3.4: Soil Water Measurement and Movement
Learning Objectives
- Become familiar with the driving forces of soil water movement and how soil properties can affect soil water movement.
- Review the key soil water contents and types of water held in soil at different tensions.
- Examine some modern instruments used to measure soil water content and describe how they function.
- Review common calculations for soil water measurement and movement.
In the Soil and Water Relationships lab we developed soil water relationships for a sandy soil and a clayey soil. These soil water relationships are essential for interpreting results from any soil water sensor. Once calibrated, those sensors can be used to quantify plant available water. They can also be used to determine the direction of water flow due to gravity, capillarity, or even osmotic potential. While it’s possible to control an irrigation system from a smart phone, a thorough knowledge of the principles that govern water measurement and movement is essential to effectively employing such technology. In this lab, we will watch a classic video on soil water movement, participate in some hands-on activities related to soil water content, and observe some state of the art soil moisture sensors.
Materials
- Soil Water Measurement and Movement Problem Set
- Computer with internet access and a projector
- Tensiometer with a vacuum gauge or pressure transducer
- WATERMARK 200SS electrical resistance blocks with user manual (Irrometer Company, Inc., Riverside, CA, U.S.)
- EC-5 Small Moisture Sensor with user manual (EC-5 Small Moisture Sensor, Decagon Devices, Pullman, WA, U.S.)
- Hydra Probe Soil Sensor with user manual (Stevens Water Monitoring Systems Inc., Portland, OR, U.S.)
Recommended Reading
- Module 4 – Water and Solute Transport in Soils (McCauley and Jones, 2005b)
Prelab Assignment
Using the recommended reading, the Soil and Water Relationships lab, and the introduction to this lab, consider the questions below. These definitions/questions will provide a concise summary of the major concepts to be addressed in the lab. They are useful study notes for exams.
- Describe why fine-textured soils are suitable for dryland farming while coarse textured soils are not.
- Rank soils with the textures clay loam, loam, and sandy loam from the soil with the highest water tension to the soil with the lowest water tension.
- Define a soil moisture retention curve (also known as a soil water relationship), and describe how it is used.
- Describe why fertilizer placement is important in irrigated systems.
Introduction
Soil water movement is an important process in soil; it controls the amount of water available to plants, how much water can be stored in the soil, and whether the root zone has excess water in. Soil water movement is classified into saturated flow, unsaturated flow, and vapor flow.
Saturated flow occurs at tensions of 0 to -0.3 bar, between saturation and field capacity. The rate of flow depends on hydraulic conductivity, which is controlled by pore size. In general, the coarser the material, the faster the flow rate. The driving force behind saturated flow is hydrostatic head, so the higher the column of water, the more hydrostatic head. This is like diving to the bottom of a pool. The deeper you go, the larger the column of water above you. With enough pressure, your ears can pop.
Unsaturated flow, also known as capillary flow, occurs between 0.3 to 31 bars, between field capacity and air dry. In general, soil water moves through capillary action from areas with the most potential energy, to areas with the least potential energy. Essentially, it moves from wet areas to dry areas. In other words, the drive force is a tension gradient. Hydraulic conductivity controls how fast the water may flow. Under unsaturated conditions, hydraulic conductivity is controlled by tortuosity, or how direct or indirect the path of water flow is. A straight path is the most direct and thus the fastest route, while the least direct path with be the slowest route.
Vapor flow is like capillary flow but is driven by gradients in vapor pressure instead of water tension.
Activity 1: Water Movement in Soils Video
Watch the Water Movement in Soils video (Hsieh et al., 1961) as a class, and then answer the following questions:
What two forces are responsible for the movement of water upward against the downward force of gravity?
When water is first added to the irrigation furrow, the movement of water outward (to the side) is ________ the movement of water downward.
- Less than
- Equal to
- Greater than
When do gravitational forces predominate?
For the demonstration using a fine soil with a layer of coarse material (sand), describe what happens initially when the wetting front reaches the sand.
For that same demonstration, what must occur for the water to pass into the coarse layer?
For the same demonstration, what happens when the saturated layer reaches the sand?
For the demonstration with a coarse soil containing a layer of fine material (such as clay), describe what happens initially when the wetting front reaches the clay.
For that same demonstration, why does a water table form above the clay layer?
What happens when free water is applied directly to a layer of coarse material? What is the driving force behind the rapid movement?
What happens when a layer of coarse material is not in direct contact with free water?
In the comparison of water movement in a sandy loam, loam, and a clay loam, which texture has the deepest penetration of irrigation water? Why?
Why is dryland farming practical on fine-textured soils, but not on coarse-textured soils?
Describe how soluble fertilizers move within the furrow in relation to both the irrigation channel and the crop root system?
Where should tile drains be placed so they carry away excess water?
Activity 2: Soil Water Movement Sponge Demonstration
Using the sponge and tray at your table, squeeze the sponge while it’s submerged under water.
Equating the sponge and the water it holds to soil and soil water, what water content does this sponge have?
- Saturation
- Field capacity
- Wilting point
- Air dry
The water that freely drained away from the once-saturated sponge is considered what type of water?
- Gravitational water
- Plant available water
- Unavailable water
- Capillary water
Now, pull the sponge out of the water, hold it above the pan, and let it drip. Notice that water will freely drain from the sponge for a while but eventually stops.
Equating the sponge and the water it holds to soil and soil water, what water content does the sponge have now?
- Saturation
- Field capacity
- Wilting point
- Air dry
Apply slight pressure to the sponge to remove some water, and let the water drip into the pan. Notice that as you apply more force, more water is removed from the sponge. This resembles how plants remove water from the soil. Initially very little energy is exerted for them to extract water. However, as the soil dries, more and more energy is required to remove water.
Equating the sponge and the water it holds to soil and soil water, what water content does the sponge have once you can no longer remove any water by squeezing the sponge?
- Saturation
- Field capacity
- Wilting point
- Air dry
Notice that the sponge is still moist. This is because even beyond the point where plants can remove water from the soil, some water still remains. This water is called what?
- Gravitational water
- Plant available water
- Unavailable water
- Capillary water
Activity 3: Devices for Measuring Soil Moisture
In the textbook, review the devices for measuring soil moisture and the principle upon which each device operates. Observe the devices on display in the lab.
Tensiometer
What does the meter actually read?
Does the tensiometer measure soil moisture content or soil moisture potential?
Electrical Resistance Blocks and Watermark-brand Sensors
What does the meter actually read?
How is moisture content determined from the meter reading?
Volumetric Water Content Sensor (Decagon EC-5)
What does the meter actually read?
How is moisture content determined from the meter reading?
Stevens Hydra Probe Soil Sensor
What does the meter actually read?
How is moisture content determined from the meter reading?
Activity 4 & Assignment: Problem Set
The problem set will be provided to you at the beginning of the laboratory session.