Water harvesting

Water availability for upland agriculture can be improved by smallscale impoundments to capture and store rainwater for irrigation.

Small-scale water harvesting is most successful when operated as a system with three components: the watershed or catchment area that generates the runoff; the reservoir which holds or collects the runoff; and the service area where the harvested water is used for production.

A catchment area of sufficient size is needed to drain water into the reservoir. The amount of runoff generated depends on the catchment characteristics and rainfall pattern (amount, duration and intensity); hence, the variability of catchment sizes. In parts of the Philippines with an annual rainfall of 1200-1500 mm, a catchment area of 0.2 to 0. 5 ha of terraced rice land yields 1000 m³ of water for storage in the reservoir. For grassland and residential areas, a catchment area of about 0.6 to 1.0 ha is enough to fill the same volume. Water harvesting is also possible in areas with low rainfall (300-500mm per year), but larger catchment areas are necessary.

Small-farm reservoir sites are suitable in elevated or depressed areas (valleys) where irrigation is possible by natural flow. Sites that are communally owned should be properly managed to ensure sharing among the intended beneficiaries. Places with springs or flowing streams to ensure a year-round water supply are good sites for reservoirs. Topography that is undulating or rolling with slopes of 2 to 18% is desirable.

Water harvesting (A straight-embankment type)

Water harvesting (A straight-embankment type)

Water harvesting (A semicircular type)

Advantages

· Improves food production (crops, fish, fruit trees, etc.).
· Promotes conservation and ecological balance.
· Involves low investment cost per hectare.
· Easy to construct.
· Provides alternative (often high-return) uses to offset sacrificed land area.
· Protects against drought.
· Allows irrigation by gravity (no additional power cost).
· Mostly individually owned; hence, minimal social problems.

Limitations

· Requires large amount of labor.
· High seepage and evaporation losses possible (depending on soil type).
· Floating vegetation may infest reservoir.
· Uncontrolled runoff in high intensity rainfall areas can overtop and damage the embankment.
· Poor design and management can lead to erosion and flooding.

Factors affecting adoption

Biophysical

· Soils that have high seepage and percolation rates may require lining.

Socioeconomic

· Farmers may be unwilling to sacrifice a portion of their land for a reservoir.

· Land tenure status can influence the investment decision.

· Labor may be insufficient.

· Funds or credit services may be unavailable.

· Engineering knowledge (both for constructing the impoundment and managing the irrigation system) is required.

Land-use planning and potential uses of small-farm reservoir system (Adapted from PC ARRD, 1993)