Skip to content

Posts from the ‘Soils’ Category

How to Protect your Soil Moisture Sensors from Lightning Surge

We occasionally see soil moisture sensors damaged by lightning.  Here’s what to do to protect them.

The secondary products of a lightning strike include electromagnetic pulses, electrostatic pulses, and earth current transients.

lighting

Surge suppression components typically perform their suppression function by temporarily short circuiting the voltage between two wires, several devices, or ground.

Electromagnetic pulses are created by the strong magnetic field that is formed by the short term current flow taking place in the lightning strike. With current flows as high as 510kA per microsecond, these currents create very large magnetic fields. These short term magnetic fields then induce voltages onto wires and cables.

Electrostatic pulses are created by electrostatic fields that accompany a thunderstorm. Any cable suspended above the earth during a thunderstorm is immersed in the electrostatic field and will be electrically charged. Quick changes in the charges stored in both the clouds and earth take place whenever there is a lightning strike. The charge on the cable must now be discharged or neutralized. Unable to find a path to ground (earth), it breaks down insulation and component in its efforts to return to earth.

Earth current transients are the direct result of the neutralization process that immediately follows the end of lightning strike. Neutralization is accomplished by the movement or redistribution of charge along or near the earth’s surface from all the points where the charge had been initially induced to the point where the lightning strike has just terminated. Earth current transients create a shift in potential across a ground plan, often called a “ground bounce”.

Read more

Soil Moisture Sensors: Which Installation Method is Best?

Patterns of water replenishment and use give rise to large spatial variations in soil moisture over the depth of the soil profile. Accurate measurements of profile water content are therefore the basis of any water budget study. When monitored accurately, profile measurements show the rates of water use, amounts of deep percolation, and amounts of water stored for plant use.

How to avoid measurement errors

Three common challenges to making high-quality volumetric water content measurements are:

  1. making sure the probe is installed in undisturbed soil,
  2. minimizing disturbance to roots and biopores in the measurement volume, and
  3. eliminating preferential water flow to, and around, the probe.

All dielectric probes are most sensitive at the surface of the probe. Any loss of contact between the probe and the soil or compaction of soil at the probe surface can result in large measurement errors. Water ponding on the surface and running in preferential paths down probe installation holes can also cause large measurement errors.

Installing soil moisture sensors will always involve some digging. How do you accurately sample the profile while disturbing the soil as little as possible?  Consider the pros and cons of five different profile sampling strategies.

Preferential flow is a common issue with commercial profile probes

Profile probes are a one-stop solution for profile water content measurements. One probe installed in a single hole can give readings at many depths. Profile probes can work well, but proper installation can be tricky, and the tolerances are tight. It’s hard to drill a single, deep hole precisely enough to ensure contact along the entire surface of the probe. Backfilling to improve contact results in repacking and measurement errors. The profile probe is also especially susceptible to preferential-flow problems down the long surface of the access tube.

Trench installation is arduous

Installing sensors at different depths through the side wall of a trench is an easy and precise method, but the actual digging of the trench is a lot of work. This method puts the probes in undisturbed soil without packing or preferential water-flow problems, but because it involves excavation, it’s typically only used when the trench is dug for other reasons or when the soil is so stony or full of gravel that no other method will work. The excavated area should be filled and repacked to about the same density as the original soil to avoid undue edge effects.

soil moisture sensors

Digging a trench is a lot of work.

Augur side-wall installation is less work

Installing probes through the side wall of a single augur hole has many of the advantages of the trench method without the heavy equipment. This method was used by Bogena et al. with EC-5 probes. They made an apparatus to install probes at several depths simultaneously. As with trench installation, the hole should be filled and repacked to approximately the pre-sampling density to avoid edge effects.

Multiple-hole installation protects against failures

Digging a separate access hole for each depth ensures that each probe is installed into undisturbed soil at the bottom of its own hole. As with all methods, take care to assure that there is no preferential water flow into the refilled augur holes, but a failure on a single hole doesn’t jeopardize all the data, as it would if all the measurements were made in a single hole.

The main drawback to this method is that a hole must be dug for each depth in the profile. The holes are small, however, so they are usually easy to dig.

Single-hole installation is least desirable

It is possible to measure profile moisture by auguring a single hole, installing one sensor at the bottom, then repacking the hole, while installing sensors into the repacked soil at the desired depths as you go. However, because the repacked soil can have a different bulk density than it had in its undisturbed state and because the profile has been completely altered as the soil is excavated, mixed, and repacked, this is the least desirable of the methods discussed. Still, single-hole installation may be entirely satisfactory for some purposes. If the installation is allowed to equilibrate with the surrounding soil and roots are allowed to grow into the soil, relative changes in the disturbed soil should mirror those in the surroundings.

Reference

Bogena, H. R., A. Weuthen, U. Rosenbaum, J. A. Huisman, and H. Vereecken. “SoilNet-A Zigbee-based soil moisture sensor network.” In AGU Fall Meeting Abstracts. 2007. Article link.

