Conjunctive use of surface and ground water
INTRODUCTION: Conjunctive use of surface and groundwaterDisadvantages of surface water development compared to groundwater In most climates of the world, precipitation, either rain or snow, and consequently peak runoff corresponding to a significant part of the total discharge of the rivers, occur during a particular season of the year which usually coincides with the smallest water demand. The water development problem therefore consists of transferring water from the high supply season to the high demand season. The most obvious and the most common solution to that problem consists of storing surface water behind dams, but storage of water in the ground may be a valuable alternative to surface storage systems, although not always systematically considered when planning water development. Yet surface reservoirs have many drawbacks, especially:
- evaporation:large open water areas are exposed, during several months and even years, to high evaporation rates leading to water losses sometimes exceeding 20 percent of the average annual runoff. Losses may be even higher when the width of the impounded valley is considerable, and induces a larger open water area.
- Sedimentation:soil erosion in the catchment results in siltation in the surface reservoirs and in the equivalent reduction of the storage capacity. The soil vulnerability to erosion, and therefore the importance of the siltation problems in surface reservoirs, grows as the vegetation cover shrinks, so the more arid the climate, the less the vegetation cover, the higher the probability of sediment accumulation in the surface reservoirs. Draining part of the mud from the reservoirs is occasionally possible through specially designed pipes placed at the bottom of the dam, but each operation is water consuming (to flush the mud) and may be detrimental to downstream environment.
- Environmental impact of surface reservoirs may often be highly undesirable for human health, flooding of inhabited or good agricultural land,
- Distribution of water from the reservoir may be expensive and requires the construction of costly canals because of the distance between dam and utilization areas.
Surface water storage always preferred to groundwater development: When looking at these advantages and disadvantages, groundwater seems to be a better alternative that should be preferred, but this not the case; large and concentrated water demand such as that from large irrigation schemes is usually supplied from surface water storage, and there are various reasons for that choice:
- groundwater aquifers seldom offer large storage capacity able to absorb large volumes of flood in a short period of time, and are unable to return them as significant discharge per unit production system of well or borehole,
- surface water storage, because of the large investments involved, is often preferred because it offers a much higher political visibility and because high construction costs give an opportunity for private profit and corruption, opening the way for improper influence on decision making.
A reasonable solution: Conjunctive use of surface and groundwater consists of harmoniously combining the use of both sources of water in order to minimize the undesirable physical, environmental and economic effects of each solution and to optimize the water demand/supply balance. Usually conjunctive use of surface and groundwater is considered within a river basin management program - i.e. both the river and the aquifer belong to the same basin. Assuming that the mixed solution is part of the national policy, several problems need to be carefully studied before selecting the different options and elaborating a program of conjunctive use of surface and groundwater:
- Underground storage availability to be determined,
- Production capacity of the aquifer(s) in term of potential discharge,
- Natural recharge of the aquifer(s)
- induced natural recharge of the aquifer(s)
- Potential for artificial recharge of the aquifer(s)
- Comparative economic and environmental benefits derived from the various possible options.