Abstract:

Objective evaluation of flow regime alteration is the foundation of river ecological restoration and integrated water resources management. Various approaches that take hydrological year types into consideration could objectively evaluate the altered flow regime. Due to climate change in the recent decades, the flow regime of upper Heihe River has altered and needs further research. Our study area was the upper Heihe River, where we employed an improved range of variability approaches to analyze the alterations of the flow regime. Firstly, we applied the Mann-Kendall method to identify the catastrophe point. We analyzed the flow regime using hydrological alteration indicator method. This method includes 33 parameters classified into five groups: The magnitude of monthly water conditions, magnitude and duration of annual extreme water conditions, timing of annual extreme water conditions, frequency and duration of high and low pulses, and rate and frequency of water condition changes. Based on these groups, we used the range variability approach to calculate the degree of alteration of these 33 parameters. We measured the difference in typical hydrologic years before and after the catastrophe point using the Euclidean distance. The degree of hydrologic alteration was calculated considering the alteration of these 33 parameters and the difference between typical hydrologic years. Our results indicated that the year 2002 was catastrophic point of the river flow regime. The main hydrological type years before and after 2002 were normal flow and wet years. The improved method calculated an alteration degree of 74.62%, while the range of variability approach showed an alteration of 54.68%, indicating underestimation of the effect. Compared to 1960-2002, the monthly mean streamflow in 2003-2015 increased to various extents in the upper Heihe River’s flow regime. The annual minima 1 day, 3 day, 7 day, and annual maxima 7 day decreased, whereas the other extreme water condition indicators increased. The frequency and duration of high and low pulses changed slightly, and the average peak flow duration decreased while the other indicators increased. Rate and frequency of water condition increased. The degree of hydrologic alteration was 74.62% considering the change of HYTs, which indicated a high alteration in the upper Heihe River’s flow regime. Climate change is the main reason causing the flow regime alteration.

Key words: alteration of river flow regime, range of variability approach, hydrological year types, upper Heihe River