Read more soil moisture sensor installation tips.

Stem Water Content Changes Our Understanding of Tree Water Use (Part 2)

This week, we continue highlighting the second of two current research projects (see part one) which use soil moisture sensors to measure volumetric water content in tree stems and why this previously difficult to obtain measurement will change how we look at tree water use.

Tamarisk tree: an invasive species dominant in Sudan and arid parts of the United States. (Photo credit: biolib.cz)

Determining Tree Stem Water Content in Drought Tolerant Species

Tadaomi Saito and his research team were interested in using dielectric soil moisture sensors to measure the tree stem volumetric water content of mesquite trees and tamarisk, two invasive species dominant in Sudan and arid parts of the United States. Mesquite is a species that can access deep groundwater sources using their taproots which is how they compete with native species. Tamarisk, on the other hand, uses shallow, saline groundwater to survive.  The team wanted to see if dielectric probes were useful for real-time measurement of plant water stress in these drought tolerant species and if these measurements could illuminate differing tree water-use patterns.  These sensors could then potentially be used for precision irrigation strategies to assist in agricultural water management.  

Temperature Calibration Was Essential

After calibrating the soil moisture sensors to the wood types in a lab, the team inserted probes into the stems of both trees.  They also monitored groundwater and soil moisture content to try and infer whether or not the trees were plugged into a deep source of water.  Interestingly, Saito found that, unlike soil, where temperature fluctuation is buffered, tree stems are subject to large variations in temperature throughout the course of the day.  This temperature fluctuation interfered with the soil moisture probes’ ability to accurately measure VWC.   The team came up with a simple method for accounting for temperature variability and were then able to obtain accurate VWC measurements.  

stem water content

Photo credit: desertusa.com

Water Use Depended on Landscape Position

Saito’s results were similar to Ashley Matheny’s study (see part 1), in that they found a lot of different patterns, even in trees of the same species.  Water-use depended on where the trees were on the landscape.  Some of them were tapped into groundwater, and the stem water storage didn’t change no matter how dry the soil became.  Whereas others, depending on their position in the landscape, were very dependent on soil moisture conditions.  

You can read the full study details here.

Implications

Saito’s study illustrates that we see everything about a tree that’s above ground, but we may have no sense of what’s going on below ground.   We can put a soil moisture sensor in the ground and decide there’s plenty of moisture available.  Or if conditions are dry, we may decide the tree is under drought stress, but we don’t know if that tree is tapped into a more permanent source of groundwater.   

Other researchers have put soil moisture sensors in orchards looking at stem water storage from a practical standpoint for irrigation management.  Their data didn’t work out so well because of cable sensitivity where water on the cable created false readings.  However, the data they were able to obtain showed that some of the trees were plugged into water sources that were independent of the soil.  Those trees were able to withstand drought and needed less irrigation, whereas other trees were much more sensitive to soil moisture.  

If we had an inexpensive, easy to deploy measurement device plugged into every tree in an orchard, we could irrigate tree by tree, give them precisely what they needed, and account for their unique situation.

What Does it All Mean?

The interesting thing about using soil moisture sensors in a tree is that stem water content is a difficult to obtain piece of information that has now been made easier.  Historically, we’ve focused on measuring sap flow, but that’s just how much water is flowing past the sensor. We’ve measured what’s in the soil: a pool of moisture that’s available to the tree. But some trees are huge in size, such as ones along the coast of California. They’re able to store vast amounts of water above-ground in their tissue.  Understanding how a tree can use that water to buffer or get through periods of drought is a unique research topic that has had very little attention. With these kinds of sensors, we can start to investigate those questions.

Reference: Saito T., H. Yasuda, M. Sakurai, K. Acharya, S. Sueki, K. Inosako, K. Yoda, H. Fujimaki, M. Abd Elbasit, A. Eldoma and H. Nawata , Monitoring of stem water content of native/invasive trees in arid environments using GS3 soil moisture sensor , Vadose Zone Journal , vol.15 (0) (p.1 – 9) , 2016.03

Get more information on applied environmental research in our

Scientists Measure Thermal Properties in Famous Japanese Tomb

Named for the tall pine tree that sits at the top of the tumulus earth mound, Takamatsuzuka Tomb is located in the Asuka village, just south of Nara, Japan. Located within the tomb are some of the most beautiful and famous Japanese wall paintings. Discovered in 1972, the paintings are believed to have been made at the end of the seventh and beginning of the eighth centuries.

Thermal properties

Mural in the inner tomb.

Though it is unknown who is actually buried in the tomb, the murals are worthy of a nobleman. They depict a small-scale universe, including star constellations, the sun, the moon, and guardian gods, for the deceased.

In 2001 this national treasure became threatened by mold growing on the interior lime plaster walls. High humidity and high water content of the lime plaster walls are believed to be the main contributor to mold growth. As a short-term solution, a cooling system was put in the structure to prevent further growth. To optimize efficiency, scientists used the transient line heat source method to determine the thermal properties of the tomb and surrounding soil.

