Mountain-Block Hydrology and Mountain-Front Recharge

In semiarid climates, a significant component of recharge to basin aquifers occurs along the mountain front. Traditionally called “mountain-front recharge” (MFR), this process has been treated by modelers of basins as a boundary condition. In general, mountain-front recharge estimates are based on the general precipitation characteristics of the mountain (as estimated, e.g., by the chloride mass balance and water balance methods), or by calibration of a basin groundwater model. These methods avoid altogether the complexities of the hydrologic system above the mountain front, or at best consider only traditional runoff process. Consequently hydrology above the mountain front is an area ripe for significant scientific advancement. A complete view would consider the entire mountain block system and examine hydrologic processes from the slope of the highest peak to the depth of the deepest circulating groundwater. Important aspects above the mountain front include the partitioning of rainfall and snowmelt into vegetation-controlled evapotranspiration, surface runoff, and deep infiltration through bedrock, especially its fractures and faults. Focused flow along mountain stream channels and the diffuse movement of groundwater through the underlying mountain block would both be considered. This paper first defines some key terms, then reviews methods of studying MFR in arid and semiarid regions, discusses hydrological processes in the mountain block, and finally addresses some of the basic questions raised by the new mountain-block hydrology approach, as well as future directions for mountain-block hydrology research.