Water harvesting and moisture conservation

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Unlike the more arid regions of the world, where water harvesting and moisture conservation measures are essential to keep farming systems operational and enable communities to survive, in the humid tropics where rainfall is abundant, there has generally been little need of such technologies.

However, this situation is now changed! With the prospect of more frequent and more prolonged El Niño-induced droughts in western Pacific countries as a result of climate change, water harvesting and moisture conservation strategies are going to be essential in this region if rural communities are to sustain their farming systems.

Water harvesting

Because of the increased propensity for extreme rainfall events in PNG, as well as droughts, the types of water harvesting technique that may be practiced here are more limited than those suitable for drier tropical regions. For instance, semi-permanent water harvesting structures such clay or rock-built bunds, although ideal for harvesting rainwater runoff during normal dry seasons, may create unwelcome water logged conditions during subsequent wet seasons. As such, only temporary water harvesting methods are suitable in this region.

In parts of the Highlands, sweet potato and other vegetable and cereal crops (eg maize) are grown on mounds. One reason for using mounds is to prevent water logging in the crop rooting zone. However, during drought events, crops planted on these raised structures are vulnerable to dehydration.
A means of enhancing moisture availability to mound-grown crops would be to erect temporary barriers between the mounds to trap rainwater runoff in the period prior to a drought, as well as any precipitation during the drought period.

Another method of enhancing moisture availability for mound-grown crops would be to create a hollow on the top of each mound so that rainwater could collect here and percolate down into the crop rooting zone. If necessary, such hollows could be refilled once the rainy season commences to prevent water logging. Having such hollows on the tops of mounds would be beneficial when watering or irrigating the crops during dry periods.

Alternatively, when a drought is expected, rather than planting crops on raised mounds, they could be planted in flat beds or even in shallow depressions on the ground where dew and rainwater could collect and provide much needed moisture. This type of cultivation would also be beneficial where irrigation is employed since water could easily be directed into the crop rooting zone. Where tree crops such as banana are concerned, shallow ditches may be made around each plant to collect rainwater or dew from their stems which subsequently can percolate into the rooting zone.

Moisture conservation

During drought periods or normal dry season, conditions in the dry lowlands, preventing moisture loss from the soil is important. Drought causes water stress and wilting of plant leaves. Low humidity and large temperature changes also affect crop reproduction cycles preventing the formation of seeds and fruit. During drought, soil is exposed to heat and wind and can become very hot during the day and may be blown away as dust. Lack of moisture also causes it to harden and crack enabling pests such as weevils to attack crop roots.

The simplest method for minimising the impacts of high temperature, low humidity and lack of rainfall on cropping areas is to apply mulch. Mulch is a layer of dead vegetation laid on the ground both to control weeds and to minimise moisture loss by evaporation.

Mulch can be spread on gardens after they have been dug and either before or after the crop has been planted. Some crops may be planted through the mulch, whereas for others, it is best to spread the mulch on the soil surface after the crop has started growing.

Mulches shade the soil, lowering surface temperatures and reducing the rate of water loss. They also protect soil from wind erosion and frost. Furthermore, in the wet-up periods following drought, they can reduce soil erosion by sheltering the soil from heavy rain and preventing excess rain water running off fields too quickly.

Cultivation under tree canopies is another option for conserving moisture in cropping systems. The trees shade the crops and the soil, lowering soil temperatures and thus minimising evaporation losses.

* Next week's article will look at innovative methods of farming to improve food security in the present climate change scenario.

Source: Dr John Bailey

* Dr John Bailey is NARI's principal scientist on natural resource management