Sierra BajaSierra Baja is a rural barrio in the municipality of Guayanilla, Puerto Rico. Its population in 2010 was 602.Features and demographicsSierra Baja has of land area and no water area. In 2010, its population was 602 with a population density of.HistorySierra Baja was in Spain's gazetteers until Puerto Rico was ceded by Spain in the aftermath of the Spanish–American War under the terms of the Treaty of [Paris of 1898] and became an unincorporated territory of the United States. In 1899, the United [States Department of War] conducted a census of Puerto Rico finding that the population of Sierra Baja barrio was 787.