generating process showed obvious differences between plots with different types of biocrusts compared with that of bare plots; The runoff coefficient of light cyanobacterial crust plot was significantly higher than that of bare land plots, while the runoff coefficient of dark cyanobacterial crust plots and mixed crust plots had no significant difference compared with that of bare land plot. No runoff generated on moss dominated crust plot within runoff duration period.2、Runoff generating process from slop lands with biocrusts and vegetation was related to vegetation distribution patterns and flow rates.1) With the increase of flow rates, time of runoff initiation of the same vegetation pattern was markedly reduced, both time of runoff recession duration、runoff velocity and water depth were markedly increased. Among the three flow rates of the study, water depth of biocrusts patch was significantly lighter than that of vegetation patch.2) Under the same flow rate, the time of runoff initiation of congregate pattern(on the top of the slope) had no significant differ from that of non-vegetation plot, while the time of runoff initiation of congregate pattern(on the bottom of the slope)、congregate pattern(on the middle of the slope)and stripe pattern were significantly longer than that of non-vegetation plot. The time of runoff recession duration of congregate pattern(on the bottom of the slope) was significantly longer than that of non-vegetation plot.3) The runoff velocity was markedly reduced by vegetation plot compared to that of non-vegetation plot. The runoff velocity was markedly reduced by congregate pattern(on the top of the slope) and congregate pattern(on the bottom of the slope) compared with that of non-vegetation plot, while that of congregate pattern(on the middle of the slope)and stripe pattern had no significant differ from that of non-vegetation plot.4) There were great differences of the runoff generating process between different vegetation patterns, and with the increase of flow rate, the differences were gradually decreased. The runoff coefficient of congregate pattern(on the top of the slope) had no significant differ from that of non-vegetation plot, while that of congregate pattern(on the bottom of the slope)、congregate pattern(on the middle of the slope)and stripe pattern were significantly smaller than that of non-vegetation plot.3、Both vegetation patterns、rainfall intensities、slope length and slope gradient exerted effects on runoff generating process from slope land with biocrusts and vegetation in the field simulated experiment.1) Under the same vegetation pattern, the beginning time of ponding and water depth of vegetation patch were significantly greater than that of biocrusts patch, while the runoff velocity was significantly slower than that of biocrusts patch. The effect of vegetation patterns on runoff generating process was not obvious under the experimental conditions.2)The area ratio of biocrusts and vegetation stem base had a positive correlation with runoff velocity and water depth. By the increase of the area ratio, the water depth of vegetation patch changed more than that of biocrusts patch.3)Under the same rainfall intensity, random pattern increased the time of runoff initiation、retarded the runoff velocity, reduced the runoff coefficient compared with control plots. By the increase of the rain intensity, The time of runoff initiation tended to decrease, both runoff velocity、water depth and runoff coefficient tended to increase.4) By the increase of slope length, the curve of both the runoff velocity、water depth and runoff coefficient had an unimodal shape, the critical slope length was 8m. When the slope gradient was 25°, both the runoff velocity and water depth were significantly smaller than that when the slope gradient was 16°.