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Abstract

Based on modeling, global climate change caused by increasing concentrations of atmospheric greenhouse gases, notably carbon dioxide (CO₂), is anticipated to be accompanied by more extreme weather such as deeper droughts and wetter storms. This paper examines one such extreme storm and the erosion caused by the associated landslides. From 7-9 December 2010, the Panama Canal watershed was affected by an enormous rainstorm that resulted in deaths, property damage, infrastructure failures, interruption of drinking water supply, and a 17-hour Canal closure (ACP, 2014). The storm was named “La Purísima – 2010,” a name that rural farmers typically give to whatever storm happens within five days of December 8, marking the end of the wet season and the beginning of the planting season.

As a research platform, the Panama Canal Watershed offers a remarkable hydrologic infrastructure that is operated today by the Panama Canal Authority (ACP) and the Smithsonian Tropical Research Institute (STRI). The hydrologic infrastructure was developed starting in 1906. The entire region is now covered by a dense network of hydrological monitoring stations: about 30 river gages, 10 lake gages, more than 70 rain gages, 20 complete meteorological stations, twice-daily weather balloon soundings, and continuous coverage by weather radar (Stallard, et al. 2010).

The western half of the Canal watershed had the greatest rain, up to almost a meter (ACP, 2014). Twenty ACP rain gauges averaged over the watershed collected about 400 mm. The storm has the greatest 3-day runoff on record and was estimated to be a 150-year to 300-year event. One of the two bridges across the Panama Canal, the Centenario Bridge, was completely closed for two months and partially closed for another six due to landslides. Sediment (up to 10,000 to 15,000 mg L^-1) associated with tremendous erosion, mostly by landslides, overwhelmed water-treatment facilities handling water from Lake Alhajuela, filling settling ponds and clogging filters. The cities of Panamá and Colón (about 1.75M people) were without reliable drinking water for about two months.

To assess the impact of landslides, in early April 2011, we flew a 42-by-5 km north-south photographic transect across the central Canal watershed over the western half of the Lake Alajuela subbasin (Stallard and Hruska, 2012). This transect paralleled the rainfall gradient for the storm, from about 200 mm precipitation in the south to almost 1,000 mm in the north (Figure 1). The transect crossed the three major stream gages (Alto Chagres, Boquerón, Pequení), several rain gages, roads, agricultural land, and mature forest. Once mapped, landslides were compared to landscape classifications. We counted more than 850 slides at 4 slides per km² and a coverage of 4,800 m² of slides per km² (0.48%). Landslide erosion within the transect (assuming complete suspension, a depth of 3 meters, and a density of 1.32 t m^-3) would provide about 37,000 mg L^-1 sediment in the runoff from that transect. The average estimated denudation caused by landslides in the transect was 19,000 t km^-2 compared to average annual rates of about 450 t km^-2 yr^-1 in the three watersheds for 1980-1996 (from Stallard and Kinner, 2005) – a factor of 27.

If representative of landslide erosion in the headwaters of the Lake Alhajuela drainage, these results are more than twice the needed amount to account for the observed maximum sediment concentrations of 15,000 mg L^-1 suspended sediment. The storm was so large that land cover made little difference: >85% of the observed slides were in mature forest. Volcanic bedrock produced about six times as many slides as did igneous-intrusive bedrock, and sedimentary rock may also produce more slides. If with climate change, huge (la Purísima-2010-size) storms become more frequent in the Canal watershed (or elsewhere), then topography and geology may become more important considerations regarding sediment supply from steep terrains than varying land cover.

Acknowledgements. This work was done with Charles Hruska and Steven Paton, who will be co-authors of the final publication. The transect was flown by LightHawk, a volunteer-based environmental aviation organization.