Assessment of collective impact of upstream watershed development and basin-wide successive droughts on downstream flow regime: The Lesser Zab transboundary basin

•Alterations ranged from −3.4% to −41.7% were linked to monthly medians.•The 1- to 90-day minimum runoffs were dropped between −33.3% and −53.8%.•A departure of about −16% was detected in the long-term median annual runoff.•Upstream water withdrawal is anticipated to increase.•Impact of successive droughts has outweighed the effect of current man-made pressures.

SummaryRapid population growth and socio-economic development coupled with climate change and variability have observably impaired the natural characteristics of hydrological regimes of most of large rivers worldwide. The Lesser Zab shared between Iraq and Iran was one of the few remaining rather intact transboundary river watersheds. The unregulated natural flow pattern, however, has been shifted mainly due to recent upstream anthropogenic factors incorporated with successive droughts. A new generic approach was introduced through integrating a subset of the Indicators of Hydrologic Alteration (IHA) into three generic empirical equations coupled with the application of two universally endorsed drought indices to assess the changes in hydrological patterns prior to, and after upstream watershed development twinned with consecutive drought spells. A departure of about −16% was detected in the long-term median annual runoff in the artificially impaired periods. Alterations ranged from −3.4% to −41.7% were linked to monthly medians. The 1- to 90-day minimum runoffs were dropped between −33.3% and −53.8% over the regulated period. More substantial shifts were perceived between 1999 and 2013. The rates of anomaly ranged from −55.6% to −73.1%. The extreme minimum flows were experienced low to high alterations, while low to moderate degree of anomalies were associated with 1- to 90-day maximum flows. This rate of increased water withdrawal is anticipated to develop and the vulnerability degree of the downstream riparian country is projected to increase. Findings reveal that the impact of successive basin-wide drought episodes has considerably outweighed the effect of current recent upstream damming and water withdrawals.

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