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., Mol&#233;nat, 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>


quantifying single reservoir functioning from the monitoring of a sample of reservoir and extrapolate the cumulative impact → cumulative impact is not the sum of single impact • Statistical analyses of the observed discharge to connect the detected changes in the statistical properties of river discharge time series with the evolution of the reservoir network within the basin → need long time series ; difficulty to decipher the potential causes of stream flow changes • Paired catchment quantifying the impact from a comparative analysis of stream flows monitored in two contrasted catchments, one with small reservoirs, the second one without small reservoirs → not appropriate for predictions of impact of new small reservoirs

2-Modelling approach
• Principle : Coupling of the small reservoir water balance model with a quantitative method to estimate stream inflow into the small reservoirs.

Current Issues Uncertainty
Review shows that the uncertainty in simulated impact may result from three main factors : i) reservoir characteristics, in particular reservoir drainage area, ii) water management estimation, especially abstraction estimation, and iii) spatial representation of reservoirs and iii)

Small reservoir characteristics
Location, connexion to the river, capacity and bathymetric relationship are among the characteristic needed for cumulative impact estimation. Along with improvements in survey and remote sensing methods, one track to improve our capability of estimating small reservoir cumulative impacts relies on storing and sharing of information collected through operational surveys and scientific studies

Abstraction estimations
Esimations of cumulative impact of small reservoirs are very sensitive to estimation of absractions from reservoir. Two way to improve abstraction estimations : i) store and make available farmers declaration (abstraction volumes and occasionally the timing) to water management agencies or state services. Empirical relations relating the characteristics of reservoirs with crop or animal needs could be one way to estimate and spatialize the water abstraction from small reservoirs more accurately than the current simple and pragmatic methods. ii) take advantage of agronomic state-of-the art in terms of crop management strategies and models. Decision rule models are now available to simulate and predict tillage, sowing, fertilization, hoeing, irrigation, crop protection, and harvesting periods.

Impact indicator
Simple indicators of cumulative impacts are needed by stakeholders and water management actors. From a scientific perspective, this operational need consists of first analysing whether cumulative impacts can be derived from properties of reservoir networks or others.
1 By storing water during wet season to support water uses (irrigation, livetsock) during dry season, small reservoirs are considered as an adaptation technique to drought, uneven rainfall and climate change. Small reservoirs are also build to prevent flooding or to store sediments in check dams.
A small reservoir has a capacity lesser than 1 million m³. The local hydrological functionning of a small reservoir depends on hydrological processes as well as man operations (Fig. 1).
The number of small reservoirs has been increasing wolrdwide, especially in rural catchments of arid and semi arid regions. Small reservoir density can exceed 100 /km² , which may increase the water resource problem in both quantitative and qualitative ways.
 Cumulative impact is mainly estimated in considering annual stream discharge  Annual stream discharge descrease due to small reservoirs network (Fig. 3) Questions 1) What is the hydrologic cumulative impact of small reservoirs network ?
2)What is the state-of-the-art in matter of methods of cumulative impact estimations ?
A litterature review was perfored to answer it.
• Modelling is currently the most common approach. However, important issues are addressed.