MOUNTAIN HYDROLOGIC SYSTEMS

For many arid or semi-arid regions, mountain ranges are the primary sources of annually renewed water supplies. They give rise to rivers which may be the only significant, sustainable source of water in the regions through which they flow. However, the nature of the hydrologic regime and the ways in which it differs from that of lowland regimes has received relatively little attention. A simple analysis of readily available data from the mid-latitude mountain ranges of the western United States of America is presented. The water balances appear to be determined by the altitude above sea level occupied by the base-ridge crest elevational interval for a given air mass. As the mean altitude of this interval increases, each mountain range will occupy a different vertical position with respect to air mass gradients of precipitable water vapour, temperature, air mass optical density, and wind speed. The resulting gradients of water exchange with altitude at the surface in a mountain range will be inversely proportional to the mean altitude of the interval for any given air mass. Above timberline wind redistribution of falling and fallen precipitation and direct shortwave solar radiation as an energy source will increase the importance of terrain aspect over elevation in determining the spatial patterns of the mountain water balance. The rapidly expanding use of both highland and lowland watersheds requires a much better understanding than now exists of the hydrologic interactions which link the two. The development of conceptual models, like that presented here, facilitating a systems approach should be a primary objective of future research on the hydrometeorology of large mountain ranges.