Archives for posts with tag: technical background article

Today I’m going to tackle one of the most important topics relating to the Ground to Sky interface. That is how our planet stays warm and what happens to heat and radiation at the Earth’s surface. I have touched on this before, explaining a little about the differences between vegetated and tarmaced surfaces. Today I will briefly describe the surface energy balance in much of its entirety.

The most important concept I will introduce here is that of the surface energy balance equation. H = K + (Sd – Sr – Su) + (Ld -Lu) – (G – λ E). This can be simplified as:

Heat = Storage + net (total including positives and negatives) Solar radiation + net longwave radiation + net heat flux.

Use the diagram below as reference as I introduce each of these terms and a small amount of discussion as to the meaning of each.

Image

Heat term (H).  This is the overall heat of the surface when all of the terms in the equation have been accounted for. This is the value we are trying to find out. How hot is our surface – how hot is it during the day? During the night? During cloudy or clear conditions? How hot the surface is affects our environment and is crucial for weather forecasting and other types of process modelling.

Storage term (K). This is how much heat is being stored by the surface. Different types of surfaces have a different ability to store heat. Man made surfaces are often capable of storing more heat than natural surfaces and this contributes to the urban heat island effect.

Net solar radiation (Sd – Su – Sr). Solar radiation (also called shortwave radiation) is the input of heat from the sun. Sd is the amount of sunlight coming down from the sun towards the patch of ground we’re interested in. Of Sd, some will be reflected by clouds and aerosols in the atmosphere before it even reaches the surface. This reflected solar radiation is called Sr. So we must subtract Sr from Sd. Some of the solar radiation does reach the surface but not all of it is absorbed by the surface and contributes to its heat balance. Su is the solar radiation that is reflected by the surface and must also be subtracted from the total incoming solar radiation, Sd. Su is controlled by the reflectivity of the surface, this is called the surface’s ‘albedo’. White surfaces reflect more sunlight than black surfaces. Some surfaces reflect only particular wavelengths of light, and this gives them their colour. For example an object that appears red to us reflects red light. The sky is blue because of the scattering of light in the ‘blue’ section of the visible light spectrum. But this is an aside. What matters here is that the amount of sunlight that reaches the surface, is absorbed and contributes to our heat term H, is reduced by reflection in the atmosphere by clouds and aerosols (Sr) and at the surface depending on surface albedo (Su).

Net longwave radiation (Ld – Lu). Longwave radiation is low frequency (infra-red) radiation. This is largely outgoing (Lu) heat from the Earth itself radiating into space. Ld is the term for the reflection of outgoing longwave radiation by clouds and greenhouse gases back down to the surface. Lu is quite close to balancing Sd overall, keeping the Earth’s atmosphere about the same average temperature. But the balance between the two varies from place to place (think the difference between summer and winter). Ld is the term that has caused such debate around the would – the greenhouse effect. The potential changes that could occur to the energy balance because Ld has been artificially altered by human activity is the subject of worldwide scientific attention.

Surface heat loss (G – λ E). These two terms represent the loss of heat from the surface itself. G is the sensible heat flux and λ E is the latent heat flux. Sensible heat (G) means that heat is being exchanged between the surface and the atmosphere and ONLY heat is being exchanged. That is, the only effect is a change in temperature. Minus G (-G) is loss of heat from surface to atmosphere (usually overnight) and +G is heating of a cold surface by warm air. When you think of air, you probably think of wind. Wind can come along in big parcels (air masses) originating from far away from our surface. Say the wind comes from a desert in the early morning? We will expect a +G as the air is hot but our surface is cool from the cold night. Interfaces of air masses and the ground can cause interesting weather patterns such as localised thunderstorms when the temperature difference between the two is high. λ E is the latent heat flux. Latent heat is different from sensible heat in that it is not only a change of temperature that is being brought about, but also a change of state. This term describes the heat that is used to evaporate water from a surface. This may or may not be mediated by vegetation depending on the surface. Plants can control evaporation using holes in their leaves called stomata. This way they can regulate how much water they have in their system to keep them alive.  More heat is required to change the state of water from liquid to gas, so more energy is used up in this process than heating the ground. This explains why conditions feel cooler after it has rained, or near a lake or the sea.

