The pressure on water resources availability has increased significantly, mainly due to population growth, migration to urban and coastal areas, climate change, and desertification, and is expected to become even higher [1
]. Therefore, the emerging issues will be even more significant as a result of water shortages in most areas [4
]. The primary challenge in urban and rural economic activities is the irregular and short duration of rainfall. Another challenge, which establishes the importance of water resources management (WRM) projects in insular clusters, is that of spatial discontinuity (distance between island land masses) [10
]. Therefore, the adoption of satisfactory socio-economic approaches, concerning water deficiency, is the solution to those severe WRM problems [12
]. Even more, spatial analysis of the specific background factors (geology, slopes, meteorological, etc.) of each insular study area is needed. In parallel, the degradation of ground water resources, related to the deprivation of their quality due to over-exploitation of aquifers, as well as of relic waters, should also be prevented [13
]. Furthermore, the use of non-renewable resources must be diminished and the rise of projects pointing at an ideal and sustainable use of surface runoff must be encouraged. Thus, a multi-dimensional view on the exploitation of water resources is considered necessary [16
Constructing projects that guarantee the exploitation of surface water through adequate, dispersed, small-scale harvesting systems (small dams, mountainous-hilly reservoirs) will possibly be more ecologically approachable than big-scale ones or those over-exploiting ground water. In addition, these systems might assist in various sustainable tasks, such as protection of natural environment, local-scale hydropower energy systems etc., while creating new opportunities for local jobs. Taking into account the evolving constraints, which arose through the recent economic crisis, inexpensive projects, in terms of economic value, become very challenging for local development [17
]. Small-scale mountainous reservoirs are those, which serve local development purposes [25
]. These are low-cost projects of high domestic local added value and should be supported in the future [26
]. The choice of constructing small reservoirs takes into account economic criteria, social imperatives, and environmental commitments [28
Developing a model that simulates natural phenomena is not an easy task. Either the same difficulty occurs when attempting to simulate the hydrological cycle, by lack of full knowledge of its internal processes or, more often, by lack of primary measured data. Geographic information systems (GIS) has made it easier and faster to process data to produce reliable simulation models. Thus, the integration of hydrological processes into a GIS environment has now reached the maturity level to allow a high degree of accuracy in simulating those processes. Many projects worldwide combine GIS and specific hydrological models to study a variety of procedures concerning dams and reservoirs. Most of these projects takes the existence of a dam or a reservoir for granted and simulates their operation under various scenarios [29
In view of the above, this paper presents a new methodological framework that can contribute efficiently to the construction of effective, low-cost systems for harvesting rainwater [32
]. It proposes a contemporary and integrated approach for selecting suitable sites within a catchment, to construct small-scale dams/reservoirs, by coupling GIS analysis techniques, SWAT (acronym for Soil and Water Assessment Tool, Austin, TX, USA) hydrological model [33
], and reservoir simulation software (RSS, Athens, Greece). The main goal is to quantify the annual runoff for a catchment, based on meteorological and spatial data (via SWAT), which then will be used as entry data in the RSS. The calculated monthly failure (in terms of meeting the water demand) by RSS qualifies the optimal positioning of the reservoir. Finally, the efficiency of the proposed methodological framework is verified in several stages throughout its application, by field measurements, the Nash–Sutcliffe index and the assessment of the simulated failure rates of the reservoirs scenarios. The verification results support the efficiency of the proposed methodology.
The methodological framework presented in this research work examines first the specific and unique characteristics of the selected study area (catchment) and then any possible intervention (reservoir) in it. The obtained results revealed that creating small-scale water reservoirs in the study catchment is feasible, provided that certain prerequisite criteria are met.
Cyclades Prefecture in Greece (a cluster of islands) experiences water scarcity, dependent on the annual and seasonal climate fluctuations and in particular rainfall. Cyclades islands are characterized by a permanent deficit in their annual spring and summer (May–September) water balance, hydrological uncertainty, and water inefficiency. These islands exhibit a lack of efficient countermeasures to deal with the rapid degradation of their water capital [59
In this work, several sites were initially examined in Andros Island to build dams and therefore to create a small reservoir. After this thorough site examination, the Afrouses catchment was chosen as the best place to build a dam. This choice was based on the spatial characteristics of the area such as slope, low-very low permeability of the geological formations, and altitude (Afrouses catchment is located on the highest mountain of Andros Island). In addition, it must be highlighted that in terms of geological/geotechnical conditions of the selected dam sites (two reservoir scenarios), a complete geotechnical research study has taken place which included stress tests for the reservoirs, geological background analysis, geotechnical slopes stability tests, etc. Also, this choice was based on its proximity to the capital of Andros Island, where most of the people of the island live, and where the highest touristic pressure occurs.
