Conifers capture the snow, but do they intercept it?

split figure with snow covered conifer on left with bare ground underneath. On right, snow covered ground with snowy deciduous forest in background.

Conifers (left) capture much more snow than grass (right foreground) or deciduous forest (right background). But will they keep the ground dry all winter? (Photo by A. Jefferson, 2017)

If you’ve walked through the forest on a rainy day and noticed that it’s drier under the trees, you’ve experienced interception.

In hydrology, interception is when water gets hung up on vegetative leaves, needles, or branches and never makes it to the ground. The precipitation gets evaporated (if liquid) or sublimated (if solid) back into water vapor directly from the vegetative surface before it gets a chance to hit the ground and infiltrate or run off. (If the water hangs out in the vegetation for a while but eventually makes it to the ground, we call it stemflow or throughfall depending on whether it ran down the tree trunk or not.)

Interception can be a pretty significant component of the water budget. In forests, the vegetation can intercept 20-40+% of precipitation. In grasslands, the numbers are in the 10-20% range. Even litter, the dead plant material covering the soil, can cause interception. Interception rates depend on plant type and density, but also how much rain you get, how fast it falls, and how much evaporation can occur during and between storms.

In the winter, interception still happens during snowfall, but now vegetation type really matters. Since deciduous trees shed their leaves in the winter, they become pretty useless for interception. In the picture above, you can’t really see the difference between the deciduous forest and the lawn — they are both fully snow-covered. On the other hand, since conifers retain their needles, they can capture a lot of snow — and you can see that in the bare ground under the trees at left.

Whether the conifers truly intercept all that snow is more complicated. Conifers can initially hold large snow loads, but wind can blow that snow onto the ground, it can be dumped off in large clumps, and melting within the snowpack on the branches can allow the water to drip to the ground. In order to effectively intercept the water and return it to the atmosphere, we’d need sublimation to happen faster than those other processes. But does that happen?

In a study in Oregon’s Umpqua National Forest (Storck et al., 2002), mature conifers initially captured up to 60% of the snowfall (up to at least 40 mm). When conditions were warm and conducive to snowmelt after the snowstorm, 70% of the water left the canopy as meltwater drip and 30% left as masses of snow falling to the ground. Only if the weather remained below freezing after snowfall, could sublimation work to reduce the snow storage in the trees. But that goes slowly, at an average rate of ~1 mm/day. If the weather got above freezing, then melting and dumping took over. Overall, the study site got about 2000 mm of precipitation in the winter and the ground in the forested areas experienced about 100 mm less than the ground in the open areas, giving a winter interception rate of about 5%.

Of course, that’s only one study and other modeling and experimental work adds more nuance and complication. Climate and solar radiation affect sublimation rates. Canopy density affects sheltering by wind and interception. And more. High spatial resolution modeling of two sites in Colorado and New Mexico gives interception values of 19% and 25%, respectively (Broxton et al., 2015). When they consider all of the processes happening to redistribute snow around a patchy forest, they conclude that the driest areas are under tree canopies and the wettest areas are <15 m from the edge of the canopy. If you get farther out into an open area, it gets drier again, though not as dry as under the forest cover. And the differences are not small, snow water input can be 30-40% higher near the edge of the canopy than underneath it. So next time you walk through a forest in the rain or snow, be impressed by the hydrologic work the trees are doing to keep you dry, and know that interception adds up to a significant amount of water. But if it's a warm winter day, don't be surprised to feel a cold meltwater drip from the pine tree above you -- or get a load of snow dumped on your head -- because even conifers can't hang onto the snow long enough to keep the ground dry forever.
Read more:
Broxton, P. D., Harpold, A. A., Biederman, J. A., Troch, P. A., Molotch, N. P., and Brooks, P. D. (2015) Quantifying the effects of vegetation structure on snow accumulation and ablation in mixed-conifer forests. Ecohydrol., 8: 1073–1094. doi: 10.1002/eco.1565. (pdf available via ResearchGate)

Storck, P., D. P. Lettenmaier, and S. M. Bolton, Measurement of snow interception and canopy effects on snow accumulation and melt in a mountainous maritime climate, Oregon, United States, Water Resour. Res., 38(11), 1223, doi:10.1029/2002WR001281. (open access)

Categories: by Anne, hydrology, ice and glaciers, photos

Speak up for NASA’s Earth Science funding

A post by Anne JeffersonIt’s Earth Science Week and Congress is still debating the budget for this fiscal year. That means that science funding is still on the line. The American Geophysical Union is running a campaign encouraging members to speak up for NASA’s Earth Science division which faced steep cuts in the White House budget proposal. Follow the link here to send a letter to your senators and representative. (You don’t have to be an AGU member to take part.) If you need some inspiration, here’s the letter I wrote to my representatives.

