Simulating The Solute Lost Via Runoff On The Loess Plateau

The Loess Plateau is the most serious region in soil and water loss in China and evenin the world. By increasingly drier climate and human activities, the region Loess Plateauwhere is also one of the important regions requiring ecological restoration for severeecological degradation. Soil erosion and solute transport on loess slope is a complicatedlyphysicochemical process. It is affected by many factors and mutually intersected withmulti-discipline, such as soil science, ecology, hydrology, hydraulics, environmentalscience and so on. The mechanisms and predictions of water and nutrient transport in soilson a slope during rainfall have played important roles in the research on the degradation ofsoil quality and the expansion of non-point source pollutions in recent years, which alsohave been an interdisciplinary field focused in soil erosion, hillslope hydrology, drylandagriculture and related environmental sciences.Investigating the mechanism and model of soil water and nutrient transport couldpredict and prevent effectively soil nutrient loss at intensively-eroded area on the loesssection. And it's good for understand and master the nature and mechanism of erosion andinternal law of solute transport during the process of erosion on loess slope. This is veryimportant in ecological environment management of the Loess plateau. The main worksand detailed results of this study are as following:(1) The initial soil moisture content influences solute loss by virtue of its impact oninfiltration, which suggests that if infiltration could be facilitated, the loss of solutes couldbe reduced. Rainfall intensity increases the mass of sediment carried away by the runoffand decreases the time required for runoff formation; the latter in turn increases the soluteconcentration in the surface layer. The slope gradient also has a significant influence on themasses of solute and sediment in the runoff; both increase as the slope gets steeper between0to25gradient. Treatment with PAM effectively increased the soil's resistance toraindrops and water flow-induced detachment. For the soil examined in this work, theoptimal PAM loading was3g/m2. Solute concentration in the runoff at the first minute when runoff take place and the average solute concentration in the runoff were positivelinear correlation with solute concentration in the soil surface when runoff take place.(2) Plant cover effect the runoff erosion ability by controlling the hydrauliccharacteristics of flow. It's different both the soil and water conservation effects and theinfluence of the nutrient loss from slope from different vegetation types under the samevegetation coverage. For the experimental area, the Stipa bungeana was better than alfalfafor soil and water conservation. The vegetation coverage increased the soil surfaceroughness, slower down the flow velocity and increase the time of water infiltration. Thevegetation coverage influenced the soil sediment in the runoff by controlling the hydrauliccharacteristics of flow and the soil cohesion. The relationship between the averagesediments in the unite width of runoff and vegetation coverage could be expressed bynegative linear equations. The relationship between the average solute concentration in theunite width of runoff and the vegetation coverage could be described by density function.The average sediments in the unite width of runoff decreased36.7percentage and theaverage solute concentration in the unite width of runoff decreased with33.2percentagewith the coverage of alfalfa increased15percentage between30and60percentage. Thestone cover decreased the runoff volume, soil sediment in the runoff, the solute loss withrunoff by increasing the surface roughness of soil surface, decreasing the flow velocity ofoverland flow, increasing the time of water infiltration and the volume of infiltration. Therelationship between the average sediments in the unite width of runoff and the stonecoverage could be expressed by density function. The relationship between the averagesolute concentration in the runoff and stone coverage could be expressed by linear equation.For the experimental area, the average sediments in the unite width of runoff decreased27.2percentage and the average solute concentration in the unite width of runoff decreasedwith14.3percentage with the coverage of stone increased10percentage between10and30percentage.(3) We developed the average solute concentration in the unite width of runoff modelwhich expressed by the runoff shear stress under the infiltration excess condition on theLoess plateau. The model expressed as Drs=a·k·(τ-τ0), where Drswas average soluteconcentration in the unite width of runoff; a was empirical parameter and which changedwith solute types and the solute concentrated in the soil. The other parameter the same with the average sediments in the unite width of runoff. The model made the modeling solutetransport with runoff easier. The developed model to control the soil and water loss,non-point pollution has a very important role.(4) The method we used to refine power functions solute transport model developedby Wang Quanjiu et al was change the exchange rate kmwith the raindrop-induced watertransfer rate er, which make the model physically-based and without the kmcalculation.The refined model was more suitable for modeling the solute loess with runoff on theLoess plateu. Test the model with experimental date. The model fitted the experimentaldata very well. Our results also showed that when the slope gradient was15°or larger than15°, the constant parameter, ρ was equal to2, ρ was equal to1while when the slopegradient was less than15°.(5) We have developed a physically-based solute transport model for estimating thesolute concentration in runoff originating from the soil surface under the infiltration excesscondition on the Loess plateau. To test the model, we carried out laboratory experimentsthat used two soil types (loam and sandy loam) and exposed them to simulated rainfall.The results simulated by the model were highly correlated with the experimental data. Thesimulated data showed a high level of correlation with the measured data for soil water,solute transport in the soil profile and runoff volume, solute concentration in the runoff.This demonstrates that the model captured the temporal behavior of the runoff and solutetransport in the runoff. The model could not predict the solute concentrations in the runoffunder severe soil erosion conditions accurately.