Método intuitivo para el cálculo de excedentes hídricos: estudio caso del área central de Córdoba, Argentina

Abstract

El cálculo de excedentes hídricos por métodos alternativos a aquellos basados en aforos permitiría obtener rápidamente información para dimensionar riesgos hidrológicos. El objetivo del presente trabajo consistió en desarrollar una metodología de cálculo de excedentes hídricos a partir de datos empíricos de capacidad de infiltración. Este método permitiría detectar potenciales riesgos hidrológicos consecuencia de incendios forestales, invasión de especies forestales exóticas, influencia del pastoreo y actividades agrícolas ocurridas en el centro de Córdoba (Argentina). El estudio y desarrollo metodológico se aplicó a tres subcuencas afectadas por los diferentes disturbios descriptos. El método utilizó como insumos un modelo digital de elevación, diversos mapas de usos de suelo, curvas de infiltración, curvas de Intensidad-Duración-Frecuencia para tres tiempos de recurrencia, y datos de cobertura arbóreo-arbustiva. Los excedentes hídricos fueron obtenidos por diferencia entre las lluvias y la capacidad de infiltración (corregida por pendiente) instantáneas cada 10 minutos, y luego integrados para un período de 120 minutos. De esta forma se obtuvieron imágenes raster de excedentes hídricos para cada situación. Estos fueron corregidos con coeficientes de intercepción y cobertura arbórea. Los resultados mostraron que los incendios forestales sobre el 45% de la subcuenca afectada incrementaron los excedentes hídricos hasta 100%; en áreas invadidas por especies forestales exóticas y con pastoreo se incrementaron hasta 50%. Por otro lado, el cese de actividades agrícolas redujo hasta 80% los excedentes hídricos en clausuras de 8 años. Con estos modelos es posible simular infinitas situaciones de cambio de uso de suelo y compararlas con las condiciones originales.

Changes of land use in hydrographic basins can lead to serious water imbalances, resulting in floods or barrages. Soils in central Cordoba have undergone significant changes over the past five decades as a result of land conversion to agricultural, industrial, or residential uses, and other anthropic disturbances such as periodic forest fires or invasive alien species. The development of expeditious methods for estimating water surpluses would facilitate rapid data gathering to estimate the extent of hydrological risks. The objective of this article is to develop a methodology for calculating water surpluses based mainly on empirical data on infiltration capacity and storms, designed for different recurrence times. This can be applied to geographic information systems to obtain graphical information on critical areas based on water surpluses. The starting hypothesis is that the method discussed would allow detecting potential hydrological risks derived from anthropic disturbances such as forest fires, invasive alien forest species, extensive grazing, and agricultural activities in central Cordoba, Argentina. The study and methodological approach comprised three hydrographic sub-basins located in the La Calera Defense Nature Reserve (RNDLC, in Spanish), Cordoba (Argentina). We selected one sub-basin used for agriculture, one with a history of fires, and one influenced by invasive alien forest species and extensive grazing, based on land-use and vegetation maps, fire events, and areas used for agriculture within the RNDLC. Based on these maps, different soil characteristics were defined in relation to infiltration capacity. From infiltration curves for the different types of soil, land use, and alterations, instantaneous rates were calculated at 10-minute intervals over 120 minutes; these values were then assigned to raster layers. The instantaneous infiltration capacity was corrected considering the mean slope. Intensity-Duration-Frequency curves were constructed for recurrence times of 5, 10, and 25 years; these values were corrected according to tree interception and tree coverage coefficients. Maximum instantaneous precipitation (calculated at 10-minute intervals) was subtracted from effective instantaneous infiltration values to define the instantaneous excess water that might potentially drain as runoff. Effective instantaneous water surpluses were then integrated over a period of 120 minutes. In this way, water surplus rasters were derived for each land-use scenario. Excess volumes were calculated for each pixel in each runoff category. In the agriculture sub-basin, 3-year closures resulted in reductions of up to 42% in water surpluses, and 8-year closures achieved reductions approaching 80%. In the fire sub-basin, no significant changes were observed when a non-fire situation was contrasted versus 3-year-old fire events, or versus a recent fire before the rainy season, although there was a significant 1.5 to 2-fold increase in water surpluses after the first rains, possibly as a result of the impact of rain drops and sediment accumulation from runoff. In the sub-basin with invasive alien forest species and grazing, reductions of up to nearly 50% in water surpluses were recorded when the replacement of invasive alien forest species by native species was simulated in scenarios of shorter recurrence times (Tr = 5 years) while maintaining livestock activity. On the other hand, restricting grazing through closure reduced water surpluses in about 47% under high rainfall (Tr = 25 years), but caused no effect under low rainfall (Tr = 5 years). Changes of land use such as agriculture, forest fires, invasion of alien species, or cattle ranching produced basins with lower water infiltration capacity; this may lead to significant runoff volumes that could cause flooding and sediment runoff with adverse environmental, social, and economic effects. Empirical infiltration capacity data, combined with data from variables that could also be empirically obtained, such as soil coverage, are the main inputs for these models, allowing the rapid prediction of potential flood events in hydrographic basins.