In general, the key outcome of this research is that with the results produced by the SWAT hydrological model as inputs, the reservoir simulation presented two acceptable and applicable choices. First, it is possible to build a dam in the selected location 1, for extracting low annual water volumes (up to 50,000 m3) and for a dam height of not less than 9 m, while second, for higher annual water volumes (up to 200,000 m3) and for heights of about 14–15 m, the selected dam location 2 is suitable.
At this point, it must be underlined that the assumptions that were made at several stages of the methodology define a specific uncertainty framework, according to which the results of this research are evaluated. It must also be emphasized that the developed methodology can be used as a standard procedure for quantifying surface runoff, which can be incorporated into a broader management framework. This means for example that on a practical level, prior to the implementation of any project (dam construction), a geological study must be carried out in order to clarify the local hydrogeological conditions. Moreover, this research work highlights the need for additional measures to avoid possible failures. In addition, in terms of meteorological data, the SWAT model provides the choice to the user to create a weather generator. Thus, in absence of historical time series, the statistical background of these meteorological data (of the weather generator) should be defined properly. Specifically, rainfall and temperature statistics (average maximum and minimum air temperature, standard deviation for maximum and minimum air temperature, average amount of precipitation, standard deviation for daily precipitation, etc.) should be valid in order for climate-change effects to be taken under considerations and co-estimated by the weather generator [37
Another crucial aspect that must be taken into consideration is the effect of sediment yield in reservoir operation. In this study, the rate of sediment yield is very low, as estimated by the SWAT model (Table 3
and Table 4
) and the assumption that the operation of the reservoir will not be affected was adopted. This was based on the fact that the geological background of the study area is not highly erodible (based on the results of the detailed geotechnical study that took place in the frame of this work) along with the fact that intense rainfall events are not frequent, thus lowering the erosion dynamics. Nevertheless, if in some period during the reservoir’s operational life increased sediment deposition is observed, thus reducing its efficiency, sediment removal actions can be considered. Furthermore, in cases of high sediment yield (e.g., calculated by SWAT), reservoir simulation can be modified accordingly.
The presented methodology offers results that can be continuously optimized as the data that are used become more and more reliable. In any case, this methodology can exceed its initial research purpose and become a very useful management tool. It can become part of a wider management plan, not only for Andros Island but also, in a more general scope, for island complexes where due to their isolation from the main continental inland, sustainable and efficient exploitation and management of their natural resources (e.g., rainfall–surface runoff) is a necessity.
Further development and optimization of the presented integrated methodology can include specific actions, which can result in the best possible approach to the real conditions of each area to be applied. Such actions can be an even more thorough calibration of the modeling and simulation processes (SWAT, reservoir simulation), based on true data from each studied site [60
], and sensitivity analysis as well [61
]. Furthermore, the surface runoff results of the applied methodology have been compared with those of other methods [62
], as well as with satellite imagery of the reservoir surface [63
], and consequently with the available water volume for extraction, and were found to converge satisfactorily.
Among the planned future actions, for testing and developing the predictive ability of the presented research methodology, simulations are included that will take into account various climate-change scenarios [64
] and/or underestimated meteorological data, while exploring and analyzing each of the estimated runoff volumes by the SWAT model [65
]. Another parameter that will be tested via various scenarios is the possible surface runoff alternations due to systematic changes in land use/cover [66
]. Synthesis of a valid and detailed soil map, based on in situ measurements and recordings, is another action currently developing, as expected, with great certainty, to contribute in producing even more reliable results from SWAT modeling [67
]. It must be mentioned here that the hydrologic analysis of the used DEM (pixel size 28 × 28 m) covers the needs of this current work, since it provides satisfactory results, as Chaplot already pointed out that there is no reason to use a very accurate DEM to obtain better predictions [52
]. Nevertheless, the used DEM cell size in this research is very close to the lower limit (better resolution) that Chaplot used in his work.
