2.4.6.2  Water Characteristics

Drainage, Tributary Patterns and Catchment

Zamboanga City is endowed with several rivers that drain to the Sulu Sea and the Moro Gulf. The Siocon-Quiniput-Taguite-Tumaga basin occupies the southern half of the Zamboanga Peninsula and encompasses the city of Zamboanga.

In terms of surface, hudrology information indicated that there are about 16 identified rivers with an average monthly discharge of 202,277 LPS. The most important rivers are the Tumaga, Manicahan, curuan, Bolong, culianan and Vitali Rivers in the east coast and the Ayala River in the west coast.

The Tumaga River forms the major drainage pattern, although smaller river systems also exist. To the south, Tumaga River assumes a dendritic pattern. The Pasonanca Watershed Reservation with a total area of 10,560 hectares serves as the catchments area for the various river tributaries. In general, Tumaga River has steeper slopes and higher in elevation and has a less dense drainage system (Table IV-4). (See Map IV-10 for the Surface Hydrology and Watershed Map.)

Table IV-4.   Major Rivers in Zamboanga City

                                             Source: DENR Regional Profile, 1995

                        Location and Extent/Size of Surface/Freshwater Resources

Of the major rivers in Zamboanga City, the Tumaga River is the most important one being the City’s source of water. Accordingly, Tumaga River has a catchments area of 101 sq. kms., a main stream length of 17 kms., and a drainage density of 64%. It has a total monthly rated capacity of 4,939,270 cubic meters. Tumaga River currently serves as a bulk of water supply of the Zamboanga City Water District (ZCWD) by means of a diversion dam and a raw water intake from the said river. Vitali River has a drainage area of 211 sq.km. with a main stream length of about 23 kilometers while Manicahan River has a main stream of 15 kilometers.

Other rivers, which are equally important, are the Bolong River and the Culianan River. The Bolong River has a catchments area of 5,440 hectares and an estimated mean annual run-off of 17,880 cu.m. On the other hand, the Culianan River has a catchments area of 3,126 hectares with an estimated mean annual run-off of 11,085,000 cubic meters. The Ayala River with an estimated mean annual run-off of 11,640,000 cubic meters and has a catchments area of 2,784 hectares. (See Map IV-11).

In addition, the presence of springs is notable in both the east and west coast, particularly in the Labuan-Pantalon sites, Qiniput and Bunguiao. Of the several springs existing in Zamboanga City, two (2) were the subject of a recent investigation and were found to have significant flows, the Mnicahan and Boalan Springs. The Manicahan Spring is located approximately 22.6 kilometers from the city proper.

SURFACE HYDROLOGY AND WATERSHED MAP

RIVERS AND TRIBUTARIES SYSTEM MAP

It emanates from a solution channel of limestone formation and believed to belong to the Oligocene-Miocene period. It is being used for domestic water supply and irrigation needs of Manicahan and vicinity and has a flow measuring 20 lps which could decrease during dry seasons. The Boalan Spring situated in Sitio Buenagatas in Barangay Boalan is 8 kilometers from the Zamboanga City. The spring emanates from 2 exposed outlets of limestone fissures and discharges into a shallow natural pond. It is situated at the foot of a low knoll in a generally rolling terrain and at an elevation of 30 meters. It is presently being used as a source of domestic water supply with a flow measuring 67 lps and could decrease during dry periods.

The wetland in Zamboanga City is located in the southwestern tip of Zamboanga peninsula and consists of fishponds, saltpans, mangrove forest and coral reefs.

The City has nineteen (19) communal irrigation systems (CIS) as shown with a service area of 2,183 hectares.

General Groundwater Information

The estimated groundwater recharge of Zamboanga del Sur is located within the Siocon-Quiniput-Taguite-Tumaga River Basin covering some 9,540 hectares with an estimated groundwater recharge of 204 cubic meters per year.

The principal groundwater reservoirs are located in the Miocene Limestone Formation and the Pilocene to recent Alluvium. The Pilocene to recent units contain aquifers composed of a sedimentary sequence of clays, sands, gravels, limestones and organic rocks.

