Posts tagged ‘temperature’

Apr 7

Green Roofs—Do They Work? (Part II)

Innovative soil scientist, John Buck, and his team have discovered that green roofs have more capacity than people imagined (see part I). Below are some of the challenges he sees for the future, and the type of measurements he suggests researchers take, as they continue to validate the effectiveness of these urban ecosystems.

Green and whited plant on a garden rooftop with orange rocks

New Challenges for Green Roofs

Green roof results are promising, but they present a new challenge: making sure the plants have enough water. The crux of the challenge is that the lightweight, expanded shale/clay substrate material, the standard in green roof design, does a good job of soaking up the water, but has some peculiar properties that are unlike typical soils. Specifically, the expanded shale and expanded clay media tend to be dominated by sand and fine gravel-sized particles that provide a high proportion of macropores, but the interior porosity of the large particles is dominated with micropores. That pore size distribution leads researchers to two important questions— How much water will be readily available for plant growth? And, will the unsaturated hydraulic conductivity be adequate to avoid starving the roots under high-evaporative demand by allowing water to flow to roots from the bulk soil? These are critical questions as green roof technologies continue to evolve.

Researchers wonder, will the unsaturated hydraulic conductivity be adequate to avoid starving the roots under high-evaporative demand.

Measurements Required for Green Roof Validation

Still, Buck has learned a great deal from his work. Considering the wild spatial distribution of summer storms, quantitative green roof performance studies require that rainfall be measured locally. Monitoring of soil volumetric moisture content measurements in concert with rainfall and soil lysimeter measurements of drainage, reveal the degree of total and capillary saturation, drainage rate, and porosity available for storage. Soil water potential sensors, placed within the capillary fringe of water ponded over subsurface drainage layers, can provide useful insights regarding the dryness of the drainage layer and overlying soil, as well as the available storage of stormwater within the drainage layer.

Direct measurement of soil drainage using lysimeters is a key supplemental measurement on green roof performance quantification projects because there is an unmeasured component of water storage where drought-resistant alpine succulents (typically Sedum species) are used on green roofs. The Sedum plants can absorb up to 10 mm of rainfall equivalent in their plant tissues.

Plants poking out of the soil in front of a house

Measurement of soil drainage using lysimeters is a key supplemental measurement on green roof performance quantification projects.

Other Projects and Future Plans

At ground level, Buck is quantifying the performance of intensive stormwater infiltration areas known as rain gardens, bioretention areas, or more generically, infiltration-based stormwater best management practices (Infiltration-based BMPs). When monitoring infiltration-based stormwater BMPs, Buck has used similar tools to those used on green roofs, but has added water-level sensors and piezometers. Buck has found that ancillary measurements of electrical conductivity, often available on water content sensors, along with surface and pore water sampling, can be used to document transformations taking place in infiltration systems. These measurements now combine to show that green roofs and infiltration-based BMPs are indeed making a difference to urban environments and contributions to CSOs. The challenge now is how to implement this technology more widely. But, with the validation now in hand, that job should be quite a bit easier.

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Apr 1

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Green Roofs—Do They Work?

Green roofs are being built in large cities to provide stormwater management, reduce the urban heat island effect, and improve air quality—but are they effective? John Buck, an innovative soil scientist based in Pittsburgh, Pennsylvania, has been trying to quantitatively answer this question in many different cities using soil monitoring equipment in order to determine the efficacy and best types of green infrastructure for managing stormwater.

Garden on a rooftop with flowers and a city around it

A green roof installation site at the Allegheny County Office Building in Pennsylvania.

Why Green Roofs?

In older cities, stormwater runoff is typically combined with sewage flows, and these combined waters are treated at a sewage treatment plant during dry weather and light rain events. Unfortunately, during more substantial storms (sometimes just a few mm of rain) the combined flows exceed the ability of the sewage treatment plant, and are discharged without treatment to surface waters as “combined sewage overflows” (CSOs). One of the ways to mitigate CSOs is to capture and store stormwater to keep it out of the combined sewer.

A green roof is essentially a garden on a roof, but rather than growing plants in soil, installers use a synthetic substrate made of expanded shale, expanded clay, crushed brick, or other highly porous, lightweight material with high infiltration rates. During a storm event, water will soak into the air-filled pore space in the substrate, which acts like a sponge to soak up the rain. Excess water will flow into a subsurface drainage layer and will leave the roof garden via existing roof drains. Because a substantial fraction of the stormwater is stored in the substrate, it can later dissipate through evapotranspiration instead of contributing to stormwater volume and CSOs.

