A. THE AGNO BASIN (THE PHILIPPINES)

by

Elvira A. Baluyut
Inland Resources Development Corporation
Suite 111 Mercantile Insurance Building
Gen.Luna cor.Beaterio Street
Intramuros, Manila
Philippines

ABSTRACT

The Agno River Basin in Luzon in the Philippines has been under considerable stress from development activities. The upper catchment has been exposed to drastic and rapid deforestation and to extensive mining, both leading to heavy erosion, high sediment load in the Agno River and siltation. In the lower basin, two dams have been constructed for hydropower and irrigation purposes, both of them functioning also as sediment traps. Sediment and heavy metal input through the irrigation system has lowered the soil quality with a resulting negative impact on agriculture of lowland plains. Fish stocks of the upper Agno have never been of commercial significance, but some subsistence fishing has been carried out in the lower Agno, especially during floods. The two reservoirs constructed on the Agno in the 1950s have good fisheries potential but, so far, there has been only a minimal effort spent on their fisheries development. Aquaculture has been the major fish producer in the Agno River Basin, with 16,452 mt (in 1982), of which 12,380 mt came from brackishwater ponds. Brackishwater pond aquaculture of the Agno River Basin contributed 9% of the total brackishwater pond production of the Philippines, with 1,200 kg/ha/yr, as compared with the national average of 870 kg/ha/yr.

Current efforts of government agencies aim at developing fisheries in the Binga and Ambuklao reservoirs, with proposals to construct a new landbased hatchery-nursery complex to provide fingerlings of selected fish for regular reservoir stocking, and to set up a floating cage culture. In spite of that, the brackishwater aquaculture will probably continue to greatly dominate the inland water fish production in the Agno River Basin.

Figure 1. The Agno River Basin.

1. INTRODUCTION

The rush towards economic development by way of resource exploitation, often without regard for the environment, has left numerous ecological systems in a state of disarray. Man, in his desire to obtain direct and readily measurable economic gains, has neglected to concern himself with the indirect, longer-term benefits that he stands to lose as a consequence of environmental deterioration.

As a result, his multitudinous activities on land and in water have caused different degrees of interference with the ecosystem. In the watersheds, his manifold economic and development pursuits have led to habitat deterioration, making him the greatest single biogenic factor affecting water quality, both directly, as a result of discharges into rivers, and indirectly, through his activities on land (Petr, 1983).

In Southeast Asia, as in the other developing parts of the world, the situation is more serious on account of the pressures arising from rapidly expanding populations - a situation that makes choices more urgent and decisions often made without a full understanding of either options or optimality (Hanson and Koesoebiono, 1977). The price of development in the region has therefore been frequently irreparable damage to the environment, brought about by indiscriminate deforestation, industrialization, and urbanization.

In the Agno River Basin in the Philippines, for example, the productivity of agricultural lands has declined by as much as 50%, and the riverine fish have been almost eliminated as a result of excessive siltation caused by extensive deforestation and the discharge of mine tailings by large mining companies in the watershed.

Experiences like these strongly suggest the need to adopt an integrated resource management strategy that seeks to maximize potential socio-economic benefits while minimizing possible ecological damage. For, while it is true that “economic development has been given the force of a moral imperative, the resources of the entire world must be developed rationally to achieve the highest quality of living for mankind” (Dasmann et al., 1973). This implies the need to shift emphasis from purely economic goals to those that take into careful consideration the natural processes and resources that may need to be sacrificed in the course of reaching such goals.

It is in this light that the integrated, multi-use approach to area development assumes greater significance. In contrast to old methods which focused on single projects in isolation from others that may be planned for the same area, the integrated approach calls for a coordinated and harmonious development of various works in relation to all resonable possibilities of the basin (UN, 1970). In addition, development planning of this type takes into account various environmental factors or ecological issues ranging from the immediate, very specific, and readily predictable to the less specific, less easily discerned, long-term, and possibly unforeseeable (Dasmann et al., 1973).

While the integrated, multipurpose development of river basins is generally considered superior to the single-use approach, mention must also be made of the possibility of conflicting resource uses and the need to minimize, if not totally resolve, these conflicts. It is also quite sad to note that from among a whole spectrum of resources within a river basin, fisheries is often relegated to a low priority or is not given any priority at all.

In some instances, it is only after the project is completed that some thought is given to the fisheries component - either because it has performed so well that its gains surpass the projected primary benefits, or because the project managers realize all of a sudden that the large volume of water in the reservoir may just as well be used for fisheries.

It is with this background in mind that this case study of the Agno River Basin is written. This paper considers the Agno River Basin System with its socio-economic, physical, and biological resources, and the major past and present development projects. It discusses issues and problems arising from river basin development as they relate to the fisheries and the environment.

