14.2 ECOLOGICAL ASPECTS OF AGRICULTURE AND IRRIGATION

Importance of floodplains for agriculture / The flood-pulse cycle and silt deposition / Rice farming "natural" deep water vs intense harvest / Rice farming and aquaculture

Agriculture and irrigation have, and continue to be, a dominant influence on the landscape and ecosystems of the Mekong River basin. Agriculture provides employment for some 85% of the basin's population, and rice is the most important food source for people in the region. An overview of land use and land cover in the lower Mekong basin is presented in the map below.

To understand the nature and scope of this influence, it is important to consider:

• Different types of agriculture occurring within the basin;
• Importance of floodplains for agriculture;
• The flood-pulse cycle and silt deposition;
• Rice farming: 'natural' deep-water rice versus intensive cultivation; and
• The relationship between rice farming and aquaculture.

Agriculture takes place in upland areas of large and small tributaries, and in lowland areas, mainly on the floodplain of the Mekong River and its major tributaries.

Upland Agriculture and Irrigation
Swidden cultivation (slash and burn). Swidden agriculture is practiced in many areas – this type of agriculture has a long history and is intensifying. Swidden agriculture is undertaken primarily in highland areas, often by minority groups, and involves clearing of land on a rotational basis.

Other differences. Upland agriculture activities vary considerably among areas often in accordance with soil and topographic conditions; differences include:

• Areas that are used for field crops usually have rich deep soils;
• Areas have that well developed markets for fruit tree crops often have indifferent soils;
• Areas that have industrial tree crops often are chosen for both site and commercial reasons.

People in upland areas in general have limited experience with irrigation and production of commercial crops. However, cropping patterns are changing in relation to demand for food in urban areas – this has an effect on water use and timing of water demand.

Issues. Shifting agriculture is practiced in some areas with serious erosion and offsite effects. Most upland agricultural lands use rain-fed agriculture. Irrigation in uplands is currently limited and improper water use has potential to affect downstream water quantity and quality. Some areas use pumped irrigation in sloping areas (water from rivers, lakes, reservoirs or groundwater).

Limitations on irrigation use include:

• Limited experience with upland irrigation;
• Limited experience with upland commercial crops;
• Limited international technology adapted to tropical conditions; and
• Limited knowledge of modern water reservoir storage systems;

Highland agricultural practices have led to erosion and other negative effects. In general, poor practices are used in upland areas for limiting floods, erosion, and pollutant runoff. Poorly developed links between market and production are a major impediment to improving agricultural production and practices.

Water use efficiency issues relate to water allocation in the dry low-river season and from underground aquifers. Improvement to water use efficiency must include water delivery and storage systems, land use practices (especially those that might lead to erosion) and optimal input for maximum sustainable outputs. High water use crops such as coffee is placing stress on groundwater supplies in cross-border areas.

Lowland Agriculture and Irrigation
Most crop production in the Mekong River basin takes place in lowland areas, primarily in the fertile floodplains of the Mekong River and its major tributaries.

Importance of Floodplains for Agriculture--
Floodplain importance. Lowland agriculture is dominated by rice production in river floodplain areas. The floodplains provide large tracts of relatively flat, nutrient-rich land. Large areas are being cultivated for production of rice and other crops in the Mekong River delta in Viet Nam and upstream along the main river and lower reaches of most tributaries. Significant crop production occurs in the major sub-basins including the Tonle Sap River in Cambodia, the Xe Bangfai River in Lao PDR, and the Mun River in Thailand, and is important for local economies.

The Mekong River is believed to have potential to irrigate several million ha of agricultural land. Full realization of this potential will undoubtedly increase existing competition for resources, such as use of water for intensive rice production versus fish production. Water demand is greater in dry season when the river is low.

Major agriculture issues
General problems faced by agriculture in lowland areas include:

• Salinity intrusion from the sea affecting agriculture and biodiversity, particularly in Viet Nam;
• Acid sulphate soils impacting on agriculture and biodiversity in Cambodia and Viet Nam; and
• Soil and water contamination from inappropriate use of manufactured chemicals – all countries.

Irrigation systems include major head works that control and divert water from the river source, primary canals that carry the water to a network of secondary/tertiary canals, and eventually to the farm users. Types and amounts of irrigation vary accord to the different agricultural production and irrigation uses in different ecological zones.

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