• Atmosphere
  • Air Temperature
    • The evaporation of water within the soil is largely driven by the local air temperature. This evaporation can play a significant role in cloud formation, air temperature, and relative humidity.
  • Precipitation
    • The soil's moisture increases after it rains and then gradually dries out from evaporation and transpiration. The amount and frequency of rain influences how stable the soil is and what plant life can proliferate there.
  • Relative Humidity
    • The relative humidity of the air influences evaporation rates within the soil as well as the soil's moisture in a particular area.
• Hydrosphere
  • Water pH
    • The soil's pH affects the pH of surrounding bodies of water. As with soil pH, water pH is a key factor for determining the types of life than can live there. Additionally, pH influences most chemical and biological processes.
  • Water Nitrates
    • The nitrate concentration in water is affected by the nitrogen content in surrounding soils and land cover. Excess nitrogen in water can lead to eutrophication in lakes and estuaries.
• Pedosphere (Soil)
  • Soil Bulk Density
    • Soil compaction or aeration will directly affect how dense the soil is. Overly compacted soils will limit plant and organism growth, while heavily aerated soils are less structurally sound and more prone to erosion.
  • Soil Fertility
    • Soil health or the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans has a strong influence on land cover and plant growth.
  • Soil Infiltration
    • How soils dry at different depths depends on the properties of the soil like composition, compaction, and moisture content. These factors, among others, determine how much water will percolate to lower soil levels.
  • Soil Moisture (Gravimetric)
    • Moist or arid soil is a determining factor for the types of plants that can grow in a given area, the occurrence of floods or droughts, and predicting future weather patterns.
  • Soil Temperature
    • The temperature of the soil heavily impacts the types of micro- and macro-organisms that can survive in the subterranean environment.
  • Soil pH
    • The soil's pH is a vital factor for the feasibility of plant life. It also influences the pH of nearby water bodies by modifying the chemical composition of runoff or drainage.
• Biosphere
  • Land Cover
    • Land cover influences soil properties like compaction, texture, fertility, organic material, which influence soil moisture retention.

Generally, scientists want to understand how water cycles through the local or regional environment. For example, they want to understand how precipitation and melting snow relate to increases in the water levels of streams, rivers, and lakes. To better understand these processes, we can take soil moisture measurements. When these measurements are gathered for an entire soil profile, they can be used to predict floods, droughts, or the optimal timing for crop irrigation. Scientists can also combine this data with soil temperature, relative humidity, and land cover data to estimate the rate at which water is returned to the atmosphere through evaporation and transpiration.

Soil moisture is also a major data factor studied by Phenology scientists. These scientists look at the effect of soil moisture on the annual cycles of plants, such as trees and annual grasses. For example, in some forested regions, tree growth begins in the spring when the soil becomes moist and then stops during the summer when the soil becomes dry. By studying how soil moisture changes over time, researchers can begin to draw conclusions about how the climate has changed over the years.

Soils can also affect the amount and characteristics of runoff into surrounding water bodies. For instance, runoff influences water pH and nitrogen concentration which, in turn, impacts the ecosystem in the water body. By monitoring water quality and soil characteristics, along with land cover and atmosphere measurements, we can develop ways to lessen pollutants entering streams and improve aquatic ecosystems.

We can also examine differences in regional or spatial patterns of soil moisture changes. Because soil moisture is a function of precipitation, soil texture, infiltration rate, and local weather conditions, researchers focus in on larger patterns rather than the absolute values of the measurements. This effort is greatly enhanced when we're able to measure soil water content over large areas, with the ultimate goal of using remote-sensing data from satellites to reach global scales. The Soil Moisture Active Passive (SMAP) satellite mission is one such project designed to meet these goals.

However, in order to verify and calibrate these satellite measurements, soil moisture data must also be gathered from observers on the ground. This is where GLOBE students, teachers, and citizen scientists play an important role in this exciting scientific endeavor. By contributing to GLOBE’s soil moisture campaign, you're helping a global effort to better understand our Earth systems.

Learn more about the SMAP mission

Student Questions

Before taking measurements out in your local community, it's beneficial to first focus your approach by creating research questions. A few examples of these questions include:

• How do high and low amounts of soil moisture influence air temperature and relative humidity?
• After a given amount of rain, how long is the dry-out period of the soil and how does this dry-out period differ for different soils?
• How does high and low bulk density influence soil moisture and infiltration?
• What is the difference in plant growth between healthy and a non-healthy soil?
• What kinds of plants do you find in undisturbed soils vs. disturbed soils?
• How does soil moisture vary for different land cover types?

To get an idea on how all these different environmental variables are incorporated into the soils bundle, check out the following case study trying to answer the question:

How does soil moisture vary in three different regions?

