Investigation of the hydrologic response of river flow to climate change (Case study: Dez Dam Basin)

Background and objectives: In the recent years due to observing evidences of climate change in meteorological and hydrological variables, concerns arisen regarding variations of renewable water resources and future water availability under climate change condition. Due to the importance of possible effects of climate change on surface water resources and flow regime, the present study investigates river flow variations during last decades as well as projections of future discharge of Dez dam basin.
Materials and methods: Nonparametric Mann-Kendal trend test and two shift change point detection tests of Pettit and Buishand were applied to the discharge time-series at the outlet of Tire, Marbore, Sazar and Bakhtiari sub-basins to identify monotonic and abrupt changes. Also to project the discharge in 21st century considering the impact of climate change the outputs of two Global Circulation Models of CCSM3 and ECHAM5-OM for three pessimistic (A2), average (A1B) and optimistic (B1) scenarios, were used. To downscale the outputs of GCMs to the study area LARS Weather Generator were applied and the downscaled data fed to the HBV hydrological model to simulate the future flow.
Results: The Mann-Kendall test showed significant negative (decreasing) trend of the flow in three sub-basins of Tire, Marbore and Sazar but no significant trend were detected for Bakhtiari. Also shift change point detection tests identified occurrence of downward shifts in the three aforementioned sub-basins; while upward shift were detected for Bakhtiari sub-basin. The magnitude of these changes is noticeable and ranges between 14.7% in Bakhtiarin to 43.3% in Marbore sub-basin. Also based on the results of the hydrological model by using the precipitation and temperature obtained for future time horizons of 2020, 2050 and 2080 as input, the annual flows show significant reductions in all sub-basins. Also, seasonal flows of Tire, Marbore and Sazar decreases in spring, autumn and winters but increases in summers. Seasonal flows of Bakhtiari basin also decreases in all seasons except for autumns. Generally, future simulation of the flow under the given climate change scenarios in the Dez Dam Basin shows drastic decrease (more than 1 billion cubic meters) of the annual potential discharge of the basin.
Conclusion: Investigation of the flow of Dez Dam Basin in the past decades shows significant monotonic and abrupt changes which is mostly toward decreasing the basin’s potential runoff. Additionally, assessment of future flow by different scenarios show reduction of flow due to impact of climate change. Considering these evidences it is likely that the basin face with discharge reduction in future and results emphasize on modification of water resources management strategies to adapt with climate change.