When all these terms are totalled up we have H. H might be a hot surface or a cool surface depending on the balance of all the different terms. Is it a hot sunny day with no cloud? Then Sd will be high and Sr will be low. If we haven’t had much rain in a while λ E will be low too. What’s the difference between the energy balance during the day and during the night? What about a shaded bit of ground compared to an unshaded bit? A city compared to the countryside? Have a think about how H might change during the year. Understanding of the surface energy balance gives us a lot of insight into our environment.

I hope that this brief overview of how energy behaves in the lower atmosphere and at the surface helps you to understand some of the everyday observations that you make whilst wandering about in the ground to sky interface. In my next article I will return to soil science, and will be discussing how soil forms with another simple equation that you can see in action every day.

Greenhouse gases are top of the worldwide green agenda and all around the world industrious groups of people are seeking to reduce emissions. Whether by legislating against irresponsible fossil fuel use, encouraging energy efficiency or creating traffic-free zones (and many, many other methods besides), the only way we’ll know whether such schemes are having an effect is by knowing the rate of emission of gases into the atmosphere.  We need to be able to do this to a fine enough level of detail to be able to tell if our reduction strategies are successful or not.

This is no mean feat. The atmosphere is notoriously complicated, as is the land surface with sinks (take up gases) and sources (emitters of gases) springing up all over the place, often as quick as you can count and varying wildly according to environmental conditions.  In spite of these difficulties, scientists, industry and policy have created methodologies and taken steps towards putting some figures on greenhouse gas emission.  There are three main methods, which I will introduce in turn.

1. Emissions inventories.

I like to call this the “let’s just tot up what we know” method.  It relies on accurate figures for the extent of a certain activity (i.e. how many cars there are, how many power stations are running and for how long) and an accurate figure for how much greenhouse gas is emitted by each of these activities. These are called activity data and the emissions factor.

Flux = sum of all (activity data x emissions factor).

gts1

I call this the “tot up what we know” method because all that goes into an inventory is the emissions sources we know about.  They also usually only include human activities or have a subjective or modelled term for natural sources and sinks.  We have several uncertainties in the inventory method and they require extensive checking:

  • is the activity data right? i.e.  is our traffic count up to date?, are the power companies telling the truth about what’s coming out of that chimney?, are we up to date with all of the new houses?
  • are the emissions factors right? i.e. are we simply estimating over a large range of possible values?
  • are we summing up all of the sources we know about? what about the sources that we don’t know about? could we be overestimating the total emissions by ignoring the effect of vegetation uptake?

Because of questions like these, inventories work best in a coarse resolution (over a large area, like a country).  They aren’t checked using measurements and are best for making broad decisions (i.e. cutting back on fossil fuel energy in a certain country) than narrow ones (i.e. implementing a neighbourhood road closure and cycling scheme).

2. Direct measurement.

Measurement of greenhouse gas fluxes can happen at all sorts of scales. From a chamber placed over a square metre of soil right up to an instrument mounted on a tall tower above a city.  For the purpose of this discussion I will focus on these tall tower instruments.  The instrument on the new tower in the drawing below is designed to capture fluxes from a wide area.  So all of those terms that we put in our emissions inventory (plus or minus the influence of the sources/sinks that we ‘forgot’) are contributing to the flux measurement.

gts2

This particular system measures the difference in concentration of greenhouse gas between two vertical air packets (I will come back to how this works some other time).  This tells us the overall flux.

To be able to interpret this, we need to know where the fluxes measured by the instrument are coming from.  That is, we need to know the area of the ground that is sending gas up to be captured by the instrument.  We don’t want to consider sources that aren’t being included in the measurement and likewise we don’t want to miss any.  To do this, we need a ‘source area’ or ‘footprint’ model.  This can calculate the area of ground we can expect the fluxes to be coming from and then we can investigate how they might be influencing this measurement.

Hm, you might think this is a little vague.  You’re right, when it comes to making decisions about changing the number of cars in your city or increasing the amount of green space, it makes sense to be able to tell more exactly where the sources really are and whether or not the action has made a difference to their extent.  But if you want to monitor what the overall flux is over the city, what it really is, through observations rather than approximations, a direct measurement scheme is what you need.

How about a middle ground between the two?  A method that includes all the knowledge we have about what is out there generating greenhouse gases (inventories) plus real greenhouse gas measurements to tell us if we’re going wrong?  This brings me to the final method, which is newest to science and very much still in development.  This is one of the things that is keeping me busy during the working day right now.

