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Irrigation and drainage
Evolution of irrigation development
Irrigation development has been undertaken by rural communities (Banawe terraces, cooperative irrigation societies (zanjera) and lowland schemes near Manila). Earlier, the major irrigation investment periods have been during the 1920s, the post-war period and the 1970s and early 1980s when public involvement in the irrigation subsector was at its maximum. In this respect, the creation of the National Irrigation Administration (NIA) in 1964 has been decisive.
Irrigation development is highest in Luzon, containing approximately 51.1 percent of the total irrigated land, followed by Mindanao with 38.7 percent and Visayas with 10.2 percent. Luzon, the major investment area for irrigation development, is considered a ‘hot spot’ for global warming and climate change, because the irrigation systems are located in high temperature areas and are annually subjected to destructive typhoons and flooding.
The irrigation potential in 2005 was an estimated 3.1 million ha, defined by the NIA local irrigation office as the land on slopes of less than 3 percent, which are considered to be the areas with minimum cost of development (Table 4). Aside from this simplified criteria, the Asian Development Bank (ADB) proposed considering additional criteria that allow the use of terraced rainfed lands covering a contiguous area of at least 100 ha, where water could easily be delivered. On the other hand, the World Bank proposed reassessing the irrigation potential to consider new settlements on agricultural land, water resources availability, development cost, need for flood control and drainage facilities.
In 2006, the area equipped for full control irrigation was an estimated 1 879 084 ha of which NIA’s national irrigation systems (NIS) represent 38 percent, NIA’s communal irrigation systems (CIS) account for 29 percent, small-scale irrigation systems (SSIP) represent 9 percent, private irrigation systems account for 9 percent and private irrigation systems irrigating crops other than rice account for 15 percent. In 1991, non-equipped cultivated wetlands and inland valley bottoms accounted for 39 478 ha, and in 2004 the non-equipped flood recession cropping area accounted for 63 814 ha, thus total water managed in 2006 was around1 982 376 ha. In 1992, the area equipped for full control irrigation was an estimated 1 532 751 ha, of which 42 percent were NIS, 48 percent CIS and 10 percent private schemes.
NIS schemes have been constructed, operated and maintained by the NIA. The construction cost is borne entirely by the NIA, while farmers pay for operation and maintenance (O&M). In 1992, there were about 150 NIS schemes throughout the country. There are three main subtypes depending on water origin:
- Three large schemes: Magat with 80 977 ha; Upper Pampanga with 94 300 ha; and Angat Maasim with 31 485 ha are backed by multipurpose reservoirs. Although classified as single entities, they are actually conglomerates served by multiple diversion structures, which also utilize supplies from uncontrolled rivers crossing the irrigated area. Parts of the service area may be too high to be commanded by the reservoir and are commanded by pump schemes. In 1989, the cropping intensity on these schemes was about 89 percent during the wet season and 78 percent during the dry season.
- Run-of-the-river diversion schemes: Most are relatively small. These diversion schemes can be fairly complicated, with several intakes and reuse systems that are often developed over time in response to observed drainage flows. The largest schemes are located in the alluvial plains. In 1989, the cropping intensity on these schemes was about 72 percent during the wet season and 54 percent during the dry season.
- Pump schemes: In 1992, there were around seven schemes irrigated only by pumps, and five large NIS schemes served mainly by gravity flow but which use pumps for a part of their equipped area.
CIS schemes have been created either by the farmers themselves over the centuries, or more recently by the NIA and then turned over to the irrigation associations for O&M. Almost half of the communal schemes are in the province of Ilocos (northwest Luzon), which reflects a long history of irrigation based on private initiative in this area. These schemes are predominantly diversion schemes; although a few are served by small reservoirs built within the framework of the SWIM projects. The average size of the communal schemes is about 115 ha, but range from 40 to 4 000 ha. The smallest schemes are found in north Luzon, while in Mindanao Island these schemes are generally large, many of them being implemented by the government settlement programmes and then transferred to farmer groups. The association bears 10 percent of the direct cost of construction, and pays back the balance within 50 years at no interest.
Private schemes are generally supplied by pumps. They originated in publicly assisted river lift and groundwater development projects.
Table 5 reflects the rice-based areas with existing irrigation facilities. The Bureau of Agricultural Statistics (BAS) estimate was larger than the NIA estimate because BAS took into account privately-owned small-scale irrigation system such as shallow tubewells.
In 2008, of a total 748 593 ha of NIS schemes equipped, the actually irrigated area was around 61.5 percent during the wet season and 71.2 percent during the dry season. In 2005, the NIA developed 10 539 ha of new areas and rehabilitated 110 865 ha of existing systems. In 2007, the agency targeted development of 17 585 ha of new area and rehabilitation of 112 534 ha of existing areas. From 1974-2007, the BSWM completed and made operational a total of 177 190 ha and rehabilitated 3 538 ha of irrigated areas.
