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Model Areas

Model Areas (size 100 ha) are used to develop Operation & Maintenance activities in cooperation with Farmers and Farmer Groups. This includes institutional organization for maintenance of the water control system, involving Water User's Associations. On this WebPage the emphasis will be put on the operation of the system. Each type of operation of the water control system depends on its physical environment of the command area.  A particular problem are the areas with high toxic ferrous iron levels in the groundwater.

	There is no Tidal Irrigation possible in the O&M Model area of Pamusiran. Water supply for rice mainly depends on rainfall. The control gates can only maintain water levels in canals below land level.
	Drainage is possible throughout the year for 40 cm below land surface and more. (Except during very high flood levels of the Batang Hari river for one or two days only.
	Without the on-farm water management system and recommended water control the rice plants suffer in this area during short dry spells when capillary rise of toxic groundwater harms the plant when it is still in the first month after planting.
	There is pyrite in the soil at 70 cm depth. There is oxidation of pyrite every long dry season. This can not be controlled by gate operation in the canals. This means new acids are formed each dry season. This acid must be leached out from the rootzone to be able to grow rice on a sustainable basis. The soils in Pamusiran became in the recent past acid and extremely low yielding, because not sufficient leaching occurred for many years. The situation further deteriorated during the three El Niņo's of 1991, 1994 and 1997.
	One rainfed rice crop is possible when there is sufficient leaching applied. There is also a very positive effect when surface soil dries out during the dry season. This must be followed by lots of leaching at the beginning of the rainy season and during planting and early growth of the rice plant. In later stages of the rice growth, the plant is more robust and strong enough to overcome more toxic conditions.
	The planting of dryland crops as a second crop is still difficult as the soil is still too acid for most dryland crops, except some acidity resistant pulses and sweet potato. In the near future it is expected that conditions for the topsoil will improve and maize and soybean would be possible.
	Yields will improve when land preparation by two-wheel tractor will be applied and land will be irrigated by pumping during land preparation. Presently we recommend pump irrigation only during land preparation and during early growth of the rice plant as long the soil is too porous and too acid. In future a second pump irrigated rice crop might be an alternative when acids are completely leached out and the topsoil becomes less porous

Operation of Gates in Tertiary canals for Model area Pamusiran

1. First :determine if rainfall >50 mm in last 24 hours. Objective operation: Maximum drainage. Open gate for 24 hours. If less rainfall: go to step 2.
2. Second: determine if rainfall is >25 mm in last 5 days. Objective operation: Controlled Drainage. Try to keep the water level at the 90 cm level for gauge TC 7 or 40 cm below land level. Provide water supply from the SPD side through the culvert near the village area. During springtide (one time in 14 days) open the gate for one full day (at least for 24 hours) and close the gate again at the 90 cm gauge level or higher. If rainfall < 25 mm: go to step3.
3. Third : rainfall will be less than 25 mm in last 5 days. Objective operation: Maximum retention and water supply. Check everyday if there is no smell of H₂S of groundwater at piezometer: 50 cm deep. If there is a strong bad smell: open gate 5 hours before low tide and close the gate again at high tide (slack water condition) when there is no flow anymore. The water level near the gate should be at about 100 cm at the gauge of TC 7 or about 30 cm below land level. When there is no bad smell : supply every day water  by opening the gate 2 hours before the expected high tide and close again at high tide and keep the water level at 100 cm near the gate.

Remaining part of the year

1. Objective of operation: Controlled drainage; Try to keep the water level at the 70 cm level for gauge SK7 or 60 cm below land level. Apply supply from the SPD side through the culvert near the village area.

Rainfall, Leaching and Water Levels in SK7 during 1999

Explanation of Chart: The Blue line shows the percolation (positive) and infiltration (negative) in meters drop (percolation) or rise (infiltration) of water levels in the soil. This means that 6 cm drop of waterlevel in one day (Corrected for rainfall and evapotranspiration) equals  6 mm percolation. (drainable pore space=10% by volume). The purple lines shows the average groundwater level  in Tertiary Unit SK7 in meter below land surface. The green line shows the water levels in the tertiary canal below average land level. The yellow line shows the water level in the secondary canal (SDU side). Note that the groundwater level drops below the canal water levels during the dry month of August. Apparently the Macro-Micro system can not maintain water levels above the pyrite layer. Leaching/percolation at the beginning of the planting season is every year essential. Leaching/percolation occurred sufficiently during October by applying controlled drainage. But there was insufficient leaching/percolation from November-January, caused by a gate operation too much focussed on water retention. The result was that farmers who planted in October found good yields, but later planting suffered from toxicity. Apparently the rice planted in October was already  strong enough to resist the toxic conditions from November-January.

On the left this field has been planted in late November and suffered during early growth. Better conditions in February did not improve the crop. The field in the middle has been planted in October with good results as it found lots of leaching during early growth. (Note the oxidized iron in the rootzone; only reduced iron will harm the plant). The best field in Pamusiran was found in SK 4, planted in October (photo on the right). This farmer uses pump irrigation during land preparation only and uses also always a two-wheel tractor for land preparation.

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