Thermal properties

Cooling system installed at Takamatsuzuka Tomb
to prevent fungal growth.

As a long term solution, the Agency of Cultural Affairs has decided to move the stone interior of the tomb to another location where the environment can be more easily controlled.

What Are Thermal Properties?

Thermal properties tell scientists important things about soil or other porous materials.  Thermal conductivity is the ability of a material to transfer heat. Thermal resistivity, the inverse of conductivity, illustrates how a well a material will resist the transfer of heat. Volumetric heat capacity is the heat required to raise the temperature of unit volume by 1℃, and thermal diffusivity is a measure of how quickly heat will move through a substance.

thermal properties

Thermal property measurements help scientists understand the effects of lasers, cauterization, or radiation on surrounding tissue.

Who Should Measure Thermal Properties, and Why?

Thermal property measurements are needed in varying industries and research fields. One example is underground power cable design. Electricity flowing in a conductor generates heat. Any resistance to heat flow between the cable and the ambient environment causes the cable temperature to rise. This can harm the cable and may even cause power outages in large sections of major cities. When cables are buried, soil forms part of the thermal resistance, and thus soil thermal properties become an important part of cable design.

Other popular applications for thermal property measurements include thermal conductivity of concrete, thermal conductivity of nanofluids, thermal resistivity of insulating material, and thermal properties of food. Unique applications range from measuring human tissue to adobe houses. 

The Transient Method is the Only Way to Measure Moist, Porous Materials

The standard technique for measuring thermal properties is called the steady state technique (guarded hot plate method). The steady state technique requires a needle to be heated until it comes to a steady state, at which time it measures the temperature gradient and determines the thermal properties of the measured material.

The transient line heat source method differs in that heat is only applied to the needle for a short amount of time, and temperature is measured as the material heats and cools.  The steady state technique is a good fundamental method because it uses the simplest equation.  However, it takes a full day to make a measurement because of the wait for steady state.  In addition, when measuring a porous material that contains moisture, heat flow will make moisture move away from the heated area and condense on the cold area.  Thus, the thermal properties of the material will change.  

This means there’s no way to measure the properties of moist, porous materials with the steady state method. The transient line heat source method, however, is able to measure the thermal properties of moist, porous materials, and it can even measure thermal conductivity and thermal resistivity in fluids.

Learn more about measuring the thermal properties of soils or other materials.

Get more information on applied environmental research in our

Electrical Conductivity of Soil as a Predictor of Plant Response (Part 2)

Salt in soil comes from the fertilizer we apply but also from irrigation water and dissolving soil minerals.  If more salt is applied in the irrigation water than is leached or taken off in harvested plants, the soil becomes more saline and eventually ceases to support agricultural production (see part 1).  This week, learn an effective way to measure electrical conductivity (EC) in soil.

Electrical conductivity

Salt in irrigation water reduces its water potential, making it less available to the plant.

How to Measure Electrical Conductivity of the Soil Solution

As mentioned above, the earliest measurements of solution conductivity were made on soil samples, but it was found to be more reliable to extract the soil solution and make the measurements on it. When values for unsaturated soils are needed, those are calculated based on the saturation numbers and conjecture about how the soil dried to its present state. Obviously a direct measurement of the soil solution conductivity would be better if it could be made reliably.

Two approaches have been made to this measurement. The first uses platinum electrodes embedded in ceramic with a bubbling pressure of 15 bars. Over the plant growth range the ceramic remains saturated, even though the soil is not saturated, allowing a measurement of the solution in the ceramic. As long as there is adequate exchange between the ceramic and the soil solution, this measurement will be the EC of the soil solution, pore water EC.

Salt in soil comes from the fertilizer we apply, irrigation water and dissolving soil minerals.

The other method measures the conductivity of the bulk soil and then uses empirical or theoretical equations to determine the pore water EC. The ECH2O 5TE uses the second method. It requires no exchange of salt between soil and sensor and is therefore more likely to indicate the actual solution electrical conductivity. The following analysis shows one of several methods for determining the electrical conductivity of the saturation extract from measurements of the bulk soil electrical conductivity.

Mualem and Friedman (1991) proposed a model based on soil hydraulic properties. It assumes two parallel conduction paths: one along the surface of soil particles and the other through the soil water. The model is

Equation 1

Here σb is the bulk conductivity which is measured by the probe, σs is the bulk surface conductivity, σw is the conductivity of the pore water, θ is the volumetric water content, θs is the saturation water content of the soil and n is an empirical parameter with a suggested value around 0.5. If, for the moment, we ignore surface conductivity, and use eq. 1 to compute the electrical conductivity of a saturated paste (assuming n = 0.5 and θs = 0.5) we obtain σb = 0.35σw. Obviously, if no soil were there, the bulk reading would equal the electrical conductivity of the water. But when soil is there, the bulk conductivity is about a third of the solution conductivity. This happens because soil particles take up some of the space, decreasing the cross section for ion flow and increasing the distance ions must travel (around particles) to move from one electrode of the probe to the other. In unsaturated soil these same concepts apply, but here both soil particles and empty pores interfere with ion transport, so the bulk conductivity becomes an even smaller fraction of pore water conductivity.

