Abstract
River regulation is challenging when there is diverse upstream and downstream interest, leading to regional and international conflict. However, quantifying the upstream-downstream flow regime changes and their causes are given less consideration in the river basin. In this study, we presented three new ratios for downstream-upstream low flow contribution (DUL), downstream-upstream high flow contribution ratio (DUH), and meteorological-hydrological drought ratio (MHD), for an integrated assessment of flow regime alteration across the river basin. To test the methods, we compared flow regime alteration upstream and downstream in the Ceyhan basin in central Turkey, which was significantly modified by agriculture between 1984 and 2018 (the irrigated area increased 2.8-fold, rainfed farming decreased by 67.6%). Our analysis revealed a clear change in the contribution of low and high flow seasons to annual flow in the last station of the river at Misis after 1984, but no considerable change in upstream tributaries. In the last decade (2005–2014) and the second half (1995–2014) of the study, the frequency of hydrological droughts increased, while meteorological droughts followed a stationary pattern. Evaluation of the impact of anthropogenic activities on river regime (by comparing flow regime characteristics after 1984 with those from 1975 to 1984 as post- and pre-impact periods) revealed low to incipient impact upstream (Hanköy, Karaahmet, and Kadirli river headwaters), severe impact below the Aslantaş dam in the basin center, and moderate impact at the last station on the Ceyhan river. The new metrics provide supplementary information on the flow regime alteration in the basin and can be introduced as a novel quantitative measure to recognize the driving factor of droughts.
Highlights
Rivers are vital for many species, including humankind, for whom they play an essential role in transportation, fishing, agriculture, in dustry, recreation, and urbanization (Paine, 2019)
Storing water in high flow season (January to June resulting in a negative trend in downstream) and releasing it in the low season is a usual function of storage dams, e.g., the Aslantasand Menzelet dams on the main channel of the Ceyhan river
We have demonstrated the application of three novel ratios: downstream-upstream low flow contribution ratio (DUL), downstreamupstream high flow contribution ratio (DUH), and meteorologicalhydrological drought ratio (MHD), for an integrated evaluation of flow regime alteration across the basin
Summary
Rivers are vital for many species, including humankind, for whom they play an essential role in transportation, fishing, agriculture, in dustry, recreation, and urbanization (Paine, 2019). Alterations in attributes of the flow regime, including magnitude, frequency, duration, and timing (Bejarano et al, 2017), can be amplified by superimposing stressors such as climate change, dam construction, and land use/land cover change (Qi et al, 2020). Such changes in the flow regime can occur on an hourly to annual scale (Ashraf et al, 2018). Developing the indices and approaches to quantify the impact of climate variability, dam and hydropower construction, and land use change to address the flow regime alteration from upstream to downstream can help decision-makers and water resources planners to make informed decisions for the future
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