2. PROFILE OF THE AGNO RIVER ECOSYSTEM

The Agno River Basin is one of the two major basins in Central Luzon. It lies within 120°00" and 121°00'E and 15°00" and 16°45'N, covering an area of 13,800 km² (Fig. 1). The Agno River with its tributaries, drains several provinces, but the major part of its catchment is situated in the Pangasinan Province. In this Province the largest municipalities are distributed mainly along the national roads and the central and eastern parts of the Province. They are lowland rice-producing areas.

2.1 Social environment

2.1.1 Population and related features:

Pangasinan Province had the third largest population count in the Philippines at 1.636 million in 1980 (NEDA, 1982). This figure represents 46% of the regional total and 3% of the national total. The population of the Province showed a 7% growth rate over the five-year period from 1975 to 1980, equivalent to an average annual growth rate of 1.4%. Population density in 1980 was 304.8 persons/km², the highest in Region I which covers the Northwest of Luzon (Table 1).

The 1975 provincial statistics placed the total labour force (15–64 age group) of the Province at 1,647, of which 81% or 862,267 come from the rural sector. Classified by major gainful occupation, 54.2% were farmers, fishermen, hunters, loggers, and related agricultural workers; 12.3% were craftsmen, production process workers; 8% sales workers; 6.6% engaged in services; and 6% in technical and related activities.

Approximately 70% of the provincial households are classified as least developed in terms of family incomes, high rate of unemployment, inadequate accessibility to social services, and infrastructural facilities.

A high level of malnutrition exists in the Province. Of 116,394 pre-school children surveyed in 1980, 41.5% had first degree malnutrition; 29.4%, second degree; and 5.8%, third degree (NACIAD et al., 1981).

In some parts of the Province, quality of water is unsatisfactory, resulting in problems of gastro-enteritis and other water-borne diseases. In many barangays (villages), the potable water supply is inadequate for the requirements of the communities. The water systems are fast deteriorating and are not being expanded at a rate sufficient to meet population growth. To compound the problem, there are no organized refuse collection and disposal facilities, except in a few cities and poblacions, the most common methods of waste disposal being composting, burning, and open dumping.

The literacy level of the Province is 81.4% and educational facilities are adequate. There are 908 complete elementary schools, 271 secondary schools, and 35 institutes of higher learning.

2.1.2 Agriculture and industry:

The primary base of the economy of the Province is agriculture, the population of Pangasinan being predominantly rural. Most agricultural activity is done in the central part of the Province where the area is mostly lowland and partly under irrigation. Portions of the eastern and western parts of the Province are also planted to rice, corn, vegetables, and fruit trees. Pangasinan is considered one of the major rice-exporting provinces. In 1980, while productivity per unit area increased, the total area devoted to rice decreased to 235,000 hectares primarily as a result of the conversion of rainfed areas to fruit orchards, expansion of residential area, and erosion. Likewise, the actual irrigated areas decreased from 133,572 hectares in 1976 to only 78,155 hectares in 1980, largely due to the siltation of irrigation canals and lack of proper canal maintenance (NACIAD et al., 1981). Serious problems have arisen from the upstream mine tailings which are at present discharged into the river and silt up the canal systems. This has prevented the expansion of the present irrigation systems and has consequently limited the possibility of expanding the present agricultural areas.

Due to the constraints to agricultural production it is predicted that the Province shall be gradually shifting from an agriculture-based economy to an industry-based one. The National Water Resources Council (NWRC) (1983) projects that the industrial sector will grow at a much higher rate, from about 26% in 1975 to 50% in the year 2000. Agriculture is then expected to drop from a share of 46% in 1975 to 30% in 2000.

2.2 Physical environment

2.2.1 Topography:

The Agno River is about 270 km in length from its source in the Cordillera mountains to its mouth in the Lingayen Gulf. It drains the southeastern half of the Central Cordillera in Benguet Province, flows across the broad Pangasinan Plain, and enters the Lingayen Gulf. Stream gradients are steep in the upper reaches and flatten appreciably downstream from San Manuel, from where the river flows south to Tayug, veers off to the southwest through Rosales, and into the Poponto Swamps, (also known as Mangabol Marsh), where it is joined by the Tarlac River. From the Poponto Swamps, the flow becomes northwestward, skirting the eastern slope of the Zambales Mountains and finally discharging into the Lingayen Gulf.

The Cordillera Central, from which the Agno River rises, is geologically a young mountain range with very pronounced erosion phenomena and a morphology characterized by steep slopes and narrow gorges. Faults, some of regional importance, tend to promote and concentrate erosion phenomena. Erosion has considerably increased as a result of recent deforestation and is still continuing at a high rate. Deforestation has progressed to the extent that primary forest has almost disappeared, except at some spots above elevation 800 m. The practice of setting fire to the grassed slopes at the end of every dry season serves only to increase the effect of deforestation (ELC et al., 1979).

The lowland plain is filled in by sediments carried by rivers and deposited in fans and bars. This sedimentation process tends to isolate low areas between relatively higher levels so that a large portion of the Pangasinan plain suffers from inadequate drainage and flooding, particularly in the lower part of the Agno River plain.