Electrical conductivity

When water evaporates at the soil surface, or from leaves, it is pure, containing no salt, so evapotranspiration concentrates the salts in the soil.

Our interest, of course, is in the pore water conductivity. Inverting eq. 1 we obtain

Equation 2

In order to know pore water conductivity from measurements in the soil we must also know the soil water content, the saturation water content, and the surface conductivity. The 5TE measures the water content. The saturation water content can be computed from the bulk density of the soil

Electrical conductivity

Equation 3

Where ρb is the soil bulk density and ρs is the density of the solid particles, which in mineral soils is taken to be around 2.65 Mg/m3 . The surface conductivity is assumed to be zero for coarse textured soil. Therefore, using the 5TE allows us to quantify pore water EC through the use of the above assumptions. This knowledge has the potential to be a very useful tool in fertilizer scheduling.

Electrical Conductivity is Temperature Dependent

Electrical conductivity of solutions or soils changes by about 2% per Celsius degree. Because of this, measurements must be corrected for temperature in order to be useful. Richards (1954) provides a table for correcting the readings taken at any temperature to readings at 25 °C. The following polynomial summarizes the table

where t is the Celsius temperature. This equation is programmed into the 5TE, so temperature corrections are automatic.

Electrical conductivity

Soil salinity has been measured using electrical conductivity for more than 100 years.

Units of Electrical Conductivity

The SI unit for electrical conductance is the Siemen, so electrical conductivity has units of S/m. Units used in older literature are mho/cm (mho is reciprocal ohm), which have the same value as S/cm. Soil electrical conductivities were typically reported in mmho/cm so 1 mmho/cm equals 1 mS/cm. Since SI discourages the use of submultiples in the denominator, this unit is changed to deciSiemen per meter (dS/m), which is numerically the same as mmho/cm or mS/cm. Occasionally, EC is reported as mS/m or µS/m. 1 dS/m is 100 mS/m or 105 µS/m.

References

Richards, L. A. (Ed.) 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Agriculture Handbook 60, Washington D. C.

Rhoades, J. D. and J. Loveday. 1990. Salinity in irrigated agriculture. In Irrigation of Agricultural Crops. Agronomy Monograph 30:1089-1142. Americal Society of Agronomy, Madison, WI.

Get more information on applied environmental research in our

 

Electrical Conductivity of Soil as a Predictor of Plant Response

Plants require nutrients to grow, and if we fail to supply the proper nutrients in the proper concentrations, plant function is affected. Fertilizer in too high concentration can also affect plant function, and sometimes is fatal.

electrical conductivity

Plant function is affected by nutrient concentration.

Most of us have had the experience of fertilizing some part of a lawn too heavily, perhaps by accident, and killing grass in that part of the lawn. Generally, it isn’t the nutrients themselves that cause the damage, it is their effect on the water. Salt in the water reduces its water potential making it less available to the plant. The salt therefore causes water stress in the plant.

Salt in soil comes from the fertilizer we apply, but also from irrigation water and dissolving soil minerals. Relatively small amounts are removed with the plants that are harvested, but most leaches with the water out of the bottom of the soil profile. When water evaporates at the soil surface, or from leaves, it is pure, containing no salt, so evapotranspiration concentrates the salts in the soil. If more salt is applied in the irrigation water than is leached or taken off in harvested plants, the soil becomes more saline and eventually will cease to support agricultural production. Thousands of acres have been lost from production in this way, and production has been drastically reduced on tens of thousands of additional acres.

electrical conductivity

Thousands of acres have been lost from over-fertilization.

Soil Salinity and Electrical Conductivity

Soil salinity has been measured using electrical conductivity for more than 100 years. It is common knowledge that salty water conducts electricity. Whitney and Means (1897) made use of that fact to measure the concentration of salt in soil. Early methods made measurements directly on a soil paste, but the influence of the soil in the paste on the measurement was not fully understood until recently, leading to uncertainty in the measurements. By about 1940 the accepted method for determining soil salinity was to make a saturated paste by a specified procedure, extract solution from the paste, and measure the electrical conductivity of the solution (Richards, 1954). The measurement is referred to as the electrical conductivity of the saturation extract. These values were then correlated with crop response.

Richards (1954) defined 4 soil salinity classes, as shown in Table 1. Crops suitable for these classes are also listed by Richards, but a much more extensive list is given by Rhoades and Lovejoy (1990). For example, bean is listed as a sensitive crop. It can only be grown without yield damage in soils with EC below 2 dS/m. Barley is a tolerant crop. It can be grown without much yield reduction in any soil up to EC of 16 dS/m.

