Water Retention – Hanging Water Column

What is a Water Retention Curve?

Soil water retention curve

A water retention curve indicates the manner in which a particular soil retains and loses water, and also the proportion of stored water that is available for plant growth. It also describes the relationship between water content and soil water tension.

To obtain a water retention curve one must start with a saturated soil. A tension (suction) is then applied to the saturated soil and the volume of water leaving the soil (outflow) at each given tension is recorded.

On a water retention curve the x axis represents the soil water tension T (remember that T = - total potential $\psi$ _t). In most soils where the dominating force is capillarity

T = - matric potential $\psi$ _m. The y axis represents the water content of the soil. Note that soil water content is unitless since it stands for the volume of water per given volume of soil (Vw / Vt).

Plant Available Water

Plants must overcome the energy of the total water potential to extract water from the soil. How do we know how much water is available for plants in a soil?

Consider the situation that occurs when a soil becomes saturated. Under these conditions, the entire pore space (macropores plus micropores) is completely water-filled and the soil is said to be at its maximum retentive capacity. Under normal field conditions, much of the water held in the macropores soon drains (before plants get a chance to take up water) in response to gravitational force. After gravity has drained the macropores, a moisture content known as field capacity is attained. The soil at this point is unsaturated and water is held by adsorption and capillary forces (responsible for the matric potential), and sometimes other forces such as the osmotic force in saline soils. The total water potential corresponding to field capacity ranges from 1 to 3 m (or –10 to –30 kPa). Plants are able to readily access the water held at these potentials. Evaporation and plant use further deplete the soil water supply to a moisture content known as the permanent wilting point. Soil water present at the permanent wilting point is held at a total water potential value of approximately 150 m (or –1500 kPa), and most plants cannot access this water. Soil water held between field capacity and permanent wilting point is termed plant available water.

Factors influencing the Water Retention Curve

Soils have very different texture, structure, porosity, salt content, etc. All these factors determine the energy at which the water is held in soil and are reflected in the water retention curve.

The curves in Fig. 6 illustrate the influence of texture on the shape and slope of the retention curve. Texture and structure influence the porosity and pore size distribution of the soil. Small particles such as clay and silt tend to give rise to small pores, whereas sand size particles favour the occurrence of larger pores. Large pores drain easily whereas small pores retain water strongly due to adsorption and capillarity.

The silt loam exhibits greater water content than the sand at all water potential values. This reflects both the high porosity (f) of the loam, probably due to its good structure, and the small pore size, due to the smaller particle size. The high porosity creates a large maximum retentive capacity and a high initial water content, while the small pore size allows the soil to hold large amounts of water at low matric potential corresponding to strong water retention force (y_t » y_m in most unsaturated non-saline soils = - T).

An inverse relationship exists between the water content of a soil and the energy with which the water is held (the less water left in a soil, the stronger it is held). This energy depends on the radius of the pore containing the water. The smaller the pore radius, the lower (more negative) the value of the matric potential of the water contained in the pore and, hence the greater the energy required to remove the water.

In the context of a water retention curve the capillary rise equation can be used to determine the relationship between the soil water tension (h) and the corresponding radius of the largest pores still filled with water (r):

A breakdown of the Soil Water Tension equation.

Air Entry Value / Air Intrusion Value

On the water retention curve for the sand we can notice two important values:

AEV = air entry value. This is the value of the soil water tension when the water content starts to drop significantly. AEV corresponds to the tension that is just large enough to begin to pull water from the largest pores and from the sample surface, and for air to begin entering the largest pores.

AIV = air intrusion value is the tension at the “flex point”, i.e. at the steepest part of the curve. More generally, AIV is the tension in the middle of the steep part of the water retention curve, where a large change of water content occurs with a small change of tension. The AIV is the tension corresponding to air intrusion into the pore size dominating the porosity

Note that beach sand has a distinct AEV and AIV because it is a well-sorted material (i.e. most pores have roughly the same size because all the particles have approximately the same size; only a few pores have larger or smaller size). The silt loam soil has an AEV of nearly zero and an indistinct AIV, because of the wide distribution of pore sizes.

How is Water Retention Data obtained?

Partial water retention curve apparatus (Hanging water column)

To obtain water retention data, a variety of equipment is used:

Table 1: Equipment used to obtain data associated with varying tension ranges.

Approximate tension range (m)	Method
0 - 1	Hanging water column device, tension table
1 - 10	Low pressure ceramic plate extractor
10 - 200	High pressure ceramic plate extractor

A partial water retention curve usually covers the tension range between 0 and 1 m. Its practical usefulness lies in the fact that its shape reflects soil texture and structure.

