Rainfall-triggered lahars at Volcán de Colima, Mexico: Surface hydro-repellency as initiation process

article Volcan de Colima is currently the most active volcano in Mexico. Since 1998 intermittent activity has been observed with vulcanian eruptions, lava flows and growing domes that have collapsed producing several block-and-ash flow deposits. During the period of heightened activity since 1998 at Volcan de Colima, pyroclastic flows from dome or column collapse have not reached long distances, most of the time less than 6k m from the crater. In contrast, rain-induced laharswere more frequent and have reached relatively long distances, upto 15 km,causing damage to infrastructureand affecting small villages.In 2007 tworain gauge stationswereinstalledonthesouthern flankofthevolcanoregisteringeventsfromJunethroughtoOctober, the period when rains are intense and lahars frequent. By comparing lahar frequency with rainfall intensity and the rainfall accumulated during the previous 3 days, lahars more frequently occur at the beginning of the rainfall season, with low rain accumulation (<10 mm) and triggered by low rain intensities (<20 mm/ h). During the months with more rainfall (July and August) lahars are less frequent and higher peak intensities (up to 70 mm/h) are needed to trigger an event. In both cases, lahars were initiated as dilute, sediment-laden streamflows, which transformed with entrainment of additional sediment into hypercon- centratedanddebris flows,withalternationsbetweenthesetwo flowtypes.Ahydro-repellencymechanism in highly vegetated areas (i.e. evergreen tree types with considerable amount of resins and waxes such as pines) with sandy soils can probably explain the high frequency of lahars at the beginning of the rain season during low rainfall events. Under hydrophobic conditions, infiltration is inhibited and runoff is facilitated at more highly peaked discharges that are more likely to initiate lahars.