Our field efforts have concentrated on parameter estimation from these two broad areas. To estimate parameters necessary to develop the model, field experiments were conducted on selected forest roads and timber harvest areas. Runoff was sampled at regular intervals to measure rate and sediment concentration from a collection trough at the outlet of the bound plot. Loose soil material was collected from small plots before the first and after the last applications of rainfall to characterize the amount and particle size distribution of this readily available sediment supply. Compliant cavity bulk density measurements were made at 30-mm depth intervals to 90 mm in or near the plots.
Concentrated flow occurs on forest roads in wheel ruts and drainage ditches.
Sediment production from concentrated flow occurs particularly during the snowmelt runoff period when snowpack protects road surfaces from raindrop detachment and overland sheet flow detachment is minimal. Under these conditions, snowmelt runoff to wheel ruts and ditches is relatively clean and, therefore, much more erosive. On each plot pair, the overland flow tributary area was identical, but one plot collected overland flow in a metal gutter for measurement of flow rate and sediment concentration, while the other plot had overland flow entering a wheel rut 50 to 80 mm deep and 0.2 m wide. The difference was a function of relative soil erodibility. Sediment production from forest roads can be reduced by gravel surfacing, mulches on cutslopes and fillslopes, rock blankets in drainage ditches, and reduced tire pressure on heavy trucks.