Runoff generation on abandoned fields in the Central Ebro Basin. Results from modelling

The results from several rainfall simulations performed on two abandoned fields of different ages in the Central Ebro Depression have been analysed by application of the physically based hydraulic model HILLFLOW 1D. The data gained by the simulations was used to parame - terise the model. Afterwards, the runoff generation characteristics were determined by simulation of different rainfall intensities and different macroporosities of the soil. The young fallow land shows a minimum inten - sity for runoff generation of 6 mm h-1. An increase of the rainfall intensity leads rapidly to a growing runoff coefficient, being the value modelled for 30 mm h-1 nearly the same as the simulated one (40 mm h-1). Another con - clusion of the simulations and their modelling is the homogeneity of the soils on the young fallow land, with no macroporosity all over the profile. On the other hand, the rainfall simulations and the modelling on the old fallow land show a high variability of rainfall-runo ff response patterns, strongly conditioned by the presence of macropores. They are distributed very irregularly throughout the soil of the old fallow land. In both cases, water infiltrates only little into the soil matrix, for that measures for enhan- cing soil hydraulic properties are needed. RESUMEN: Se han analizado los resultados de numerosas simulacio - nes de lluvia en campos abandonados de diferente antuguedad en la Depresion central del Ebro aplicando el modelo hidrologico de base fisi ca HILLFLOW 1D. Los datos de las simulaciones se utilizaron para para - metrizar el modelo. A continuacion, las caracteristicas de la generacion de escorrentia se analizaron modelizando diferentes intensidades de lluvia y volumenes de macroporos en el suelo. En el campo recientemente aban - donado se pudo calcular una intensidad de llluvia minima para la gene- racion de escor rentia de 6 mm h-1. Una intensidad de precipitacion cr e - ciente produce un incremento muy fuerte del coeficiente de escor rentia, de modo que precipitaciones de 30 mm h-1 se diferencian poco de las simu- ladas con 40 mm h-1. Otra conclusion es que los suelos del campo recien - temente abandonado son muy homogeneos y estan caracterizados por la ausencia de macroporos. La respuesta de la generacion de escor rentia a la precipitacion es en cambio muy variable en el campo abandonado anti