5.04 - Orographic Precipitation, Freshwater Resources, and Climate Vulnerabilities in Mountainous Regions

Abstract

About half the world’s freshwater resources come from precipitation (rain and snow) in mountainous regions. Understanding how climate and environmental change can impact monsoon rainfall, which is the dominant freshwater source on mountain slopes and adjacent lowlands in the tropics and subtropics, is therefore an important question of global implications. The diurnal cycle of orographic rainfall (e.g., where it rains, how much it rains, how fast it rains, how long it rains, and at what time of day it rains) varies from one mountain region to another, and in the same region it can vary greatly with elevation and landform. Heavy rainfall is associated with landslides and flash flooding that cause short-term landform and vegetation disturbances and often loss of life and property. By contrast, because of its ubiquity and space–time persistence independently of the variability in large-scale forcing, light rainfall (<2–5 mm h−1) is de facto water lifeline of mountain environments. The vulnerability of headwater catchments and cloud forests to aridification presents a key challenge to the sustainability of mountain environments and freshwater resources. Changes in fog and rainfall intensities affect canopy harvesting and interception of precipitation, infiltration, and runoff processes, and consequently evapotranspiration, surface energy balance, and boundary layer conditions, which ultimately feedback into cloud formation and rainfall processes.