Surface water development for irrigation is in the form of dams or reservoirs while groundwater development is through pumping from deep and shallow aquifers. Groundwater irrigation development, particularly from deep aquifers, is relatively expensive including O&M. This is one of the reasons why less groundwater is used for irrigation, aside from the fact that groundwater withdrawal is being reserved for municipal/drinking purposes owing to its inherent good quality. In 2007, surface water was used to irrigate 78.6 percent of the area equipped for irrigation, 5.7 percent by groundwater and 15.7 percent by mixed surface water and groundwater (Figure 3).
Surface irrigation is the major technique practiced in the Philippines owing to rice, which accounts for 1 863 664 ha or over 99 percent (Figure 4). Lowland paddy fields are flooded to prevent weeds and ensure yields. Sprinkler and localized irrigation systems, 4 500 ha and 10 920 ha respectively, are used on privately-owned large plantation areas such as for banana, pineapple and sugarcane. Their use is constrained by their relatively high investment cost and the skills required to operate and maintain them. Currently, the use of sprinkler and drip is being promoted, even for small-scale production systems, particularly in water-scarce areas. These include greenhouses producing high-value commercial crops, such as vegetables, where investment costs could be recovered over a shorter period.
Irrigation schemes can be differentiated according to size of the service area (SA). In 2006, small irrigation systems (< 100 ha SA) accounted for 625 360 ha, medium irrigation systems (100-1 000 ha SA) for 548 978 ha, and large irrigation systems (> 1 000 ha SA) for 704 746 ha (Figure 5). In 1999, the average farm size was 2.2 ha.
Irrigation water from NIA dams, SWIP and diversion dams is distributed by gravity system, conveying the water through open lined main canals to lateral ditches into the farm paddy fields. Farm lands, at elevations higher than the canal, use pumps to siphon water into the paddy fields. Pumps are also used for extracting shallow groundwater.
Role of irrigation in agricultural production, the economy and society
There are two cropping seasons in the Philippines. All schemes, equipped area and canal capacity, have been designed to provide supplementary irrigation to the entire irrigable area during the wet season. The area actually irrigated during the season should be 100 percent. In practice, this level is never reached owing to many reasons, such as over-optimistic design of service areas, flooding and waterlogging in the wet season, complexity of the irrigation system, pump performance, and conflicts between water supply, power and irrigation. The actually irrigated area varies significantly from one season to another, but it is always much lower than the area equipped for full control irrigation.
In 2006, the harvested irrigated crop area covered around 2 695 825 ha, of which 89.8 percent was for rice, 3.6 percent for maize, 2.4 for sugarcane and 1.4 percent for vegetables (Table 4 and Figure 6).
Paddy is cultivated throughout the country during the wet season and in some areas during the dry season when other crops with higher added value are also grown. The yields are much lower (30-40 percent) in the communal schemes than in the national schemes, because the water supplies are uncertain in the small catchment areas where communal schemes are located. On average, the 1992 yield for irrigated paddy was an estimated 3.34 tonnes/ha per season, which was 2.9 times the average yield of irrigated paddy in 1961. For rainfed paddy, the 1992 average yield was an estimated 2.07 tonnes/ha, which is twice the 1961 average yield. Irrigated rice paddy has a 65 percent higher gross return compared to non-irrigated. The sustained increase of water supply for rice production helps significantly satisfy the ever increasing food demand as well as improve food security.
Meanwhile, because of lack of finances to develop new land for irrigation, to help close the food gap, the focus has been on the development and improvement of technology and support services to improve rice production on rainfed land. The actual harvested rice area covered by irrigation facilities increased by almost twice from 1.43 million in 1970 to 2.42 million ha in 2006. The package of production technology, besides water provision, has made the country close to self-sufficient for rice, which is now placed at 97 percent, based on the average annual rice requirement of 118 kg per capita.
Under the BSWM and the NIS schemes, the average cost of irrigation development is about US$3 277/ha for new schemes, while the cost of rehabilitating existing schemes is US$1 608/ha, and the annual cost of O&M is US$98/ha. The average cost of irrigation development in private schemes is around US$556/ha; and the cost for rehabilitation of existing schemes is US$156/ha. The average cost of sprinkler irrigation and localized irrigation for on-farm installation is US$1 556/ha and US$2 222/ha respectively.
Status and evolution of drainage systems
In most schemes, drainage water from one field goes into another field downstream either through the irrigation canal or directly. It is, therefore, it is difficult to estimate the drained areas. In 1993, total drained area was an estimated 1 470 691 ha (Table 4).