Introduction

Different soils with the same porosity and the same amount of water present can vary significantly in the value of Soil Water Content, and understanding whether the values measured are reasonable or not is easier if the soil characterization protocols have been done for a horizon.

Soils are expected to show an increase in water content after a rain or during snowmelt, if the soil is not frozen or saturated. Soils gradually dry out during times with little or no precipitation. How the soil dries at different depths depends on the properties of the soil in each horizon. In some cases, water enters the soil from below, when the water table rises. The water content in these soils may be more variable lower in the soil profile than at the surface.

If it rains, some of the rainfall is expected to soak or infiltrate into the ground and increase soil moisture. This infiltration starts happening immediately and can continue for several hours if water continues to be available from a steady rain or puddles. If infiltration continues until all the pore space is filled, then the soil becomes saturated. Most soils drain rapidly, usually within hours or days. The field capacity of a soil is the amount of water a soil will hold without downward drainage or redistribution.

As the ground dries from evaporation and transpiration, soil moisture decreases slowly, with the soils closer to the surface usually drying faster than deeper soils. Soil moisture decreases from field capacity to a water content known as the wilting point, (the point at which the soil holds the water too tightly for plants to take it up). Depending on the soil properties, soil temperature, air temperature, and relative humidity, it may take from days to weeks for the wilting point to be reached. A general picture of how soil water content changes in a single horizon with time is illustrated in Figure 1, however, there are times when the actual data do not follow this pattern.

Figure 1: Example of soil wetting and dry-down period

Moisture content is affected by rainfall variation and soil properties. In a soil profile some horizons retain more water and have a greater porosity than others, affecting the flow of water from one horizon to another.

For example, if a sandy horizon is located above a clayey horizon, water moving through the sandy horizon will enter the clayey horizon very slowly because of the difference between the large pores in the sandy soil and the very small pores in the clayey soil. The small pores act as a tight layer that only lets water move gradually, so that the sandy soil may actually be much wetter at a given time than the clay.

Examining graphs of data collected at three locations will help demonstrate the process to determine whether data are valid or not. The following graphs are used for this demonstration: Valdres, Norway (61.13 N, 8.59 E): Figure SO-GR-2, Stowe, Vermont, USA (44.48 N, 72.708 W): Figure SO-GR-3 and Norfork Arkansas, USA (36.19 N, 92.26 W): Figure SO-GR-4. Each data set includes rainfall, new snow rain equivalent, and soil moisture.

Data Collection Analysis

For the first two schools, the classes chose to take weekly measurements for three months. In this case, the protocol calls for taking measurements during periods when the soil moisture is changing. The students in Valdres, Norway knew from experience that melting winter snow would result in wet soils initially, then drying out gradually as summer approaches. Of course, near-surface soil moisture can also increase during spring rains (as happened on May 28 and later in July).

The students in Stowe, Vermont decided to monitor their soil moisture as it changed from dry summer conditions to wet fall conditions. Again, the near-surface soil moisture appears more variable, drying significantly for a short period early in October 2001. Conversely, the deeper 10 cm soil moisture shows fewer extreme changes.

The class in Norfork, Arkansas decided to take monthly measurements for 12 months to investigate the seasonal cycle of soil moisture in their area. Despite having precipitation off and on throughout the year, the soil moisture shows a gradual dry-down, particularly at the surface. The soil moisture at 10 cm shows less variation for most of the year.

Conclusion

All three of these are interesting data sets. Comparison with precipitation has helped explain some of the variability while applying basic climatic knowledge has helped explain some of the longer-term trends. Knowing the soil characterization properties (texture, bulk and particle density, etc.) helps scientists and students understand more about how water is moving or stored in the soil.

Soils are an essential component for life on Earth. They provide a medium for plant growth, habitat for many insects and other organisms, act as filtration systems for surface water, and serve as significant carbon reservoirs. Additionally, the soil moisture in the the top soil layers is a vital measurement for flood prediction, droughts, plant growth, and weather patterns. When big changes happen to any of the soil's characteristics, it can have significant consequences for all life.

Because soils can vary drastically from place to place, citizen scientists play an important role in piecing together a more thorough picture of our world's soil resources. In an effort to engage more GLOBE students and citizen scientists in the interaction of soils, the atmosphere, and the ecosystem at large, we've compiled a group of relevant GLOBE protocols and other scientific resource into the Soil Protocol Bundle .

Acknowledgements

Compilers:

• Dr. Erika Podest
• Mr. Olawale Oluwafemi (Femi)
• Dr. Rebecca Boger

Editors:

• Dr. Dixon Butler
• Dr. Costantinos Cartalis

Additional thanks to all the members of the GLOBE Science Working Group and Brian Campbell of NASA Wallop for improving the quality of the work.