Runoff generation processes during the wet-up phase in asemi-arid basin Runoff generation processes during the wet-up phase in a semi-arid basin in Iran Runoff generation processes during the wet-up phase in asemi-arid basin

Abstract

Understanding the hydrological processes in catchments is important for water re-sources management, particularly in semi-arid regions of the world. To contribute to this field, dominant runoff generation processes in a semi-arid basin (283 km 2) in Southwestern Iran were investigated using analysis of hydrometric data in combination 5 with natural isotopic tracers through the wet-up phase of a rainy season. The analysis of seven rainfall–runoff events during the rainfall dominated period illustrated the role of antecedent base flow and cumulative rainfall for explaining the hydrological response. Three distinct storm events and the corresponding discharge were collected and ana-lyzed for oxygen-18 and deuterium isotope composition. The results show that during 10 the wetting-up cycle, the runoff ratio during storm events increased progressively from 1 to 10 %. Higher event runoff ratios following catchment wet-up were shown to be di-rectly linked to changes in soil moisture, which in turn controlled the runoff generation processes. In line with the hydrometric results, the two-component hydrograph separa-tion using δ 18 O and δ 2 H demonstrated a clear connection to the antecedent wetness 15 conditions. The results suggest that the runoff ratios during storms and the partitioning of event and pre-event water fractions are sensitive to the amount of catchment wet-up and could hence be strongly impacted by changes in the timing, duration and amount of precipitation in the future.