Reservoir management

5.2.1. Need for reservoirs

Most water supply schemes need to incorporate reservoirs. These may be surface storages or sub-surface aquifers. The function of the storages is to smooth out the natural variability of the hydrological system to allow human activity to be supported by a constant, or a regular, seasonally varying supply. Where water is scarce it is most unlikely it will be possible to take water on demand from the natural system. In times of high flow (either surface or subsurface) it is often possible to extract whatever water is required. However in drier times the natural flow is likely to be significantly lower than the expected extraction rate. Hence surface or subsurface reservoirs serve as temporary storages, capturing high flows whose water can then be available for use during periods of low natural flow.

Reservoirs need to be sited to minimise non-beneficial leakage or removal of captured water. For example a sub-surface reservoirs (aquifer) should be located where there can be large inflow but minimal outflow of water. Making effort to encourage recharge of an aquifer which quickly transmits water to the sea makes little sense. One needs to be found where water is trapped in the local region, and which can then be

extracted for later use.

Surface reservoirs need to be sited to minimise evaporation and seepage. This means the geology needs to be investigated to attempt to ensure seepage will be small.

Evaporation can realistically only be minimised by keeping the surface area of the reservoir as small as possible. Reservoirs should be located to have a maximum volume/surface area ratio, otherwise a large fraction of the captured water will be lost by evaporation. In a region of Australia where the annual pan evaporation is about 4 m, one of the reservoirs providing water for a town of 30,000 people makes beneficial use of only 20% of all its stored water, the remainder (80%) evaporates. If evaporation could be reduced by just 25%

the water supply could be doubled. However at this time, no practical strategies are available for reducing evaporation from large reservoirs. The only possibilities are at the planning stage to choose a site for the reservoir where the storage will be deep and have minimum surface area.

Silting is another opponent of maximizing water use. Over time surface reservoirs capture almost all the sediment carried by inflowing streams. Care is needed to site reservoirs where the sediment load of streams is small or provide a sediment trap upstream of the reservoir. The sediment trap will need regular cleaning if it is to function effectively.

This is quite possible for a small reservoir which is supplied from only one stream but for large reservoirs it is usually unrealistic because inflow rates may be very large and from many streams. In this case the only means of preventing reservoir silting is to improve soil conservation practices by good land management and channel protection.

5.2.2. Water scarcity management

There are many opportunities for management of water in an environment of general shortage. Some of these possibilities for good management of a scarce resource are dependent on availability of capital for investment in fixed infrastructure. These opportunities usually need to be grasped at the planning stage. Their capital intensive nature means they are fairly inflexible and therefore are unlikely to be changed significantly in the near future. This means that good long term planning is needed, but this planning needs to avoid focusing on a narrow range of water use opportunities. The chances of major socio-economic or technological changes in the short-to-medium term are high and therefore narrow range or single use capital infrastructure may be a serious impediment to beneficial water management in an as yet unforeseen socio-economic and technological environment.

For example there is a move away from having large storage reservoirs as the major investment of new water development projects. Moves are towards larger numbers of smaller scale water storages since these offer many more management options and particularly localised management, which is rarely possible with projects dependent on a single large storage. In overall terms a single large reservoir may provide for maximum control of water, but ultimately the inflexibility of operation of such a system often means that only a very limited sector of the population can benefit, whereas with multiple small systems the potential for flexible management and spread of benefit to a wider sector of the community is large.

Another benefit of smaller systems is that stratification and eutrophication problems can be more easily managed in small reservoirs than large ones. Overriding all these considerations must be the topography of the location. Some landscapes lend themselves to

multiple reservoirs, others do not. Large reservoir development depends on having a suitable dam site in the region.

At the operational management level there can be many opportunities for improving water availability and for improving security of supply. These two benefits are usually considered to be opposed – to increase security means more water must be held in reserve and so supply must be decreased. In a general sense this is true but with careful consideration of all the factors which influence supply, sensible flexibility of management can lead to a significant increase in one of these benefits, with no reduction of the other.

