Mountains, Freshwater Resources and Climate Change
About half of the world’s freshwater resources come from precipitation (rain and snow) in mountainous regions. Whereas the impacts of climate change on the stability of mountain glaciers and year-to-year reliability of snowpacks have received much attention, understanding how climate change will impact monsoon rainfall that is actually the dominant freshwater source on mountain slopes and adjacent lowlands in the tropics and subtropics remains a challenge.
With worldwide distribution and strong environmental and climatic gradients, mountains are elevated observatories where the impacts of climate variability and change can be first detected. Yet, mountainous regions remain among the least observed regions in the planet.
In the Fall of 2011, a science-grade network of 10m towers was placed at high elevations (1,400 to 4,000 m) to measure above-canopy precipitation on the envelope orography of the Central Andes, more precisely in the Kospiñata river valley in the vicinity of Parque Nacional del Manu a collaborative project funded by the Nacional Science Foundation including Duke University, Wake Forest University, and the University of Cusco in Peru. These stations are to be the core of an observing system to understand cloud forest and wet puna hydrometeorology, and in particular climate controls of fog-cloud-rainfall interactions. It is expected that such observations will also provide valuable ground validation data to improve the performance of satellite-based rainfall estimation algorithms (e.g. NASA’s upcoming Global Precipitation Measurement mission) in the region and elsewhere. Ultimately, satellite-based observing systems are the realistic path toward achieving high density continuous observations of mountain precipitation at global and regional scales.
The diurnal cycle of rainfall (e.g. where it rains, how much it rains, how fast it rains, how long it rains, and at what time of day) varies greatly from one mountain region to another, and in the same region it can vary greatly with elevation and landform. For example, a comparison among observations on the envelope orography of the central Andes and the central Himalayas indicate that whereas the wet season (monsoon) rainfall totals are about the same, rainfall amounts peak in the early afternoon whereas rainfall intensity peaks in the evening at low and high elevations in the Central Andes, but rainfall amounts and intensity peak during in the evening and very early morning in the southern facing slopes of the central Himalayas at all elevations below the treeline. Interestingly, despite their location in the cloud forest, recent observations from the Peru show that rainfall intensities exceeding 100 mm/hr are not uncommon at roughly 2,700 m elevations, where rates as high as 200 mm/hr were measured a high elevation valley location. These high rainfall rates are associated with landslide activity and debris flows that can cause much landscape damage and loss of life.
Rainfall intensity as a discriminant of rainfall regime is critical for assessing the sustainability of freshwater resources in mountainous regions. Light rainfall is dominant at high elevations in cloud forests and wet grasslands in the Andes. Although light rainfall is expected to decrease due to changes in fog and low level cloud regimes, lower relative humidity and higher low level temperatures in the troposphere, morning light rainfall is likely to be more sustainable than afternoon light rainfall depending on how changes in temperature are reflected in the diurnal cycles of temperature in a warmer climate. Changes in fog and rainfall intensity affect canopy harvesting of fog and interception of rainfall on the one hand, and infiltration and soil moisture. Lower surface soil moisture leads to increased soil and boundary layer temperatures, in a positive feedback loop, that would tend to further increase surface layer temperatures and the dryness of upper soil layers, thus affecting understory and forest floor vegetation at first, and consequently albedo and surface temperature, depleting soil moisture from the root zones, ultimately altering runoff production mechanisms and groundwater recharge, and streamflow regimes. On the other hand, if heavy rainfalls were to increase in frequency or in intensity with climate change, this would have major implications for hillslope stability and overall ecosystem resilience besides the human social and economic toll on mountain populations. Vegetation disturbances caused by landslides for example can facilitate the progression of invasive species, and threaten the biodiversity of mountain ecosystems.
This blog post was written by Ana P. Barros, Senior ECPA Fellow, of Duke University and the Energy and Climate Partnership of the Americas for Climate Conversations.
Image: Alpamayo, a peak in the Cordillera Blanca mountain range of the Peruvian Andes. (Wikimedia/RedWolf)