The Energy Balance of American Deserts

Darren Moring

8 May 2016

The Energy Balance of American Deserts

Energy balance studies are conducted worldwide in an effort to better understand how

climate areas influence and impact an area’s precipitation and temperature. There are multiple

facets to energy balances as well as a multitude of factors that can change how these balances

interact with the local atmosphere. Even within the same desert climate zone, areas of shrubs or

grasslands will see varied atmospheric conditions from areas with little to no vegetation. This

paper will delve into the deserts of the American southwest, and how energy is dispersed in this

environment. This energy balance is key to the survivability of the desert ecosystem. While

desert plant and animal life have adapted to such harsh conditions, small adjustments can mean

huge implications for living organisms in the environment. What is the balance of energy in the

desert ecosystem and how could small changes affect the climate in the future?

The “desert” climate category is a broad topic that includes a large portion of the world’s

landmass. Desert can be defined as a dry area, with low precipitation. Unfortunately, this

definition includes climates from a multitude of locations with varying temperatures and

precipitation patterns. The Saharan Desert receives extremely low precipitation and holds some

of the world’s records for highest temperature. Antarctica is also classified as a desert, although

its temperatures hardly get above the freezing point. The annual precipitation that Antarctica gets

comes in the form of snow, amounting to a small amount measureable liquid precipitation. This

general definition spawns from the impact of global climate circulations. Most non-pole deserts

are found around 30 degrees on either side of the equator. This occurs because the Hadley and

Ferrel Cells, two major global climate circulations, intersect above this region and cause a

general downward motion. This downward motion inhibits a majority of convection over this

area, leading to lower amounts of precipitation. At the poles, the poleward end of the Polar Cell

has downward motion, like the intersection of the Hadley and Ferrel Cells, so precipitation and

convection are very limited in this region as well. It is easy to see that we need something to

differentiate the different regions where limited precipitation occurs.

Besides separating areas based on temperature differences, climates can be broken down

into areas of different plant types or topography. The Koppen Climate system helps to divide

generic climate definition terms into smaller, more definable sections. According to Koppen,

“Deserts” are part of the B climate zone, which denotes dry climates (Fraedrich 2001). Koppen

differentiates the poles from deserts by defining a category for Ice climates (F climate zone). In

this classification, the poles remain dry areas, but their cold temperatures and lack of vegetation

classify them differently than other deserts near the midlatitudes. However, deserts are also

differentiated from steppes, which include desert lands with shrubs or dry grasslands (Fraedrich

2001). American deserts best fit into this category, as they mostly contain shrubs and

occasionally some groves of saguaro cacti. This type of climate is what this paper will generally

focus on, but it is important to note the differences between the American deserts and other

deserts, such as the Sahara, which is classified as a Koppen desert, with little to no vegetation.

The energy balance of a climate system is difficult to conceptualize as there are many

aspects to consider. Net radiation is a measure of how much energy, shortwave and longwave, is

reaching the surface. Shortwave radiation from the sun enters the atmosphere and is either

absorbed by the atmosphere or ground, or is reflected away from the surface. The Earth

constantly emits longwave radiation, the amount of which is determined by the temperature of

the Earth. This relation, where E is related to T4, is known as the Steffan-Boltzmann Law. The

longwave energy that Earth emits is what is primarily responsible for warming the lower part of

the atmosphere and keeping the atmosphere from cooling off too much during the night. Latent

heat flux is a measurement of how much energy goes toward changing the phase of water, either

evaporating or condensing. Although this tells us about the moisture content of the ecosystem,

this measurement can also tell us directly about the carbon flux. Other facets to the energy

balance include conduction through the soil and photosynthesis (which is also related to latent

heat flux). The data used for this study is from tower data from a site near Tombstone, Arizona,

right in the heart of the Chihuahuan Desert of the southwestern United States and northwestern

Mexico. Figures used are direct measurements from 2015 and this is what my findings will be

based off of.

