The cumulative impacts of small reservoirs on hydrology: A review

Abstract

The number of small reservoirs has increased due to their reduced cost, the availability of many favourable locations, and their easy access due to proximity. The cumulative impacts of such small reservoirs are not easy to estimate, even when solely considering hydrology, which is partially due to the difficulty in collecting data on the functioning of such reservoirs. However, there is evidence indicating that the cumulative impacts of such reservoirs are significant. The aim of this article is to present a review of the studies that address the cumulative impacts of small reservoirs on hydrology, focusing on the methodology and on the way in which these impacts are assessed. Most of the studies addressing the hydrological cumulative impacts focused on the annual stream discharge, with decreases ranging from 0.2% to 36% with a mean value of 13.4% ± 8% over approximately 30 references. However, it is shown that similar densities of small reservoirs can lead to different impacts on stream discharge in different regions. This result is probably due to the hydro-climatic conditions and makes defining simple indicators to provide a first guess of the cumulative impacts difficult. The impacts also vary in time, with a more intense reduction in the river discharge during the dry years than during the wet years. This finding is certainly an important point to take into consideration in the context of climate change. Two methods are mostly used to estimate cumulative impacts: i) exclusively data-based methods and ii) models. The assumptions, interests and shortcomings of these methods are presented. Scientific tracks are proposed to address the four main shortcomings, namely the estimation of the associated uncertainties, the lack of knowledge on reservoir characteristics and water abstraction and the accuracy of the impact indicators.

Distribution of small seasonal reservoirs in semi-arid regions and associated evaporative losses

“…At present, in some catchments the density of such small reservoirs may reach 10 reservoirs per km 2 [16], yet a systematic assessment of the global distribution of such an important class of small reservoirs remains lacking. With development of remote sensing techniques for retrieving salient information from satellite imagery [17][18][19], the identification of locations, sizes, and usage of small water reservoirs now becomes feasible.…”

Hydrological Characterization of Watering Ponds in Rangeland Farms in the Southwest Iberian Peninsula

“…Some authors that addressed this issue in semi-arid regions [31,34] found extraordinarily high values of seepage rates (up to 80 mm/day) that led the ponds to keep water only for short periods of time, which questions their usefulness. Nevertheless, these seepage rates contrast to regular values reported for this type of pond, which rarely exceed 5 mm/day [11,35,36]. Therefore, there is limited knowledge about the effectiveness of these ponds operating under normal conditions.In this study, the structural and functional attributes of a sample of representative livestock watering ponds in the southwest of the Iberian Peninsula and the temporal patterns of their water regime were investigated, with the objectives (1) to determine the influence of temporal rainfall variability on water availability in the ponds; (2) to assess the capacity of the ponds to conserve stored water (i.e., their effectiveness) during the regular dry season and during drought events; (3) to examine the influence of factors such as pond size and catchment area on the effectiveness of the ponds; and (4) to test whether simple analyses, primarily based on precipitation data and aerial photographs available from public sources, can be used reliably in these and other rangeland areas to suggest minimum values for the above factors, which can serve as a guide for the design of watering ponds.It is expected that the conclusions of this study may be helpful for a better understanding of the hydrological behavior of watering ponds of Iberian rangelands, as well as the factors that determine their efficiency in terms of water supply.…”

“…Some authors that addressed this issue in semi-arid regions [31,34] found extraordinarily high values of seepage rates (up to 80 mm/day) that led the ponds to keep water only for short periods of time, which questions their usefulness. Nevertheless, these seepage rates contrast to regular values reported for this type of pond, which rarely exceed 5 mm/day [11,35,36]. Therefore, there is limited knowledge about the effectiveness of these ponds operating under normal conditions.…”

“…A similar consideration can be made in respect to seepage losses. Although in this type of infrastructure seepage losses are usually considered to be small relative to other inputs and outputs [35,36,77], some studies revealed cases in which seepage was the main loss of the ponds [31,34]. Therefore, the assumption of small seepage losses should be carefully checked for each case.…”

Challenges in assessing the regional feasibility of local water storage

“…Although the impacts of on-farm storage are not considered in their study, the authors suggest that their assessment could be helpful in comparing the storage potential of large reservoirs with on-farm storages. Habets et al (2018) review modelling approaches to assess the hydrologic impacts of small reservoirs. As their focus is on modelling river flow, they relate to water extraction for irrigation as an exogenous factor.…”