Dear Senators Portman and Brown and Representative Ryan,

In honor of Earth Science Week (8-14 October 2017), I’m writing to urge you to provide robust support for NASA’s Earth Science Division, which is invaluable to our nation and our local community.

As recent hurricanes and wildfires have vividly demonstrated, millions of Americans depend on NASA’s eyes in the sky to keep them informed and safe. Our economy also benefits massively from being able to prepare in advance for weather disruptions.

Closer to home, NASA satellites are providing important data on the harmful algae blooms in Lake Erie and our inland lakes. These satellite images are giving us important clues as to how lake conditions, river contributions, and weather interact to produce these toxic blooms.

As a hydrology professor at Kent State University, I use NASA products in my research and teaching every week. The new soil moisture active passive (SMAP) mission will be hugely valuable for understanding how changes in land use and climate conditions influence flooding and drought. The GRACE mission, which recently came to an end, was an important tool for understanding large scale groundwater declines and ice sheet changes. As there is now a gap until a replacement satellite can be launched, we are losing critical data on our changing planet.

It’s important to keep continuous and robust funding for NASA’s Earth Science division in order to keep NASA missions on track, so that we don’t fly blind in the face of severe weather, algae blooms, droughts, and all of dynamics of our home planet.

Thank you for your leadership and continued support of science.

Sincerely,

Anne Jefferson

Categories: by Anne, hydrology, public science, society

Hurricane Harvey and the Houston Flood: Did Humans Make it Worse? (Part 2: Urbanization)

A post by Anne JeffersonThere’s been a lot of speculation and discussion about the role of urbanization in contributing to the flooding from Hurricane Harvey in Houston. Fortunately, urban hydrology is my specialty, so even though I’ve never been to Houston, I feel like I can offer some insights. (Also, read part 1 on the climate change aspects of this disaster.)

Many of the damaged buildings were outside designated “100-year” floodplains. Why is that?
First, given that Harvey produced more rainfall than the contiguous US had ever before recorded from a single storm (51.88″ in one location), I think it’s safe to say that this storm has historically had a <<1% chance of occurring. In other words, based on historical data, we wouldn’t expect this storm to occur 1 time in 100 years in Houston, because nothing anywhere near has been has ever occurred before. But, history of course, is not a very good guide to a climate-changed present and future (see below).

Second, the FEMA maps are out of date in many areas. Not only do they not consider climate change, but they only account for the level of urbanization that has occurred at the time the map is made. They don’t take into account planned or projected future urbanization, levee building, etc.*

Yet, these maps aren’t updated very regularly and can be a decade or more out of date in some areas (mostly rural). Plus, local politicians don’t want to show that a large part of their community is in a floodplain (and require more expensive buildings and flood insurance), so there’s local pressure to keep delineated floodplains as small as possible. Finally, it doesn’t help, that the flood mapping program has always been under-funded and is a popular target for funding cuts.

We need to wrap our head around the scale of the damage. There are at least 90,000 residential structures damaged in 3 Texas counties, based on an early FEMA estimate. Harris County (where Houston is) officials are estimating up to 136,000 homes destroyed. I’ve seen one estimate that 40-50% of the damaged homes are located outside the mapped floodplain.

Did urbanization and impervious surface make flooding worse?

No. Impervious surface (pavement and rooftops) prevents infiltration. But there isn’t a landscape in the world that can take 50” of rain and infiltrate it without flooding (or landslides). Even if your landscape can infiltrate whatever rainfall is landing on the surface, the soil will wet up from the bottom as water tables rise toward the surface. And when the water table is at the surface, it doesn’t matter what your infiltration capacity is, water can’t move down.

Furthermore, Houston is in the Coastal Plain, which is made up of lots of fine-grained sediments, notoriously poor for infiltration. Southern Harris County’s soils are classified as group D, meaning that they are the worse of four classes of soil for infiltrating water.