The data source near Tombstone, Arizona is located near 32 degrees North latitude, and

110 degrees W longitude. This location is near the area of general atmospheric subsidence

generated by the convergence of the Hadley and Ferrel Cells, perfect for the formation of a

desert climate. The solar zenith angle peaks on the summer solstice at around 82 degrees, giving

the area nearly direct insolation during the summer months. Around the winter solstice, the

zenith angle drops to around 35 degrees. The lower zenith angle provides a less direct amount of

insolation to the area. Because of this, less solar radiation is absorbed by the Earth and its

atmosphere per square meter. Long wave radiation drops as temperatures fall during the winter

months.

However, the data shows something rather peculiar about the solar radiation. Plotting the

net radiation alone, the data shows a maximum and minimum near each respective solstice. This

is what one would expect when mean temperatures don’t vary a lot from day to day during the

summer and winter months. However, when the incoming solar radiation is plotted, the

maximum occurs earlier than expected, around day 130. The increase in incoming solar radiation

from the winter months to the solar radiation maximum is very similar to the slope of the net

radiation increase over the same period. However, around the time of the net radiation

maximum, incoming solar radiation has already begun falling. Except for a few days, flux

density levels are lower for incoming solar radiation than net radiation. Clearly something is

going on in the atmosphere that is blocking the solar radiation from reaching the surface.

The phenomenon that is occurring is an annual monsoon that affects this particular part of

the continent. The summer monsoon begins around June and will last through the summer into as

the early fall months (Corrie 1998). Seasonal winds bring additional moisture into the region.

Because of this, monsoon season spawns a large percentage of the region’s precipitation and

clouds for the year. Increased cloud cover from monsoonal moisture would reflect a portion of

the incoming solar radiation without necessarily affecting the net radiation adversely. From the

incoming solar radiation plot above, it is easy to see that some of the days during the monsoon

season stayed dry and cloud-free, producing flux densities around their geographical standard for

that time of the year. However, a majority of the days during this season saw some reduction in

the incoming solar radiation. The monsoon topic will repeatedly appear in later sections of this

paper as it influences many of the facets of the energy balance covered in this paper.

Terrestrial radiation can also manifest itself in the forms of sensible heat and latent heat,

both of which are quite important in terms of energy balances. Sensible heat flux refers to the

energy used to heat a medium, in this case the atmosphere, without changing phases. This is the

heat that is felt when one walks over a blacktop surface from a grassy patch. It feels warmer on

the blacktop because the amount of sensible energy passing through each square meter of

blacktop is greater than the amount of sensible energy passing through the grassy area. The

greater the temperature of an area, generally, the higher the sensible heat flux. Studies have

shown diurnal patterns in sensible heat flux in the desert southwest, including one by Helene

Unland et al (1996). Her research showed that the sensible heat flux peaks daily around the

middle afternoon, near the time when temperature maxes out during the day. At night, the study

also showed low and even negative sensible heat flux, especially when latent heat flux rose to

higher levels.

The data from the Tombstone, AZ tower shows the range of sensible heat fluxes

diurnally, but it also shows the variance of sensible heat over the 2015 year. Each night, sensible

heat at the tower station fell below 0 W/m2, with some nights measuring much higher rates of

sensible heat loss (cooling). The daily maximum sensible heat flux increases as daily

temperatures increase throughout the year. The only discrepancy is that the diurnal sensible heat

maximum seems to level off or lower around the summer solstice, similar to the incoming solar

radiation plot. This can be related to the monsoonal moisture advected into the region. The more

energy devoted toward changing the phase of the moisture, or latent heat flux, the less energy

that is devoted toward sensible heat flux.

The latent heat flux measures the amount of energy allocated toward changing the phase

of water or water vapor. This flux term is usually more important in maritime or wet climate

locations such as a rainforest or coastal location. In dry areas like deserts, the latent heat flux

term is much smaller than other radiative terms as very little moisture is available in these

regions. The data from the tower shows that a majority of the time, the latent heat flux remains

well under 100 W/m2, which is typical for a desert measurement.

Then the monsoonal moisture again makes an appearance in the data. Besides the spike in

latent heat flux early in the year (around day 80), latent heat flux seems to increase over the

summer months, during the time of the monsoon. The additional moisture advected by the

monsoon takes energy to condense into water. This is the purpose of latent heat flux, to convert

water between phases. As mentioned previously, when energy is used to change the phase of

water, it reallocates some of the energy that was originally going to heat the atmosphere.

