Atmospheric Streams Subsidize Valley Forests

I invented a new term to describe small-scale flows of water in the atmosphere.  Just as atmospheric rivers are large flows that transport tropical moisture thousands of miles to the mid-Latitudes, atmospheric streams share the moisture of the mountains with the valleys.

Example of an atmospheric river: Hurricane Priscilla projected track from October 7, 2025.  The remains of this storm brought copious moisture to the desert Southwest.

I first starting thinking about this when I noticed that the new weather station in the Watson Woods Riparian Preserve was often colder in the mornings than weather stations on the surrounding hills.  

Note the 40 degree temperature swing from cool (30's!) temperatures at night, to warm (80's) temperature during the day.

This is caused by katabatic winds from the mountains:

"On clear nights with calm winds, the ground cools rapidly. Air in contact with the colder ground cools by conducting heat to the ground. When this cooling process occurs along mountain slopes, the cooling air becomes colder and denser than the air away from the slopes, which causes the cold air to sink downslope. The dense cold air flows downslope in streams (called katabatic winds) following the steepest slopes. When the cold air flows into a relatively flat area (a mountain or river valley, for example), the streams of cold air slow down. This causes the valley to fill with cold air, much like streams filling a lake. "(MountWashington.org)

Hubbard Brook Experimental Forest, a good example of cold air drainage.

Atmospheric streams are distinct from the riparian drainages they follow, because air flows differently than water:

"Air flows in much larger volumes relative to the topographic surface. Water, even in hillside gullies, flows in volumes that are small relative to the scale of the landscape, and hence topography is the major control on the flow. Air masses are generally much larger relative to the landscape. This can lead to rather different effects. When a shallow cold air flow is moving slowly or is strongly stratified, it can become trapped by topographic barriers that would not trap water. Conversely, when the cold air flow is rapid or has lower stratification, it can flow over barriers, rather than go around them and so minimize friction.” (Research Meteorology)

Cold air flows are an important part of riparian ecology.  A study at the Coweeta Long Term Ecological Research (LTER) site found that cold air drainage subsidizes valley ecosystem productivity.  The study observed lower temperature air from the mountains cooling riparian forests, which lowered their carbon loss due to plant respiration.  The cool air must be a welcome respite for plants during the heat of summer.

Image from Coweeta LTER site in the South Carolina Appalachian mountains.

Cool mountain air can also be moister than valley air, especially in arid regions like Arizona.  Riparian streams carry water from mountains to valleys, while invisible atmospheric streams carry water in the form of humidity.  The extra boost in humidity only becomes visible (as fog) when the temperature drops below the dew point. The studies I looked at did not measure humidity, but it makes sense that higher elevation forests would have moister air than the hotter valleys.  When they share their air, they share their water.

Atmospheric streams are an important, but often overlooked, part of the global water cycle that carries moisture from the land to the ocean.  The recycling and transport of water from one part of the land to another part is sometimes called the "small water cycle".  We still have much to learn about the way our planet works!

El autobus magico: viaja por el agua

More Forest Thinning Science

1. You need Water for Ecohydrology

I previously wrote about the effects of thinning Southwestern ponderosa pine forests on forest hydrology.  AE Brown et al (Journal of Hydrology, 2005) summarized the last 50 years of hydrology research around the world on exactly this question.  Their conclusion was that yes, thinning increases water availability by decreasing evapotranspiration (ET).  However, at drier sites there is less of a difference.

In the figure above, the difference between the grass and forest curves represents the change in mean annual water yield for 100% conversion of one vegetation type to the other.  Partial conversion (i.e. thinning) was shown to have a proportional partial response.  The lack of difference between grass and forest in drier climates (below 500mm or 20 inches precipitation/year) indicates that most ET is actually just evaporation in these areas.  Therefore, because transpiration does not play a large role, reducing transpiration via thinning would not be expected to generate a large increase in water availability.

2. Don't Miss the Forest for the Trees

This classic forestry study found that thinning ponderosa forests increased growth of the remaining trees, but decreased total wood production.  In other words, the increase in vigor didn't compensate for the decrease in trees.  This even includes the decrease in disease (bark beetles) in thinned forests. So the question becomes, do you want a healthier forest or more wood?  

Data is from the The Level-of-Growing-Stock (LOGS) study on thinning ponderosa pine forests in the US West: A long-term collaborative experiment in density management.  A 2020 follow up provides a summary review of this study that started in 1962.  The The AZ portion of the study was conducted at Fort Valley experimental Forest just north of Flagstaff.   PDF with much more info is available from https://www.fs.usda.gov/rm/pubs/rmrs_p055.pdf.