3. Measurements + Model (the “inversion” method)

Inversion methodology is where we start to get really clever.  I will explain this step by step.

a. Take the inventories and make it as accurate as you possibly can for your chosen area.

b. Place an instrument on a tower that measures concentrations of greenhouse gases.  (Note: this is concentration not flux.  The instrument tells us what the concentration of gas is, not the change in that concentration).

c. Use a meteorological forecast model to tell you what the winds are doing throughout the surface km of atmosphere.

d. Place the inventories and the wind data (plus some other information such as the height of the boundary layer and the surface roughness) into a chemistry-transport model.  This model can give you an idea of what the concentration of the greenhouse gas in question is EXPECTED to be.

e. (And here’s the clever bit) Use the real data from the atmosphere, plus the expected concentrations and then run the model BACKWARDS.  This “inversion” of the method uses everything we know to ‘go back to source’ and tell us where we should be checking our inventories a little more closely.

photo

This last method has a lot of potential. But right now it is in its pilot stages.  New science is consistently being published in developing this method and industry is becoming aware of the economic opportunity in providing governments with such a service.

The method that an international group/national govenment/local authority will choose depends on what they wish to get out of the investigation. Benchmark figures? Real measurements? In-depth analysis? Participating in new development of new scientific technology? The future is bright for estimation methodology, so don’t let “how will we even know if it works?” put you off taking your bike to work instead of your car.

In the next article at Ground to Sky, I will be discussing the ups and downs of wading through huge piles of scientific literature and providing some tips from my experience.  Following this I will return to the technical science and will be discussing some of the methods that we can use to cool down our hot cities during the summer months, without compromising winter warmth.

Imagine you’re out walking in your local big city on a bright and very hot day. Before you set out from your suburban home, you noticed a light breeze on the air but here, upon exiting the underground station, the air is heavy and still. You have a look around you. Where did that nice breeze go? A nearby tall building captures your attention, you look up and there on the roof is a pair of small wind turbines, furiously turning in the wind that has been deflected above your head.

You frown, why is the wind all the way up there, it’s down here on street level that you need to feel its benefit. Because it is a nice day, you decide to walk to the big city park. A short distance later, you turn off of the wide road and into a side street. Suddenly, the breeze hits you but it is much stronger than you remembered, whipping around your clothes and your hair. Looking around, you realise that this street is much more narrow than the road with the train station on it. Because it is facing a different way, the wind is channeled directly into it and is being buffeted by the tall buildings on either side.

You’re thankful when you reach the end of this street. You turn again into a street running parallel to the first. It isn’t quite as wide as the first street. Once you turn the corner, the wind has gone again and you walk a little while before you have to cross over. When you do, you can feel the wind again, pulling at your clothes and the stack of papers you were taking to the park to read. Better not lose them.

A few turns later, through narrow streets with hardly any wind at all and a hot, humid feel to them, you reach the park. It feels cooler here and the light breeze has returned. You settle yourself in the shade of a tree and begin to read.

The route of this hypothetical journey through the city is shown in the below drawing. Take a moment to look at the airflow that is depicted on each stage of the trip.

20130709-090908.jpg

The street with the train station on (1) is wide and runs perpendicular to the direction of the wind. The wind hits the tall buildings and is deflected upwards, into the flow of the air turbine which is positioned to capture the most consistent wind direction. The narrow street which is parallel to wind direction (2) funnels any wind that has not been deflected by tall buildings, decreasing flow at the edges but increasing it down the centre of the road. The next road perpendicular to wind direction (3) is not quite as wide as the first and the buildings are not quite so tall. The walker was first in the ‘wake’ of the building (the area in which the building has interrupted the air flow) but when they crossed over, they exited the wake and were once more affected by the air stream. The narrow streets that are perpendicular to wind direction (4) do not receive much replenishment of air on this day. The air feels warm here and a bit difficult to breathe thanks to the traffic. If the air is not getting replaced due to the flow simply ‘skipping’ over the top of this street – what might be the consequence for air quality? It’s a relief for the walker to reach the park (5), where the trees are having a cooling effect on the warm air and that breeze has returned as a result of the wide open space without building interference.

