Effects of seasonality and cascading reservoirs on evaporative water loss in a tropical river basin: A case study from the Deduru Oya River Basin, Sri Lanka

In the tropics, the strong seasonality of monsoon precipitation with recurring droughts leads to large uncertainties regarding evaporative water loss in regional water balances. To address these uncertainties, this study investigated the evaporation/inflow ratio (E/I) in the Deduru Oya basin in Sri Lanka. The investigation approach relies on a revised Craig–Gordon model with stable water isotopes (δ¹⁸O_H2O). A high-resolution survey was carried out, where river water samples were collected every two weeks near the river mouth during a hydrological year from November 2022 to October 2023. These fortnightly data show an overall trend of enrichment in ¹⁸O up to -1.4‰ due to the evaporation of surface waters in the basin. Based on these data, the calculated evaporation/inflow ratio (E/I) resulted in an evaporation loss of 10 ±1.2% for the entire catchment. This corresponded to 403 ±48 million m³ for 2022-2023. Meteorological factors such as temperature and humidity, as well as surface water storage and conveyance systems with many small reservoirs, were primary regulators of evaporative loss in the Deduru Oya River basin. This study provides an important dataset for a better understanding of the effects of the strong seasonality of the tropical climate and river morphology on water loss through evaporation. It lays a basis for further considerations of CO₂ uptake via water use efficiency.

Keywords: Craig–Gordon, evaporation, stable isotopes, surface water, seasonal effect