GLOBE Soil Moisture Hierarchal FAQ Sheet

1. What are soils? Learn more …

2. Why are soils important? Learn more …

3. What is the GLOBE program, and what does it have to do with soil moisture? Learn more …

3.1. How to I find out more about the GLOBE program? Learn more …

3.2. Where do I find out more information about GLOBE protocols and activities? Learn more …

3.3. How do I get involved in GLOBE? Learn more …

4. What is soil moisture? Learn more …

1. What are soils?

Soils are made of macrorganisms, microorganisms, minerals, organic matter, air and other gases, and water.

2. Why are soils important?

Soils provide the minerals and nutrients necessary to sustain plant and animal life. Soils are necessary to grow the food we eat. Additionally, soils are also important for producing and storing gases such as carbon dioxide which regulate Earth’s climate. Soils are also important for decomposing wastes and filtering water.

3. What is the GLOBE program, and what does it have to do with soil moisture?

The Global Learning and Observations to Benefit the Environment (GLOBE) program is a worldwide network of students, teachers and scientists working together to study and understand the global environment. GLOBE improves student understanding of science via student participation in scientific activities by taking simple measurements and analyzing data. GLOBE students from 105 countries around the world contribute to data collection for scientific research while helping teachers and students achieve state and local educational goals and standards. One of the many GLOBE areas of emphasis is the study and data collection of soil moisture.

3.1. How to I find out more about the GLOBE program?

To find out more about the GLOBE program, visit http://www.globe.gov.

3.2. Where do I find out more information about GLOBE protocols and activities?

To find out more about GLOBE sampling and data reporting protocols, visit http://www.globe.gov and click on “Protocols” under the heading “Education and Science.”

3.3. How do I get involved in GLOBE?

There are many different ways to get involved in GLOBE. While it is typically necessary for teachers and other educators who wish to lead students in GLOBE to attend special workshops, this is not the case with the GLOBE Soil Moisture Campaign (SMC). The SMC is currently the only project that does not require GLOBE workshop certification! To find out more about how to get involved in GLOBE, visit http://www.globe.gov and click on “How to Join GLOBE”. To learn more about how to participate in the GLOBE SMC, visit http://www.hwr.arizona.edu/globe/sci/SM/SMC/

4. What is soil moisture?

Soils adsorb water from both rainfall and snowmelt and hold the water on soil particles surfaces and in between granular particles. This water held within soils which is not part of the minerals that compose the soil, is termed soil moisture.

4.1. How do scientists quantify soil moisture?

One way that scientists quantify soil moisture is by using the ratio of the dry soil mass of a particular volume of soil to the wet soil mass of the same of volume. This is called the gravimetric soil moisture, and the measurement units are grams per grams, or unitless.

4.1.1. What is the typical range of soil moisture values on planet Earth?

Average monthly soil moisture values range from about 0.05 to 0.40.

4.1.1.1.Where are soils the wettest on planet Earth?

The wettest soils on planet Earth are found in areas that receive the most rainfall, and in areas were the ground is frozen for most or part of the year.

4.1.1.2.Where are soils the driest on planet Earth?

The driest soils on planet Earth are in deserts, most of which are located between 30° N and 30° S latitude where the energy from the sun is the strongest. Deserts are areas in which the evaporation rate of water from soils is high because of elevated day-time temperatures, and where precipitation is low. The largest desert in the world is the Sahara desert in northern Africa which covers almost 3.5 millions square miles. Other major deserts with very low soil moisture include the Arabian desert located mostly in Saudi Arabia, the Gobi desert in central Asia, and the Great Sandy and Great Victoria deserts in Australia, and the Namibi desert located in South Africa and Angola. The Atacama, the driest place on earth, is a coastal desert in Chile. In the United States, there are four desert regions: the Chihuahuan, Great Basin, Mojave and Sonoran.

4.2. What variables affect the soil moisture of a particular location?

In addition to the time since the last precipitation event, soil moisture at a particular site is affected by the slope of the ground surface, the vegetative canopy cover, water retention properties of the soil, and sun exposure.

4.3. How do soil moisture values change throughout a year?

Soil moisture values are highest during the rainy season, and furthermore when the temperature is lower which results in reduced evaporation rates. This period varies throughout the year depending what climate and which hemisphere you live in. For example, in the American Southwest the majority of the precipitation occurs during two periods, during the winter and the summer monsoon season. Soil moisture values are highest during the winter when evaporation rates are low. In an average year, after the winter rainy season, soils slowly dry out until the summer monsoon season when the soil begins to wet up again.

4.3.1. How does soil moisture change when it rains or snows? Do soil moisture values depend on recent weather?

In most places soil moisture will increase following a rain or snowfall events because soils will absorb the water from rain and snowmelt. After the precipitation event, the soils will slowly dry out and lose soil moisture until the next precipitation event. The only circumstance in which soil moisture is not affected by precipitation is where the soil is already saturated with water prior to a precipitation event, or where the ground is frozen. In these cases, water ponds and trickles off the land surface and soil moisture does not increase because it was already at 100% saturation, the maximum possible soil moisture value.