Researcher kneeling testing soil with a soil sensor

Researchers are using soil moisture sensors for measuring temperature, bulk electrical conductivity and volumetric water content in green roofs and green infrastructure.

Finding Answers

Designers and regulators want to know how well green roofs work and if they are being over-engineered. They want answers to questions such as: “What sort of substrate should I be using? What type of plants can survive green roof conditions? Will I need to irrigate the green roof when there are no storms to water the plants?” and, “Will the green roof work as well during a one-inch storm that occurs over a half hour versus a five-inch storm that occurs over five days?”

Buck is using soil lysimeters and modified tipping bucket rain gauges to measure the quantity, intensity, and quality of water coming into and going out of the green roofs. He also tracks weather parameters and calculates daily evapotranspiration of landscapes. Using soil sensors, he measures electrical conductivity (dissolved salts), volumetric water content, and temperature. He has installed data loggers that send data to the web via GSM cellular connection, allowing stakeholders access to the data in real-time. This data telemetry provides additional data security, immediately updated results, instant feedback of system problems, and an easy way to share data with others.

Visualized data of the 87% annualized runoff reduction at Phipps Conservatory green roof site in Pittsburgh, PA.

What Has Been Learned?

Buck discovered that green roofs have much more capacity than people ever imagined. At The Penfield Apartments in St. Paul, Minnesota, the green roof retained enough water to reduce runoff to about half of a conventional roof, and the peak intensity of the runoff was about one-quarter of what it would have been without the green roof. At Phipps Conservatory in Pittsburgh, there was an 87% annualized runoff reduction and almost no runoff from typical summer rain events. Buck comments, “Interestingly, on the Penfield project, we expected better hydrologic performance where soils were thicker, but there was no difference, or results were slightly the reverse of expectations. That reversal was likely due to the confounding influence of irrigation, which was probably non-uniform and not metered or measured by the rain gauge.”

Next week: Read about some of the challenges John Buck sees for the future, and what kind of measurements he suggests researchers make, as they continue to validate the effectiveness of these urban ecosystems.

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Jan 28

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The Quest for Accurate Air Temperature (Part 2)

In the conclusion to last week’s blog, Mark Blonquist, chief scientist at Apogee Instruments and air temperature measurement expert, explains the complexities of some proposed solutions to the problems that challenge accurate air temperature measurement.

A aspirated raditation shield by Apogee Instruments

An aspirated radiation shield manufactured by Apogee Instruments in Logan, Utah. Multiple models of passive and active shields are available from several manufacturers.

Solution: Passive Radiation Shield

In addition to an accurate sensor, accurate air temperature measurement requires proper shielding and ventilation of the sensor. Passive shields do not require power, making them simple and low-cost, but they warm above air temperature in low wind or high solar radiation. Warming is increased when there is snow on the ground due to increased solar radiation load from higher albedo and increased reflected solar radiation. Errors as high as 10 degrees C have been reported in passive shields over snow (Genthon et al., 2011; Huwald et al., 2009). The figure below shows the differences in error for the two conditions.

Corrections for Passive Shields

Equations to correct air temperature measurements in passive shields have been proposed, but often require measurement of wind speed and solar radiation, and are applicable to a specific shield design. Corrections that don’t require additional meteorological measurements have also been proposed, such as air temperature adjustment based on the difference between air temperature and interior plate temperature differences. Others have suggested modifying traditional multi-plate passive shields to include a small fan that can be operated under specific conditions, but using natural aspiration when wind speeds are above an established threshold.

Solution: Active Shields

Warming of air temperature sensors above actual air temperature is minimized with active shields, which are more accurate than passive shields under conditions of high solar radiation load or low wind, but power is required for the fan. The power requirement for active shields ranges from one to six watts (80-500 mA). For solar-powered weather stations, this can be a major fraction of power usage for the entire station and has typically required a large solar panel and large battery. Power requirement and cost are disadvantages of active shields (Table 3), and they have led to the use of less accurate passive shields on many solar-powered stations.

Also, the fan motor can heat air as it passes by. Active shields should be constructed to avoid recirculation of heated air back into the shield. There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect.

Advantages and disadvantages of passive and active radiation shield diagram

Table 3: Advantages and disadvantages of passive (naturally-aspirated) and active (fan-aspirated) radiation shields.

