The South Central Texas Regional Water Planning Area, also known as "Region L", reaches from the Gulf Coast, to the Hill Country, and encompasses portions of nine river and coastal basins, the Guadalupe Estuary, the San Antonio Bay, and includes all or parts of Atascosa, Bexar, Caldwell, Calhoun, Comal, DeWitt, Dimmit, Frio, Goliad, Gonzales, Guadalupe, Karnes, Kendall, La Salle, Medina, Refugio, Uvalde, Victoria, Wilson, Zavala, and part of Hays Counties.

The largest cities in the region are San Antonio, Victoria, San Marcos, and New Braunfels. The region's largest economic sectors are tourism, military, medical, service, manufacturing, and retail trade. 

The region contains the two largest springs in Texas: Comal and San Marcos. Water planning in the region is particularly complex because of the intricate relationships between the region's surface and groundwater resources.