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Several options exist for the design of the on-farm water management system (Micro-level). Open field ditches, spaced 100 m apart, are constructed for all O & M Model areas. They cover an area of about 1500 ha in total, divided over 15 schemes in three provinces. (Jambi, Riau and West Kalimantan). Beside these open field ditches AARD (Agricultural Research Institute for Food Crops) recommends shallow field ditches 25-m apart with a depth of 30 cm and a shallow field ditch 40 cm deep surrounding the whole field.
On this WebPage, we show a micro-design, which has been applied in two areas in Jambi province, Sumatra, totaling 9.5 ha. One area is in the Pamusiran Scheme with Javanese settlers and one area is in the Sungai Raya Scheme with Buginese settlers. The proposed lay-out is based on a subsurface drainage system, using woodtwigs. See WebPages System for the reasoning to use woodtwig subsurface drainage.
Pamusiran Scheme. The main problem in Pamusiran Scheme is Iron toxicity (pH4.5) and lack of water supply with rice yields of 0.5-1.2 ton/ha. In 1997/1998 when El Niņo had its effect, farmers did not have any yield. During dry spells usually the rice dies by capillary rise of the toxic groundwater.
Sungai Raya Scheme. The main problem in the Sungai Raya Scheme is H2S toxicity (pH5.5) and organic matter in strongly reduced conditions with rice yields of 1.5-2.0 ton/ha. There is ample water supply as fields receive here extra water from higher located swamp areas.
The costs to install subsurface drains, using wood-twigs, are in Indonesia for 9.5 ha, separated over two areas, about US$250/hectare. Transport and cutting of wood-twigs appears the most important cost item and consists of 46% of the total costs. It is thought that economics of scale would considerable reduce the costs per hectare.
It is not sure how long the wood-twigs sub-surface drains will last for Indonesian conditions. But in Holland they are still working 60-70 years after construction. In Indonesia the decomposition rate of wood is higher, but as long most of the year the wood-twigs are below the groundwater level, there is not much decomposition. For instance: Large Mangrove roots and swamp tree trunks can be found within 80 cm from the surface. They must be at least several hundred years old, if not more than 1000 years, and show still little decomposition.Only time will learn how long the presently installed wood-twig drains will last in the poorly drained conditions of the swamp areas.
The effect of tidal fluctuations has been measured for the sub-surface drains. Trials, executed 6 months after installation, showed that daily tidal fluctuations of 50 cm in the adjoining tertiary canal will have an effect of 15 cm tidal fluctuation at 50 m away from the tertiary canal and 7 cm tidal fluctuation at 150 m away from the tertiary canal. Also the out-flow from the sub-surface drains at low tide has been calculated in the culvert connection near the tertiary canal for a field of 1 ha in size. This out-flow was measured as 2.5 liter/sec.It proves that leaching by subsurface drainage has a good effect on iron toxicity for 100 m away from the tertiary canal and infiltration by the subsurface system for 50 m.during extreme dry periods.
Photo (above): Field ditches just filled with wood twigs (left picture) and next are covered by the peeled-off coconut skin.
Photo (left): Pamusiran; Rice field in sub-surface drainage field with wood-twig drains. First reasonable yield after many years of hardly no yield. There is no micro-system of open ditches in this field, only wood-twig sub-surface drains are present. The farmer hardly used any fertilizers for the local variety he planted; No land preparation, direct planting of rice seedlings (only manual weed removal). Essential for the good result in this field was the high leaching quantity during planting and early growth. The leaching quantity was about 6mm/day in this period, calculated from daily observations.
Photo (left): Sungai Raya; Rice field in sub-surface drainage field with wood-twig drains. Initial conditions in Sungai Raya were much better than in Pamusiran. Close to the main tertiary canal they are presently even excellent.(see photo) In Sungai Raya it became however very clear that the wood-twig drains do not have a positive effect for more than 100 m from the canal. That means another tertiary canal would be required parallel to the original canal at 200 m distance for optimum effects of the sub-surface drains. There are no open ditches in this field.
Design Criteria for On-Farm Watermanagement system (open ditches)
Fields with frequent tidal irrigation
Objective is to grow two times rice per year; for the dry season crop some additional pump irrigation might be required for an optimum yield. The existing conditions for supply of water should never be changed. For all new connections of tertiary and sub-tertiary canals with the secondary canals, care should be taken that all drainage can be controlled by the farmers.
In principle the quaternary ditches are made at the borders of the fields with a distance of 75-100 m between the ditches.
Fields with no tidal irrigation (rainfed)
Objective is to grow one rice crop in the rainy season and one dryland crop in the dry season. For that reason improved drainage is required to grow the dryland crop. In the wet season a balance should be found between the need for leaching of the toxic surface soil and to control the waterlevels to grow a wetland rice crop.
Quaternary ditches should be made at a distance of 37 - 50 m, both at the borders and in the middle of the field.
For an example see Webpage Design Macro
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