The river is torrential, high sediment transport capacity in the mountains, and meandering and sharply reduced flow velocity and sediment transport in the plains.

2.2.2 Meteorology:

The climate of the Agno River Basin is characterized by two seasons: a dry period from November to April, and a wet period during the rest of the year.

The mean annual rainfall is about 3,300 mm (Fig. 2). The maximum annual precipitation of 9,038 mm was recorded at the Baguio City station in 1911 and is the highest ever recorded for the Philippines. The area of lowest precipitation is along the river valley upstream of Binga where rainfall averages about 2,000 mm/yr. Throughout the year, precipitation varies sharply between the dry season when zero rainfall can occur over two or more consecutive months, and the wet season when practically the entire annual rainfall occurs. The rainiest period is the four months span between June and September, when about 70 to 80% of the annual rainfall occurs.

Differences of several degrees in mean annual air temperature occur between mountain and river valley sites, with the mean temperature in Baguio City (altitude 1,500 m) being 19.2°C and that in Dagupan City at sea level being 27.7°C. In the Agno Valley, at Ambuklao (altitude 900 m), mean annual temperature is about 25°C.

2.2.3 Soils:

The soils in the Basin area represent a wide range of development stages - from young soils of recent alluvial deposits to old soils in the mountainous areas of the northeastern section. Soil textures range from moderately coarse to fine.

Figure 2. Isohyetal map of rainfall (in mm) (from ELC et al., 1979)

Several soil series and land types have been established by the Bureau of Soils. The San Manuel series, which occurs mainly on the extensive alluvial plain, is characterized as deep and poorly drained to moderately well-drained. It has moderate natural fertility and is medium to low in organic matter content. Surface soil reaction is medium acidic and permeability is moderate, with medium water capacity.

The San Fabian group is predominantly clay-loam with hardpan subsoil and generally underlain by volcanic tuff. The Alaminos group in the upland and rolling hills are predominantly loam and are susceptible to erosion. The last group is composed of soils which are developed on the sedimentary rocks, generally on limestone.

Soil quality in the Province is mainly low to moderate, with 53% of the total area of low fertility and 31% of moderate fertility. Also, about 14.2% of the soil in the Province is highly susceptible to erosion and 21.4% is moderately susceptible to erosion. The National Environmental Protection Council (NEPC) and the National Council on Integrated Area Development (NACIAD), in a joint report on the Ecological Profile of Pangasinan (1983), attribute the high percentage of low to moderate soil quality and the erosion susceptibility to the extensive deforestation/ denudation of the watershed leading to siltation, deposition of mine tailings, unstable land use, and improper cultivation practices.

2.2.4 Surface water:

2.2.4.1 River discharges and floods:

In the Pangasinan Province, the Agno River and its tributaries are the main sources of surface water in the Province for use in irrigation and for inland fisheries (Table 2).

The discharges of the Agno River basically follow the rainfall cycle over the Basin, the low period being from January to May and the high flows in the remaining months, with the highest from July to October when the cumulated discharge is more than 70% of the annual volume (Fig. 3).

The floodplain of the Agno River lies in the northeastern portion of the Basin and covers 1,810 km². Directly affected by floods are 27 municipalities involving population of more than 800,000 (based on 1975 census). During the wet season the Agno River and its tributaries overflow their banks and flood adjacent lowlands, usually causing extensive damages to life and property. Also, unguided development on the floodplain, reduced conveyance capacities of waterways due to excessive sediment load, and river meanders that destroy valuable chunks of land adjacent to the river banks further aggravate the flood situation in the Basin.

An ecological survey of flood risk areas by the NEPC and NACIAD (1983) showed three types of flood-prone areas: 1) areas near the headwaters of Agno River as it approaches the flat alluvial plains of Pangasinan; 2) areas situated near the banks or included in the river's drainage; and 3) areas on the coast, especially those affected by the tributaries of the Agno as it changes its course and enters the sea.

2.2.4.2 Water quality:

Ecological mapping for water quality shows that about 5.2% of the total land area of the Province has a very high pollution level. About 2.3% is heavily polluted with a severe impact on aquatic life; about 8.3% moderately polluted with tangible impacts on aquatic life; 33.6% slightly polluted; and the remaining 50.6% is relatively undisturbed. The extremely polluted areas, which are mostly heavily silted farmlands, are concentrated in the northeastern part of the Province. Those areas traversed by the Agno River are all moderately polluted.

Figure 3. Monthy runoff in the Agno River Basin shown as percentages of the annual runoff (from ELC et al., 1979)

Based on the river classification of the National Pollution Control Commission (NPCC), the waters of the upper Agno are classified as Class A while the lower Agno is Class C (Table 3). The upper Agno is used for domestic water supply, irrigation, bathing, and power production and the lower Agno for fisheries and irrigation.

