A Method for Estimating Soil Loss due to Runoff

Chamcteristics of soil erosion caused by rain water flow (III)

Abstract

Numerous problems in relation to the quantitative analysis of cohesive soil erosion by surface water flow still remain unsolved from a practical point of view since the resistance mechanism of the soil layer to the tractive force of surface water flow ha. s not yet been clarified. When the resistance of viscous soil such as farmland soil is supposed to be represented by the clay ratio (C_r*) and dry density (γd), the quantity of soil erosion (q_e) and dimensionless tractive force of a surface water flqw (τ₀/τ_c=u*²/u*c²) are able to be represented by the equation, q_e/u*d_k=a exp [αu*²/(γd/ρ) gd_k]. At the same time, when the depth (h) of surface water flow (q_w) was expressed by equation, h= (N. q_w/√sinθ) ^P, the hourly and widthwise quantity of soil erosion (q_e) by the surface water flow was able to be obtained by the following equation:
q=ad_k √Kb qw^P/2 exp [αKbqw^P/(γd/ρ) gd_k](provided that Kb=gN^P/(Sinθ) ^(P/2)-1)
Wherein a and α represent the coefficient and exponent, respectively, to be determined by the conditions of the native matter of soil and soil layer, p the constant (=3/5), d_k the average clod size of the surface soil layer, N the equivalent roughness of catchment area, ρ the water density, g the gravity acceleration and θ the gradient of the slope. An application of this equation was attempted using the N value adapted to the runoff analysis, to the sloped standard plot for soil erosion where the actual amount of surface water flow and loss of soil caused by natural rainfall had been measured. As a result, it produced a relatively close correlation between the value obtained by the calculation on the basis of the above equation and the quantity of soil loss by surface water flow actually measured at the time of the rainfall. This, therefore, indicated the possibility of estimating the loss of soil on sloped farmland, if the appropriate surface water flow on the slope of the farmland and the N value were able to be determined from the precipitation and runoff analysis.