P Wave and S Wave Velocity

Manning's Equation

Famine and Feast: Recent Changes to the Global Nitrogen Cycle

The global human population exploded from 1.5 to over 7 billion during the last century. By the middle of the century, the need for increased of crop production was urgent order to meet the growing demand for food. The invention of synthetic nitrogen fertilizer, coined the ‘Green Revolution’ achieved this goal. Crop yields, or the amount of food produced on a given piece of land, increased by 30% globally, saving many people from starvation. So much nitrogen has been added to the fields since then, however, that much has leaked out into streams, lakes, and estuaries and caused major pollution problems that jeopardize human health and the economic value of fisheries. After drastically altering the global nitrogen cycle to relieve a shortage, society is now grappling with an overabundance. My own research seeks to understand how to remove excess nitrogen from watersheds and remediate the issues related to over-fertilization. 

Nitrogen is the most abundant element in Earth’s atmosphere, yet millions of dollars are spent manufacturing nitrogen-based fertilizer every year. What drives this lucrative industry? The answer is the form of nitrogen in the atmosphere. Nearly 80% of the atmosphere is composed of pure nitrogen gas, which is un-useable to most plants. Harnessing nitrogen from the sky requires a lot of energy. The Haber-Bosh process uses man-made energy (oil, gas, nuclear power) to add hydrogen atoms to nitrogen gas, creating massive amounts of ammonia. Some of this ammonia may end up in household cleaning products, but most is used as fertilizer.

Of course, humans did not invent this process. Several different types of bacteria and algae have special enzymes that allow them to harvest nitrogen from the sky at low temperatures, without the excess energy that the Haber-Bosh process requires. A small number of plants (mostly peanuts and beans, and some common flowers) formed alliances with these bacteria, allowing them to live in tiny holes in their roots. These plants enjoy the advantage of abundant nitrogen, and microbes accept a safe place to live and a small share of the sugary products of photosynthesis in return.

The majority of plants are not so lucky to cohabitate with nitrogen harvesting bacteria. Instead, nitrogen is only available as a recycled organic matter – broken down leaves, wood, or animal scat. For the entire history of plants the small number of organisms controlled the abundance of bio-available nitrogen in the world, and whole ecosystems organized around the principle of recycling. Agricultural fields played by the same rules, and farmers had to carefully recycle crop and animal waste in order conserve nutrients in the soil.  

Haber-Bosh tipped this natural balance. While this invention revolutionized agriculture and increased crop yields around the world, many natural, non-agricultural ecosystems were fertilized in the process. Aquatic ecosystems, especially estuaries, are particularly sensitive to fertilization and respond with a flurry of algae growth. Fertilized algae in an estuary grow like weeds in a garden, choking out other life forms such as fish, mollusks, and even crabs. This phenomenon, known as a Dead Zone, happens regularly in many coastal zones today.

The Haber Bosh process has doubled the rate of nitrogen fixation from atmosphere to land. This excess has shown up in water bodies and caused major ecological problems. 

Dead zones present a major challenge for managing chemical cycles globally. Human activities have doubled the rate of nitrogen fixation since 1950, and synthetic fertilizer is still widely applied to agricultural fields, despite widespread impacts on ecosystems. Unfortunately there is no mechanized process to convert ammonia back to nitrogen gas. Certain microbes can convert ammonium back into nitrogen gas. This process is very slow compared to the Haber-Bosh process, though. Significant progress has been made to mitigate nitrogen pollution in certain critical regions, however. In the past twenty years, efforts to curb agricultural fertilizer applications have reduced pollution. Scientists such as myself have dedicated entire careers to studying the bacterial communities that remove nitrogen from waterways. One way to remove nitrogen is to create or restore wetlands where these bacteria live and provide the conditions necessary for rapid nitrogen removal. Widespread implementation of different ecosystem restoration practices has potentially curbed the load of nitrogen entering the Chesapeake Bay, for instance.

