Identification of the degree of impact of dams on the hydrologic regime of semi-arid rivers: an evaluation of the DHRAM method

ABSTRACT The flow regulation that results from the implantation of dams causes consequences to the river ecosystems due to the modification on the characteristics of the hydrologic regime. The investigation of these changes become relevant, mainly in semi-arid regions where there is a great amount of these hydraulic structures and lack of such analyzes. Considering the above, this paper aims to evaluate the Dundee Hydrological Regime Alteration Method (DHRAM) through the classification of the degree of impact of dams located on rivers Itapicuru, Paraguaçu and their tributaries, verifying the adequacy of its use to represent the semi-arid hydrologic regime. Thereby, the DHRAM was applied in three versions: considering the thresholds that define the scores to classify the degree of impact in its original set (accordingly to Black et al. (2005)); with the adjustment of those thresholds to local conditions; and, with the regrouping of variables and adjustment of thresholds. The results showed that the method in its original set is applicable to semi-arid rivers, however it tends to be very restrictive against the high natural hydrologic variability characteristic of these rivers, and it ends up pointing to a high degree of alteration for dams that are known for not causing a very siginifcant flow regulation. The DHRAM with the regrouping of variables and the adjustment of thresholds presented the classification that approached the most to the known characteristics of the studied dams, being useful for the evaluation of the impact of dams still in project, and also to guide the adoption of operating rules that minimize the most significant hydrologic alterations that are identified.

摘要 大坝修建引发的径流调节作用会通过改变水文情势特征，对河流生态系统造成负面影响。对这类变化开展研究具有重要意义，尤其是在水工构筑物数量众多且此类分析较为匮乏的半干旱地区。鉴于上述背景，本研究旨在通过对伊塔皮库鲁河（Itapicuru）、帕拉瓜苏河（Paraguaçu）及其支流上大坝的影响程度进行分类，评估邓迪水文情势改变法（Dundee Hydrological Regime Alteration Method, DHRAM），验证该方法适用于表征半干旱区水文情势的合理性。据此，本研究采用三种版本的DHRAM开展分析：一是沿用原始阈值体系划分影响程度评分（参照Black等人2005年的研究）；二是针对当地条件调整阈值；三是对变量进行重新分组并调整阈值。研究结果表明，原始版本的DHRAM可适用于半干旱区河流，但由于这类河流本身具有较高的自然水文变异性，该方法往往过于严苛，最终会将那些实际径流调节作用并不显著的大坝判定为高度改变等级。经过变量重新分组与阈值调整后的DHRAM，其分类结果与研究坝体的实际特征最为契合，可用于评估尚处于规划阶段的大坝的影响程度，同时也可为制定运行规则以最小化已识别的显著水文改变提供指导。