Table 1: Salinity classes for soils

Two other columns are shown in the table. The “salt in soil” column shows how much salt is required to salinize a soil. In terms of the total soil mass, only a small percentage change is needed to make a big difference in salinity, but this would still represent a large addition of fertilizer. A 200 kg/ha addition of fertilizer would represent a fairly high rate. If this were incorporated into the top 15 cm of soil, it would represent

electrical conductivity

This wouldn’t cause much change in soil salt percentage.

The other column shows osmotic potential of the saturation extract. To give some reference for this number, remember that the nominal permanent wilt water potential of soil is -1500 kPa. Osmotic potentials of plant leaves vary widely depending on species, but -1500 kPa is a kind of median value. The values in the table may seem small compared to the permanent wilt (PW) value, but remember that these are values at saturation. When a soil is saturated, water quickly drains to a “drained upper limit” (UL) water content which is around half the saturation value. The useful water storage of the soil is between the UL and the PW or lower limit water content, which, again, is about half the UL. The concentration of salts at the UL is about the same as at saturation because the water drained away, but the water loss between the UL and PW is typically by evapotranspiration, so little or no salts are lost. The concentration at the lower limit is therefore twice that shown in Table 1, which is significant compared to the permanent wilt water potential. Likewise the osmotic potential of the soil solution after fertilizing with 200 kg/ka and mixing wouldn’t change much, but the same amount of fertilizer concentrated in a band near seed would have a much larger effect.

Next Week: Read part 2 of Electrical Conductivity as a Predictor of Soil Response.

Get more information on applied environmental research in our

 

Avocado Growers in Kenya Fight Drought with Recycled Water Bottle Irrigation (Part 2)

Dr. Brent Clothier, Dr. Steve Green, Roberta Gentile and their research team from Plant and Food Research in New Zealand are working in Kenya to alleviate the poverty of the many small-holder farmers who grow avocados in the Central Highlands of Kenya (see part 1). This week, read about an inexpensive irrigation solution for these farmers and how the researchers are developing a plan to manage nutrients.

avocado

The period of water stress in October is at the time of main flowering.

Recycled Water Bottles Provide a Solution

When the team was visited Kenya in early March, the Long Rains had not arrived, and the trees were under water stress. The researchers sought to reduce the impact of drought by using a prototype of a portable drip-irrigation system they developed. They used ‘old’ 20 liter drinking water bottles to deliver water to the trees at 4 L/hr.

avocado

20 L water bottles used for tree irrigation.

The bottles can be refilled and moved from tree to tree. By measuring water content in the soil, the team found that the 20 L of drip irrigated water lasted in the soil about 2 days. When the period was increased to 4 days, the root water uptake was reduced over days 3 and 4 after wetting. Thus they recommended the bottle be recharged and reapplied every two days. This enables the bottle to be used on another tree on the intervening day and should help the farmers to reduce the worst impacts of the drought while waiting for the Long Rains to arrive.

avocado

Refilling the water bottles.

Replacing Low Soil Nutrients

In another phase of the experiment, Dr. Clothier’s team surveyed soil and plant nutrient contents in the main avocado production regions to assess the current fertility status of the farms. Soils in this region are classified as Nitisols, deep red soils with a nut-shaped structure and high iron content (Jones et al. 2013). These soils have low levels of organic matter and low pH. Soil sampling revealed a decrease in pH and increase in organic matter with altitude in the Kandara valley. This observed gradient is likely attributable to the higher amounts rainfall received in the higher altitudes of the valley, which can increase organic matter production and leach base cations from the soil. Soil and leaf nutrient analyses of the monitoring farms showed similar trends in nutrient availability. There are also low levels of the macronutrients nitrogen and phosphorus and the micronutrient boron in these soils. These nutrients are essential for avocado growth and production. One challenge to improve avocado productivity is finding ways to improve soil nutrient availability and tree nutrition.

avocado

An example of the benefits of a secure revenue-stream: One farmer purchased a new cow, which enables him to meet the nutrient requirements of more avocado trees.

A Plan for Managing Nutrients

The majority of the small-holder farms supplying avocados to Olivado use organic production methods. This means organic amendments such as plant residues, composts and animal manures are required to replenish the nutrients that are exported from the farms and improve soil fertility. Livestock have the potential to provide nutrient amendments for a considerable number of avocado trees. Even better, the input of organic materials will build-up soil organic matter levels, which benefit soil conservation, water holding capacity, pH buffering, and soil biological activity.

The researchers are developing simple nutrient budgets for these avocado trees using yield and fruit nutrient concentration data to assess the quantity of nutrients being exported off-farm in the harvested crop. Using the nutrient concentrations of locally available organic amendments, they will provide recommendations on the amount of organic material needed to sustain soil fertility.

Nutrient balances will be incorporated into a decision support tool to assist small-holder farmers in enhancing their soil and plant nutrition. These budgets will be enhanced by further characterizing the nutrient composition and quantities of available organic matter amendments in the region. The researchers are working to improve these nutrient budget estimates with data specific to the avocado farms in the region. They will also set up demonstration farms to evaluate the production responses to recommended nutrient management practices.

