HESSD Hydrology and Earth System Sciences Discussions HESSD Hydrol. Earth Syst. Sci. Discuss. 1812-2116 Göttingen, Germany 10.5194/hessd-11-3787-2014 Runoff generation processes during the wet-up phase in a semi-arid basin in Iran Zarei H. https://orcid.org/0000-0003-3414-1816 1 Akhondali A. M. 1 Mohammadzadeh H. 2 3 Radmanesh F. 1 Laudon H. https://orcid.org/0000-0001-6058-1466 4 Department of Hydrology and Water Resources, Shahid Chamran University of Ahvaz, Ahvaz, Iran Department of Geology, Ferdowsi University of Mashhad, Mashhad, Iran Groundwater Research Center, Ferdowsi University of Mashhad, Mashhad, Iran Department of Forest Ecology and Management, SLU – Swedish University of Agricultural Sciences, 90183 Umeå, Sweden 02 04 2014 11 4 3787 3810 Copyright: © 2014 H. Zarei et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://hess.copernicus.org/preprints/11/3787/2014/hessd-11-3787-2014.html The full text article is available as a PDF file from https://hess.copernicus.org/preprints/11/3787/2014/hessd-11-3787-2014.pdf

Understanding the hydrological processes in catchments is important for water resources 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<sup>2</sup>) in Southwestern Iran were investigated using analysis of hydrometric data in combination 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 analyzed for oxygen-18 and deuterium isotope composition. The results show that during 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 directly linked to changes in soil moisture, which in turn controlled the runoff generation processes. In line with the hydrometric results, the two-component hydrograph separation using δ<sup>18</sup>O and δ<sup>2</sup>H demonstrated a clear connection to the antecedent wetness 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.

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