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<p>The hydrology of tropical mountain catchments is important to study because it plays a vital role in ecological and geochemical processes in the downstream regions. Sri Lanka is a unique tropical region that provides excellent experimental sites for studying water resources and climate variability due to its orographic and climatic differences within a small land area. The spatial and temporal variations of δ<sup>2</sup>H and δ<sup>18</sup>O, deuterium excess (d-excess) and orographic effects were studied in a small mountain tributary catchment of Atabage Oya, whose elevation varies between 600 and 2250&#xa0;m within a 44 km<sup>2</sup> catchment area. Samples were collected before (September), during (November), during the transition month (January) and after (March) the north-west monsoon. The isotope fractionation due to evaporation was calculated based on the Craig-Gordon model. The isotope values of the river water samples followed the local meteoric water line in all studied months. Nevertheless, evaporative enrichments occurred in the drier months of March and January, which experienced a change in weather from rain to dry. The d-excess values varied downstream, ranging from 11.9‰ (September) to 15.0‰ (January) with generally decreasing trends towards the river confluence. Based on the non-steady-state fractionation model, the estimated evaporation loss varied from 0.2% at the upper basin boundary to 6% at the confluence. Our study suggests minimal water loss during downward flow for tropical mountain catchments.</p>

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Evaporation losses of surface water in a small mountain catchment with a higher elevation gradient in the humid tropical terrain of Sri Lanka

  • 期刊论文

  • 发表日期：

    2025年10月21日

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  • Nilmini Ekanayake,
  • Iresha Sumudumali,
  • J. M. C. K. Jayawardana,
  • Sachintha Senarathne,
  • Tilak Hewawasam,
  • Rohana Chandrajith

摘要

The hydrology of tropical mountain catchments is important to study because it plays a vital role in ecological and geochemical processes in the downstream regions. Sri Lanka is a unique tropical region that provides excellent experimental sites for studying water resources and climate variability due to its orographic and climatic differences within a small land area. The spatial and temporal variations of δ2H and δ18O, deuterium excess (d-excess) and orographic effects were studied in a small mountain tributary catchment of Atabage Oya, whose elevation varies between 600 and 2250 m within a 44 km2 catchment area. Samples were collected before (September), during (November), during the transition month (January) and after (March) the north-west monsoon. The isotope fractionation due to evaporation was calculated based on the Craig-Gordon model. The isotope values of the river water samples followed the local meteoric water line in all studied months. Nevertheless, evaporative enrichments occurred in the drier months of March and January, which experienced a change in weather from rain to dry. The d-excess values varied downstream, ranging from 11.9‰ (September) to 15.0‰ (January) with generally decreasing trends towards the river confluence. Based on the non-steady-state fractionation model, the estimated evaporation loss varied from 0.2% at the upper basin boundary to 6% at the confluence. Our study suggests minimal water loss during downward flow for tropical mountain catchments.