In the past decade or so, however, the Agno River has been receiving significant loads of sediments coming from erosion of natural slopes and from several mines operating at the upper reaches. The river water has been shown to be contaminated with toxic metals such as iron, copper, zinc, lead, and mercury at levels exceeding permissible limits for indigenous fish species like carp, tilapia, and milkfish (Table 4). Also, the water immediately offshore of Lingayen Gulf shows the presence of noxious heavy metals in concentrations considerably above any acceptable standard (Table 5). These are at least partially attributable to contaminants flowing through the Agno River system (NACIAD et al., 1981).

2.2.5 Ground water:

Ground water is the main water source for household consumption and is used by a small number of farmers for irrigation. The main aquifer consists of sand, gravel, and silt, and is recharged by rainfall in the Sierra Madre range and through numerous creeks and streams. Not much is known of the quality of groundwater resources in the Basin. The NEPC/NACIAD (1983) Report on Pangasinan states that based on aquifer transmissivity and soil permeability, ground water in Pangasinan is, for the most part, of high quality except for very small patches of problem areas, such as in Dagupan City (for inferior bacteriological quality) and in the coastal fringes of Agno, Bolinao, and Alaminos where salt water intrusion has been observed.

In Dagupan City, a high incidence of gastroenteritic diseases, with a five-year average morbidity of 439/100,000 population (representing 32% of communicable disease deaths) has been reported (NACIAD et al., 1981). These may have been due to the high soil permeability and high pollution levels of surface water bodies in the Dagupan estuary, particularly in the immediate city area and in the mid-reaches of all tributaries of the Malabago-Sinocalan system, which may have resulted in the contamination of groundwater supplies, especially for drinking water.

2.3 Fisheries resources of the Agno River Basin

Apart from agriculture, marine and inland fisheries constitute another major economic activity in the Province. Pangasinan is a coastal province, the length of its coastal area extending along the Lingayen Gulf from San Fabian in the north to Infanta in the south. Other important coastal fishing grounds in the province are Tambac Bay, Dasol Bay, China Sea, and Sual Cove. Fourteen municipalities covering 129 barangays are classified as coastal.

The Province is also endowed with inland waters like the Agno River, the Mangabol Marsh, Mabulitec Marsh, and Bued River, among others. The Agno River is considered the longest river in the Province, originating from Benguet and flowing into Lingayen Gulf, cutting through sixteen municipalities on both sides.

Notes:  ARIS = Agno River Irrigation System
bdl = below detectable limit
NPCC recommended safe levels for fish:
Iron: 0.095 ppm for carp and tilapia
Manganese: 0.007 ppm for carp and tilapia and milkfish

Nickel: 0.054 ppm for carp and tilapia

* Maximum allowable concentration for fish
** Guidelines for acceptable environmental quality of water

2.3.1 Marine fisheries:

About 15,000 fishermen derive their livelihood from marine fishing, with the following groups of fishes being the most common: groupers, snappers, mullets, goatfishes, sardines, swordfishes, roundscads, tuna, and mackerel (BFAR, 1981). In 1981, total marine fishery production from the Lingayen Gulf was 19,449 mt, of which 7,692 mt came from commercial fishing and 11,757 mt from municipal fishing (Table 6). Of the various fish groups, the most abundant were those that belong to the ISCAAP Group No. 36 (mackerels, tunas, bonitos and billfishes), followed by the jacks, scads, mullets, and garfish (ISCAAP No.34), and the perches, breams, snappers, and eels (ISCAAP No.33).

*   Commercial fishing is done using a licensed vessel over three grosstons in weight and operating in areas more than seven fathoms deep.
** Municipal fishing uses vessels three gross tons or below and operatesin areas less than seven fathoms deep.

ISCAAP -       International Statistical Standard Classification of Aquatic Animals and Plants.
ISCAAP Group-  includes species of fish belonging to the same group and sub-class.

2.3.2 Inland fisheries:

2.3.2.1 Riverine fisheries:

From limited information available on the fisheries of the Agno River itself, it is evident that the river has not supported a commercial fishery since fishes in it are rare. The fishermen along the banks are only parttime fishermen who fish when the river water is high, as in the rainy season (Gracia and Natividad, 1979). In a recent survey conducted by the BFAR in August 1983, it was noted that the lower Agno is not exploited anymore since there are no fish to catch (Gracia and Magsumbol, 1983).

In 1979, experimental fishing by the BFAR showed that the dominant fishes in the river are the mullets (Mugil sp.), common carp (Cyprinus carpio), tilapia and mudfish (Channa striata) (Table 7). Other minor fishery resources include freshwater shrimps (Macrobrachium sp.), snails, and crabs, which were abundant in areas in Lingayen, Bugallon, and Urbiztondo, where the river had muddy bottoms (Gracia and Natividad, 1979).