Are Increasing Flood Losses Due to Climate Change?

Federal aid in response to the damage caused Hurricane Sandy currently amounts to over $60 billion dollars, while total economic damage will likely be over $80 billion dollars. This follows just a year after 2011’s Hurricane Irene and tropical storm Lee, which caused damages of over $16 billion. Much of the damage caused by these storms was due to extensive flooding from the large amounts of rain delivered by the storms. Figures from the National Weather Service show an increasing trend in flood damages in recent decades.

The consensus within the scientific community, and increasingly in the public opinion, is that climate change is causing more frequent and intense storm events. But climate change is not the whole story behind the increase in economic costs from storms. A significant factor behind increased losses from floods is that more people live in high-risk areas such as floodplains and coastal areas. A related cause is that the value of our assets has increased in the past several decades. To illustrate this, compare the value of everything you own in your house to what your grandmother would have owned at your age.

Why have we allowed so much development in high-risk areas? Ironically, much of the reason is because of federal policy originally designed to reduce the cost of flood damages. The National Flood Insurance Program was established in 1968 in response to rising concern over the amount of federal aid given to victims of floods. The program mandated flood insurance for homeowners in designated high-risk areas. The standard chosen for delineating these high-risk areas was the “100-year” flood.

The “100-year” flood is a term that is often misunderstood, because it does not refer to the flood that only occurs once every hundred years. Rather, it is the magnitude of flood that has a 1% chance of occurring in any given year. Stated another way, over a very long period of time, this is the magnitude of flood that would occur on average every 100 years.

Hydrologists determine the magnitude of the “100-year” flood at a given location by using available streamflow data from the U.S. Geological Survey. Hydrologists assume that the chance of a future flood of a certain size will be the same as the chance that size flood occurred in the past, which can be estimated using the streamflow data. This method assumes the conditions causing floods in the pasts will be the same as future conditions. There are two major problems with this assumption. The first is that the watershed and stream channel may not have stayed the same over time. In many places we have modified rivers by building dams (decreasing flood magnitude), and levees (decreasing flood risk locally but increasing flood magnitude downstream). Also, urbanization creates more impermeable surfaces, which increases the amount of runoff after storms and leads to larger floods.

As mentioned before, the frequency and intensity of storms is likely changing, such that the climatic conditions that led to flood events in the past will be different from those in the future. This means that the “100-year” estimates underlying the National Flood Insurance Program are not accurate reflections of true flood risk in many areas, and has led to inappropriate development in high-risk areas. Scientists such as myself are interested in better quantifying the relationships between climate and large floods in order to support improved risk management policies. 

Haven’t we seen this before? Understanding the 100-year flood

After hurricane Irene in 2011 and “superstorm” Sandy in 2012, it seems like every year has storm that causes the “100-year” flood or greater. This term can be confusing because it refers to a flood that has an average recurrence interval of 100 years, which means that in a given year, it has a 1% chance of occurring not one that only occurs every 100 years. This is why hydrologists prefer to talk about the 1% annual exceedance probability flood.

So, how do hydrologists determine how large the 1% annual exceedance probability flood is? In part because our streamflow records are relatively short, it is impossible to determine what is the “true” 1% annual exceedance probability flood. But, we can use the available annual maximum streamflow records to fit a statistical probability distribution. From this frequency curve we can estimate the magnitude of annual maximum streamflow for a given exceedance probability, as shown in the example below.

The key assumption of this method is that the historical data given an accurate representation of the distribution of future floods. Within the debate over the effects of climate change, this assumption of stationarity has been questioned, if not outright rejected. Even ignoring the climate question, this assumption is often problematic because of land use changes or modifications to stream channels that affect flood flows.  For example, flood control structures such as dams reduce annual peak flows. Conversely, urbanization increases the amount of impervious surfaces, increasing surface runoff and annual peak flows.