5. What do we learn about the Earth from soil moisture?

By looking at soil moisture values we learn about a specific part of the hydrologic cycle. Studying soil moisture helps us to better understand the distribution of plant and animal life on Earth. Changes in soil moisture also help scientists to develop better weather forecasts and to better predict climate change. Soil moisture is also important for flood aversion. For example, agricultural lands adjacent to rivers that periodically flood can be used as storage areas for the flood waters. To learn more about flood aversion and soil moisture visit http://www.dartmouth.edu/~floods/.

6. How do we collect soil samples for soil moisture measurement?

Using a digging tool such as a garden trowel, collect two samples at two different depths: from the ground surface to 5 cm depth, and from 8 to 12 cm depth. For each sample, collect enough soil to fill half of an 8 ounce cup. Place each sample into a foil pouch or a tin can with a snug-fitting lid and then seal your sample in a zip-loc-type plastic bag to prevent evaporative moisture loss. Each sample needs a unique identification label that includes the location, date and time the sample was taken. By noting adjacent roads, or approximate distances to adjacent roads, the location can be converted to latitude and longitude from maps.

6.1. Where are the best places to take soil moisture samples (and why)?

Pick a flat uniform area about 90 meters by 90 meters that is not shaded by a vegetation canopy and that is representative of the larger landscape. Avoid irrigated areas like lawns and roadside ditches which tend to be wetter than the surrounding landscape.

6.2. What kinds of containers are effective for storage and transportation of soil moisture samples (and why)?

For an inexpensive approach, we recommend collecting soil moisture samples in heavy-duty aluminum foil packets. Place the sealed aluminum foil packet in a zip-loc-type plastic bag to minimize evaporative moisture loss. Label each packet - both the foil and the plastic bag - with the date, time, and location of the sample. For more instruction on how to build a foil packet visit http://www.hwr.arizona.edu/globe/sci/SM/SMC/AGIfoilpouch.pdf.

7. How do we measure soil moisture?

Soil moisture is measured by taking the mass of a soil sample both after drying the soil sample, and with the moisture when the sample was collected. The ratio of the mass of the dry to wet sample is called the gravimetric soil moisture. Soils can be dried in many ways, as long as the soil samples are not dried at too high a temperature, or for too long, in which cases water molecules which are part of the clay found in the soil matrix can be burned off, resulting in inaccurate soil mass measurements.

7.1. What is the standard procedure for measuring soil water content? If we want to measure soil moisture, why are we drying the soil samples?

In order to measure soil moisture we need to know the mass of the soil both when it is wet and dry. Soil moisture is typically measured by comparing the mass of the dry soil with the mass of the wet soil. The difference in mass before and after drying is the mass of soil water that was present in the soil sample at the time of data collection. The ratio of the mass of water to the mass of dry is known as the soil water content. This method is called the gravimetric method, which is the standard scientific procedure for measuring soil water content.

7.2. How do we dry the soil samples?

Many different methods can be used to dry soils. The conventional method for drying soil samples is to heat the samples to 100-110˚Celsius in a laboratory oven for a 24-hour period.

7.2.1. Can we dry soil samples in a microwave oven?

In the microwave oven, soil samples can be dried at 30-second intervals to avoid overheating. If the samples are overheated, the water molecules in clay particles in the soil will be evaporated. In this case, the soil moisture measurement would be inaccurate. We recommend that if you choose to dry soil samples using a microwave, that you avoid using a microwave that is used regularly for food because microwaved soil samples tend to emit strong organic odors that are not compatible with most food scents.

7.2.2. Can we dry soil samples in a kiln commonly used for ceramics?

It is possible to dry soil samples in a ceramics kiln by running two 12-hour drying cycles at a low firing temperature (around 105˚Celsius). However, we recommend that you weigh a few soil samples after the second 12-hr cycle, and then let all of the samples dry for another hour or so. Then weigh the same few samples again. If one or more of the samples have lost more than 0.1 g of water, then continue to dry all of the samples for another 12-hour cycle. Continue this process until the samples stop losing moisture.

7.2.3. How do I build a 55-gallon drum light-bulb-powered oven?

Another way to dry soil samples is to use a light bulb heat source within an insulated metal drum. Detailed instructions for building a 55-gallon drum light-bulb-powered oven can be found at http://www.hwr.arizona.edu/globe/sci/SM/SMC/OvenHowto3pg.pdf.

7.3. How do satellites measure soil moisture?

Satellites measure how different frequencies of energy are reflected from the surface of the earth. Differences in reflected energy from the Earth’s surface can be related to soil moisture. Four main factors that influence the soil reflectance are mineral composition, soil moisture, organic matter and soil texture. Satellites measure the radiation or microwave energy at the surface of the earth in units of brightness, temperature and power. Satellite-measured soil moisture reflects an average soil moisture value for the top 5 to 10 cm only. An important part of interpreting soil moisture measurements from satellites is “ground-truthing” or comparing satellite determined soil moisture to measurements collected by hand in the field, which is why collecting soil moisture samples for the GLOBE program is a very valuable scientific effort.