2.3.2.2 Fish ponds:

The production from the inland fisheries sector comes mainly from brackishwater fish ponds, found mostly along the coast and in the delta of the Agno River, and from a few freshwater fish ponds in the lowlands. The brackishwater fish ponds are mostly privately owned, with the biggest number found in the municipalities of Binmaley, Lingayen, and Bani (Table 8). The biggest total hectarages of fish ponds are found in Binmaley, Bolinao and Anda (Table 9). Their size distribution by number of farms and by hectarage is shown in Figure 4.

In 1981, a total production of 16,452 mt of fish was obtained from 13,444 hectares of fish ponds (both brackishwater and freshwater) in the Province (Table 10). The total brackishwater fish production of 12,380 mt accounts for about 95% of the total production of Region I and about 9% of the total brackishwater pond production of the Philippines in 1981. About two-thirds of this production is supplied by the municipalities of Binmaley and Lingayen, where the fish ponds rely on the Agno River for their water supply (Gracia and Natividad, 1979). It is interesting to note that on a per hectare basis, the average production of brackishwater fish ponds in Pangasinan, which is equal to 1,200 kg/ha/yr, is higher than the national average of 870 kg/ha/yr.

Figure 4. Size distribution of fishponds in Pangasinan by (a) number of farms and (b) by hectarage, (from GOPA, 1983)

Of the volume of fish produced in the Province, however, only about 50% is consumed locally. The rest is sold mainly in the markets of northern Luzon and about 2% in Metro Manila (Fig. 5) (GOPA, 1983).

3. AGNO RIVER BASIN DEVELOPMENT: PAST AND PRESENT

Since the early 1950s, the waters of the Agno River have been harnessed for electricity and irrigation - the Agno River being the first river in the Philippines to be dammed for hydropower production. The Ambuklao and Binga projects in the upper Agno River, fully operational in 1956 and 1960, respectively, have since then provided electric power to the province of Pangasinan and most of Central Luzon. The Agno River Irrigatin System (ARIS), built in 1957, uses Agno River water to irrigate some 40,000 hectares of agricultural lands in the Province.

Figure 5. Distribution of the fish produced in Pangasinan (from GOPA, 1983)

Continuing extractive activities in the watershed, however, have now put these projects in serious jeopardy. Logging, deforestation, shifting cultivation, and mining in the upper Agno district, have severely affected the Binga and Ambuklao reservoirs and the ARIS. At this point in time, a solution to the siltation problem in the Basin is sought in the construction of the San Roque Multipurpose Dam in the lower Agno. Apart from that, a number of proposed short-term and long-range ameliorative measures is designed to be undertaken to alleviate the present situation.

3.1 The Binga and Ambuklao dams

3.1.1 Physical features:

Both dams are located in the upper reaches of the Agno River, in Benguet Province, within 19 km of each other (Fig. 6). Ambuklao Dam is located in the elevated areas of the municipality of Bokod, about 36 km northeast of Baguio City. Binga Dam is in the municipality of Itogon, 19 km downstream of Ambuklao, and retains water spilled from Ambuklao.

Figure 6. Binga and Ambuklao reservoirs and the proposed San Roque Dam. Also shown are their respective drainage areas. (From ELC et al., 1979)

The Ambuklao reservoir is supplied by five tributaries: Lalay and Bantay Rivers in the west and northwest, Pesick River in the north, and Agno and Bokod Rivers in the northwest and east (Fig. 7). It is also a 617 km² watershed which is largely devoid of trees. The reservoir has a maximum surface area of 750 hectares at the highest water level and a maximum depth of 124 m. The dam has an installed capacity of 75 MW, and supplies electricity to Central Luzon.

The Binga Dam, also known as Agno River Project No. 2, has a capacity of 100 MW and supplies Central and Southern Luzon. The reservoir has a maximum surface area of 425 hectares, maximum depth of 40 m, and drainage area of 860 km².

Figure 7. Ambuklao Reservoir.

3.1.2 Hydrobiological and fisheries aspects:

A recent hydrobiological survey conducted at the Ambuklao and Binga dams by Aypa et al., (1983) showed physico-chemical parameters to be within acceptable limits for aquatic production (Table 11). Of the two reservoirs, Binga has slightly higher values of pH and dissolved oxygen. Recent values are lower than those obtained from surveys conducted at the early stages of the reservoirs. The two reservoirs have an abundant phytoplankton composed of diatoms, dinoflagellates, blue-green and green algae, and zooplankton consisting of rotifers, copepods, and cladocerans (Table 12). In Ambuklao, zooplankton formed 30 to 40% by volume (m1/m³) of the total plankton, while in Binga it formed 20 to 30% of the total (Aypa et al., 1983).

Very little is known about the fisheries of the Ambuklao and Binga reservoirs. Gracia (undated) noted the presence of gobies (Glossogobius giurus) and eels (Anguilla sp.) in both reservoirs, and the Japanese fish “doyo” (Misgurnus anguillicaudatus) in Ambuklao at the initial stages of operation.