Cutting Trees for Water: Are Thinned Forests Wetter or Drier?

Forest thinning can be controversial.  Concerned citizens want to know when logging counts as restoration;  can thinning a forest have beneficial ecological effects beyond reducing the risk of stand-replacing wildfires?  Will cutting trees increase soil moisture because there are less "straws sucking up water", or does it decrease soil moisture due to increased windspeed and more sunlight drying out the forest understory?

April 2017 - views of Rogers Lake, AZ overlooking untreated (left) and treated (right) areas.  Photos by Conor Flynn.  Click this link to play with the slider.  

Whether thinned forests are drier or wetter is complicated.  The excellent paper "Adapting western North American forests to climate change and wildfires: 10 common questions" by Prichard et al provides a good introduction to this question:

"Decreasing canopy bulk density can change site climatic conditions (Agee and Skinner 2005). Wildfire ignition potential is largely driven by fuel moisture, which can decrease on drier sites when canopy bulk density is reduced through commercial thinning (e.g., Reinhardt et al. 2006). Reduced canopy bulk density can lead to increased surface wind speed and fuel heating, which allows for increased rates of fire spread in thinned forests (Pimont et al. 2009, Parsons et al. 2018). Other studies show no effect of thinning on surface fuel moisture (Bigelow and North 2012, Estes et al. 2012), suggesting that thinning effects on surface winds and fuel moisture are complex, site specific, and likely vary across ecoregions and seasons."

Anecdotally, some people have noticed springs beginning to flow again after thinning and prescribed fire in AZ.  My research in NM pinyon noted increased soil moisture at thinned sites (unpublished data), however this could be due to the specifics of how thinning was accomplished at those sites.  Thinned slash was chipped and the chips were left on-site without follow-up prescribed fire.

In addition to water quantity, water quality should also be considered.  Prichard et al point out that "Treatments in watersheds that are distant from the WUI and protect municipal and agricultural water supplies are critical to minimizing high-severity fire impacts that can jeopardize clean water delivery (Bladon 2018, Hallema et al. 2018). For example, post-fire erosion and debris flows may cause more detrimental and longer-term impacts to watersheds than the wildfires themselves (Jones et al. 2018, Kolden and Henson 2019)."  However, even carefully managed thinning and prescribed fire can generate excess erosion from new roads, decreased large woody debris, and increased mobility of light charred wood.  Charcoal washing into local lakes can cause fish kills, even when not generated by catastrophic wildfire.  Creating erosion-control structures as part of forest thinning work could help to mitigate these risks.  

Further research is needed to ensure that large thinning projects adequately account for water cycle restoration in addition to natural stand density and fire interval restoration.  

Riparian Restoration in a Time of Changing Hydrographs: La Jencia 2008-2014

A talk presented at the 2015 Tamarisk Coalition Conference in Albuquerque, New Mexico.
 

Ecohydrology of Wet Semi-Arid Climates versus Dry Semi-Arid Climates

Semi-arid climates are exquisitely balanced on a range of ecosystem properties. Most famously, these savanna ecosystems, which border the truly arid deserts, are susceptible to desertification. Ecologists say that they have a bimodal stability: they can exist in two very different states, either grassland or shrubland, and small changes in land management and climate can 'flip' them from one to the other. In Huxman, T.E. et al. Ecohydrological Implications of Woody Plant Encroachment. Ecology, 308-319 (2005) ecohydrology researchers hypothesize that the response of semi-arid ecosystems differs between wet semi-arid sites and dry semi-arid sites, with subtle, yet important differences between them if they flip from grassland to shrubland.:



"The relative contribution of T to ET will increase for systems dominated by woody plants as compared to those dominated by nonwoody plants in more mesic climate zones, with the opposite effect in semi arid systems. Four zones can be delimitated where changes in the T/E T are likely to occur. In Area 1, there is little change in T/ET because leaf area remains about the same and ET is dominated by E from large expanses of bare ground. In Area 2, E increases substantially in woody plant systems as a result of the loss of herbaceous cover in intercanopy spaces. Area 3 is a transition zone (no further changes in T are being caused b woody plants, and herbaceous vegetation still dominates intercanopy spaces). In Area 4, differences in T/ET are due to increased T by shrubs (shrubs are using ‘‘extra’’ water that, in a grassland system, would become groundwater recharge)."