Development of a revised method for indicators of hydrologic alteration for analyzing the cumulative impacts of cascading reservoirs on flow regime

The impacts of reservoirs, especially multiple reservoirs, on the flow regimes and ecosystems of rivers have received increasing attention. The most widely used metrics to quantify the characteristics of flow regime alterations are the indicators of hydrologic alteration (IHAs) which include 33 parameters. Due to the difference in the degree of alteration and the intercorrelation among IHA parameters, the conventional method of evaluating IHA parameters that assigns the same weight to each indicator is obviously inadequate. A revised IHA method is proposed by utilizing the projection pursuit (PP) and real-coded accelerated genetic algorithm (RAGA). Data reliability is analyzed by using the length of record (LOR) method. The projection values reflecting the comprehensive characteristics of the evaluation parameters are calculated. Based on these methods, a scientific and reliable evaluation of the cumulative impacts of cascading reservoirs on the flow regime was made by examining the Jinsha River. The results showed that with the increase in the number of reservoirs, the alteration degrees of IHA parameters gradually increased in groups 1, 2, 3 and 4 but decreased in group 5 (each group addresses the magnitude, timing, frequency, duration and rate of change in turn), and the flow duration curves showed a declining trend at the high-flow part and an increasing trend at the low-flow part. The flow regime alteration of the outlet section was more stable than before. This change had a negative impact on downstream fish reproduction and ecological protection. An attempt at ecological regulation was made to simulate the natural rising process of water, and four major Chinese carps have a positive response to the flood peak process caused by manual regulation.

Cumulative impact of small reservoirs : a review of estimations and methods

<p>Due to a reduce cost, availability of many favorable locations, easy access due to proximity, the number of small reservoirs has increased, especially in arid and semi-arid regions. The cumulative impact of reservoirs in a catchment is considered as the modifications induced by the reservoir network taken as a whole. The impact may exert on the flow regimes and sediment, nutrient and contaminant transfer, and thereby modify the ecological behaviour of the aquatic environment, the continuity of rivers and the habitats of organisms living in them. The cumulative impact is not necessarily the sum of individual and local modifications, because reservoirs may be inter-dependent. This is the case for instance in cascading reservoirs along a stream course. The cumulative impact is not straightforward to estimate, even solely considering hydrological impact, in part due to the difficulty to collect data on the functioning of those reservoirs. However, there are evidences that the cumulative impacts are not negligible.</p><p>This work is dedicated to a review of the studies dealing with the cumulative impact of small reservoirs on hydrology, focusing on the methodology as well as on the way the impacts are reported. It is shown that similar densities of small reservoirs can lead to different impacts on the quantitative water resource in different regions. This is probably due to the hydro-climatic conditions, and makes it difficult to define simple indicators to provide a first guess of the cumulative impact. The impacts vary also on time, with a more intense reduction of the river discharge during the dry years than during the wet years. This is certainly an important point to take into account in a context of climate change.</p><p><em>Habets, F., Molénat, J., Carluer, N., Douez, O. and Leenhardt,D, 2018, The cumulative impacts of small reservoirs on hydrology: A review, Science of The Total Environment, 643, 850-867, doi.org/10.1016/j.scitotenv.2018.06.188</em></p>

Spatial and temporal variation of the phytoplankton structure in cascading oligotrophic reservoirs of southeast Brazil

Aim Structural changes in phytoplankton was evaluated in one ultraoligotrophic and two oligotrophic cascading reservoirs during two climatic periods (austral winter and summer) of 2014. Changes in phytoplankton biomass, species composition, and codons (functional groups) were evaluated along a vertical and a longitudinal profile. Methods Four sampling sites were selected for the first reservoir in the cascade (Cachoeira do França) and three for each of the other two reservoirs (Cachoeira da Fumaça and Serraria). Samples were collected along a vertical and a longitudinal profile. Species biomass and codons ‘sensu’ Reynolds were identified and expressed in terms of biomass. Results 18 codons were identified and associated to the reservoirs’ limnological conditions. Composition and biomass of descriptor species changed among seasons. Codon B consisting mainly of Discostella stelligera was the most frequent in all reservoirs in both climatic seasons. Conclusions Our findings showed that seasonality was the key factor for the species composition and phytoplankton codons changes. However, the cascade effect on phytoplankton structure was shown by the continuous contribution of Ceratium furcoides (codon LM) during the winter, and of Discostella stelligera (codon B) in both sampling periods in all three reservoirs.

