Abstract
Ephemeral streams are highly dependent on rainfall and terrain characteristics and, therefore, very sensitive to minor changes in these environments. 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 semiarid 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 nonlinear 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 ~ 1.2-day and <1.5 mm event was not enough to create new flows. At least a 4-day event ranging from 4 to 20 mm, depending on the watershed was needed to ensure new flow at a high probability (95 %). While these thresholds represented low return periods (from 4 to 10 years), the great irregularity of annual precipitation and rainfall characteristics, makes prediction highly uncertain. Almost a third part of the rainfall events resulted in similar or lower flow than previous day, emphasizing the importance of lithological and terrain characteristics that lead to differences in flow generation between the watersheds.
Highlights
25 Precipitation plays a paramount role on drainage of the watersheds, especially in those depending on rainfall for the persistence of the flows, considered intermittent streams
Ephemeral streams are drainage networks remaining completely dry during a variable period of the year and, owing to rainfall events of certain magnitude, they can discharge relatively high flows (Donglioni et al, 2015) that can persist for some time
3.1 Characteristics of flows and precipitation Average daily flows (Q) in Algeciras and Mula were relatively low in both watersheds (0.29 and 0.15 m3/s, respectively) and 135 these values were distant from the median of each month (Figure 3), denoting their great irregularity
Summary
25 Precipitation plays a paramount role on drainage of the watersheds, especially in those depending on rainfall for the persistence of the flows, considered intermittent streams. Western Mediterranean area is especially prone to accommodate watersheds with these types of streams because of the high irregularity of precipitation, both in space and time (Tockner et al, 2009; Thibault et al, 2017) In ephemeral streams, this irregularity turns into a great uncertainty in flow generation affecting the stream and to other parts of the system. The fickleness of flows alters the actual ecological functioning of the watershed at variable scales and, affects the agricultural systems covering lowlands, that usually require infrastructures to retain water Understanding how these watersheds react to precipitation is fundamental for prediction and forecasting of droughts and floods (Döll and Schmied, 2012; Arnone et al, 2020), and for erosion potentiality depending on the type of lithology under the soil and the type of vegetation or land cover at surface. Previous research in ephemeral watersheds on Western Mediterranean (e.g., Camarasa and Tilford, 2002; Camarasa, 2016) showed that rainfall-runoff relationships drive hydrological mechanisms and the dynamics of the rest of the system at basin scale, and that they can be modelled to forecast flows based on the rainfall events of different magnitude
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