Abstract
The goal of this study is to demonstrate the usefulness of the Lombard method for analyzing dam-induced hydrologic impacts. The method was used to accurately detect the effects of the construction of a diversion dam in 2003 on annual and seasonal maximum and minimum daily flows in the Manouane River, Quebec, Canada, measured from 1980 to 2014. The Lombard method yields results that are nearly identical to results obtained using the monitoring (Kruskal-Wallis test) and long-term trend (Mann-Kendall test) methods. The Lombard method revealed a shift in mean values of annual and seasonal minimum daily flows in 2003, the year the dam was built. This shift is sharp for all four seasons. The dam induced a significant decrease in minimum daily flows in all four seasons. As far as maximum daily flows are concerned, unlike the monitoring method, the Lombard method detected a significant decrease only in the mean values of annual and spring maximum daily flows. This decrease occurred two years prior to the construction of the diversion dam. Instead, this decrease is interpreted to be the result of a significant decrease in spring precipitation after 1997. These hydrological changes are different from those induced by other types of dams in Quebec.
Highlights
Four methods are generally used for analyzing the impacts of dams on streamflow [1]: The monitoring station method, which consists of comparing the streamflow or water level before and after the construction of a dam
In the case of gradual hydrological changes, the effects of dams may occur later, making it difficult to link them directly to dam construction; They do not accurately distinguish the effects of various factors in a watershed; They do not allow the detection of multiple hydrological changes that may occur after the construction of a dam, including some that may not be induced by the dam itself
The Kruskal-Wallis test brought out a significant difference in the mean values of annual and seasonal minimum daily flows before and after the construction of the diversion dam, which reflects a significant decrease in the minimum daily flows after dam construction (Table 1 and Figure 2)
Summary
The monitoring station method, which consists of comparing the streamflow or water level before and after the construction of a dam This is the most widely used method in the scientific literature to constrain the impacts of dams, and it is considered to be the most precise (e.g., [2,3,4,5,6,7,8,9,10]); The control station method, which consists of comparing flows downstream from a dam with flows upstream from it or from another undammed natural stream (e.g., [11,12,13,14,15,16,17]). These four methods, are plagued by several shortcomings both from a hydrological and a statistical standpoint: They do not allow the accurate determination of the sharp or gradual nature of dam-induced changes in streamflow, a very important consideration in ecohydrology, because it affects the capacity of aquatic and semi-aquatic organisms to adapt to such changes. Later climate change may dampen or amplify dam-induced hydrologic impacts
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