* From Aypa et al., 1983
** From Gracia, undated

After the initial introduction of fish (Table 13), the BFAR continued stocking fingerlings of tilapia, milkfish, mudfish, and other freshwater species in both reservoirs but detailed records of these introductions are lacking. Detailed fish catch statistics are missing, but the presence of large tilapia and carp in the catches of the 500 or so fishermen in the vicinity of the reservoirs show that these two species have established themselves successfully (Aypa et al., 1983). Also, casual observations by some people and reports of part-time fishermen in the area indicate that the fisheries in the reservoirs have stabilized and are providing about 50% of the protein requirement of the fishing families in the area (Aypa et al., 1983).

On the recommendations of BFAR, the National Power Corporation (NPC), which oversees the operation of the Ambuklao and Binga projects, shall implement shortly the establishment of a pilot fish cage culture project at selected sites within the two reservoirs (Castro, pers. comm.).

3.1.3 Sedimentation problems:

The physical conditions in the watershed of the two reservoirs, coupled with the human activities of mining, agriculture and settlement, have resulted in high sedimentation rates which in turn have led to reduced storage capacities and shorter life spans of both reservoirs, thus minimizing the planned benefits from the development of the river system.

For example, the large inputs of silt have drastically lessened the volume of water available for fish production. Although at present the water is reported to have little suspended matter (Aypa et al., 1983), therefore allowing the possibility of cage culture, this may not be the case for long, if steps are not taken to correct the existing situation.

The problem of sedimentation has been the subject of several studies. Before the Ambuklao reservoir was completed, the Harza Engineering Company of the United States, the consultant for the feasibility study phase of the project, conducted sedimentation studies of the Agno River from 1954 to 1956. In 1954, prior to damming the Agno, another sedimentation study was done. In 1967, eleven years after the damming, the National Power Corporation conducted surveys to determine the extent of sedimentation in the Ambuklao reservoir. Results of these studies are compared in Table 14. Since 1967, the rates of erosion and sedimentation have increased considerably, mainly on account of the continued deforestation in the watershed. Some estimates suggest that the Ambuklao reservoir will function for only five more years (Aypa et al., 1983).

At Binga reservoir, the original 1961 capacity of about 92 10⁶m³ at elev. 575 m, is said to have decreased to some 80.5 10⁶ m³ corresponding to a yearly sedimentation rate of about 1.9 10⁶m³/yr inclusive of bed-load transport (ELC et al., 1979), lower than Ambuklao's average 2.4 10⁶m³/yr. Total soil losses in the Agno watershed are estimated at 8.9 10⁶m³/yr (ELC et al., 1979).

* Total reduction in reservoir volume after 11 years of operation is 2710⁶m³ or a deposition of 2,455 × 10³, equivalent to 2.4 10⁶m³ sedimentsper year.

The possibility of undertaking corrective and rehabilitative measures like desiltation of the reservoirs has been considered, but seems to be more costly than building a new hydroproject (Castro, pers.comm.). Instead, the San Roque Multipurpose Project in the lower Agno, originally scheduled for implementation in 1982 and later moved to 1986, is now being started as an alternative project under instructions of the President of the Philippines (Anon, 1982).

3.2 The Agno River Irrigation System (ARIS)

3.2.1 Current situation:

The Agno River Irrigation System is one of the biggest national irrigation systems being operated and maintained by the National Irrigation Administration (NIA). Formally inaugurated on 17 May 1957, it serves most of the larger-scale irrigated areas in the Province, mostly in the lowlands of eastern and central Pangasinan, covering approximately 41,569 hectares out of about 95,000 hectares of irrigated land in the Province (NWRC, 1983).

As in the Ambuklao and Binga projects, however, the ARIS is plagued by heavy sedimentation problems which started barely two years after its construction. These problems have grown into serious proportions over the years as a result of the excessive soil erosion in the watershed and the discharge of large volumes of mine tailings from big mines situated in the upper Agno district.

Out of an estimated volume of 8.9 10⁶m³ of soil lost through erosion of the watershed annually, about 2.5 10⁶m³ of sediment goes through the ARIS diversion weir located at the head of the extensive delta system between the San Roque dam site and the confluence of the Ambayabang River. In addition to these erosion-derived sediments, mine tailings together with decant water from, and diversions around, tailings dams operated by three companies, all flow into the ARIS. As a result, not only have the irrigation canals silted up; their water has also been contaminated with toxic metals deleterious to agricultural crops and aquatic life in areas affected by discharges of the irrigation canals.

In 1978, the Agno River was reported to have received an average of 26,380 mt of mine tailings daily from the Benguet Consolidated Inc. (which produces 50% of the country's gold output), the Philex Mining Corporation, and the Itogon-Suyoc Mining Company, all in the Benguet district (Tamayo-Zafaralla, 1983). It is not exactly known, however, how much of this volume enters the ARIS intake and is distributed by the main and lateral canals into the irrigated areas. It is also not known adequately what quantity of accumulated sediments are due to past mining activities and to incidental tailings dam failures, particularly between the Binga Dam and the ARIS diversion weir and intake structure (NACIAD et al., 1981).