There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect. Radiation-induced temperature increase was analyzed in long-term experiments over snow and grass surfaces by comparing temperature measurements from three models of active radiation shields (the same temperature sensor was used in all shields and were matched before deployment). Continuous measurements for one year indicated that mean differences among shield models were less than 0.1 C over grass and less than 0.3 C over snow. Differences increased with increasing solar radiation, particularly during winter months when there was snow (high reflectivity) on the ground.

Air Temperature: a Complex Measurement

The properties of materials and nearly all biological, chemical, and physical processes are temperature dependent. As a result, air temperature is perhaps the most widely measured environmental variable. Accurate air temperature measurement is essential for weather monitoring and climate research worldwide. The road to accuracy is complex, however, and will continue to be challenging given the trade-off between accuracy and power consumption with passive and active shields.

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Jan 22

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The Quest for Accurate Air Temperature (Part 1)

Mark Blonquist, chief scientist at Apogee Instruments and air temperature measurement expert, explains the difficulties of obtaining accurate air temperature.

Air Temperature gauge at the foot of a tree

The accuracy of air temperature has come a long way.

Accurate air temperature measurements are challenging, despite decades of research and development aimed at improving instruments and methods. People assume that they can use a static louvered radiation shield along with a temperature sensor and start measuring accurate air temperature. That assumption is good if you are at a site where the wind blows all the time (roughly greater than 3 m/s). However, if the wind at your field site is below that, you’re going to see errors due to solar heating (See Figure 1).

Figure 1: Passive Shield Error: Data for 3 different models are graphed.

Challenge 1: Accurate Sensors

Over the years, thermocouples, thermistors, and platinum resistance thermometers (PRTs) have been used for air temperature measurement, each with associated advantages and disadvantages. PRTs have the reputation as the preferred sensor for air temperature measurement due to high accuracy and stability. However, thermistors have high signal-to-noise ratio, are easy to use and low cost, and have similar accuracy and stability to PRTs. Thermocouples are becoming less commonly used for air temperature measurement because of the requirement of accurate measurement of reference temperature (i.e., meter temperature, data logger panel temperature).

Challenge 2: Housing Air Temperature Sensors

The challenge of accurate air temperature measurement is far greater than having an accurate sensor, as temperature measured by an air temperature sensor is not necessarily equal to air temperature. Temperature sensors must be kept in thermal equilibrium with air through proper shielding in order to provide accurate measurements. To do this, housings should minimize heat gains and losses due to conduction and radiation, and enhance coupling to air via convective currents. They must shield it from shortwave (solar) radiant heating and longwave radiant cooling. A temperature sensor should also be thermally isolated from the housing to minimize heat transport to and from the sensor by conduction. The housing should provide ventilation so the temperature sensor is in thermal equilibrium with the air. Also, the housing should keep precipitation off the sensor, which is necessary to minimize evaporative cooling of the sensor. Conversely, condensation on sensors can cause warming. When condensed water subsequently evaporates, it cools the sensor via removal of latent heat (evaporational cooling).

Challenge 3: Size of Sensor

The magnitude of wind speed effects on air temperature measurement in passive shields is highly dependent on the thermal mass (size) of the sensor. Many weather stations have combined relative humidity and temperature sensors, which are much larger than a stand-alone air temperature sensor. Air temperature errors from larger probes are greater than those from smaller sensors. One study, Tanner (2001), reported results where a common temperature/RH probe was approximately 0.5 degrees C warmer than a common thermistor in a weather-proof housing.

Thermal mass of temperature sensors also has a major impact on sensor response time. Sensors with small thermal mass equilibrate and respond to changes quicker and are necessary for applications requiring high-frequency air temperature measurements.

Challenge 4: Proper Shielding

In addition to an accurate sensor, accurate air temperature measurement requires proper shielding and ventilation of the sensor. Active, fan aspiration improves accuracy under conditions of low wind but requires power to operate the fan. Passive, natural aspiration minimizes power use but can reduce accuracy in conditions of high solar load or low wind speed. Radiation shields for air temperature sensors should be placed in an environment where air temperature is representative. For example, air temperature sensors and radiation shields should not be deployed on the tops of buildings or in areas where they will be shaded by structures or trees. Conditions in microenvironments have that potential to be very different from surrounding conditions. Typical mounting heights for air temperature sensors are 1.2 to 2.0 meters above the ground. Typically, radiation shields should be mounted over vegetation.

