Rainfall-runoff relationships at event scale in western Mediterranean ephemeral streams

Abstract

Ephemeral streams are highly dependent on rainfall and terrain characteristics and, therefore, very sensitive to minor changes in these environments. The western Mediterranean area exhibits a highly irregular precipitation regime with a great variety of rainfall events driving the flow generation on intermittent watercourses, and future climate change scenarios depict a lower magnitude and higher intensity of precipitation in this area, potentially leading to severe changes in flows. We explored the rainfall-runoff relationships in two semi-arid watersheds in southern Spain (Algeciras and Upper Mula) to model the different types of rainfall events required to generate new flow in both intermittent streams. We used a non-linear approach through generalized additive models at event scale in terms of magnitude, duration, and intensity, contextualizing resulting thresholds in a long-term perspective through the calculation of return periods. Results showed that the average 1/41.2d and <1.5mm event was not enough to create new flows. At least a 4d event ranging from 4 to 20mm, depending on the watershed, was needed to ensure new flow at a high probability (95%). While these thresholds represented low return periods, the great irregularity of annual precipitation and rainfall characteristics makes prediction highly uncertain. Almost a third of the rainfall events resulted in similar flow to or lower flow than the previous day, emphasizing the importance of lithological and terrain characteristics that lead to differences in flow generation between the watersheds.