The structure of the city (i.e. where are all the tall buildings and how are they oriented) has a large influence on the city’s microclimate. The effect of the city buildings on the meteorology of the region has lots of important conseqences for city planning, building design, placement of energy services such as wind turbines and structures such as tall masts and for air quality and greenhouse gas dispersion. The climate of the city and the breathability of the air is vastly important for the quality of life of city residents and workers and the experience of visitors.

In my next blog article, I will take a step outside of the city and look at things on a much larger scale, talking about the many methods in which a country can estimate its greenhouse gas emissions and the current science on which of these methods is closest to giving us a good answer.

Atop most of the world’s land surface, there is a layer of soil varying in depth from a mere few centimetres through to over ten metres in well established peat bogs. The majority of the world’s cities were founded in the most sensible places for human inhabitation – fertile lands close to a source of water. As time advanced, so did the extent and population density of many of these cities and towns and gradually bare and vegetated soils came to represent the minority land use within city borders.

Today, in many cities, parkland and other greenfield sites are considered premium resources. Townhouses bordering parks fetch the highest house prices and nearby green space is prerequisite for the majority of buildings of state and homes of the rich. Urban vegetation takes the form of trees planted by the roadsides, parks large and small, turf playing fields, urban farms and forestry and, importantly, residential gardens.

Across the world, city planners are coming to value the importance of greenery and soil within the city bounds and seek to maximise it. Why have they come to this conclusion? Not only to drive up the house prices and provide a pleasant environment to live in for city citizens but also because of the key influence that soils and vegetation have on natural cycles of heat, water and pollutants in the surface to atmosphere interface. These ‘services’ that the soils provide are termed “ecosystem services” and they are as important in the city as they are in natural environments. Ecosystem services might be cultural (house prices, space for recreational activity, aesthetic value) or provide habitats for animals and plants. They might be productive (city farms and allotments) or they might regulate the environment (take up carbon, capture pollutants such as heavy metals, cool the city down).

20130530-161702.jpg

In the bottom panel of the above graphic I have drawn three different land surfaces and how they impact three important things; heat, water and carbon dioxide. The size of the arrow is a rough indicator of the magnitude of the movement of that commodity. Outbound heat is much greater from the tarmac surface than from the vegetated or soil surfaces for several reasons. Firstly, tarmac is a dark surface and it absorbs a larger amount of incoming solar radiation. But soils can be dark too – so why no large heat loss from soil surfaces? Because soils are often wet as well as dark, in fact quite often a darker soil is a wetter one. And heat is required to evaporate this water, therefore less heat is absorbed and the warming effect is reduced. On vegetated soils the plants mediate this evaporation through their water-upake systems, a process known as transpiration. Back to the tarmac. The second reason it is a heating surface is that it does not absorb water, it pools on the surface. So instead of a consistent and steady evaporation flux you have either rapid evaporation from a pool on the surface, or if there is drainage, little cooling effect from water. In fact the tarmac is a barrier to downward water flow (infiltration) which is why urbanisation in areas that are naturally floodplains can cause serious flooding issues if nowhere is provided for displaced water to go. Finally, another important ecosystem service is the regulation of carbon dioxide by vegetation. This effect in some city neighbourhoods could be substantial enough in the summer to balance out the carbon dioxide from human sources (UC Santa Barbara, 2012) though it would take a lot of vegetation to do this at the city scale – to the extent that such an environment might no longer qualify as an urban space (University of Helsinki, 2012).

For further information on this fascinating and complex subject, I recommend this detailed review by Pataki et al (2011) which is available open-access here. This article provides analysis of key ecosystem services provided by urban ecosystems and their related costs (here termed ‘ecosystem disservices’).  It also provides a useful summary in Table 1. of the magnitude of impact of each of the discussed ecosystem services and the level of uncertainty associated with current knowledge.

In the next technical article on Ground to Sky, I will describe in more detail the influence of land surface type on heat, including the startling impact that hot cities have on the lower atmosphere. Before this, I will write the first of what I hope will be many insight articles into my life in science, focusing on taking field measurements of soil properties and greenhouse gases and why measurements are critically important now, as environmental science moves towards an entirely modelled world.

