TOPOGRAPHY

Mountain Ranges

The Province is relatively uneven in its physical features.  The relief changes from level to hilly and mountainous.  The warm coastal plain is level to undulating with several low hills, scattered at random all throughout the entire length of the province.  The eastern portion which borders Abra, Benguet and the Mountain Province is hilly and mountainous.  The highest mountain in the province are the peaks of the Malaya Range located southeast of the town of Cervantes, the two highest peaks of which is the Malaya Mountains (2,352 ft) and the Mount Libo (1771 ft.).  Ilocos Sur is an elevated coastal tract in some places comprised of raised coral while in other places, overlaying an alluvian sediments.  The eastern boundary of the plains is an escarpment, which has been caused by a fault and formed at the edge of audesitic extrusives.  The hills and low mountains are gently folded sandstones and shales.

Slopes

Ilocos Sur is considered to be unique when we speak of physical surface or condition of the earth surface. As one strolls along the western part of the municipalities of Sinait, Cabugao, San Juan, Magsingal, Sto. Domingo, San Ildefonso, Bantay, San Vicente, Sta. Catalina, Caoayan, Vigan, and Santa he notices that the slopes vary from 8% to 18%.  It can be noted also that the slope on the eastern tips of Magsingal, Sto. Domingo and Bantay is approximately 50%.

As we go south within the province, from Narvacan down to Tagudin, the western, central, eastern or the entire areas of the said section, the slope is still from 0-8%.  The hilly to mountainous sections with slopes varying from 30% to 50% can be located at the eastern part of Sta. Cruz, Lidlidda, southern and eastern part of San Emilio, eastern part of  Sta. Cruz, Suyo, Alilem, northern and eastern part of Sta. Cruz, Suyo, Alilem, northern and eastern part of Sugpon, Cervantes, and Quirino. Sections with slopes varying from 50% and above are located in the western and southern parts of Sugpon, eastern part of Alilem, western part of Cervantes up to Sigay and del Pilar, western part of Quirino and eastern part of San Emilio.

The province’s land slope is categorized into six groups.  The first category is the 0-3 percent slope or level to nearly level.  These are irrigable lands  highly suitable for agricultural, urban, industrial, and other related uses.

The second category is the 4-8 percent slope, which are gently sloping to undulating lands.  These are also irrigated lands, suitable for agriculture.

The third category is the 8-18 percent slope.  These are undulating to rolling lands.  These lands have a wide variety of uses, with agricultural options ranging from seasonal to permanent crops.

The fourth category is the 18-30 percent slope.  These are rolling to moderately steep lands.  These are generally considered marginal lands for most of the agricultural crops requiring tillage.  Some have deep friable soils which when planted with trees can be productive given the ideal environmental conditions.

The fifth category is the 30-50 percent slope, or steep hills and mountain areas.  These are reserved for forest trees, to attain the required balance between forest and agriculture areas.

The sixth and last category covers areas with more than 50 percent slope.  These comprise very steep and extremely rough mountainous areas, wherein extraction of trees is difficult and or economical.

Homogenous Ecological Zones (HEZ)

A homogenous ecological zone is the spatial unit for an area-focused development planning in the region.  This approach defines two (2) contiguous ecological zones in Ilocos Sur, namely the lowland and the upland zones.  The upland zones consist of municipalities which are predominantly 18 percent and above in slope (more than 50 percent of the total land area), while those municipalities which are predominantly less than 8 percent in slope comprise the lowland zone.

Based on the Geographic Information System Data, there are 12 municipalities identified as predominantly upland areas in the province.  The total land area of these municipalities is 1,497.40 square kilometers occupying about 58.05 percent of the total provincial area.  The remaining 22 municipalities with a total land area of 1.082.20 square kilometers are considered lowland areas.

Water Basins

SURFACE WATER RESOURCES

Precipitation is the source of all surface and sub-surface water in the province.  These are distributed mainly by the rivers on the area, namely:  Badoc River, San Ildefonso River, Abra River, Narvacan River, Sta. Maria River, Bucong River, Oaig River, Buaya River, and the Chico-Amburayan River.  All these rivers drain to the Luzon Sea.

GROUND WATER RESOURCES

Ground water resources are categorized into shallow well, deep well and difficult areas. 

In the shallow well areas, wells within 20 meters below ground surface (mbgs) may be developed for rural water supply for levels I and II services.  The static water levels in these areas are generally within 6 meters below ground surface.  In some cases, deep wells may also be constructed particularly for higher demands like the Level III system.

Deep well areas are generally sedimentary formations, 90% of which are water carriers.  These are usually located in slopes reaching up to 10% at elevation more than 50 meters above sea level.  The chances of constructing shallow wells in deep well areas are slim although there are chances due to the fact that water-bearing formations are deeper than 20 meters.

The water in deep well areas is usually of good quality.  However, care must be exercised in limestone formation where calcium carbonates are the major constituents.  The aquifers are generally susceptible to pollution caused by the activities of man, animals, birds, and other creatures.  This is so because said geologic formation normally has solution channels/cases where water flows as underground creeks/rivers, and therefore it has no considerable filtration and/or purifying properties.

Difficult areas have varying slopes, elevations and water depths.  The water supply sources are mainly replenished by way of sheared rocks, through fissures, cracks and crevices.  They generally consist of several formations of geologic ages.

Springs are generally found in these difficult areas.  Although the yield from springs may be minimal, they may well be the only viable source in the area.

CLIMATE

Type

The Hernandez type of classification under PAGASA defines the climate of the province as generally arid (Type E) wherein there are more dry than wet months.    At most, there are only 4 ½ wet months in a year with Q ranges from 1.00 to 1.67, where Q is the ratio of the number of dry months to the number of wet months.

However, the southernmost portion of the province (part of the municipality of Cervantes) was observed to be humid (Type B), where rain is evenly distributed throughout the year with at most 3 dry months and the eastern part of Sugpon is dry (type D) with rain not sufficiently distributed with at most 6 dry months.

Rainfall

The total annual rainfall in Ilocos Sur is 2,552 millimeters, with a monthly average of 213 millimeters.  Maximum rainfall is observed during the month of August with a mean rainfall of 710 millimeters, with rainfall means of 3 and 6 millimeters, respectively.

Temperature 

The mean annual air temperature in the province is 27°C.  April and May were observed to be the hottest months with temperatures of 28.3°C and 28.7°C, respectively, while the coldest month is January with a temperature of 25.4°C.

Physical Constraint

The province is located within a region of the country where either typhoon or prolonged drought prevails yearly.  The low portions of the province are flooded during the rainy season, however, damage is not great.  The rivers and the sea sometimes pose a threat of flooding and erosion in some coastal municipalities of the province more particularly Caoayan, Santa, Narvacan, Sta. Maria and Sta. Cruz.

Geophysical

Landslides and/or erosions are to be considered only in mountain areas or southern part of the province, and occur only along the roads, rivers and/or creeks.

LAND USE

Land use is the dominant activity or function on a specific area or location.  The province has 6 different land uses namely:  the settlement areas, croplands, grasslands, shrublands, woodlands and miscellaneous land uses.

Of all these land uses, the grasslands have the largest area with 738.10 square kilometers covering about 28.6 percent of the total provincial area, while settlement areas comprised only of 77.11 square kilometers, the least among the different land areas.