LABORATORY DETERMINATION OF INTERRILL SOIL ERODIBILITY

Laboratory soil pans and rainfall simulators have been used to study f undamental erosion processes affecting interrill soil loss. Problems e xist when laboratory results are extended to those determined under fi eld conditions. Experimental methodology influences a soil's interrill erodibility and interrill erodibility ranking among soils. We compare d interrill soil loss and erodibility data from two erosion pans with field data from identical soils. Three plot size-rainfall simulator me thodologies were used: (1) 0.14-m2 lab pan under a constant drop size (4.6 mm) rainfall simulator, (2) 0.32-m2 lab pan with soil border area s under an oscillating nozzle (2.3 mm median drop diameter) rainfall s imulator, and (3) 1-m2 field plots under the same rainfall simulator a s used in Method 2. Soil loss was measured at 5-min intervals. Interri ll erodibility (K(i)) was calculated from two equations (E = K(ii)I2 a nd E = K(iq)Iq) using measured soil loss (E), rainfall intensity (I), and flow discharge (q) values. The interrill erosion equation, methodo logy, time, and initial water content influenced calculated K(i) value s. Soil loss and K(i) values from Method 1 did not correlate with and were greater than corresponding values from Methods 2 and 3. Soil loss and K(i) values from Method 2 were correlated (r = 0.56 to 0.79) with corresponding values from Method 3. The K(iq) values decreased with t ime and were a function of soil properties related to soil detachment and sediment transport. The K(ii) values (i) increased with time, (ii) were primarily a function of soil properties related to soil detachme nt only, and (iii) did not account for infiltration and runoff differe nces among soils.