20130523-175020.jpg

There’s no hiding the impact of human life on the planet and for those in the know, the same thing goes for human impact on the soil. The way we use the land is a strong control on the ability of soils to sustain plant and animal life and the maintenance and development of those soil resources. Take the well-known issue of desertification for example: some soils are high in sand and low in organic matter and therefore don’t naturally bind together very well (organic matter acts a bit like glue for the mineral soil particles). Combine this with large trampling pressure from livestock and people and/or constant cropping whereby no residues are returned to the land and the outcome is loose soils, easily carried by air or water, leaving behind infertile bare rock.

A different example arises from my memories of my PhD studies on peatland environments. How we manage peatlands has a powerful influence on the capabilities of carbon storage, peat accumulation and fertility. Peatlands might be drained or excavated for a variety of reasons including agricultural, forestry, horticultural or fuel demands. When we drain a peatland, we allow respiring organisms to use the carbon locked away in that soil as ‘food’ and they release it as carbon dioxide. When we excavate a peatland we take away the ability of the landscape to create more peat (and therefore locking away more carbon). Striking a balance between the use of peatlands for exploitation and conservation is important for the future of these landscapes.

Soil management is particularly important for farmers, who are the only group of people (save perhaps very keen gardeners) that might be said to have an in-depth relationship with the soil, requiring substantial knowledge of the resource available on their land. Farmers on the whole are aware that it is a foolish move to disrespect the soil by overworking it. Sometimes the pressures of making a living from the land and the high reliance on factors outside of the control of the farmer (i.e. the weather, market prices, disease) can lead to a soil becoming more taxed than it can handle. This is particularly the case for intensified farms, where the land is in continual use and overworked soils are compensated with fertilisers. Fertilisers usually focus on the key nutrients (nitrogen, phosphorous, potassium) and exclude other important nutrients that are also required by plants (to a lesser degree than the aforementioned ‘macronutrients’). So follows a reduction in crop yield, for reasons that seem inexplicable without tests to confirm what is missing. In light of our understanding of the soil systems, few realise the careful balancing act that farmers must contend with when managing their soils. In the UK, there are a substantial range of consultancy business and governmental advice organisations that focus on providing a service to the farmer to help them to understand the impact they have on their soils and how to preserve them for future harvests.

Finally, what about the soils in the cities? We may pave over the land but below the tarmac, the soil is still there. In city parks, soils provide a valuable service; they feed plants that help to soak up some of the bustling city emissions (and give city residents a cool and pleasant space for recreation). These important functions are often termed ‘ecosystem services’ and even in our urban centres, they provide utility that has an influence on the surrounding environment and human quality of life. In the next article, I will focus on these ecosystem services in urban environments and talk a bit more about what makes the city different from the countryside in terms of understanding our ground to sky interface.

Soils are an important part of the complex and carefully balanced chemical cycles that keep the planet functioning in a manner that is comfortable to humans and human development. Soils are stores of key nutrients such as carbon, nitrogen and phosphorus which they pass on to plants and microorganisms to use. Most soils receive nutrients from elsewhere; from dead plant and animal matter, from the atmosphere, from rivers and groundwater, from the bedrock below and from animals moving about.

Some nutrients are used by microorganisms to produce or consume greenhouse gases. Organisms that live in the soil need a range of nutrients to survive and, like humans, they ‘breathe out’ waste products. Microorganisms that ‘breathe’ like us are called heterotrophic microorganisms. They survive in soils with plenty of oxygen which they use to decompose carbon forms in the soil and they release carbon dioxide. These organisms like drier soils, with a nice moisture film to give them the water they need (see my previous post) but enough oxygen to respire. When a soil is very rich in carbon, e.g. peat, allowing these organisms to function by draining the land means that more of that carbon is released into the atmosphere in the form of carbon dioxide and that an important carbon store is lost. Rather than taking in oxygen from the air, some other heterotrophic microorganisms use oxygen that is attached to nitrogen in a common nutrient form called nitrate. These are called denitrifying microorganisms and they can turn a nutrient that is useful for plants into a greenhouse gas which is far less useful in terms of land productivity.

20130523-172600.jpg

On the other hand, some microorganisms like to live without oxygen, and they ‘breathe’ in a different way, using chemical respiration (chemotrophs). Some examples of these are bacteria that produce methane (a greenhouse gas 25 times more powerful than carbon dioxide) and some that produce nitrous oxide (300 times more powerful a greenhouse gas than carbon dioxide). These bacteria are called methanogens (methane generators) and nitrifiers (bacteria that ‘breathe’ by turning ammonium ions into nitrous oxide) respectively.