Yes. Urban development that encroaches on natural water storage areas – floodplains and wetlands – makes flooding worse elsewhere. (And more to the point, it puts people and property in harm’s way.) If you fill in the low areas, install drainage pipes to shunt water elsewhere, or build levees to keep water off the floodplain, then that water has to go somewhere, and in fairly flat areas like Houston, that somewhere will become everywhere. Houston and its suburbs have faced criticism for rampant wetland destruction and for building whole neighborhoods in designated floodplains. The New York Times has a fantastic set of animated graphics that show how development overlays with floodplains in the Houston area (and how flood damages from Harvey aren’t restricted to mapped floodplains. I’m taking the risk of reproducing two screenshots below that I think are particularly good example of this phenomenon.

Development on a Houston area floodplain. via New York Times (click the link above and read their article too)

Won’t insurance pay for the damage to people’s houses?
Only if you have flood insurance, because standard homeowners and renters policies don’t cover flood damage. If you live in the 100-year floodplain and have a mortgage on your house, you are required to buy flood insurance, which can run $1000s per year. If you don’t live in a designated 100-year floodplain, flood insurance is totally optional. It’s also less expensive, since your risk of flooding is lower, but who among us says “Sure, I’ll buy totally optional insurance at a cost of several hundred dollars per year for something the government says isn’t going to happen at my house?” In all likelihood, the majority of Houston-area flood victims will not have any flood insurance to pay for their lost possessions and houses.

For those who do have flood insurance, the National Flood Insurance Program (NFIP) will help pay for them to repair and rebuild – in the same risky location. One percent of properties insured by the NFIP take up 25-30% of its payouts. The program loses money every year, and US taxpayers foot the bill.

Can we engineer our way out of disasters like this?
Stormwater management is not designed for extreme events. Most stormwater control measures are designed for the type of storm that happens a few times per year. In Harris County, stormwater controls have what’s called a “water quality volume” that is designed to treat 1.5″ of rain in 24 hours. Anything above that does not have to be detained or infiltrated by the stormwater control. Remember, the whole Houston metro area got >30″ of rain over 5 straight days and a new contiguous US record of 51.88″ was set in one location. Why aren’t stormwater controls designed for bigger storms? It would cost more and they would take up more valuable real estate. And most of the time, that extra storage wouldn’t be put to use. (Of course, it doesn’t help if a large number of your stormwater controls aren’t performing up to design standards.)

Flood control reservoirs work, to a point, but then they run out of storage volume and might even make things worse. Flood control reservoirs might be designed to reduce the effects of the 100-year flood, which has a 1% chance of occuring in any given year. For the Houston area, the 1% rainfall is 13” of rain in 24 hours. But that’s happened >8 times in last 27 years, so clearly the math is a bit off (likely out of date, thanks in part to climate change). Houston has two flood control reservoirs, Addicks and Barker, built in the 1930s, that sit dry until a flood comes along. Of their 10 highest levels, 9 have happened since 1990 and 6 have happened since 2000 (and that was before Harvey). When Harvey roared in, the reservoirs filled up completely and over topped for the first time in their history.

After flood control reservoirs are full, they can’t help store any additional rainfall and runoff. Plus, management operations will swiftly turn from storing the flood to protecting the structural integrity of the reservoirs. Meaning that the dam operators will start to release extra water downstream – even though that makes downstream flooding worse – because they don’t want the dam to fail. You don’t want a 100-foot high wall of water rushing towards downtown Houston either. The Houston reservoirs were forced to release water during the height of the flooding, making things worse in downstream neighborhoods. But dam failure is a risk that no engineer or hydrologist is willing to take.

What Barker Reservoir looks like empty. (US Army Corps of Engineers photo, public domain.)

Just like with stormwater controls, we don’t build flood control reservoirs for the most extreme events, because designing bigger capacities to accommodate a rare storm increases costs dramatically. This is particularly true in a flat area like Houston, where every additional foot of height on a dam means a much larger footprint for the reservoir, locking that area away from development.

Levees – walls built between the river and neighborhoods – might work locally, but they make flooding worse elsewhere. And like every other form of engineering, they can be under-designed for extreme events. And they can fail.

So what can be do to avoid a disaster like this in the future?