It is the opinion of the Bureau of Mines, however, that with the stricter controls imposed on mine tailings disposal by them in co-ordination with the National Pollution Control Commission, the actual volume of mine tailings and washings that flow into the Agno River is now less than it was over a decade ago (Obra, pers.comm.). Moreover, it has to be acknowledged that the erosion-derived sediments resulting from denudation and poor land use in the highly mineralized upper Agno may also contain heavy metals in solution and/or adsorbed to soil particles, particularly clays (NACIAD et al., 1981).

Whatever the major sediment source is, the fact remains that the ARIS has been severely affected by siltation. Heavy sedimentation of the intake structure and the entire canal system has reduced discharge capacity to only 7.5 m/sec from the original design capacity of 28 m/sec (UNESCO/UNEP, 1979). Current estimates of the areas adversely affected by sedimentation in the Agno River catchment are 27,000 hectares of land, of which 18,500 hectares are NIA-administered areas and 8,120 hectares are served by the communal irrigation systems in the lower Agno (NACIAD et al., 1981).

Crop yields have been significantly reduced by about 20 to 50% and crop losses estimated to cost approximately P14.8 million/yr due to a number of factors (UNESCO/UNEP, 1979). First, the accumulation of silt discharged together with irrigation water (Table 15), causes a cementing action on the soil and stunts growth of crops by inhibiting root absorption. Secondly, the high proportion of mine tailings reduces the fertility of the soil, as indicated by low organic matter content and reduced available P₂0₅. Thirdly, the highly-polluted irrigation water impedes with nutrient availability and uptake by the crops.

Similarly, the NIA has been incurring losses in the past several years as a result of the following:

1. non-payment of partial payment of farmers" irrigation fees, equivalent to about P12 million between 1960 and 1977;

2. payment of the farmers" claims for damages wrought by extensive siltation of their rice paddies; and

3. periodic desiltation/rehabilitation of ARIS canals and laterals, costing over P5 million since 1967 (NACIAD et al., 1981).

The growing magnitude of the ARIS siltation problem and the resultant losses in revenue for both the farmers and the government, have prompted various agencies to take a closer look at the situation and initiate projects meant to alleviate the problem. Both long-term and interim mitigating measures have been proposed and are now lined up for implementation.

The NACIAD et al., (1981) have proposed that the following immediate and long-range measures be adopted:

1. Immediate:

Clear Water Development Program;

2. Long-term:

a)   Construction of the San Roque Multi-purpose Project whose main feature is the establishment of a reservoir that shall act as a trap for mine tailings discharged in the upper Agno;

b)   ensuring an adequate design, construction, and operation of tailings dams throughout the economic life of all existing and future mining companies in the upper Agno basin;

c)   implementation of an extensive reforestation program with priority on the residual catchment above San Roque dam site and downstream of Binga Dam;

d)   rehabilitation of the ARIS infrastructure including construction of a redesigned intake system, and canal desiltation and improvement; and

e)   requiring all mining companies to implement comprehensive tailings dam bank stabilization measures.

3.2.2 Clear Water Development Project:

As the periodic desiltation of irrigated canals and the installation of settling basins at the farm level have been observed to be losing strategies and in view of the fact that the proposed San Roque Multipurpose Project shall take from six to seven years to complete, the NIA, in cooperation with the NACIAD, has embarked on the Clear Water Development Project for the ARIS. The project is aimed at developing small-scale diversion schemes to provide sediment-free supplementary irrigation water to about 11,000 hectares of agricultural land in the wet season.

In essence, the project involves diverting the clear water of small creeks and tributaries of the Agno River through canals and into the farmlands. Work on the diversion dams of three clear water sources has been completed. The completed project, though of a much smaller magnitude than the San Roque Project, is expected to provide immediate relief to many farmers in the Agno River Basin area (Nietes, pers.comm.).

3.3 San Roque Multipurpose Project

3.3.1 Features and purposes:

The project, to be located in the lower Agno River, some 30 km from Baguio City, shall have drainage area of 1,250 km², reservoir capacity of 990 10⁶m³, active storage of 670 10⁶m³, dead storage of 320 10⁶m³, maximum surface area of 14 km², and maximum normal water level of 290 m a.s.l. (Table 16, Fig. 8).

The San Roque reservoir would regulate inflow for both power generation and irrigation use. Releases for power generation would be made through a tunnel extending from the reservoir to a power plant located immediately upstream from the existing ARIS diversion dam. An afterbay reservoir would be constructed adjacent to the diversion dam to re-regulate discharges from the power plant.