Scientists in Turkey conducted a study in an irrigated basin. They measured the sensible

heat flux and latent heat flux over a plot of land, once when it was dry, and again when it was

irrigated. They found that when dry, the sensible heat flux was stronger than the latent heat flux,

which was nearly non-existent. When the land was irrigated, the latent heat flux became much

greater than the sensible heat. In fact, the sensible heat dropped to near 0 W/m2 (Bastiaanssen

2000). While this situation took a very dry location and doused it with water, the scenario can be

applied to a desert such as the Chihuahuan Desert, our test area.

Outside of monsoon season, the latent heat flux is nearly non-existent, with the sensible

heat flux dominating. While the moisture added to the desert during a monsoon season is not

nearly the same as irrigating a field, the principle is comparable. The added moisture to the

environment increases the latent heat flux and condenses water which the shrubs and other desert

plant species can use to thrive. The more moisture is advected, and the stronger the latent heat

flux is, the sensible heat flux will not be as great.

One of the consequences mentioned when latent heat is increased is plants can receive

more moisture. Plants use photosynthesis to produce energy and to thrive. Photosynthesis

involves taking water and CO2 and combine it to form energy, but this process is highly

dependent on the soil moisture. In order for the stomata to open, the soil moisture content and

potential must be high enough (Nobel 1976). In other words, without precipitation, no

photosynthesis happens, and thus CO2 concentrations are not lowered. The tower in Arizona

measures CO2 concentration with each measurement it takes (every 30 minutes). The patterns in

the CO2 data match with patterns in other relevant measurements.

Since photosynthesis helps to assimilate atmospheric CO2 into the plant, an increase in

photosynthesis should lead to a decrease in the CO2 concentration. An increase in moisture

caused by an increase in latent heat flux will produce more photosynthesis considering the health

of the plants is good. At first glance, the CO2 concentration data doesn’t seem to show

photosynthesis, as during the monsoon season when moisture is more available to the plants

measurements of greater CO2 concentration are recorded. But besides the high outliers, the

minimum CO2 concentrations decrease during the monsoon season, showing some signs of CO2

assimilation into the desert plants. It is important to remember that the moisture the plants

receive even with the increased latent heat flux is still extremely small. So even though

photosynthesis may be occurring, the “fuel” runs out very quickly. Another feature to note is the

increase in CO2 concentration after day 300. Once the desert starts receiving low temperatures

below the freezing point, the growing season comes to a close, and plant functions begin to shut

down. If photosynthesis shuts down, the CO2 assimilation stops as well. When CO2 is not

assimilated, it accumulates back in the atmosphere. What the data could be showing is the end of

the growing season and the decay of photosynthesis until the whole process repeats itself next

year.

Finally, conductance is an important issue to discuss when talking about energy balance.

Conductance is a measurement of the direct transfer of energy from molecule to molecule

through the layers of soil. Several factors can influence the conductivity of the soil. Not only can

the composition of the soil change the conductance, but soil water content can as well by

changing the specific heat capacity of the soil. Water has a very high specific heat capacity, so

adding it to soil, which has a low specific heat capacity, will increase the amount of energy

needed to change the temperature of the soil.

Measuring this conductance is more of an issue, as methods used in the past have been

proven to be not as effective. A soil study in 1997 in the Negev Desert of Israel found that

conventional radiometer methods were inadequate for the dry desert soil, primarily because the

dampening depth was very low and temperature gradients were very steep (Heusinkveld 2004).

The conductance for these types of desert floors with little vegetation and abundant sunshine

would provide a large portion of the energy balance. In the United States, the deserts are mostly

covered in a layer of brush, which won’t take a lot of radiation away from being absorbed and

conducted through the soil, but it’s enough of a layer that some radiation will be absorbed by the

plant. The desert floor is impermeable, meaning water won’t easily penetrate the soil and

contribute to raising the specific heat capacity. Because of the low specific heat capacity, vertical

temperature gradients will be steep, causing the dampening depth to be lower. According to

Heusinkveld (2004), this can mean that on sunny days, conductance can account for up to 50%

of the net radiation in the heat of the day.