There are far more bacteria types, that have all kinds of intesting metabolisms (iron oxidisers, sulphur oxidisers etc.) but I will continue to focus on the greenhouse gases for now. So, once a bacteria has made some gas – what happens to it? And how does this relate to soil porosity? Well, if a gas is produced in a wet pore – it may well stay there for some time, building up in a bubble until development of a pressure or concentration gradient releases it. Sometimes, if a gas stays in the soil long enough, other bacteria with different metabolisms from those that made the gas might want to use it. For example, some denitrifying bacteria can use nitrous oxide in their metabolism and they put out nitrogen gas instead. Nitrogen gas makes up nearly 80% of the atmosphere and is preferable in terms of global warming potential than nitrous oxide. Other bacteria consume methane, these are called methane oxidisers, so they need oxygen to function. So imagine you have a soil with a groundwater level of 10cm below the surface. In here, methane is being produced. On top, methane is being oxidised. So how much methane actually makes it out of the soil and into the atmosphere to contribute to greenhouse gas accumulation? Well, this depends on the extent of the oxidising contition and in turn on the soil porosity.

In my next article I will talk about how human land use change can affect the porosity of the soil and the nutrients that are available to bacteria. Soil’s contribution to greenhouse gas production is a natural process that can not (and should not) be artificially halted, however, when humans make substantial changes to the way the soil system naturally operates then the soil’s contribution is no longer in balance with other processes. This is where problems can arise.

When it comes to soil, space matters. The gaps between soil particles and aggregates provide channels for aeration, hydration and living space for organisms.

Soil porosity is a measure of this space between the particles. It is usually measured in cm^3 per cm^3. This is the volume of pores in cubic centimetres per cubic centimetre of total soil volume.

Porosity can vary massively between soil types. Take a clay for example, in this soil the pores are very small and not very well connected. When it rains the water takes some time to drain through the soil. This leads to features such as mottling – the green of reduced iron oxides speckled with patches of red oxidised iron – as water is not evenly distributed. Another issue with tight packed, non-porous clays is the difficulty roots have to penetrate through. It takes a water-loving plant with a shallow and efficient rooting system to thrive in clay. A sandy soil on the other hand has large and well connected pores for water, gas, plants and animals to travel through easily.

Soil pores can be full of either air or water or a mixture of the two. The water-filled pore space (WFPS) is a useful measure of how much of your soil’s pore volume is full of water. Even very dry soils retain some residual moisture, which clings to the particles in a film as a result of surface tension. This slick particle surface is the ideal living space for micro-organisms. As a soil drains, it becomes more and more difficult to pull more water out of it as dry particles hold on to the water very tightly. This is why plants start to struggle in a very dry soil, and why we water them.

20130523-170131.jpg

The gases that make up the atmosphere in the soil pores is quite different from the air that we breathe. There is far more carbon dioxide for example, thanks to the respiration efforts of millions of aerobic micro-organisms. There is also a little less oxygen, and sometimes there are higher amounts of nitrous oxide, methane and ammonia gas than in the Earth’s atmosphere.

These gases, after being produced by micro-organisms, have various ways of getting out of the soil. The key transport pathway is diffusion – simple passing of the gas through air or water according to a concentration gradient. The gas will always travel from a region of high concentration to a region of low concentration – hence the carbon dioxide making its way towards the surface where the concentration is much less.

Another way that gases travel is through convection. This is bulk flow that is driven by a difference in temperature or pressure from the pores to the surface. On a very windy day, pressure in the soil might end up higher than at the surface, as air is pushed away by the wind, and so, thanks to this gradient, the soil gas will travel out seeking equilibrium. In wet soils, gases that do not readily dissolve can accumulate in large bubbles, which are forced out of the soil by gradients in pressure or temperature. This process is known as ebullition.

Soil gases also travel through plants, particularly some reeds and sedges that have tissues specifically adapted to help them survive in wet soils where oxygen is limited.

So for transport of gases, soil porosity is very important, primarily in determining the rate of transport through diffusion. The amount of water in the soil pores is also a strong control on whether a gas produced at depth will make it to the surface and when that will happen.

In the next article, I will discuss some gas forming processes and talk a little bit more about what happens to a gas trapped in the soil by high amounts of water and/or low levels of pore space/connectivity.