Geographic Location and Area
Vigan, the capital town of Ilocos Sur is situated along the western coast of Northern Luzon. It is bounded on the north by the municipality of Bantay; on the east by the municipality of Santa; on the south by the municipality of Caoayan; on the west by the municipality of Sta. Catalina and lastly, on the southwest by the China Sea. It is 408 kilometers from Manila; 80 km. from Laoag City and 139 km. from San Fernando, La Union.
It has a total land area of 2,886.085 hectares or 28.86085 sq. km. The heart of Vigan is located approximately between the meridian 120 23’15” east longitude and the parallel 17 34’30” north latitude.
Vigan has 39 barangays, the poblacion barangays consist of nine barangays,with a total land area of 192.45 hectares and the adjoining 30 barangays make 2,693.635 hectares.
Topography
The land area of Vigan is generally flat with slopes of 20 which are found at Brgy.V, Tamag, and Paoa. Out of the 2,886 hectares of the land area, 36.60% is residential and 45.18% is agricultural.
The drainage network of the eastern margin of Vigan-Bantay plain consist of deeply cut valleys whose pattern is strongly controlled by main faulting trends, particularly those in the northerly and easterly directions.
Vigan is being drained by the Vigan River and Bantay River, respectively, on the north and on the south by Mestizo River whose water discharge comes from Bantaoay head water.
The relief of Vigan-Bantay plain is actively level to undulating with a slope ranging from 0% to 8%. Vigan in particular is generally level with several hills in Brgy. Tamag, Bulala and Salindeg, sometimes reaching about 50 meters in elevation and has a slope ranging from 3% to 8%.
The eastern margin of the plain characterized by a steep to a very steep slope, about 40% to 60%, the relief often reaches an elevation of more than 600 meters especially in the northerly and easterly portion.
The dominant topographic feature in the eastern margin of the town is the Vigan Gap Hill where the continuous shifting Vigan River cuts its way. The Gap Hill is located approximately 10 kilometers east of Vigan.
Hydrogeology
Folding and faulting apparently are absent in Vigan-Bantay plain except in the eastern and southern margin of the plain. In the eastern margin, a north-south trending fault occurs transecting the Pre-Quaternary rocks. The fault line runs from Sinait down to Bantay and Santa, while the southern extension of the plain is bounded by another structural line that trends northwest and southeast occurring from Santa down to Narvacan and Sta. Maria.
Surface Drainage
The Mestizo River
A sixteenth century map indicates that Vigan used to be completely surrounded by water that separated in from the mainland like a triangular-shaped island. Geological observation confirmed this, because the bodies of water surrounding Vigan were traced to have been the main channels then flowing in a westerly direction and branched as the river dissecting the Vigan ridge.
At present, the Mestizo River is just a mere river draining Vigan. It is almost devoid of fish and is even at the verge of reaching intermittency.
A brief study was carried out to gather preliminary geological information necessary for the plan of rejuvenating and developing the Mestizo River. The geologists of the National Museum estimated that from the mouth of the river to the bridge of Caoayan with a distance of about 2.5 kilometers long, with a width of 90 meters and at a depth of 2 meters, the volume of sediments to be dredged is approximately 450,000 cubic meters.
Climate and Rainfall
Vigan falls under the first climate type in the Philippines which is characterized by two pronounced season. The dry season normally commences from November to April reaching its peak during the torrid months of April and May. The wet months are from June to October, with the month of August as the rainiest period. The average annual rainfall is 190.683 mm. and the heaviest registered rainfall is 693.3 mm.
The average temperature is 26 °C. The average warmest temperature is 30.9 °C and the average coldest temperature is 21.1 °C. An average of 7-10 typhoons visits Vigan annually. The average relative humidity is 81%.
Soil Type and Suitability
The soils of the area have been classified by the Bureau of Soils into three main groups based upon their topographic positions namely: (1) soil of the plains; (2) intermediate upland soils; and (3) soils of the mountainous area.
The first group consists of Bantog Clay Loam, San Miguel Clay Loam, Beach Sand and River Wash. The intermediate upland soils are represented by Bantay Clay Loam and Umingan Sandy Loam. The mountain soil is represented by rough stony land.
The soil type of the city is classified into five (5). Vigan’s land resource is mostly made up of the san manuel clay loam type of soil which is 49.55% of the total 2,886 hectares land area of Vigan. This type of soil is best suited for planting bananas, cotton and vegetables. The umingan soil on the other hand shares 47.44%, identified to be suitable for upland rice and vegetables. The Bantog clay which is considered the most significant type of soil in Vigan because this is used as raw material in the making of pots, jars and all earthen wares (burnays) only makes up 1.26% of the total land area of Vigan. The last two types of soil, the bantay loam which comprises 1.46% is considered as a pastureland and the beach sand that covers only 0.29% of the city’s land resources.
Geologic Base and Mineral Resources
The sedimentary and metamorphic rocks that are present in the city are alluvium, fluviatille, lacuatine, pludal and beach deposits namely: coral reefs, stools, and beach rock. These are predominantly found along the coastal areas of Vigan. An important non-metallic mineral resource found in Vigan is the kind of clay that is used in making earthen jars locally called “burnay”. Earthenware of different uses and sizes are made of this kind of clay. Known as Bantog Clay, the mineral is the basic material in making bricks. These bricks are the original material in building the old Hispanic houses of Vigan.