The Carlsbad Hydrologic Unit (HU) is approximately 210 square miles in area extending from the headwaters above Lake Wolhford in the east to the Pacific Ocean in the west, and from Vista and Oceanside in the north to Solana Beach, Escondido, and the community of Rancho Santa Fe to the south.  The cities of Carlsbad, San Marcos, and Encinitas are entirely within this HU.  There are numerous important surface hydrologic features within the Carlsbad HU including four unique coastal lagoons, three major creeks, and two large water storage reservoirs.  The HU contains four major, roughly parallel hydrologic areas (HAs).  From north to south they are the Buena Vista (901.2), the Agua Hedionda (904.3), the Batiquitos (904.5), and the San Elijo (904.6) HAs.   Two smaller HAs, the Loma Alta (904.1) and the Canyon de las Encinas (904.4) are also within the Carlsbad HU.