Groundwater recharge in a sandstone aquifer near Los Angeles was studied using a detailed computer model. The researchers aimed to understand how water from rainfall moves through the ground. They found that recharge varied greatly across the area, from almost no water entering the aquifer to over 1000 mm per year in some spots. This recharge mainly happened at the end of the rainy season, with water moving down through the soil layers. The study used water isotopes to confirm these findings, showing that the timing of recharge events matched the model's predictions. This research provides valuable insights into how water moves through semiarid regions and can help in managing groundwater resources effectively.