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.