Up next: Mark Blonquist explains the complexities of some of the proposed solutions to the above challenges in part 2.

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Dec 4

Understanding the Influence of Coastal Fog on the Water Relations of a California Pine Forest

Forests along the California coast and offshore islands experience coastal fog in summer, when conditions are otherwise warm and dry. Since fog-water inputs directly augment water availability to forests during the dry season, a potential reduction of fog due to climate change would place trees at a higher risk of water stress and drought-induced mortality. Dr. Sara Baguskas completed her Ph.D. research in the geography department at UC Santa Barbara on how variation in fog-water inputs impact the water relations of a rare, endemic tree species, Bishop pine, located on Santa Cruz Island in Channel Islands National Park. The goal of her study was to enhance our ability to predict how coastal forests may respond to climate change by better understanding how fog-water inputs influence the water budget of coastal forests.

Dr. Baguskas’ study seeks a better understanding of how fog-water inputs influence the water budget of coastal forests.

Fog Manipulation

Santa Cruz Island supports the southern extent of the species range in California, thus it is where we would expect to see a reduction in the species range in a warmer, drier, and possibly less foggy future. To advance our mechanistic understanding of how coastal fog influences the physiological function of Bishop pines, Dr. Baguskas conducted a controlled greenhouse experiment where she manipulated fog-water inputs to potted Bishop pine saplings during a three-week drydown period. She installed soil moisture (VWC) sensors horizontally into the side of several pots of sapling trees at two different depths (2 cm and 10 cm) and exposed the pines to simulated fog events with a fog machine.

In one group of plants, Baguskas let fog drip down to the soil, and in another treatment, she prevented fog drip to the soil so that only the canopies were immersed in fog. She adds, “Leaf wetness sensors were an important complement to soil moisture probes in the second treatment because I needed to demonstrate that during fog events, the leaves were wet and soil moisture did not change.” Additionally, Baguskas used a photosynthesis and fluorescence system to measure photosynthetic rates in each group.

The fog events had a significant, positive effect on the photosynthetic rate and capacity of the pines.

Results

Dr. Baguskas found that the fog events had a significant, positive effect on the photosynthetic rate and capacity of the pines. The combination of fog immersion and fog drip had the greatest effect on photosynthetic rates during the drydown period, so, in essence, she determined that fog drip to the soil slows the impact of drydown.

“But,” she says, “when I looked at fog immersion alone, when the plant canopies were wet by fog with no drip to the soil, I also saw a significant improvement in the photosynthetic rates of these plants compared to the trees that received no fog at all, suggesting that there could have been indirect foliar uptake of water through these leaves which enhanced performance.” An alternative interpretation of that, Baguskas adds, is that nighttime fog events reduced soil evaporation rates, resulting in less evaporative loss of soil moisture.

Dr. Baguskas says her “canopy immersion alone” data are consistent with other research: Todd Dawson, Gregory Goldsmith, Kevin Simmonin, Carter Berry, and Emily Limm have all found that when you wet plant leaves, it has a physiological effect, suggesting the plants are taking water up through their leaves and not relying as much on soil moisture. (These authors performed different types of experiments, but their papers serve as reference studies). Baguskas says, “My results suggest that is what’s going on, but it’s not as definitive as other studies that have actually worked on tracking the water through leaves using a stable isotope approach.”

Lessons Learned

Though Dr. Baguskas did not monitor soil temperature in this study, she says that in the future, she will always combine temperature data with soil moisture data. She comments, “Consistently, the soil moisture in the “canopy-immersed only” plants was slightly elevated over the soil moisture in the control plants. It made me wonder if this was a biologically meaningful result. Does it support the fact that if plants are taking up water through their leaves, they don’t rely on as much soil moisture? Or did my treatment change soil temperature, and is that having a confounding effect on my results? What I’ve learned from this, is that in the future I will always use soil probes with temperature sensors because you may not know until you see your results if temperature might be important.”

Future Fog Studies

Baguskas is a USDA-NIFA postdoctoral Research Fellow working with Dr. Michael Loik in the Environmental Studies Department at UC Santa Cruz. She continues to study coastal fog, but now in strawberry fields. Her current research questions are focused on integrating coastal fog into water-use decisions in coastal California agriculture. She loves the work and continues to rely on soil moisture sensors to make meaningful and reliable environmental measurements in the field and greenhouse.

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