Most significant ground water recharge in the southwestern USA takes place in mountain blocks such as the San Gabriel Mountains that provide water supplies for Los Angeles and Antelope Valley. However, the quantification of recharge in semi-arid mountain blocks remains a great challenge for hydrologists worldwide. A case study is presented on the quantification of ground water recharge in the San Gabriel Mountains using a combination of the Distributed Parameter Watershed Model (DPWM) with the remote sensing algorithm Mapping EvapoTranspiration at high Resolution using Internal Calibration (METRIC) that used Landsat imagery as its main input. The average annual ground water recharge rate is quantified on the basis of DPWM simulations spanning a 30-year period using representative Landsat images from the years 2003, 2005, and 2007.