When it rains in the desert of Patagonia: heavy rains, sediment laden flash floods and hazard management challenges.

Ephemeral streams in dry environments can produce high intensity sediment-laden flash floods. The research of these events is quite difficult due to far locations with limited accessibility, unexpected events, and rough weather conditions. Hence combined data of rainfall, flood hydrograph and sediment transport is rather scarce in the world. Recent extreme events (RI > 550 years) in arroyos (ephemeral streams) of the central Patagonia (Argentina) has called the attention of public and scientific community. The Sagmata basin (total drained area of 129 km2), located in the Lower Valley of the Chubut River (Province of Chubut, Argentina) has been equipped with 4 raingauges, a weather station, four cameras and sediment sampler in four reaches of the arroyos. Data from a two-year monitoring provides valuable insights about the hydrological response of basins in Patagonia, such as the time of response, the shape of hydrographs and its relations with the hyetographs and duration curves. The arroyos are wetted a short period of the time (8 to 21 hours a year). Single-peak, multi-peak and flat top hydrographs have been observed. The lag-time decreases with the increase of the rainfall intensity and the maximum water stage. Moreover, the bore front moves with mean speed which value is between 0.8 to 1.6 m s-1. The wash load in the arroyos depends on geological settings and vegetation cover of the basin. Mean values are between 23 – 46 gr l-1. The complex hydrograph shape produced by the basin has been linked to the rainfall features (duration, mean intensity and patchiness) and the basin physiography of the basin. Findings from the present study provide valuable information both for the comprehension of arroyos in drylands and hazard management as well.

Habitat ephemerality affects the evolution of contrasting growth strategies and cannibalism in anuran larvae

Ephemeral streams are challenging environments for tadpoles; thus, adaptive features that increase the survival of these larvae should be favored by natural selection. In this study, we compared the adaptive growth strategies of Bombina orientalis (the oriental fire-bellied toad) tadpoles from ephemeral streams with those of such tadpoles from non-ephemeral streams. Using a common garden experiment, we tested the interactive effects of location (ephemeral vs. non-ephemeral), food availability, and growing density on larval period, weight at metamorphosis, and cannibalism. We found that tadpoles from ephemeral streams underwent a shorter larval period compared with those from non-ephemeral streams but that this difference was contingent on food availability. The observed faster growth is likely to be an adaptive response because tadpoles in ephemeral streams experience more biotic/abiotic stressors, such as desiccation risk and limited resources, compared with those in non-ephemeral streams, with their earlier metamorphosis potentially resulting in survival benefits. As a trade-off for their faster growth, tadpoles from ephemeral streams generally had a lower body weight at metamorphosis compared with those from non-ephemeral streams. We also found lower cannibalism rates among tadpoles from ephemeral streams, which can be attributed to the indirect fitness costs of cannibalizing their kin. Our study demonstrates how ephemeral habitats have affected the evolutionary change in cannibalistic behaviors in anurans and provides additional evidence that natural selection has mediated the evolution of growth strategies of tadpoles in ephemeral streams.

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

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.

Time-lapse graphical representation methods for mapping of Intermittent Rivers and Ephemeral Streams (IRES)

Flow data visualizations describe runoff, flooding or drought, showing the interconnectivity and complexity of water data issues or water management problems. Intermittent Flow Rivers constitute more than half of the length of the global river network and their presence is expanding in response to climate change. A new approach is developed to visualize the flow of the Intermittent Rivers and Ephemeral Streams (IRES) based on the creation of time-lapse videos. Two statistical methods, the Natural Breaks and Equal Interval one, are used and evaluated for the creation of the mapping content. The flow dataset of IRES for the island of Crete (in Greece) is used as a case study for a six-year period. The results of both methods are used as an input to create the time-lapse videos of IRES. The videos show the flow fluctuation and cessation during a six-year period and the differences between the two methods.

Stormwater harvesting in ephemeral streams: how to bypass clogging and unsaturated layers

To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.