To find out more about Kenyan avocado research contact Brent Clothier: brent.clothier@plantandfood.co.nz .

(This article is a summary/compilation of several articles first printed in WISPAS newsletter)

References:

Jones, A., Breuning-Madsen, H., Brossard, M., Dampha, A., Deckers, J., Dewitte, O., Gallali, T., Hallett, S., Jones, R., Kilasara, M., Le Roux, P., Micheli, E., Montanarella, L., Spaargaren, O., Thiombiano, L., Van Ranst, E., Yemefack, M., Zougmore, R., (eds.) 2013. Soil Atlas of Africa. European Commission, Publications Office of the European Union, Luxembourg. 176 pp.

Get more information on applied environmental research in our

 

Avocado Growers in Kenya Fight Drought with Recycled Water Bottle Irrigation

Dr. Brent Clothier, Dr. Steve Green, Roberta Gentile and their research team from Plant and Food Research in New Zealand are working in Kenya to alleviate the poverty of the many small-holder farmers who grow avocados in the Central Highlands of Kenya. These farmers have old and very large avocado trees. The fruit from these trees are purchased by the company Olivado EPZ who presses over 1300 small-holders’ avocados for oil. Dr. Clothier and his team are investigating how to increase the productivity of the farmers’ avocado trees and increase the quality of the fruit so they yield more oil.

avocado

Small-holder farmers grow avocados in the Central Highlands of Kenya.

Reducing Leaf Area to Avoid Water Stress

Because of the age and size of these trees, harvesting of the avocados is difficult and time consuming, and through dropped fruit, the quality of the avocados can be comprised. In addition, any dry season water-stress negatively impacts fruit filling. The research team performed some initial remedial pruning of these trees to develop a more manageable and productive tree form. They sought to assess whether the reduced leaf area would enable the trees to avoid water stress during the dry season of January through March between the short and long rainy seasons. They removed 30-40% of the central limbs of the avocado tree to create a more open canopy form.

The team instrumented two trees with heat-pulse sap-flow probes. One tree was left unpruned and the tree in the photo above was pruned. The tree that was pruned was using between 300-400 liters per day, as expected for a tree of that large size. The unpruned tree was smaller in size, and it was using between 150-250 liters per day during May and June. The selective limb pruning resulted in the rate of water-use dropping to 200-300 liters per day, a drop of 100 liters per day.

avocado

The more open canopy form of the pruned avocado tree.

Determining Tree Water Use During Rainy and Dry Seasons

The team also measured the water-use of four trees of different sizes during the entire season using the compensation heat-pulse method and soil water content. They found the trees’ water-use doubled with the arrival of the Short Rains and then began to decline in early January after the rains ended. The trees were under a degree of water stress prior to the arrival of the (short) Short Rains, and as the weak Short Rains ended early, the trees again went into water stress with only occasional respite due to isolated rainstorms in January and February.

This pattern of water stress presents a challenge for sustaining high levels of avocado production. The period of water stress in October is at the time of main flowering, and researchers who were there noted a carpet of aborted flowers on the orchard floor. They also noticed that the fruit were smaller at one farm than those higher up in the Central Highlands where rainfall is higher and more frequent. Thus, to improve production it is imperative to mitigate the impacts of drought, and this needs to be done without reference to any infrastructure for irrigation.

Next week: Read about an inexpensive irrigation solution for these farmers and how the researchers are developing a plan to manage nutrients.

(This article is a summary/compilation of several articles first printed in WISPAS newsletter)

Get more information on applied environmental research in our

 

Soil Moisture Sensors: Why TDR VS. Capacitance May Be Missing the Point (Part 2)

Dr. Colin S. Campbell discusses whether TDR vs. capacitance (see part 1) is the right question, the challenges facing soil moisture sensor technology, and the correct questions to ask before investing in a sensor system.

It’s easy to overlook the obvious question: what is being measured?

What are You Trying to Measure?

When considering which soil water content sensor will work best for any application, it’s easy to overlook the obvious question: what is being measured?  Time Domain Reflectometry (TDR) vs. capacitance is the right question for a researcher who is looking at the dielectric permittivity across a wide measurement frequency spectrum (called dielectric spectroscopy). There is important information in these data, like the ability to measure bulk density along with water content and electrical conductivity. If this is the desired measurement, currently only one technology will do: TDR. The reflectance of the electrical pulse that moves down the conducting rods contains a wide range of frequencies.  When digitized, these frequencies can be separated by fast fourier transform and analyzed for additional information.

The objective for the majority of scientists, however, is to simply monitor soil water content instantaneously or over time, with good accuracy. There are more options if this is the goal, yet there are still pitfalls to consider.

capacitance

Considerable research has been devoted to determining which soil moisture sensors meet expectation.