Zamboanga City is fortunate to have groundwater sources identified to be located in 13 Barangays as reflected in Table IV-5. Of these, 10 are noted as potable while 3 are not potable but are being used for domestic purposes.

Table IV-5.   Location of Identified Groundwater Sources

As per report of the ZCWD, there are currently six-groundwater operated by the WD with a monthly total rated capacity of 387,417 cubic meters.

Information on groundwater availability indicated that along the coastal areas of Zamboanga City are potential shallow and deep well areas with depths of 18.17 and 48.21 meters, respectively and static water level of 6.75 meters for the shallow well areas and 20.42 for the deep well areas. In Zamboanga City, the groundwater flow moves in a southerly direction with groundwater level expected to fluctuate in accordance with the rainfall occurrences, which affects the recharge of the aquifer. The groundwater cycle could be best described as follows:

·         Groundwater recharge takes place mainly through direct infiltration from rainfall and from the underflow of creeks and rivers into exposed permeable beds of alluvial formation as well as though loose talus mantle and fractures in the mountains north of the city

·         Infiltration from irrigated lands provides additional discharge during flooding of the lands so that considerable amount of water infiltrates and consequently recharges the aquifer.

·         Direct groundwater occurs at a number of scattered locations along the flanks of the rivers, springs and at a depth through permeable formation of numerous wells being utilized in the city and nearby areas.

Availability of groundwater in Zamboanga City is based on some measured parameters such as rainfall (precipitation), temperature, transmissivity, hydraulic gradient ads well as infiltration and evapotranspiration. The total net precipitation surplus for surface run-off and groundwater infiltration is approximately 204 mm per year. Thus, using an estimated surface area of 75 square kilometers in the Zamboanga City Plain, the equivalent surface run-off and groundwater infiltration rate has been calculated to be about 485 lps. With the assumption that 15% of this value corresponds to the amount of water that will infiltrate into the aquifer, the aquifer recharge is estimated to be 73 lps with an estimated long term sustained yield of the aquifer is approximately 373 lps. In addition, an estimated 52 lps can be withdrawn via river bed infiltration. Thus, the allowable estimated abstraction rate amounts to 425 lps. By setting aside an allowance of 20% or 85 lps of the total aquifer yield for users other than domestic water supply utilities, the remaining capacity of the aquifer that can be abstract would be 340 lps (Map IV-12 shows the Groundwater Availability Map.)

                        Water Use and Regulation

Generally, surface waters are being used for socio-economic purposes considering that the city is largely and agricultural area. Rivers serve as a source of water for agricultural purposes such as irrigation as well as for domestic usage including industrial purposes. This is being tapped for domestic water usage. The most common source of drinking water is springs, deep well and reservoirs with pipe connections.

The Zamboanga City Water District (ZCWD) is one of those few water districts that rely heavily on surface water (Tumaga River) to supply the needs of the city.

As of 1997, the ZCWD reports that out of the total population of 388,035 only 186,089 are served by the WD or about 48% covering some 30,638 connections. The combined total water demand for domestic, industrial, institutional and bulk is 46,558 cubic meters per day. Per capita water consumption has been computed at 2.5 cubic meters per day.

Total water production is 79,920 CMD wherein 70,000 CMD is being sourced from surface sources, mainly from the Tumaga River with the balance supplied by the six- (6) groundwater wells owned by the ZCWD. Three (3) of the wells are standby wells used only when the supply from Tumaga are insufficient to meet the demands. Data from the ZCWD indicated that of the total 103 wells, 59 are owned by the government while the 44 are privately owned. These wells vary in type from shallow and dug wells to bored wells used for domestic, commercial and industrial water supply including irrigation.

GROUNDWATER AVAILABILITY MAP     

Surface Water/Stream and Groundwater Conditions

In general, surface water is exposed to the atmosphere and thus, subject to pollution and contamination. Water from surface sources are usually loaded with suspended and are often turbid, particularly, during the rainy season. Even during the dry season when the river flow is low, the water is still turbid.

