Simple irrigation systems for PNG

 Irrigation simply means the supplying of water to land and crops, especially by means of specially constructed channels or pipes. 

It includes the process of water withdrawal from sources and distribution to application sites. Many countries practise irrigation. 

In PNG most areas have enough rain all year round to make irrigation unnecessary, except for a few places that use traditional irrigation systems.

However irrigation becomes necessary during prolonged dry spells and droughts.

NARI has focused on systems that could use ground water through shallow wells and tapping surface flows from streams and rivers. 

These systems are appropriate for smallholder farmers who cannot rely on engine-powered pumps that are costly and hard to run and maintain. 

The adaptation of such intermediate technology pumps that deliver more water than simple hand pumps would prove more economical and reliable for delivering irrigation water. 

Such simple technologies are more likely to be accepted and adopted by small-scale farmers.

Water withdrawal systems

Water withdrawal refers to the removal and transportation of water from a groundwater or surface water source to a place of use. This is referred to as “water lifting” because most systems lift water from streams, ponds or wells. Systems studied by NARI include the gravity-flow, rope and washer pump, treadle (pressure) pump, hydraulic ram pump and coil pump.

Water distribution systems

NARI has recommended three feasible waster distribution systems. The rope and washer pump for water withdrawal or lifting, the micro tube drip irrigation system for distribution and the gravity flow system for both the lifting and distribution of water. These systems are cheap to set up and maintain. This means most parts can be locally made; operation of the system be easily understood and managed by almost anyone; system suitability not be restricted to only one type of water source; water volumes lifted be sufficiently large with pump use being reasonably efficient; low labour requirement for set-up and use; and that the system be portable. 

1. Rope & washer pump

A nylon rope, washers, a pipe and a pulley are used to make this pump. As the pulley is rotated the nylon rope is pulled up through the pipe. The washers attached to the rope push the water up through the pipe. This device can lift water 20m high and costs around K500 if made commercially. It is most suitable for pond, rivers or underground sources of water. This pump can be manufactured using local materials with low costs of acquiring and maintaining it. It can be used upright for well-water sources or set up at an angle for river sources.

2. Micro tube (drip) system

A drip or trickle system is a network of perforated piping, installed at or below the ground surface, releasing a trickle of water close to the plant roots. This minimises evaporation and seepage, and brings 80-90 per cent of the water to crops. Similar to the drip or trickle concept already in use, the micro-tube system is being promoted as simple bucket and drum kits. 

The bucket kit consists of a bucket with a lateral line (hose) fitted with micro tubes, ideal for kitchen gardens. The drum kit differs by the inclusion of a sub-main pipe to which several lateral lines with micro tubes are connected, and seen ideal for small commercial farmers. It is not only cost effective but also water efficient. 

The drum and bucket kits cost around K221 and K24 respectively. Alternatively, hoses can be replaced with perforated bamboo pipes. 

3. Gravity-flow (flood) systems

This is the channelling of water through ditches or pipes from a source by gravity flow, similar to some traditional practices already in use. Such systems would be costly for areas where water has to be channelled a great distance with the use of plastic or metal pipes along with reservoir tanks. Labour costs are incurred to make ditches to point of irrigation. 

However where the topography is suitable, such systems are attractive as they only involve construction of diversion weirs and channels from the upslope water source to the area to be irrigated.

Distribution of water through flooding of fields or furrows made in gardens, though cheap, delivers far more water than needed for crop growth with only 50-60 per cent of the water reaching crops because of evaporation, seepage and runoff. 

Water sources

Water sources need to be easily accessed for these systems to be useful. In most cases, many of the sources would have dried up during a drought, so others will have to be found. Wells can be dug in dry river or swamp beds and water can be drawn and supplied to gardens using the portable systems recommended. Areas with green grass may also indicate the presence of water. 

Holes can be dug to the depth at which water is found and with a water lifting system, or use of buckets (if the well is too shallow), water can be drawn to feed the irrigation system.

The systems chosen are suitable for most water sources except bore water. The water distribution system is adaptable to any source, even if the drum needs to be filled manually. A combination of these simple irrigation systems is possible, where bamboo pipes can bring water to a drum set that can be used to irrigate a garden, especially down a hill or mountain.

• Next week’s article will focus on Water and Fire Management.