MODELING INTERRILL SEDIMENT DELIVERY

Shallow surface runoff is a primary transport agent for interrill sedi ment delivery. Runoff, rainfall intensity, and slope interactively aff ect interrill erosion. We hypothesized that the inclusion of a runoff factor in an interrill erosion model can reduce the dependence of the interrill soil erodibility (K-i) on soil infiltration characteristics as well as improve model predictability. A complete factorial rainfall simulation experiment with two soils (Cecil sandy foam, a clayey, kao linitic, thermic Typic Kanhapludult, and Dyke clay, a clayey, mixed, m esic Typic Rhodudult), four rainfall intensities, four slopes, and two replicates was conducted under prewetted conditions to measure runoff and sediment delivery rates. Tap water with electrical conductivity < 0.2 dS m(-1) was used in all the runs. Rainfall intensity I, unit disc harge q, slope S, soil type, and their interactions significantly affe cted sediment delivery per unit area (D-i). Sediment delivery had the greatest correlation (r = 0.68) with unit discharge; however, neither discharge nor rainfall alone adequately predicted sediment delivery. T he equation D-i = K(i)Iq(1/2)S(2/3) was proposed. The linear intensity term (1) represents detachment of soil by raindrop impact and enhance ment of transport capacity of sheet flow, while the product of q(1/2)S (2/3) describes sediment transport hr sheet flow, Validation with inde pendent data showed that the model predicted soil erodibilities well, The mean r(2) for four validation soils was 0.93 when the proposed mod el was fitted to validation data to predict interrill erodibility (K-i ), The better estimation of K-i indicates that interrill erosion proce sses were adequately described by the model.