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In topographically complex terrains, downslope movement of soil organic carbon (OC) can influence local carbon balance. The primary purpose of the present analysis is to compare the magnitude of OC displacement by erosion with ecosystem metabolism in such a complex terrain. Does erosion matter in this ecosystem carbon balance? We have used the Revised Universal Soil Loss Equation (RUSLE) erosion model to estimate lateral fluxes of OC in a watershed in northwestern Mexico. The watershed (4900 km2) has an average slope of 10 degrees +/- 9 degrees (mean +/- SD); 45% is >10 degrees, and 3% is >30 degrees. Land cover is primarily shrublands (69%) and agricultural lands (22%). Estimated bulk soil erosion averages 1350 Mg x km(-2) x yr(-1). We estimate that there is insignificant erosion on slopes < 2 degrees and that 20% of the area can be considered depositional. Estimated OC erosion rates are 10 Mg x km(-2) x yr(-1) for areas steeper than 2 degrees. Over the entire area, erosion is approximately 50% higher on shrublands than on agricultural lands, but within slope classes, erosion rates are more rapid on agricultural areas. For the whole system, estimated OC erosion is approximately 2% of net primary production (NPP), increasing in high-slope areas to approximately 3% of NPP. Deposition of eroded OC in low-slope areas is approximately 10% of low-slope NPP. Soil OC movement from erosional slopes to alluvial fans alters the mosaic of OC metabolism and storage across the landscape.

Este estudio evalúa los impactos de los ciclones tropicales (CT) que tocaron tierra en áreas pobladas a lo largo de la costa del Pacífico mexicano. El periodo de interés abarca de 1970 a 2010 y se utiliza una base de datos internacional de desastres para identificar dichos impactos. Se registraron más de 30 eventos durante el periodo de referencia, de los cuales se examinan los 25 que acumularon mayor precipitación, así como los 10 que causaron mayores daños a la población afectada, que en estos casos varió de 20 000 a más de 800 000 habitantes. Vientos fuertes y precipitación intensa en periodos de uno a tres días se asociaron con daños materiales y pérdida de vidas humanas. Los resultados del estudio indican que las acumulaciones excesivas de lluvia y su intensidad diaria son elementos importantes vinculados con la ocurrencia de desastres en áreas densamente pobladas. Seis de los primeros 10 CT asociados con los mayores desastres ocurrieron en condiciones de El Niño y tres en condiciones neutrales; sin embargo, al examinar los 25 CT asociados con la precipitación acumulada, se observó que 10 de éstos ocurrieron en condiciones de El Niño y 10 en condiciones neutrales. Tres casos ocurridos en eventos correspondientes a El Niño (Liza en 1976, Pauline en 1997 y Lane en 2006) se documentaron con más detalle debido a que afectaron áreas con densidades poblacionales diferentes en las costas del sur y noroeste de México. ABSTRACTThis study evaluates impacts from tropical cyclone (TC) landfalls on populated areas located along the Pacific Ocean coast of Mexico. The period of interest is from 1970 through 2010 and an international disaster database is used to identify the impact from the landfalling TCs. More than 30 landfall events occurred during the period; we examined the top 25 TCs based on rainfall accumulation, as well as the top 10 TC-related disasters based on the affected population. Each event resulted in affected population from 20 000 to more than 800 000. Strong winds and heavy rainfall, during periods of one to three Atmósfera 26(2), 163-182 (2013) 164 L. M. Farfán et al. days, are associated with property damage and loss of lives. Our results indicate that excessive rainfall accumulations and daily rates, over highly populated areas, are important elements associated with the occurrence of disasters. Six of the top 10 TC-related disasters occurred during El Niño and three during neutral conditions; however, looking at the top 25 events, 10 occurred during El Niño and 10 during neutral conditions. Three case studies that occurred during El Niño events (Liza in 1976, Pauline in 1997, and Lane in 2006 are documented in more detail as they affected areas with different population densities in the southern and northwestern coasts of Mexico.

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