Understanding of natural processes such as the occurrence of precipitation in time has enabled the development of statistical techniques which better reflect the natural processes and which can be used in flexible modes to mirror local operations and increase the benefits to be obtained from a resource. For example conservative thinking combined with only simple analytical techniques often suggests that the best management policy is to keep a reservoir as near to full as possible at all times. However it may be that more adventurous management, which may increase water use by 25% (and reduce losses of water from the reservoir by seepage and evaporation by 10%) may lead to crop failure one year in 10. The overall increase in productivity from such management may be anywhere from 15 to 50%.

However, introduction of such a scheme would need to be accompanied by an ongoing education program to encourage water users to build up a reserve from their increased returns, to allow them to survive in the years of failure.

There are many reservoirs around the world which are full or near-full most of the time, yet almost all reservoirs were originally designed to become near-empty more than once during their life. Unfortunately there is too little communication between reservoir designers and reservoir operators, and much of that communication is via politicians who tend to see empty reservoirs as some indicator of evil, rather than the near empty reservoir as an item of infrastructure providing optimal performance. A whole science of “risk management” has developed in recent years. Its aim is to analyse risky operations so that all risks and potential benefits are considered and the best overall outcome is achieved. The key to good risk management is consultation with all interested parties and complete openness in all decision making. Success of reservoir planning, management and operation depends very much on education of, and ongoing communication with affected parties, be they water users, system operators, local landholders and business operators or neighbours.

Water resources systems are often seen by politicians and the population as preventers of drought. They are not, and water resources planners and managers must educate their “masters” and “customers” of this fact. Water resources projects can supply drinking water over drought periods but they are rarely, if ever designed to supply irrigation water during prolonged droughts. Rather, water resources projects aim to improve human comfort and productivity in regions of water scarcity. However there must be the recognition that every few years all regions will experience much drier conditions than usual. During these droughts water availability will be very restricted and the normal productivity of the land will be suspended. However, the very much increased productivity that the water resources project has provided in other years (perhaps 8 or 9 out of 10) will more than cover the losses of the drought, provided all water users manage their own resources (bank balance, food bank, stock numbers) sensibly, with this reality as a central feature of their management strategy.

Improved analysis of water resources statistics and development of optimal management strategies has been mentioned here. The detail of these approaches is not covered in this book but readers should refer to standard texts on this topic (e.g. Salas, 1993; McMahon, 1993). Analysis and management strategies are continually improving and so the above list may be appropriate in 2000, but better materials are likely to be available in later years.

5.2.3. Operation of single and multiple reservoir systems

Here we should use the word reservoir in its broadest sense. That is we will include all water storages including surface reservoirs, natural lakes and groundwater aquifers. These reservoirs may be replenished naturally, or they may be pumped systems or even storages built up from desalting of brackish or sea water.

Without doubt the operation of several linked water sources, rather than a single reservoir system, offers the water manager many options and flexibilities with numerous opportunities to maximise the potential availability of water. This applies to all water resource situations, not just to regions where water is scarce. However the cost of this increased flexibility of management is a very large increase in the complexity of the operation of the system and a very much increased possibility of management of the system being sub-optimal. However there are very large advantages to be gained from multi-reservoir systems, particularly where the multi-reservoirs within the system have different water sources. For example a system with a river-fed surface reservoir, whose storage level depends on recent precipitation and a groundwater system which varies only with long term variations in precipitation has many advantages. High rates of extraction can be obtained from the surface reservoir but only for limited periods, whereas the groundwater can provide a medium level of extraction for very long periods. Similar advantages can be obtained from inclusion of desalinated water, renovated waste water, or urban stormwater.