Effect of Cascading Reservoirs on the Flow Variation and Thermal Regime in the Lower Reaches of the Jinsha River

We analyzed the alteration of discharge and water temperature caused by two newly established reservoirs in the lower reaches of the Jinsha River. In comparison to longer-term observations from the pre-impoundment period, the seasonal flow variability was significantly affected by the larger, upstream-located Xiluodu reservoir, with higher discharge in spring and reduced discharge in summer. The smaller, downstream located Xiangjiaba reservoir did not contribute significantly to the total hydrological alteration caused by the reservoir cascade. Thermal stratification occurred in spring and summer in the Xiluodu reservoir, but was not observed in the Xiangjiaba reservoir. The vertical structure and seasonal dynamics of thermal stratification were mainly governed by the water temperature of the inflow and the depth of the water outlet. Despite the different thermal structure, both reservoirs reduced the amplitude of annual temperature variations and delayed the seasonal temperature cycle in the downstream river water. In contrast to discharge variation, thermal effects were cumulative along the cascading reservoirs. Homogenization and delay effects can be expected to increase further with the completion of ongoing reservoir construction upstream of the two studied reservoirs. Based on our findings, we discuss the larger-scale impacts of cascading hydropower developments and emphasize the need for taking water temperature and its variation into account when developing optimized operation or mitigation strategies for these systems.

Climate factors and limnological conditions shaping phytoplankton community in two subtropical cascading reservoirs

Aim: A series of dams along the course of a river forms a complex cascading reservoir system. The interconnectivity of the dams is perceived with the reduction of turbidity and phosphorus and the increase of specific richness throughout the system. Designing a management system that promotes the integration of biotic and abiotic data is essential to these resources of recognized importance for the population, the country's power generation and watersheds management. The Jacuí River is an important water resource in the state of Rio Grande do Sul, along its upper course, five water reservoirs comprise a cascade system tapped for energy generation and agriculture, but the effects on the phytoplankton community of cascading dams are still unknown. Thus, the present study aimed to investigate spatial and temporal patterns of phytoplankton associated with climatological and limnological characteristics of the Ernestina and Itaúba water reservoirs, located in the upper Jacuí River, RS, Brazil. Methods Sampling campaigns were carried out seasonally from January 2012 to February 2013, with four sampling stations within each dam and at three different depths. The sampling units were determined according to the three zones of the reservoir: river areas, transition and lake. Results: A total of 91 phytoplankton species were identified in Ernestina and 130 in Itaúba. There was no significant difference between vertical and horizontal patterns, since the community responded to seasonal changes and the specific characteristics of each dam. Conclusions The specific richness and water transparency increased downstream of the system and changes in water retention time were important for cell density and composition.

Sensitivity Analysis of Hydroelectric Power Generation from Cascading Reservoirs

The cascading reservoirs in Perak, Malaysia, were used to test the sensitivity analysis of hydroelectric power generation during refill and deplete period of the reservoirs. The cascading scheme comprises four reservoirs namely Temenggor, Bersia, Kenering and Chenderoh. The test was conducted after the analysis of water balance and stage-storage relationship of each reservoir in the cascading scheme. The result showed that power generation from the smaller reservoir, Bersia, is more sensitive to the change of headrace level, while the larger storage capacity and rated head reservoir is the most sensitive to the change of release. Therefore, to maximize the power generation from the cascading reservoir, the refill operations should be ranked according to the increasing order of the reservoir storage capacity and a reverse order should be followed during deplete period.