Accurate estimation of the 1% annual exceedance probability flood is important because it is the standard used in the designation of high-risk areas for the National Flood Insurance Program. As shown in the figure from the National Weather Service below, floods cause billions of dollars of economic damage each year.

The National Flood Insurance Program was established as a way to help pay for and mitigate these costs. Homeowners living in the 100-year floodplain are required to purchase flood insurance, which is not covered under standard homeowner insurance policies. Often municipalities restrict development in the designated floodplain or have stricter building codes to make structures more flood-proof. Incorrect designation of these high-risk areas may mean that many of these areas are inappropriately developed and people living there are unknowingly at risk. When floods inevitably do occur the damages are then unnecessarily costly, and taxpayers are often left to cover the costs through emergency relief funds, Thus, properly quantifying flood risks is in everyone’s best interest.

References:

Holmes, R.R., Jr., Dinicola, K., 2010, 100-Year flood–it's all about chance: U.S. Geological Survey General Information Product 106, 1 p. http://pubs.usgs.gov/gip/106/

National Flood Insurance Program: http://www.floodsmart.gov/

Hydrologic Information Center http://www.nws.noaa.gov/hic/

Famine and Feast: Recent Changes to the Global Nitrogen Cycle

Nitrogen is the most abundant element in Earth’s atmosphere, yet millions of dollars are spent manufacturing nitrogen-based fertilizer every year.  What drives this lucrative industry? The answer is the form in which atmospheric nitrogen resides.  Nearly 80% of the atmosphere is composed of N₂ gas, which is unavailable to most land plants.  Nitrogen is a vital nutrient for biological organisms, and therefore plant growth tends to be limited by nitrogen in most natural environments.  Some plants have formed alliances with certain families of bacteria (i.e. Rhizobium), which possess key enzymes used to ‘fix’ N₂ into biologically available forms such as NO_x (NO₂, NO₃) and NH₄, to harvest nitrogen from the sky. Some forms of algae, such as Cyanobacteria can also fix nitrogen as well. But for the majority of plants, nitrogen is only available as a recycled organic matter – broken down leaves, wood, or animal scat. For the entire history of plants, nitrogen-fixing organisms controlled the abundance of bio-available nitrogen in the world, and whole ecosystems organized around the principle of recycling. Agricultural fields played by the same rules, and farmers had to carefully recycle crop and animal waste in order conserve nutrients in the soil.  

Vitousek et al. (1997) Comparative timing of a number of global changes. Alterations to the global nitrogen   cycle are on par with deforestation, population growth, and Carbon Dioxide release.

 

Haber Bosh process

N₂ + 3H₂ à 2NH₃

Things have changed recently. Without enzymes, nitrogen fixation takes a lot of energy and combustion engines in cars and coal fired power plants regularly reach temperatures hot enough to transform significant amounts of N₂ into NO_x. The nitrogen fixed from these processes is usually deposited near its source (roads, parking lots), or concentrated in areas downwind of coal plants.  The invention of the Haber-Bosch process, which uses energy from oil, rather than enzymes to fix N₂ into NH₃, harnessed nitrogen in a way that cars and coal did not. This invention revolutionized agriculture and increased crop yields across the world by as much as 30% in a period of agricultural history ironically coined the “Green Revolution”.  The vast toll of this prosperity for ecosystems is still being realized today.  Human activities have doubled the rate of nitrogen fixation globally¹, so much that this limiting nutrient has become over-abundant in agricultural fields and some forests.  Where more is added than plants and soil can absorb, nitrogen has leached into waterways at alarming rates tipped the delicate chemical balance of many estuary ecosystems.  In the past twenty years, efforts to restore estuaries and watersheds have curbed some agricultural nitrogen application, but a problem of this magnitude will undoubtedly take a long time to resolve because the only way to permanently remove nitrogen, is via a slow microbial process entitled denitrification. Scientists such as myself have dedicated careers to studying the controls on this elusive process, and are working to determine ways to enhance the natural ability of ecosystems to permanently remove nitrogen from waterways. 