  • Stop filling wetlands. Support wetland restoration and mitigation efforts.
  • Don’t (re)build in floodplains. (Even beyond the official 100-year floodplain.)
  • Consider the likelihood of enhanced flooding from increased precipitation intensity and sea level rise when making rebuilding and future development decisions.
  • Develop smart, staged evacuation plans for the most flood-risky areas and for vulnerable populations.
  • Advocate for enhanced funding for FEMA’s flood mapping program.
  • Revamp the National Flood Insurance Program.
  • Take immediate action (individually and collectively) to reduce the greenhouse gas emissions that are causing climate change.

 


*A reader alerted me to this 2001 FEMA rule on “Future Conditions Hydrology” which says that at the request of a community, for information purposes only, the maps can reflect future conditions data, as generated by the community, in addition to the current 100-year (1%) “base flood elevation.” Areas outside the current 1% floodplain, but inside the future 1% floodplain are designated as area X. In those areas, homeowners are not required to buy flood insurance by federal rules, though a private lender could make that requirement. Similarly, under federal rules, structures would not have to be elevated, though local laws might require flood production in these areas. “Future conditions hydrology means the flood discharges associated with projected land-use conditions based on a community’s zoning maps and/or comprehensive land-use plans and without consideration of projected future construction of flood detention structures or projected future hydraulic modifications within a stream or other waterway, such as bridge and culvert construction, fill, and excavation.” Note that “public works in progress including channel modifications, hydraulic control structures, storm drainage systems and various other flood protection projects” had been considered in the FEMA maps since 1995. The TL;DR of all of this is that IF a community requests future conditions be displayed, and does the legwork of figuring out what those future conditions are, they do appear on maps generated since 2001, but they are not federally enforceable.

Categories: by Anne, geohazards, hydrology, society

Hurricane Harvey and the Houston flood: Did Humans Make it Worse? (Part 1: Climate Change)

A post by Anne JeffersonThis Friday at noon, the Kent State University Department of Geology is hosting a panel discussion on the human role in the catastrophic flooding experienced by Houston and surrounding communities in the wake of Hurricane Harvey. I will be one of five faculty participating in the discussion. Since I know that many of you aren’t local, I thought I’d summarize my talking points below.

What’s the connection between climate change, hurricanes, and rain?

Established Physics: Warmer sea surface temperatures lead to more intense hurricanes. And the Caribbean and Gulf of Mexico are really warm right now. That’s also one of the reasons that Irma has been able to maintain its Category Five status for such an unprecedented length of time. You can read more about how hot waters fuel hurricanes in this NASA Earth Observatory post on Irma.

NASA Earth Observatory image of Irma's path (as of September 6) and sea surface temperatures,

NASA Earth Observatory image of Irma’s path (as of September 6) and sea surface temperatures, September 3-5.

Simple Math: Sea level rise contributes to a higher storm surge. Storm surge wasn’t the problem in Houston, but it was a contributor the major damage that occurred in the coastal communities where Harvey made landfall and it’s a huge piece of the story in the devastating destruction that Irma is causing in the Caribbean right now. Of course, higher storm surges aren’t a problem if you have buildings higher relative to the sea level. But our buildings aren’t rising as fast as sea level, so higher sea levels and higher storm surges mean more storm surge damage.

Established Physics: Warmer atmospheres can hold more water, which leads to more big rains, which leads to more big floods. The physics of increasing saturation capacity with warmer air are taught to every meteorologist, via the Clasius-Claperyon equation. If you are not a meteorologist, Climate Central has a basic primer in the context of hurricanes. When that warm saturated air cools down, that’s when we get tremendous rains, because now the air isn’t warm enough to hang onto all that water vapor. In hurricanes, the cooling of the air (and rain generation) occurs simply because of warm air rising up into cooler parts of the atmosphere.

Climate Central’s graphic illustrating how warm air can hold more water and how that leads to more rain.

Possible: Climate change may lead to more rapid intensification of hurricanes. Harvey was one of the most rapidly intensifying hurricanes on record.

Possible: Climate change creates more ”stationary weather patterns” that hold weather in one place for days. The thing that really turned Harvey into an epic catastrophe for Houston was that it stayed in one place for days, rather than moving on to new places to drench. Michael Mann has a really nice explanation of how stationary weather patterns were in place during Harvey in this piece in the Guardian. Stationary weather patterns have also previously been blamed for terrible peat fires in Siberia and deadly heat waves in Europe, so they are not just a problem for hurricanes.

