6 Conclusions

In this paper, the impact of Chaersen Reservoir (Northeast China) on downstream runoff and baseflow recession characteristics is studied using a pre‑post comparison method, and the influence of climate change is excluded using two upstream sub-watersheds as control basins. In addition, the impact mechanism of the reservoir is further explored. The main conclusions are as follows:
1) After the construction of Chaersen Reservoir, the regulation of runoff directly leads to the reduction of peak flow (Qmax decreased by 35%) and the increase of baseflow (7Qmin increased by 235%) of downstream Zhenxi station, and the increased direct and indirect consumption of reservoir storage leads to the reduction of average or total streamflow by about 14%.
2) After the construction of the reservoir, a relatively constant flow (about 1m3/s) is added to the baseflow recession process of downstream Zhenxi station, which causes the log (|dQ /dt |) vs. log (Q ) scatter points to shift to the right in the late recession stage, and finally causes the recession coefficient a to decrease by about 60% andb to increase by about 24%. This change also means the upward shift of the MRC, the slowing down of the recession rate, and the increase of the active water storage in the basin.
3) After adding a constant flow, the log (|dQ /dt |) vs. log (Q ) scatter points no longer satisfies a strict linear relationship. The MRC obtained by the traditional linear parameterization method is only an average approximation, which will overestimate the streamflow in the middle recession stage and underestimate the streamflow in the later stage, but the deviation is not obvious as a whole.