How could slight changes impact the plant and animal species in these climate zones? We

have already seen the impact that climate change has on an established community. The biggest

changes to the energy balance that could cause a catastrophic change to this desert community is

changes in temperature and moisture availability. Both of these changes lead to similar

outcomes, with the possibility of severe plant stress, or extinction of species. A recent example is

the discovery of woody plant species increasing in density in the Sonoran Desert, more than

likely due to climate change (Walther 2002).

Increasing the temperatures either by increasing the net radiation or by increasing the

sensible heat flux would limit moisture production due to the balance between the sensible and

latent heat fluxes. Since one flux thrives while the other goes away, adding more temperature

would have to mean taking out what little moisture there is in the desert ecosystem. Also as

temperature increases, the more evaporation of water that occurs, leaving plants less liquid water

to use for photosynthesis. Another possible outcome is the change of the monsoon season.

Besides winter precipitation, the desert’s primary source of precipitation comes from the inflow

of monsoonal moisture into the region. Taking this away or limiting it would severely stress any

desert plant life. As the plants cannot survive in these conditions, the animals that depend on the

plants for shelter and food will either have to adapt to survive, or die from the elements. These

types of changes are not hard to imagine, as the Earth currently continues to steadily warm.

The positive feedback from these changes would only further any damage to the eco

system. An example of a possible outcome deals with the decrease in photosynthesis and an

increase in CO2 concentration. If photosynthesis assimilates CO2 into plants, then if less

photosynthesis occurs, then there is little to keep the CO2 from continuing to build. Because CO2

is a greenhouse gas, the outgoing terrestrial radiation is trapped, keeping the heat in the

atmosphere. Meanwhile, incoming solar radiation continues to pour into the atmosphere which

creates more longwave radiation which in turn raises atmospheric temperatures. This begins the

cycle over again, and unless the process is stopped, it can only get worse.

The energy balance of the American deserts resides heavily on the relationships between

incoming solar and outgoing terrestrial radiation, sensible and latent heat fluxes, as well as the

conductance of the desert soil. Within this balance, relationships between temperature in the

sensible heat flux and moisture in the latent heat flux, for example, provide us insight on the

intricacies of the desert ecosystem, and how fragile it is. It is clear to see that for some aspects of

this balance, should small changes occur, large consequences could be just around the corner.

Our knowledge of the desert ecosystem and its energy balance is not only for knowledge sake,

but for the protection of an extremely diverse biological community.

Work Cited

AmeriFlux Data Site: Lucky Hills Shrubland, Arizona, 2015

Bastiaanssen, W., 2000. SEBAL-based sensible and latent heat fluxes in the irrigated Gediz

Basin, Turkey. Journal of Hydrology 229, 87–100. doi:10.1016/s0022-1694(99)00202-4

Comrie, A., Glenn, E., 1998. Principal components-based regionalization of precipitation

regimes across the southwest United States and Northern Mexico, with an application to

monsoon precipitation variability. Climate Research Clim. Res. 10, 201–215.

doi:10.3354/cr010201

Fraedrich, K., 2001. Climate Shifts During the Last Century. Climactic Change 405–417.

Heusinkveld, B., Jacobs, A., Holtslag, A., Berkowicz, S., 2004. Surface energy balance closure

in an arid region: role of soil heat flux. Agricultural and Forest Meteorology 122, 21–37.

doi:10.1016/j.agrformet.2003.09.005

Nobel, P.S., 1976. Water Relations and Photosynthesis of a Desert CAM Plant, Agave deserti.

Plant Physiology 58, 576–582. doi:10.1104/pp.58.4.576

Unland, H.E., Houser, P.R., Shuttleworth, W.J., Yang, Z.-L., 1996. Surface flux measurement

and modeling at a semi-arid Sonoran Desert site. Agricultural and Forest Meteorology 82,

119–153. doi:10.1016/0168-1923(96)02330-1

Walther, G.-R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J.C., Fromentin, J.-

M., Hoegh-Guldberg, O., Bairlein, F., 2002. Ecological responses to recent climate change.

Nature 416, 389–395. doi:10.1038/416389a