May 21, 2019
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    The water buffer

    March 04, 2019

    Groundwater is already our largest reservoir of fresh water and stores more than 90% of the global fresh water, ice (polar ice and glaciers) excluded. Freshwater stored in rivers, lakes, large reservoirs and as soil moisture is less than 1%.The discussion on groundwater often focuses on overuse and control, which is indeed of great concern in many areas. The clear need for better groundwater management should include maximizing recharge and storing rainwater where possible, storing water from floods, managing water levels and ensuring water quality to make reuse possible. Water storage is analogous to savings in a bank, and has been the basis for the sustainable development of economies.

    The climate change dimension

    In many parts of the world, both in dry and wet climates, Integrated Water Resources Management (IWRM) is a key challenge under the present, highly variable climatic conditions. Each region requires appropriate IWRM concepts that take both natural resources and socio-economic conditions into full account.

    In humid regions, climate change may cause monsoons to become more erratic, arriving later and with longer dry spells in between, and placing a premium on groundwater management, soil moisture management and supplementary irrigation. Managing groundwater, and the water buffer in general, is at the heart of climate change adaptation in arid and humid areas alike.

    Scope for water quality improvement

    Sub-surface storage has several advantages including low (if any) evaporation losses, relative protection against water pollution, and improved water quality. Suspended solids are absorbed by the soil, temperatures are moderated and with sufficient detention time in warm aquifers many pathogenic bacteria, viruses and unicellular micro-organisms (protozoa) are eliminated (Dillon, et al. 2009). Furthermore, the soil can reduce acidity, remove inorganic and organic compounds through adsorption, and chemical as well as biological processes can change and neutralize hazardous compounds. In addition, underground storage of surface waters and the purification potential of capturing rainwater where it falls have the advantage of providing a clean safe source of water, thus avoiding the need to purify water in the first place.

    Support functions for ecosystems and agriculture

    Managing the water buffer has some important beneficial side effects. These are not always taken into account, but they can have a substantial impact. High groundwater tables assist in maintaining adequate soil moisture, thereby making an important contribution to ‘green water management’. Green water management is the management of soil moisture (as opposed to the management of blue water which is the water in rivers, lakes and reservoirs).

    Guaranteed soil moisture is directly related to high productivity in rain-fed agriculture both in arid and humid areas. Green water management is linked to fertility as it also benefits from better tillage, composting and mulching, and physio-chemical and biological processes such as nitrogen fixing, nitrification and denitrification, and oxidation.

    The 3R techniques

    Several techniques, the main elements of which are the 3R’s of Recharge, Retention and Reuse, can increase storage capacity at the scale of a sub-basin and improve its management. Some of these techniques are ancient and time-tested, others are new and innovative. 3R is required in arid and humid areas alike. It is important to recognize that water should not only be managed when it is scarce, as in arid areas, but also when it is abundant. The manner in which this fact is addressed may differ in arid and in humid areas, but better water management and climate change adaptation are necessary everywhere.

    Recharge

    By adding water to the buffer, recharge contributes to water circulation. Recharge can come from the interception of rain and run-off water (natural recharge), from increased infiltration of natural processes by manmade interventions (managed aquifer recharge – MAR) or can be a by-product of some other factor (i.e. inefficient irrigation or leaking pipes in water supply systems).

    Retention slows down the lateral flow of groundwater. This helps pond up groundwater and create a large wet buffer in the subsoil. Under such conditions, it is easier to retrieve and circulate water. Retention makes it possible to extend the chain of water uses. With retention, the groundwater table is heightened. Slowing down, or even controlling lateral outflow affects the water table and the soil moisture and soil chemistry. This has led to improved yields of rain-dependent agricultural areas. Some argue that in some cases it is better to control soil moisture from below than to provide surface irrigation water from above because of lower losses through evaporation and less development of salt crusts on the top soil.

    Reuse

    Reuse is the third element in buffer management. The biggest challenge of 3R is making water revolve as much as possible. Scarcity is resolved not only by managing demand through reduction in use, but also by keeping water in active circulation. Three processes are important in managing reuse. The first is management of (non-beneficial) evaporation. Water that evaporates ‘leaves’ the system and can no longer circulate within it. There is a fine balance between keeping good soil moisture (which is also achieved by agronomic practices, shade trees and the like) and avoiding evaporation losses from the soil.

    The second process in managing reuse is managing water quality. The possibility for reuse depends on the quality of the water, with different functions putting different demands on the water quality. It entails avoiding the mixing of reusable water with lower quality water, and preventing up-coning or lateral flows from lower quality sources.

    Ensuring that repeated reuse of water and frequent circulation do not move water quality beyond safe thresholds requires significant effort.The third element of optimizing reuse is ensuring that water does not move to an area from which it is difficult to retrieve and reuse. The difference between wet and dry buffers is relevant here. Water which is recharged in a dry unsaturated buffer is difficult to retrieve and, though not lost, is difficult to bring back into circulation. When the buffer is saturated, on the other hand, it can be readily retrieved. An important challenge in 3R is to increase the ‘wet water buffers’ and successfully manage the existing uses. By ponding up groundwater and slowing down lateral movement, retention can create or enlarge such saturated zones.

    An understanding of ‘what lies beneath’, or the characteristics of the groundwater buffer, is essential to all of these techniques. Not all buffers are the same. They differ in size, in hydrological interaction, in storage capacity, and in vulnerability. There is no standard approach to determining ‘buffer strategies’, as different socio-economical and environmental conditions set different starting points. Much is to be gained, however, from tailoring 3R to local opportunities and preferences, as identified by the different parties living ‘on top of the buffer’.

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