MODELING INVESTIGATION OF MOUNTAIN FRONT RECHARGE FROM TYPICAL MOUNTAIN BLOCKS

Mountain front recharge (MFR) is a significant component of recharge to adjacent basin aquifers in arid and semi-arid climates. Traditionally, MFR considers only focused stream channel recharge at the mountain front, and assumes that the bedrock underlying the mountain block is essentially impermeable, suggesting negligible groundwater movement through the mountain block. However, for some blocks there is evidence for diffuse, as well as focused, recharge to mountain blocks, groundwater movement within blocks, and discharge from the blocks. Bedrock permeability is one major controlling factor determining whether or not the diffuse processes are significant. Understanding the role of mountain structure and bedrock geology on groundwater flow through mountain blocks is a key to understanding the characteristics of MFR. Based on typical basin-and-range geology and saturated-unsaturated flow equations, archetypes of mountain blocks are numerically simulated considering bedrock matrix and fractures, and the role of faults. Different MFR patterns are observed for different mountain block archetypes. The results indicate a threshold bulk mountain block permeability needed for diffuse processes to become important.