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A typical Gabion structure with retaining walls to control erosion and flooding on the banks of a fast-flowing river.

In the article ‘The hidden role of water on earth’ featured in the January edition of Stockfarm, we discussed the crucial but often ignored role that water plays in making earth habitable. This article, the second in the series, provides a brief overview of the steps that can be taken to ensure that enough water is available in the right places to have a stabilising effect on the climate, which will also make the land more drought-proof.

Water is the most precious substance on earth, a fact which has been undisputed throughout history. Water has unique properties that are required for life. This article focuses on the essential components required for ensuring that enough water is available to stabilise the climate at local and regional level.

An interrelationship exists between the climate, water availability, vegetation, and land use. Large areas are drained of water through deforestation, agricultural activities, and cities. This drainage leads to decreased evaporation from that area, resulting in an increase in the transformation of incoming solar radiation into heat. This causes changes in the energy flows in the area, water circulation on land, and increases the incidence of extreme weather events.

However, by applying simple rainwater harvesting measures, it may just be possible to turn this dire situation around.

Vegetation: Keeping things cool

Vegetation plays a significant role in protecting the soil, water storage, and the controlled release of water into the atmosphere.

The amount of water present in a landscape influences how much of the incoming solar energy will be used for heating the area and how much will be used for water evaporation. If water is only available in small quantities, a greater part of the solar energy will be changed into sensible heat, thereby increasing the temperature of the environment dramatically.

Vegetation cools an area by providing shade and through the transpiration of water.

Dehydration of the land

Dehydrated soil only starts absorbing water more than ten minutes after rain has started falling. In the first minutes it acts like an impermeable surface. Therefore, during extreme downpours, rapid runoff and concentration of water occur, consequently leading to erosion and flooding, resulting in little water infiltration into the soil.

Accelerated rainwater runoff from an area, decreased water infiltration into the soil, and a lack of vegetation all cause warming of the surface of the land and a gradual change in the microclimate of entire regions.

The gradual, but direct, effect is that the total volume of water available for evaporation from the land decreases, subsequently causing the volume of water available for precipitation through the small water cycle to decrease. A greater portion of the incoming solar energy is changed into sensible heat, resulting in temperature extremes between day and night, between neighbouring areas, as well as more intense seasonal extremes. These extremes rouse strong winds which carry the available water vapour away from the area.

Higher air temperatures also increase evaporation from the oceans, which intensifies the large water cycle. The small water cycle is weakened, and the large water cycle begins to dominate. Moreover, light and frequent precipitation decreases, and there is an increase in intense and less frequent precipitation originating from evaporation from the ocean. The periods between these precipitation events are characterised by intense heat waves and drought.

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Figure 1: Expansion of arid areas due to a weakened small water cycle.

Alternative management approaches

The general perception is that water is lost when it is not visible anymore, with people often wanting to prevent these ‘losses’ as far as possible. This applies to water that evaporates into the air or infiltrates into the soil to the groundwater table.

In order to ensure that the ecosystem in a river downstream of a reservoir remains alive and healthy, a certain stream flow is allocated and managed according to the ecosystem needs of the remaining length of river.

It is also necessary to consider the water that needs to circulate in the entire ecosystem in order to maintain it. This pertains to not only visible water, but also the water in the remainder of the water cycle, namely the atmosphere, soil, and vegetation.

The water lost from the continents can be returned by capturing rainwater where it falls on land and storing it in the soil, especially in those areas that are drying out due to human activities. Interventions to restore moisture aim to increase the contact time between rainwater and the soil. This is done by slowing runoff, spreading it over a specific area, and allowing it to infiltrate into the soil. Only surplus surface water should be allowed to flow away from the area.

This will support a gradual recovery of groundwater reserves and vegetation cover. Moreover, streams will start flowing again, the volume and frequency of precipitation will increase, and extreme weather events will decrease.

Benefits of harvesting rainwater

The individual producer or city plot owner who implements rainwater harvesting measures will experience the full value of the rainwater that falls on his or her property, as well as from the water that flows onto the property from neighbouring upslope areas.

This will be visible in:

  • Thriving vegetation.
  • Increased soil moisture.
  • Rising groundwater levels in boreholes.
  • Greater drought resilience.
  • Seasonal springs and streams may become permanent again.
  • Wetlands may be rejuvenated.

When a community or producers in a region implement rainwater harvesting in their area, the entire region will enjoy the aforementioned benefits, but to a greater extent.

Additional benefits include:

  • Precipitation could increase and occur more regularly.
  • The surrounding areas could also start to benefit from this increase in moisture.
  • Jobs will be created in constructing and maintaining the different structures and revegetation of the area.
  • Socio-economic advantages due to the abundance of water.
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Figure 2: The beneficial effect of water conservation on neighbouring arid areas.

If rainwater harvesting measures should be implemented on a sub-continental or continental basis, the results will be even more spectacular. For instance, in addition to the benefits already mentioned, thermal extremes as well as climatic weather extremes will be mitigated, especially regarding droughts and floods.

In addition, an increase in vegetation density will regulate temperatures and evaporation to a greater extent, thereby cooling the local climate, as well as increase CO₂ removal from the atmosphere. It could also be possible to revive semi-desert and desert areas, although the process will be slow and difficult. As water is of tremendous social and financial value, it adds to a country’s wealth.

Small steps to conservation

Every roof and yard are a micro catchment in which a surprisingly large volume of water falls annually. This is an asset the individual can utilise to improve his or her life in a variety of ways. For instance, start with collecting rainwater for a patch of home-grown vegetables and plant a few indigenous trees or fruit trees.

The next step could be to motivate one’s neighbours or the community to find a way to restore the water which once existed in the area and which, throughout the years, has been lost due to human activities.

Water conservation interventions

Water harvesting techniques are practiced in the arid and semi-arid areas with insufficient annual rainfall. Erosion control and recharge of groundwater are additional advantages of water harvesting.

There are different methods to conserve rainwater – these methods depend on the purpose for which it is to be harvested, on the surrounding topography, and on the available resources. The interventions employed can be divided into three main groups and make use of low-tech, low-cost materials which are available locally.

Technological measures include:

  • Shallow infiltration ditches on the contour.
  • Sloped catchment and reservoir areas.
  • Depressions and infiltration pits.
  • Surface treatment to enhance infiltration of rainwater.
  • Small dams or ponds in watercourses.
  • Discharge and spreading of runoff water into flood plains.
  • Small weirs.

Biotechnical measures include:

These measures are similar to technological measures, but here vegetation is used:

  • Vegetation belts.
  • Areas with undisturbed natural indigenous vegetation.

Preventive measures include:

  • Soil protection through contour ploughing, conservation agriculture, and no-till cultivation.
  • Controlled grazing.
  • Replacement of impermeable surfaces with permeable ones.
  • Protecting indigenous vegetation. Fanie Vorster (Pr Eng), ARC-Agricultural Engineering

     

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Figure 3: On-contour swales used to conserve water.
Figure 4a and b: Typical cascading bunds as a water conservation intervention. (Source: www.greener.land)

Detailed information on the different rainwater harvesting interventions is available in the manual Changing climate, the role of water and what you can do to drought-proof your land available from the author. For more information or a list of references, phone the author on 012 842 4052 or send an email to vorsters@arc.agric.za.