Following the aforementioned conceptual logic for evolving the presented integrated methodological framework, it is of equal importance to investigate specific/individual flood phenomena through daily simulations, in order to develop flood design maps, as well as scenarios for protective actions and works against catastrophic events [68
]. This investigation also makes sense in relation to climate-change scenarios, as for example a small increase in temperature will lead to an increase in evaporation and consequently a decrease in surface runoff [69
In the present study, as mentioned above, the assessment of sediment yield that was attributed by the model along with the other results was not taken into consideration in the evaluation process. This is also a parameter that is important to be considered (as long as there are reliable soil data), as it plays a critical role in the reservoir’s operation, in terms of availability of water volume for extraction [70
In addition, the results of the reservoir simulation can be explored in light of different scenarios regarding changes in the water demand distribution rates per month, as well as different extraction volumes from the reservoir depending on variations in demand. For the second case, comparisons can be made with other research or applied works on reservoir management that use control curves [71
Another point that could be explored further is the comparison of the produced results for the selected dam locations, with the results of other similar works that relate to the location and size of the reservoir [72
]. Also, the size of the dam, compared to more classical methods used for its determination, could be explored additionally [73
]. Finally, in the present work, it is possible to measure/quantify the sustainability of the examined reservoirs using a methodology for measuring it with specific indicators, such as resilience, reliability, vulnerability, and relative vulnerability of the project [74
All previous suggestions aim at improving the reliability, completeness, and predictive ability of the methodology proposed herein and constitute a satisfactory framework for developing an integrated decision-making system. It could be a policy-making tool towards the research and application of sustainable and optimum reservoir construction, in various mountainous and semi-mountainous areas in Greece, but worldwide as well. In this context, research projects funded by national bodies, such as local authorities, aim at advancing local development. It must be also highlighted that in the research project “Utilization of surface runoff in Andros Island via the creation of mountainous water reservoirs” (2009–2014), the possibility of applying the developed methodology in catchments of similar scale in the Aegean islands and beyond was examined, as the application in Andros Island was only a pilot. This project has been accepted by the municipality of Andros and is in the process of finding the necessary funding to implement it.
In the present paper, an integrated and contemporary methodological framework, based on geoinformation technologies, was introduced for exploiting surface runoff, by creating small mountain and semi-mountain water reservoirs. With this approach, descriptive and spatial information are coupled in the best possible way, to produce reliable results, which will lead to the adoption of the best and most sustainable practices. The presented methodology can be a part of a general and holistic management framework for viable water resources exploitation. This was demonstrated using as a case study the island of Andros, in Greece. Andros Island (and specifically Afrouses catchment) was selected as the pilot study area for applying the research methodology, due to its specific characteristics of particular interest. The fact that the island experiences sufficient rainfall in the period from September to April, combined with the increased needs of the dry season and the lack of primary meteorological data, led to its selection as the study area (it is considered important to apply this research in areas with limited data adequacy).
Results showed that by using the proposed scheme it is possible to create small water reservoirs, if certain criteria that have been set from the beginning are met. A small prediction uncertainty was present, resulting from the various simulation uncertainty and the assumptions made in some stages of the research. On the other hand, the validation process and the comparative analysis of the results, along with the comparison with similar works found in the literature, managed to minimize (or even eliminate in some cases) these uncertainties. In any case, this research work highlights the need for adopting additional measures to avoid potential failures in the design and/or operation of a small-scale reservoir. Furthermore, it must be mentioned that the results of the presented methodology can be continuously optimized as the data used become more and more reliable.
Taking into consideration all the above, it must be underlined that by selecting the best available sites for constructing small-scale reservoirs, it is easy to create a network of them, using local human resources and materials, thereby creating low-cost projects with high socio-economic benefits in local scale. This is a huge step towards sustainable exploitation of surface runoff and an ideal countermeasure for areas suffering from water scarcity.
In conclusion, the potential that rises by coupling geoinformation technologies, such as GIS, with simulation process modeling offers a potentially promising roadmap towards improving our ability to provide a mathematical representation of our natural environment. As the proposed methodology can be automated to simulate both present and future conditions, it may offer a very promising tool to the scientific and wider community for a better understanding of our dynamically changing physical environment. As such, it could assist research and practical applications alike.