Projected to be the biggest multipurpose dam in Southeast Asia (Anon, 1982), the dam shall be built primarily for irrigation, power generation, flood control, and water quality improvement (ELC et al., 1979). It will render possible the year-round irrigation of 70,500 hectares in the Pangasinan plain with a partially diversified crop in the dry season. It shall also generate 780 GWh of primary energy plus up to 434 GWh of secondary energy to be made available at a distance of 150 km from Metro Manila. In addition, the project is expected to attenuate the flood problem in the Pangasinan plain and improve the quality of water in the lower Agno River by trapping all mine tailings produced in the watershed, the volume of which is expected to exceed 150 10⁶m³ in the next 30 years.

3.3.2 Project rationale and justification:

The San Roque Multipurpose Project has been considered by the NACIAD et al. (1981), to be the most economical and the best technical alternative to reestablishing the viability of the Agno River Irrigation System which, at present, is suffering from heavy sedimentation problems and resultant economic losses.

The strongest justification for the San Roque Project arises from its projected capacity to entrap mine tailings and erosion-derived sediments from the upper Agno, and thus release clarified water to serve the irrigation and other requirements of the Province.

Figure 8. Map showing drainage area of San Roque project (from ELC et al., 1979)

The lower Agno River receives a heavy load of mine tailings and washings from three big mines located in the upper Agno district. These gold mines - Philex Mining Corporation, Benguet Consolidated Inc., and Itogon-Suyoc Mining - all exploit ores to obtain gold bullion and copper concentrates by cyanidation and flotation. Each of these mines also has tailings dams within their vicinities. At present, Itogon-Suyoc Mining, which is predicted to have an entire lifespan of 10 years, uses more than 600 kg/day of cyanide and produces a mine tailings output of up to 600 m t/day, which are disposed in tailings ponds formed along the banks of the nearby creeks. Benguet Consolidated, with an expected lifespan of 10 more years, uses over 300 kg of cyanide per day plus large amounts of flotation agents and mixes. It has a daily output of 300 mt of mine tailings which are impounded in tailings ponds formed with the help of rockfill dikes in the vicinity of the mill. Philex Mining, the largest mine in the Agno watershed with a projected lifespan of 40 more years, uses 400 kg of flotation agents daily to produce 25,000 m t of ore per day and tailings which comprise 90% of the overall tailings volume produced in the Agno watershed. Its tailings are impounded in a series of large tailings dams which are being gradually built up on a right bank tributary of the Agno, that meets the main stream in the stretch to be occupied by the future reservoir of the San Roque plant.

The tailings produced by the processing plants are conveyed by gravity launders to the tailings ponds formed by damming nearby creeks. To prevent the overtopping of these tailings dams, the natural waterflow from the river or creek upstream of the ponds is diverted by diversion works and tunnels. These structures notwithstanding, several cases of tailings dam collapse due to overtopping have been reported, resulting in the release of mine tailings in large amounts into the Agno River downstream of Binga reservoir. (The water released by the Binga Dam is practically pollution-free).

The sedimentation problem of the lower Agno River has prompted government agencies to look for possible measures to remedy the situation.

In 1975, a study was conducted by ELC (as cited in ELC et al., 1979) to compare two alternatives: 1) construction of tailings ponds near each of the mines; and 2) establishment of a 45-km conveyance along the Bued River (another long river in Pangasinan) and ending shortly before reaching the coastline, with the tailings to be accumulated in a sacrifice area.

In 1978, a study was carried out by the Japan International Cooperation Agency (JICA) (as cited in ELC et al., 1979). The study recommended the establishment of a tunnel-launder-pipe (TLP) system to convey the tailings to a sacrifice area in the Gulf of Lingayen. At one point in the past, the Bureau of Mine even proposed that all mine tailings be dumped into the Bued River which would then discharge them into the sea (Obra, pers.comm.).

The construction of tailings ponds near each of the mines is seen as an unattractive solution to the problem, considering that these would represent a no-return investment for the mining companies. The stability of the tailings ponds is therefore deemed very uncertain since the mining companies would settle for minimum expenses just to comply with the requirements.

The establishment of conveyance channels ending up near the Lingayen Gulf does not also represent an attractive alternative because of the possible impacts on the sacrifice area which would have a dump capacity of 150 10⁶m³ (ELC et al., 1979). Neither does the proposal of dumping the tailings into the Bued River, which is a long way from the sea.

In view of the projected primary role of the San Roque Multipurpose project in sediment entrapment and water quality improvement for irrigation, in addition to hydropower generation and flood control, its establishment has been considered the best option for the Philippine Government, especially since preliminary assessments of its environmental impacts showed that the project would indeed to ameliorate the existing problems.

The Feasibility Study on the project, which was prepared by ELC et al. (1979), claimed that since the reservoir shall trap the suspended sediments, the resulting silt-free water shall regain its scouring capability and tend to erode away the present obstructing sediments previously deposited in the Pangasinan plain, thus clearing out new channels and reinstating a good flow across the plains.