I believe this quote from Tsai (2007) answers your question (link to article below): "The results show that in spite of the rainfall duration and the rainfall pattern, the rainstorm with less than the minimum landslide-triggering rainfall amount will not trigger landslide."  In other words, surface flow can be ignored when the quantity is less than what is needed to trigger a landslide.

 https://www.researchgate.net/profile/Tung-Lin_Tsai/publication/225660951_The_influence_of_rainstorm_pattern_on_shallow_landslide/links/543d1a990cf24ef33b766f9c.pdf

Article The influence of rainstorm pattern on shallow landslide

No, I would not ignore.  True, surface flow is often not connected, but may be a contributing factor in instances such as where unstable soils develop soil cracks to surface before failure.  A few of my papers deal lightly with some landslides from primarily Oregon, and in discussing this many times with geologist Joe Cornell and others in Oregon, the geology and soils indicators are often the defining components to slope failures.  Roads were often contributors, but other significant disturbances also possible, such as wildfires, storm damage or clearing operations of surface vegetation such as forest, and loss of root support and transpiration leaving slope with already some hazard susceptable to being hydrologically overloaded.  Stream bank and toe slope erosion may also contribute, and I have seen instances of log jams causing channel plugs under severe events and toe slope erosion.  But roads in SW Oregon were often a contributory factor in funnelling water to fill materials, overloading slopes with road drainage and removing materials in cutbank that effectively removed slope support and altered subsurface flow path and saturated cut slope.  I have another paper on Hazel Pistol erosion study that found surface flow after clearcutting and severe fuel burning that produced surface erosion and in the right circumstance, could overload a slope with too much water.  Generally though, I would suggest that conditions of surface flow across the land can be a problem, it does not necessarily cause a slope failure unless the materials are unstable or have been disturbed such as fill material.  If materials are erodible on landscape with surface flow, there is often a development of combining into concentrated flow, and rilling to gullying is sometimes the problem.        

 I thought I could find a research study that confirmed some of the things we found on the Siskiyou National Forest, south of this study area.  I was not in research at the time, but met with Dr. Fred Swanson a few times and appreciated his research on slope stability on the Siuslaw National Forest.  

http://www.fs.fed.us/pnw/pubs/pnw_rb084.pdf

I believe this quote from Tsai (2007) answers your question (link to article below): "The results show that in spite of the rainfall duration and the rainfall pattern, the rainstorm with less than the minimum landslide-triggering rainfall amount will not trigger landslide."  In other words, surface flow can be ignored when the quantity is less than what is needed to trigger a landslide.

 https://www.researchgate.net/profile/Tung-Lin_Tsai/publication/225660951_The_influence_of_rainstorm_pattern_on_shallow_landslide/links/543d1a990cf24ef33b766f9c.pdf

Article The influence of rainstorm pattern on shallow landslide

Dear Prof. William F. Hansen & Dear Meghan,

Thanks so much for your valuable explanations! Yes, i believe that the overland flow does have influence on the initiation of shallow landslides. In fact, surface flow and subsurface flow are often occur in a coupled manner. The overland flow caused by a rainfall may act like an additional pressure head to the ground surface affecting the infiltration capacity. HOWEVER, it is quite difficult to model this coupled process. SO I wonder that is there any reasons related to soil property, slope angle, rainfall pattern... THOSE WE CAN USED TO IGNORE THE INFLUENCE OF RAINFALL PATTERN when dealing with landslide modeling?

Best regards,

The hazard is often read by Landform, geology, soil indicators or tests.  The driving forces however may be involved also with rainfall and movement of groundwater and perhaps instances when surface water is high or at some extreme.  In my watershed restoration after calamity, I included a mention of using welding rods to define subsurface flow path, and then by horizontal drilling and tapping that subsurface flow and bringing to surface as a stability practice recommended by Joe Cornell, Geologist.  This worked.  The old adjacent to this slide may have contributed to failure, but it was ultimately the accumulation of water that exceeded the ability of the slope to stay in place, possibly adding some lubrication effect to a subsurface contact.  Not all geology, soils, climates, rainfall, vegetation, etc. Should be expected to function exactly the same.  I would probably agree, surface flow in itself is an unlikely threshold, but it may be that instances of surface flow also correlate well with subsurface saturation.  But I have also seen instances of dry soils from drought below saturated or ponding from extreme event, so surface flow and subsurface saturation or groundwater flow are not always in perfect agreement, and groundwater flow as evidenced in contacts with springs is also possible when soil surface is dry.  But in numerous instances, I have seen where road drainage was diverted onto unstable slope, initiating failure, and some of these may have been associated with road fill, but also some not.  Directly or indirectly, rainfall seemed to be involved in those recent ones I analyzed

I think physical and chemical properties (porosity, permeability and leaching effect) of a soil and rock have a key role for surface flow or run-off to infiltrate into or leach down the subsurface. Water retention capacity of the material (soil/rock) is an other factor to trigger a landslide. Capillary action of the material is an other important property to accelerate the problem. Duration of surface run-off is also very important. When water attains its level of saturation, shear strength of soil reduces and failure of soil mass occur in the form of landslide.