Quality of surface water, in particular, rivers are considered an important factor in the physiological process of human and animals. As such, selected rivers are being subjected to periodic monitoring to determine the quality of its water. The main reason for the seemingly poor water quality of rivers could be attributed to the indiscriminate disposal of domestic wastes (both solid and liquid) into rivers, creeks and streams.

Recent results of chemical and physical water testing of water samples indicated that in the case of Tumaga River, color and turbidity of the samples exceed that of the limits set by national standards. The excess in color may indicate polluted water while that in turbidity may be caused by presence of colodial or larger suspended materials. The same results were noted from the samples taken from springs (Manicahan and Boalan).

The groundwater in Zamboanga City is characterized by a relatively high calcium and manganese content. Calcium and manganese are non-toxic substances and are not harmful to human unless in excessive quantity. The groundwater availability map is reflected in Map IV-11.

Watershed Importance

Watersheds refer to an area drained by a river system. It serves as a water reservoir on which the existence of rivers and other fresh water bodies are largely dependent. As such, watersheds paly a very vital role in the socio-economic development of an area. Seven watersheds play a delineated in Zamboanga City namely: Pasonanca Watershed; Ayala River Watershed; Curuan River Watershed; Vitali River Watershed; Manicahan River Watershed; Culianan River Watershed and; Bolong River Watershed. Of these, only Pasonanca Watershed has been proclaimed. The rest are still awaiting formal proclamation pending the completion of necessary requirements for the proclamation of a watershed reserve. Table IV-6 provides information on the identified watersheds in Zamboanga City.

Table IV-6.   Identified & Delineated Watershed Area in Zamboanga City

                                Source: Provincial Profile of Zamboanga del Sur, 1995

2.4.6.3    Atmosphere

Climate

Zamboanga City has a third type climate. Seasons are not very pronounced but relatively dry from November to April and wet during the rest of the year. The climatic control in the City Is influenced by the alteration of the wind direction due to the presence of high elevation forested mountains and its geographic location. The northeast monsoon prevails during November to February while the southwest monsoon occurs during the

Months of June to October. Its climate is characterized using 1951-1985 data from Zamboanga City weather station.

Rainfall

The rain period starts in June and last up to November while the three-month period from January to March which receives less than 500 mm of rain per month is relatively dry. December and April may be considered transition months. The mean annual total rainfall in Zamboanga City is 1211.8 mm.

There are 132 rainy days per year in Zamboanga City, 97 of which have thunderstorms. Precipitation from localized thunderstorms accounts for a large portion of the total rainfall in the area. The climatic normals for Zamboanga City are shown in Table IV-7.

Table IV-7.   Climatological Normals fro Zamboanga City

(Period of Record: 1950-1985, Source: PAGASA)

Relative Humidity

Humidity is consistently high in Zamboanga City with an annual average value of 82%. The lowest mean monthly value is registered in March at 78%.

Cloudiness

Based on the BSWM study, high cloudiness (7.2) can be observed during the month of June while minimum cloudiness of 5.3 often occurs in March.

Wind Speed

The wind prevailing over the area is generally westerly. The wind speeds are mostly at 2.0 meter per second. Local circulations such as the land and sea breezes have minimal effect on the prevailing winds in the area. This wind speed is not considered to cause severe damage to plants.

2.4.6.4  Coastal and Marine Environment

General Conditions

The Department of Environment and Natural Resources (DENR) – Region IX and the Zamboanga State College of Marine Science and Technology (ZCMST), note Zamboanga City for having one of the diverse and dense concentrations of marine resources specifically on the east coast where a current study is being conducted. The total coastline of Zamboanga City including its immediate islands is computed at nearly 396.684 kilometers using a GIS computer. (Map IV-13 shows the map of the marine resources in Zamboanga City.)

MARINE RESOURCES MAP

Prevailing Tidal Fluctuation

The tidal station of Zamboanga City is located at Isabela, Basilan. The station contained the lowest average heights of a