Caracterización del peligro por inundaciones en el oasis La Purísima, Baja California Sur, México

ResumenEn el estado de Baja California Sur existen solo cuatro oasis con extensiones mayores a 2.0 km2; La Purísima (2.25 km2) representa el tercero en extensión. En el pasado el oasis fue frecuentemente afectado por inundaciones resultantes de los eventos extremos de precipitación, que generaron graves daños en la cuenca.Se realizó un diagnóstico del oasis La Purísima, con respecto a posibles inundaciones como consecuencia de lluvias extremas, lo cual incluye el análisis estadístico de la precipitación para la elaboración de un modelo hidrológico e hidráulico, con el fin de calcular el caudal máximo, el volumen y la frecuencia de crecidas en los cursos del arroyo Cadegomo bajo diferentes escenarios de lluvias extremas. Finalmente se elaboró un mapa de peligro por inundación según los escenarios elaborados.Se efectuó un análisis de la ocurrencia de inundaciones en el pasado y sus efectos en el oasis por medio de evidencias históricas de inundación de la zona, para verificar su posible aplicación en la calibración de los modelos.Los caudales resultantes de los eventos de precipitación extremos generan un incremento importante de los niveles en el drenaje de la cuenca. La respuesta del arroyo es un incremento en el área de inundación de hasta 14% respecto al tiempo de retorno de 20 años. Un evento de 1 000 años generaría un incremento en el área de inundación de 67% y bajas, afectando las zonas de la región urbana, así como todos los lotes de cultivo.AbstractIn the Mexican state of Baja California Sur there are only four oasis of more than 2.0 km2; La Purísima (2.25 km2) is the third largest of them. In the past this oasis was frequently affected by flooding, resulting from extreme rainfall events, which in several occasions generated severe destruction in the oasis. The first permanent settlement in the oasis of La Purísima was the foundation of a Jesuit mission at Cadegomó River in 1720. Since then the oasis repeatedly has been affected by floods; for example hurricanes caused severe damages in 1770 and in 1828, when finally the mission was abandoned. It took ten years to revive the mission, which remained in operation for several years after independence of Mexico. After its secularization, La Purísima became village in 1858. Major inundations occurred in 1957 and 1959, as well as in 2008 and 2009 which affected all farmers in the oasis. Parts of garden areas were lost, many fields covered with sand and the irrigation channels were severely damaged. The destruction included plantations of avocado, mango, guava, banana, plum, apple, black fig, orange trees, sugar cane and severe livestock losses.The mayor part of the La Purísima watershed, with a total area of 1,728.6 km2 is dominated by a very arid, warm climate (BWh (x’) after Köppen), with an average annual temperature between 18° and 20° C, whereas the lowlands, near the Pacific Ocean are influenced by a very dry and warm weather (BW (h>) (x>)), with a higher annual average temperature of 22° C.The main extreme weather events that aífect the area are the tropical cyclones that originate in the Northeast Pacific, and aífect the peninsula of Baja California during the months May to November. In the last decade there has been an increase of extreme rain events in the area, caused by tropical cyclones.A hydrological analysis was conducted, regarding to possible flooding levels, caused by extreme rainfall events. This included a statistical analysis of precipitation data in order to define the relation between rainfall intensity and return period. The data were entered into hydrologic and hydraulic models, using HEC-1 and HEC-RAS software in combination, which permitted to calculate the volume and impact of floods in the Cadegomo River under diíferent scenarios of extreme rainfall events. According to the evaluated scenarios of extreme runoíf events, a hazard map of flooding areas under diíferent return periods was elaborated.The results indicate a significant rise in the water levels of the drainage basin, caused from extreme rainfall events with an increase in the flooded area by 14%, estimated for the return period of 20 years and a 1 000 year event would generate an increase in the flooded area by 67%, which would aífect important areas of the urban region, as well as nearly all gardens of the oasis.The occurrence of floods in the past and their eífects on the oasis La Purísima were analyzed through historical evidences which permitted to verify the scenarios and calibrate the models. In three occasions runoíf volumes of higher magnitude were documented at the hydrological station ”Ojo de Agua”, run by the National Water Commission (CONAGUA): in September 1962, December 1972 and September 1974. The rainfall recorded in 1972 was 69 mm, which caused a total runoíf of 29.48 million cubic meters, observed during the next 230 hours; this is comparable to a model event with a return period of 10 years.In Baja California Sur, due to the lack of direct runoíf measurements, it is of utmost importance to continue ela-borating climate and hydraulic models to quantify the local eífects that could be expected before the end phenomenon of runoíf, in order to install measures to limit the economic damage and impacts on society