Surface water can often be supplied at low cost. Groundwater may involve larger costs for pumping. It may be efficient to design a system for surface water supply only, with high-cost groundwater only to be used in emergencies, such as prolonged droughts, when the surface supply is exhausted.

Multiple reservoir systems also offer the advantages of redundancy. Provided there are multiple water delivery pathways the system will not be shut down by any single failure of a reservoir, a pipeline or a control valve, and therefore the security of supply can be very high.

5.2.4. Groundwater recharge

Where groundwater is an important source of water there can be considerable advantages in encouraging recharge of the aquifer. Care is needed to protect aquifer recharge areas from land use changes that can decrease the recharge. It is well established that increase in quality of vegetation, by reforestation, improvement of grazing vegetation or in some cases by introduction of cropping, generally increases infiltration, but also increases transpiration and therefore reduces recharge and runoff. Therefore care is needed to allow for the effects of such surface changes on the groundwater availability or to ensure that the vegetation characteristics of the recharge area are not changed. Change from native vegetation to

cropping may lead to increased recharge because cropping involves leaving the soil fallow for part of the year. However the actual effects in any location will depend on the vegetation and crop types and rotations, the soils and the climate of the region.

Surface reservoirs can be used advantageously to increase recharge by holding water over a recharge area for some time, or by diverting water to spread it widely over a recharge area. However care is needed to ensure this does not encourage increased evaporation and a large net loss of water from the system. Precipitation is the source of all fresh water and in most situations the surface and groundwater resources are interdependent. Encouragement of recharge may mean a reduction in available surface water further downstream. Conversely impounding surface water may diminish recharge direct from the stream-bed downstream of the reservoir and hence may reduce the groundwater resource.

5.2.5. Design and management of water resource systems

Design and management of water resource systems in areas of water scarcity need particular characteristics. Since water demand in such regions will always exceed supply, and since social, economic and technological conditions are continually changing the systems need to be developed for maximum flexibility of operation. Installation of a single large reservoir to serve a large number of diverse uses in a region where new ideas and adventurous attempts to increase productivity per unit of water need to be encouraged is unlikely to provide for the real needs. There will always be some inflexibility of thinking among those given responsibility for water releases and if this responsibility lies with a single manager or management body this inflexibility tends to become entrenched.

However with numerous managers it is likely that one or more will be flexible and encourage experimentation. When the results of flexible thinking are seen, users of water controlled by other managers will demand more progressive practices.

Another need for water projects is the careful integration of design and management.

Management flexibility will always be limited by the fixed assets of the system, and so there is a need for managers to be involved with design as much and as early as possible.

Management plans need to be part of the original design. Similarly ongoing education programs for both managers and users also need to be set up as part of the initial design.

Participatory management of a resource by all interested water users should begin at the planning stage. Such arrangements can be very cumbersome and so some kind of consensus or representation scheme needs to be devised that is sympathetic to local cultural expectations. It is common for education programs to be run at the inception of a new capital project. However it is difficult to maintain enthusiasm for such programs once a project begins operation. Without an ongoing commitment to education projects tend to have fixed operational programs and slowly run down. Continuing education is needed for newcomers to the project and to continually introduce new ideas and new technologies to the community. Flexible thinking regarding water needs to be part of school education since the children will be the users and managers of the next generation. It is much easier to educate the young than the middle aged who have become fixed in their ways.

As time passes and management (release rules) of reservoirs changes little by little there is a need to re-analyse the whole management system. Unfortunately this is a rare

practice. However a system is usually designed to maximise water use with a given set of release rules. If these rules change over time, and they usually will, the system will then probably make less than optimal use of the available resource. The system should periodically be re-analysed with the current operating rules, so that adjustments can be made to these rules, to make better use of the water. Wastage as a result of changes to operating rules, without a complete re-analysis of the system is common. The work needed to re-analyse the system is quite small, but persuading users to adopt slightly different practices to maximise benefits from the system is a much larger task which must not be (but often is) omitted from the implementation plan.