Each Technology Has Challenges

Why would a scientist pay $100+ for a soil volumetric water content (VWC) sensor, when there are hundreds of soil moisture sensors online costing between $5 and $15? This is where knowing HOW water content is measured by a sensor is critical.

Most sensors on home and garden websites work based on electrical resistivity or conductivity. The principle is simple: more water will allow more electrons to flow. So conductivity will change with soil water content. But, while it’s possible to determine whether water content has changed with this method, absolute calibration is impossible to achieve as salts in the soil water will change as the water content changes. A careful reading of sensor specs will sometimes uncover the measurement method, but sometimes, price is the only indication.

Somewhere between dielectric spectroscopy and electrical resistance are the sensors that provide simple, accurate water content measurement. Considerable research has been devoted to determining which of these meet expectation, and the results suggest that Campbell Scientific, Delta-T, Stevens, Acclima, Sentek, and METER (formerly Decagon Devices), provide accurate sensors vetted by soil scientists. The real challenge is installing the sensors correctly and connecting them to a system that meets data-collection and analysis needs.

Installation Techniques Affect Accuracy

Studies show there is a difference between mid-priced sensor accuracy when tested in laboratory conditions. But, in the field, sensor accuracy is shown to be similar for all good quality probes, and all sensors benefit from site specific soil calibration. Why? The reason is associated with the principle upon which they function. The electromagnetic field these sensors produce falls off exponentially with distance from the sensor surface because the majority of the field is near the electrodes. So, in the lab, where test solutions form easily around sensor rods, there are differences in probe performance.  In a natural medium like soil, air gaps, rocks, and other detritus reduce the electrode-to-soil contact and tend to reduce sensor to sensor differences. Thus, picking an accurate sensor is important, but a high quality installation is even more critical.

capacitance

Improper installation is the largest barrier to accuracy.

Which Capacitance Sensor Works Best?

Sensor choice should be based on how sensors will be installed, the nature of the research site, and the intended collection method. Some researchers prefer a profile sensor, which allows instruments to be placed at multiple depths in a single hole. This may facilitate fast installation, but air gaps in the auger pilot hole can occur, especially in rocky soils. Fixing this problem requires filling the hole with a slurry, resulting in disturbed soil measurements. Still, profile sensor installation must be evaluated against the typical method of digging a pit and installing sensors into a side-wall. This method is time consuming and makes it more difficult to retrieve sensors.

New technology that allows sensor installation in the side of a 10 cm borehole may give the best of both worlds, but still requires backfill and has the challenge of probe removal at the end of the experiment.

The research site must also be a consideration. If the installation is close to main power or easily reached with batteries and solar panels, your options are open: all sensors will work. But, if the site is remote, picking a sensor and logging system with low power requirements will save time hauling in solar panels or the frustration of data loggers running out of batteries.

capacitance

Often times it comes down to convenience.

Data Loggers Can Be a Limitation

Many manufacturers design data loggers that only connect to the sensors they make. This can cause problems if the logging system doesn’t meet site needs. All manufacturers mentioned above have sensors that will connect to general data loggers such as Campbell Scientific’s CR series. It often comes down to convenience: the types of sensor needed to monitor a site, the resources needed to collect and analyze the data, and site maintenance. Cost is an issue too, as sensors range from $100 to more than $3000.

Successfully Measure Water Content

The challenge of setting up and monitoring soil water content is not trivial, with many choices and little explanation of how each type of sensor will affect the final results. There are a wealth of papers that review the critical performance aspects of all the sensors discussed, and we encourage you to read them. But, if soil water content is the goal, using one of the sensors from the manufacturers named above, a careful installation, and a soil-specific calibration, will ensure a successful, accurate water content measurement.

For an in-depth comparison of TDR versus capacitance technology, read: Dielectric Probes Vs. Time Domain Reflectometers

For an understanding of how capacitance sensors compare to other major contemporary sensor technologies, watch our Soil Moisture 201 webinar.

Improved Methods Save Money in Future Borehole Thermal Energy Storage Design

Globally, the gap between the energy production and consumption is growing wider. To promote sustainability, University of California San Diego PhD candidate and ASCE GI Sustainability in Geotechnical Engineering committee member, Tugce Baser, Dr. John McCartney, Associate Professor, and their research team, Dr. Ning Lu, Professor at Colorado School of Mines and Dr. Yi Dong, Postdoctoral Researcher at Colorado School of Mines, are working on improving methods for borehole thermal energy storage (BTES), a system which stores solar heat in the soil during the summer months for reuse in homes during the winter. Baser says, “We are running out of finite energy resources. We need to come up with new strategies to use free and renewable energy resources such as solar energy for a sustainable future.”

borehole thermal energy storage

Baser’s BTES design.

