A novel comprehensive hydrologic alteration evaluation method
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Abstract:
Reservoir construction and operation disturbance effects on downstream can not be ignored. Hydrologic alteration analysis is an effective way to reflect the impact of human activities on the hydrologic regime. Currently, the widely used hydrologic regime analysis methods are mostly derived from hydrologic alteration (IHA) indicators. However, each method has its advantages and disadvantages. Different evaluation results by these methods indicate that the hydrologic variability analysis is subject to a high degree of uncertainty. Due to the different characteristics of the assessment units, it is still uncertain to determine which method is the most reasonable. Therefore, it is necessary to find a reliable evaluation method for hydrologic regime alteration.Four evaluation methods of hydrologic regime alteration, i.e., the range of variability approach (RVA), histogram matching approach (HMA), histogram comparison approach (HCA), and revised range of variability approach (RRVA) were selected for analysis and comparison. The natural daily flow series from 1954 to 2021 and the measured flow from 1974 to 2021 at Huangjiagang hydrological station were used to analyze the hydrologic regime alteration after the operation of Danjiangkou Reservoir. Application results of the four methods are similar and could confirm each other, but also had their own merits and limitations. The RVA has mathematical drawbacks and ignores the specific variability of the IHA within and outside the target range. Both the HMA and HCA use frequency histograms to describe IHA, which could show the full range distribution of IHA values. HCA, as an improved version of HMA, has more reasonable and comprehensive results. Based on considering frequency difference, RRVA takes into account the morphological changes of IHA, including multiple characteristics such as time, magnitude, etc., so its evaluation results are the largest.Therefore, a novel comprehensive hydrologic alteration evaluation method is proposed, which is the combination of RRVA and HCA methods with different weights based on subjective and objective information. The comprehensive evaluation results of the hydrologic variability degree are analyzed in more detail by the frequency histogram of each indicator. It has the following four advantages: preserve the distribution information of all intervals of IHAs through frequency histogram; avoid abnormal extreme results of hydrologic alternation; consider the morphological change and spatial-temporal differences of the hydrologic regime; reduce the result uncertainty effectively. The comprehensive evaluation results showed that the changes after the construction of Danjiangkou Reservoir conformed to the operation law of flood storage and dry compensation. The flow increased significantly in the dry season and decreased significantly in the flood season. The change degree of monthly average flow was 41.28%, and the change degree of extreme flow conditions was 64.83%. Moreover, the rate and frequency of water conditions changed highly, which was 80.53%. After the implementation of the middle route of the South-to-North Water Transfer Project, the flow showed a downward trend, indicating that the available water resources in the middle and lower reaches of the Han River were reduced. The overall hydrologic regime alteration at Huangjiagang hydrological station was 55.70%, which represented a moderate change.The comprehensive hydrologic alteration evaluation method could reduce the uncertainty of the results, capture more information on IHAs before and after human impact, and provide reliable data for ecological restoration and operation, etc. The operation of the Danjiangkou Reservoir has changed the natural flow regime and caused pressure on the natural habitat in the middle and lower reaches of the Han River, which requires measures to restore its ecology.
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