Physical Profile
Location and Land Area
The Municipality of Irosin lies at the foot of Bulusan Volcano at the southern tip of Luzon Island. It is nestled in a caldera with a valley floor that is surrounded by mountains and peaks making the town the only landlocked inland municipality of the province. Irosin has a gross land area of 15,110.6725 hectares or 151.106725 square kilometers. Irosin is comprised of 28 barangays and five are classified as urban barangays occupying 2.11% of the total land area while 23 are rural mostly farming barangays encompassing 97.89% of the total land area.
Topography
Irosin has a very irregular terrain characterized by the plain on the valley floor, secluded plains and valleys on the mountaintops and undulating hills and mountain peaks. The municipality is in the lowlands with 79% of the total land area located in elevation range of 0-300 meters, 13% in 300-500 meters and 6% hectares in 500-800 meters elevation range. About 17% of the total land area has a slope range of 0 to 3% or level to moderately level, 13% of land area is gently sloping to modulating (slope range 3 to 8%), 17% of land area is moderately sloping to rolling (slope range of 8 to 18%), 29% of land area is strongly sloping to moderately steep (slope range of 18 to 30%), steep hills and mountains (slope range of 30 to 50%) occupies 17% of the land area and very steep hills and mountains is about 7% of the total land area.
Table 1. Total Population, Number of Households, Average Household Size, Gross Area and Gross Density by Barangay as of May 1, 2020
Vegetation Cover
Coconut, woodland and rice are the prominent vegetation cover occupying 62%, 15% and 14% of the gross land area of the municipality respectively. Other agricultural crop is planted in 1% while grass/shrubs cover 2% of the total land area. A total of 94.47% of the municipality’s land area has vegetation cover. Rice occupies the broad alluvial plains with an area of 2,192.14 hectares and is planted in 18 barangays. Coconuts cover 9,431.83 hectares and can be found over the four points of the compass in 23 barangays. Coconut is interspersed with other crops such as abaca, citrus, fruit trees, pili and other crops. Woodlands cover 2,082.52 hectares and can be found in elevations from 90 to 1,060 meters.
Hydrogeology
Water resources are abundant in Irosin. The sources of the creeks and streams, which are the main feeders of the river network, are springs located in upland and hilly landscapes of volcanic hill landforms. The Irosin riparian system consisting of creeks and rivers has an approximate total length of 127.66 kilometers. Cadac-an River is the biggest and longest river and has 4 main river tributaries and 8 collateral creek feeders with a length of 16.59 km in the territory of Irosin. This river originates from the slope of Bulusan Volcano and the Mount Mara-ot Banwa ranges and empties in Juban before it flows to Sorsogon Bay. There is one lake in Irosin, the Danao Lake located in barangay Patag with an approximate area of 5.05 hectares.
Cadacan Watershed is the largest watershed in the province of Sorsogon. Headwaters of this watershed is the Bulusan Volcano and the hills and mountains of an ancient crater surrounding the Irosin caldera. Numerous tributaries in Irosin caldera converges into the main Cadacan River which then drains north into Juban and to Sorsogon Bay. The Cadacan Watershed covers 10,504 hectares of Irosin or 70% of the total land area and all barangays except Liang and Casini are inside this watershed.
Figure 2. Watershed and Water Bodies Map
Existing Land Use
Agricultural lands dominate the landscape of the municipality with a total area of 12,025.38 hectares or 79.58% of the total land area and is found in all barangays except in the urban barangays of San Julian, San Juan and San Agustin. The second dominant land use is forest with an approximate area of 2,336.71 hectares or 15.46% of the total land area. Lands with forest cover include NIPAS/E-NIPAS areas with 1,132.9 hectares located in Cogon and non-NIPAS forest have an estimated area of 1,291 hectares. Urban use areas comprise only 474 hectares or 3.14% of the total land. Residential areas cover 321.7 hectares, followed by roads and infrastructures with 90.9 hectares, institutional areas with 47.9 hectares, commercial areas with 10.5 hectares, parks and playgrounds with 3.1 hectares and industrial sites with 0.06 hectares. The banks or easement of rivers and creeks are categorized under water uses and covers 257.7 hectares or 1.71% of the total land area. There are only 5 hectares each of land used for agro-industrial and tourism purposes. Categorized under other uses are cemeteries with 2.7 hectares, dumpsite/sanitary landfills with 1.8 hectares and vacant lots with 1.7 hectares.
Climate
The climate of Irosin belongs to the Type II in the classification of Philippine Climate and is characterized of having no dry season or not a single dry month, a very pronounced maximum rain period from December to February and minimum monthly rainfall occurring during the period from March to May. Northeast Monsoon or “amihan” and Southwest Monsoon or “habagat” are the two main air streams which influences the climate pattern of the municipality. Irosin experiences rain almost all year round. Rainfall is observed at an annual average of 234 days a year. Maximum rainfall comes in November to January caused by the intensification of the Northeast Monsoon.