How it works

BTES systems are an approach to provide efficient renewable resource-based thermal energy to heat buildings. They are configured to store thermal energy collected from solar thermal panels during the summer and discharge the heat to buildings during the winter. They function by circulating a fluid within a closed-loop pipe network installed in vertical boreholes to inject heat collected from solar thermal panels. During winter, cold fluid is circulated through the heat exchangers to recover the heat from the subsurface and distribute it to the buildings. Baser explains, “The subsurface provides an excellent medium to store this heat due to the relatively lower thermal conductivity and lower specific heat capacity especially when the soil layer is in the vadose zone. Lower thermal properties allow us to concentrate the heat in a specific array and the heat losses to the environment are potentially low. These systems typically include an insulation layer and a hydraulic barrier near the ground surface to reduce heat and vapor losses to the atmosphere.”

borehole thermal energy storage

BTES construction.

Why do we need improved methods?

Baser and her team are trying to improve the understanding of heat storage mechanisms and evaluate changes in the rate of heat transfer and heat storage in the vadose zone where the soil is unsaturated. The goal of the project is improve conventional methods by generating models to fit different soil types and situations.  She says, “The European community introduced us to the borehole thermal energy storage systems to provide heat specifically for domestic use, but there is still a chance for us to design them more efficiently by having a full understanding of the thermal response of these systems that is specific to the ground material and subsurface conditions. The primary objective of this research is to understand the mechanisms of coupled heat transfer and water flow in unsaturated soil profiles during the heat injection and subsequent heat extraction into these different arrays and different dimensions of borehole heat exchangers.”

borehole thermal energy storage

Solar panels.

Baser and her team working on designing numerical models based on finite element method which improve some of the numerical models in the literature used to characterize the thermal response of the systems. The new models add new considerations, such as the heat pipe effect in different soil types. Baser explains, “Because thermal and hydraulic properties of soils are highly coupled and are specific to soils, the thermal response of a BTES system will be different when it is installed in different types of soils. For example, you see the heat pipe effect where there is evaporation and subsequent condensation in fine grained soils rather than coarse soils because in coarse grain soils the pore characteristics are different. The duration of the heat pipe effect (or convective cycle) is longer in fine grain soils. We conclude that considering coupled heat transfer and water flow in the thermal response of Borehole Thermal Energy Storage system is important.”

borehole thermal energy storage

In-ground heat exchanger

Experiments in the field and in the lab help verify the new models

To fully understand heat transfer mechanisms and water flow in unsaturated soils, the research team installed two different SBTS systems at different scales, one in Golden, Colorado School of Mines campus, and the other at the UC San Diego research campus.  Baser says, “The subsurface characteristics of both sites are different, and this gives us the opportunity to investigate the impact of the different soil layers on the thermal response experimentally in a full scale. In addition, the scales of each Borehole Thermal Energy Storage system are different, and we also apply different heat injection rates. We have used these data to further validate our coupled heat transfer and water flow model so that we can use it for design purposes.”

borehole thermal energy storage

Soil moisture sensor locations.

Baser started with laboratory heating experiments, in which soil in a large tank is heated by heat exchangers. She installed soil moisture sensors to measure volumetric water content and the temperature and then used the KD2 pro thermal property analyzer to monitor thermal properties during heating experiments to characterize the coupled thermo-hydraulic relationships. For the field experiments the team uses soil moisture sensors equipped with temperature sensors and the KD2 pro to monitor subsurface temperature fluctuation because during the summertime the air temperature is higher, thus ambient air temperature fluctuation and penetration may become significant.

Baser also uses thermistor strings that include six thermistors at different depths and thermistor pipe plugs, voltage input modules, and flow meters.  She says, “Thermistor pipe plugs and flow meters are used in the manifold to monitor the inlet and outlet fluid temperatures and flow rates in each loop to calculate heat transfer rate into the ground. Flow meters were installed to control flow in each loop because you don’t want to over or underload the borehole loops. The amount of energy that you collect from the solar loop and the amount of energy that you inject into the ground can be used to define the efficiency of the system.” Baser says thermistor strings help monitor the ground temperature during the summer heat loading at different depths. They’re also used to monitor borehole wall temperature over time. The team installed one thermistor string 9 meters away from the heat storage array to see if far field is affected by the heat transfer within the array.

borehole thermal energy storage

Insulation prevents heat loss to the environment.

The new models will save money in future Borehole Thermal Energy Storage design

Baser says building numerical models and solving them was very complicated and time consuming, but they’ve had good results. She explains, “We’ve recently proved, both experimentally and numerically, that considering coupled thermal and hydraulic relationships are very important for thermal response analysis. Thus, our recommendation is that it’s fine to use the analytical models and user-friendly numerical models that consider constant thermal properties in the design analyses for saturated soils. However, in unsaturated soils, there is a very high possibility that the contribution of heat transfer evaporation and condensation would be missing and the Borehole Thermal Energy Storage system would be oversized, costing a significant amount of money. When dealing with soils in the vadose zone, coupled thermo-hydraulic constitutive relationships in the modeling efforts need to be considered.”

You can learn more about Tugce Baser’s research here.

Get more information on applied environmental research in our

 

%d bloggers like this: