The flood-pulse cycle and silt deposition--
Silt deposition
The flood-pulse cycle is initiated by climatic conditions (see Section 2) and brings an annual rhythm of nutrient addition and ecological activity in the floodplain. In their natural state, these lands are subject to seasonal flooding that results in deposition of nutrient-rich silt brought down the river from upstream sources. The annual silt deposition renews soil nutrients providing a form of natural fertilization to agricultural land. Upstream changes to the extent or timing of the floodwaters will alter the extent or timing of the waterborne silt and associated nutrients, and their availability to support crop cultivation.

Although most floodplain areas are exposed to the natural cycle of flooding, many lowland agriculture areas also have large public irrigation systems.

Irrigation issues
Irrigation issues often relate to water use, salinity, flood risks and water flow/quality in relation to cropping patterns. A change in crop type induces a change in timing of water use, possibly increasing water demand at times of low river flow.

Poor water use efficiency in paddy irrigation is related to:

• Poor management of water, especially after water leaves the main head works and between the system managers and farmers;

• Unfinished secondary and tertiary systems;

• Inefficient chemical inputs (fertilizers and pesticides);

• Lack of attention to alternative crops that may improve returns from water use; and

• Lack of attention to environmental impact, such as salinity and acid drainage.

Significant differences in water use exist among rice growing areas; these include the differing needs of heavily flooded areas, delta areas and predominantly pumped-irrigation areas.

Rice Farming: "Natural" deep-water rice vs Intense Harvest--
Rice farming practices in lowland areas are diverse: wet season production includes 'natural' deep-water rice and rain-fed production sometimes supplemented with irrigation; dry-season production is undertaken in areas of receding flood water and/or with full or partial irrigation. Cropping intensity is once per year in most of NE Thailand, Lao PDR and Cambodia, and two times per year in many parts of the delta in Viet Nam, with three crops per year in some locations.

Deep-water cultivation takes place in low parts of the landscape usually around small lakes and in depressions or channels. Land is plowed and seeds planted at the beginning of the wet season; plants must survive and grow using local rainfall for at least six weeks in order to be strong enough to withstand subsequent flooding as the river water level rises and inundates the floodplain. Deep-water productivity is often low and is dependent on numerous water-related factors such as the timing and sufficiency of pre-flood rainfall; onset of flooding; rate of water rise; maximum water depth; duration of submergence; water recession; and water currents.

Rain-fed production takes place in shallow water areas during the wet season, comprised of early-duration forms in high fields, medium-duration forms in middle fields, and late-duration forms in low-fields. Some land used for rain-fed production may be irrigated.

Dry season production is highly variable in terms of method of cultivation, cultivation time, and topographic and soil conditions in the growing areas. Production may be reliant on full irrigation or on partial irrigation that augments natural water conditions. In some areas, one rice crop is produced during the wet season and a second crop is produced during the dry season using full or partial irrigation. Recession farming takes place in areas where water can be blocked as floodwaters begin to recede. Crops are planted progressively along the edges of flooded areas as water recedes during the dry season, so that there is a succession of crops being produced in the recession zone. Where possible water is pumped back onto the crops.

Fertilizers used for rain-fed production may include farmyard manure or chemical fertilizers, with higher elevation fields usually receiving more fertilizer than lower fields. Chemical pesticides are used to control insects in many rice cultivation areas.

Rice Farming and Aquaculture--
In many rice-farming areas, fish are grown in small ponds and/or are captured in the rice fields. During much of the wet season, fish are captured by a variety of methods including nets, hooks, spears and gaffs, and traps. During the dry season, trenches and trap ponds are usually dug in the rice fields so that fish can use these as refuges as water levels drop; fish are captured from these ponds at a later date. Rice fields that are higher in elevation than the surrounding floodplain may be stocked with fish from other areas, sometimes using trap ponds both as nurseries for fingerlings to be stocked and as dry season refuges.

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