Denitrification

2 NO₃⁻ + 10 e⁻ + 12 H⁺ à N₂ + 6 H₂O

Measuring nitrogen removal via denitrification in groundwater near streams, where nitrate concentrations are high and anaerobic microbes can convert NO₃^- back into N₂ gas. We use a stable isotope tracer to determine the rate of N₂ production in stream banks and created wetlands to identify "hot spots" of nitrogen removal in coastal watersheds. 

Citations

Vitousek, P.M. et al. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7, 737-750 (1997).

Observations of the Moon's Interior

 The Moon: A possible interior of the moon from outside to inside: Crust, Mantle, Core.

The Moon became the second celestial body that humans set foot on when Neil Armstrong and Buzz Aldrin landed there in 1969 and remains the only body that we have been on besides Earth. Due to the costs and limited number of trips to the Moon, we do not have a very extensive knowledge of the interior of the Moon. However, we were able to place four devices on the Moon from 1969 to 1972 that measure vibrations from Moonquakes to observe the interior of the Moon. Moonquakes are the equivalent of Earthquakes, except that they tend to release thousands of times less energy. The deep (700+ km below surface) Moonquakes observed in this paper are caused by the gravitational pull from the Earth causing stresses and fracturing inside of the Moon. There are other sources of Moonquakes, but they are not relevant to these deep Moonquake studies. These Moonquakes can be more easily felt because the Moon is colder and more brittle, which makes it more rigid, so it will vibrate more easily. This is similar to the comparison between hitting a metal bell with a hammer and hitting a pillow with a hammer. The bell will vibrate much more and much longer. The vibrations that the devices are detecting from the Moonquakes are actually Primary (P) Waves and Secondary (S) Waves because Primary Waves move faster and get detected first. Primary Waves vibrate in a motion towards and away from you (compressional), like the sound of someone's voice being projected towards you. Secondary Waves move in a direction that is at a right angle to the P-waves and they can also not pass through a liquid. The data from these stations was analyzed in the 1970s and was reanalyzed to find new results using new methods and technology in 2001 to make different observations. The results found that there appears to be a dislocation similar to the motion of the S-waves (shearing motion), which shows the source of these deep Moonquakes is internal, as opposed to meteorites. The results also show that there seems to be a general direction that the waves propagate, which could imply some sort of preferred orientation in the deep fractures of the Moon.

 

Conversions: This figure shows the S-p and P-s conversions at the lithosphere-asthenosphere boundary and also shows how multiples influence P-s conversions by reflection off of the surface and Crust-Mantle Boundary (Moho).

This data was analyzed using a method different from the method that was used when the data was originally gathered.  The method that was originally employed was a method used where the waves observed are waves that began as P waves and got converted to S waves before they reach the device.  The new method used was one where the wave began as an S wave and was converted to a P wave before it reaches the device.  This method is used because a wave can be converted from P to S or S to P when it passes through a structure, such as a fracture.  New layers, where the physical properties of the interior change, can also cause the conversion before the wave reaches the device that detects vibrations at the surface.  The older method is more effective for observing shallow structures in the moon because if you try to observe deep structures using the P to S method, it will be covered up with multiples.  Multiples create an effect of there being a lot of incoherent noise before the wave you are trying to observe arrives, similar to the effect of trying to pick one specific persons voice out in the middle of a crowd.  Using this alternate method instead allows for you to observe the first wave before the noise gets there or listen to the person you want to hear before everyone else starts talking.  This allows for observations of deep features and can allow for different interpretations about the sub-surface structure of the Moon. 

The first image is from http://www.daviddarling.info/images/Moon_interior.jpg

The information from this paper is from:

Vinnik, L., Chenet, H., Gagnepain-Beyneix, J. & Lognonne, P.  First seismic receiver functions on the Moon. Geophysical Research Letters, volume 28(15). pp. 3031-3034, August 1, 2001.