In my view, the science is incontrovertible, human-caused climate change contributed to the severity of Harvey’s impacts on Texas. I’m sure there are rapid attribution studies already kicking off that will be able to give us some sense of how much worse the wind and rain were as a result of our green house gas emissions.

Read on for Part 2: Did urbanization make the flooding in Houston worse?

Categories: by Anne, climate science, geohazards

August climate impacts stories: Hurricane Harvey, other climate change fueled-floods, and more

August 30th: Harvey reminds us that we should treat climate change as we treat other public health threats. That’s the argument in this New York Times op-ed: Harvey, the storm that humans helped cause.

Holthaus-harveyAugust 29th: The most sobering hot-take on Harvey is by Eric Holthaus: Harvey is what climate change looks like.

August 27: Michael Mann offered a clear explanation of the climate change-Hurricane Harvey connection.

Climate change boosts hurricanes through higher and hotter seas say @KHayhoe @AndrewDessler: The relationship between hurricanes and climate change.

August 26: Rapid hurricane intensification, like we’ve seen with Harvey, is consistent with climate change, writes Chris Mooney.

August 25: First tanker crosses Arctic from Europe to Asia-without icebreaker help. Fleet of 15 coming soon.

August 24:Alaska’s permafrost is thawing, and that has huge implications, locally, regionally, and globally. (My colleagues Beth Herndon and Lauren Kinsman-Costello are studying how permafrost melt, wetlands, carbon, and phosphorus interact in the Alaskan tundra.

2017WarmNights_cleveland_en_title_lgAugust 23: As the number of warm nights is increasing, it makes it harder to recover from heat waves.

August 22: Britain’s seabird colonies face catastrophe as warming waters disrupt their food supply. But we don’t know exactly how big problem is because the UK government won’t fund a new seabird census. The last one was done in 2000.

August 20: High Ground Is Hot Property as Sea Level Rises: Climate Gentrification in Miami.

August 19: Climate gloom and doom? Bring it on. But we need stories about taking action, too. (via @ClimateCuddles)

August 16:Humid heat waves that can kill healthy people in hours will affect millions in South Asia in decades.

August 15:
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August 14th: 91 volcanoes discovered under the West Antarctic Ice Sheet!!!! This is so exciting on multiple levels, from the pure geo-nerdery to the potential climate change impacts. Geeking out. (Read Andrew Freedman’s article for all of the details, but its possible that (1) geothermal heating associated with the volcanoes will contribute to melting and destablization; (2) melting the ice sheet could enhance volcanic activity as the pressure is released; and (3) the volcanoes will help anchor the ice sheet in place and reduce the possibility of catastrophic collapse. These are not mutually exclusive possibilities.)

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August 13th: Flooding in Miami now happens every spring tide. Rainfall & storm surges don’t help out either. I liked the headline in the Washington Post: “Flooding in Miami is no longer news – but it is certainly newsworthy.”

August 12th: Sometimes you just have to laugh, so you don’t cry. Talking about climate politics, with humor. Thanks, Stephen Colbert.

 

August 11th: Climate change fueled a mega-rainstorm that flooded Louisiana 1 year ago, and the vulnerable people caught in the flood are still picking up the pieces. Special and important reporting by Climate Central.

August 10th: A new paper in Science shows that European floods have shifted in timing, up to 2 weeks earlier per year, since 1950. What’s particularly intriguing about the study is that the timing shifts haven’t just occurred in snowy regions, but also in places where rainfall is causing water tables to rise and soils to become more saturated earlier in the spring.

August 9th: Erratic weather affects subsistence rice farmers in Madagascar, further proof that some of the worst climate change impacts will be felt by those least responsible for causing it.

August 7th: Peru’s glaciers have made it a laboratory for adapting to climate change. It’s not going well. This is a really nice feature story from the Washington Post.

August 5th: A new report says that extreme heat & weather could kill 50x more people per yr in Europe by 2100 than today. That may be overestimate, but a more reasonable number in the report is that 2 out of every 3 Europeans will be affected by weather disasters (per year?) compared to 1 in 20 today. Heat waves will account for 99% of all the excess deaths predicted, and it’s awfully hard to relocate away from a heat wave.

August 4th: If you are not yet listening to smart & human commentary on climate change, what’s stopping you?

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August 2nd: A visualization that condenses space and time to tell a powerful story about the local and global impacts of climate change through the 20th century and beyond.

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Categories: by Anne, climate science, hydrology, ice and glaciers, links