Step-by-Step Process

This protocol details the investigation of water retention properties of two soils of different texture:

a medium sand (light colour), and
a fine sand (dark colour)

A saturated soil sample of known bulk density is placed on the porous plate prior to this exercise.

At complete saturation T= 0. Note that at saturation the volume of soil water (Vw) is equal to the pore volume (Vf), while the volume of air (Va) = 0. As T increases, Vw decreases and Va increases (remember: Vw + Va = Vf).

Table 2: Schedule of recommended soil water tensions and equilibration times for medium and fine sand.
T (cm)	Equilibration time, t (min)	T (cm)	Equilibration time, t (min)
Medium sand (0.25 - 0.50 mm)		Fine sand (0.15 - 0.25 mm)
0	0	0	0
10	3	20	3
20	8	40	3
25	15	50	5
30	15	60	15
40	5	70	12
60	3	80	8
-	-	90	When water stops dripping

Required Supplies

10 porous cups with hanging columns and outlets attached (some are cracking)
10 watch glasses to cover porous cups between lab sessions
10 hose clamps
#3 filter paper
10 stands for the hanging column
10 long (1m +) bars to attach the outlet
10 meter sticks
Multiple attachment pieces
8 (50 mL) graduated cylinders
8 – 25 mL graduated cylinders
8 – 10 mL graduated cylinders

Preparation Notes

Step 1: Set up the porous cup

Setting up the porous cup

Because the sand can be reused from lab to lab, the porous cup may have been stored containing sand. Empty the porous cup and set aside the sand for use later.
A filter paper is used at the base of the porous cup. Cut a #3 filter paper to an appropriate size. The filter paper cannot be reused because the pores become clogged. Discard the old filter paper. Note: Filter paper #1 or #5 can be tested for use. Common filter papers #41 and 42 are too thin for this use.
Wet the paper to ensure it adheres to the porous plate before replacing the collar.
Tighten the collar.

Step 2: Prime the porous cup

Priming the porous cup

The hanging water column must be primed prior to setup as it will not function with an air lock.
Fill a bin with water on the lab bench and place an empty bin on the floor.
Place the porous plate in the water and drop the hanging column into the empty bin on the floor. Open the back valve on the pressure plate to allow water to enter.
If water does not begin to flow through the hanging column, force water backwards through the tube, taking care not to create an air lock.
Test that water will flow by gravity prior to filling the cup with the sand. Once confirmed, clamp the hanging water column to maintain water in the tube.

Step 3: Saturate sand

Two sands are used: a fine sand (0.1 – 0.25 mm) and a medium sand (0.25 – 0.50 mm). The sand can be reused between labs; DO NOT DISPOSE of the sand after use.
Because the sand can be reused from lab to lab, the porous cup may have been stored containing sand. Use the sand that was set aside in a beaker in step one. Saturate the sand with tap water and mix to eliminate air bubbles.
Pour the saturated sand into the porous cup. The sand must be level with the top of the cup.
The surface of the sand will be glistening with no pooled water at the surface. Blot excess water if required.
When rewetting between labs, take care to mix the sand.
Prior to attaching the plates to the stations, test the water flow in the buckets to ensure no air lock.
Once water is flowing freely through the plates, clamp the hose at the base. The outlet port can now be lifted above the pressure plate without causing an airlock.

Setting up the apparatus

Step 4: Setup the Apparatus

Not order of setup in the image below.
The meter stick measures “0 cm” at the top of the porous cup.
The outlet port is marked with a pen line to show where it lines up with the measure on the meter stick as it is moved down.

Step 5: Reset Between Labs

Slowly add water to the sand, taking care not to spill.
Mix the sand and water taking care not to scratch the filter paper at the base.
Have additional sand on hand in case some was spilled to add to the surface. If there is an air lock, set up one of the two“spare” units for the lab to have sufficient time to fix the issue.
Place a watch glass on top of the porous cup between labs to minimize evaporation. The samples should be glistening at the beginning of the lab or students may not obtain any water at the first reading which will lead to confusion.

Best Practices

Ensure that the tubing is clamped before moving the outlet back up to the top. This eliminates the risk of an air lock.
Ensure that the samples are glistening prior to beginning of lab. Cover with watch glass.

Soil Samples in the porous cup

Additional notes:

Do not clamp tubing between measures as this forces additional liquid through the tube.
Ensure that the secondary poll is long enough to accommodate the longest measurement (i.e. 90 cm for fine sand and 60 cm for medium sand).Check that there is sufficient sand for the lab well in advance. If new sand is required, it will need to be prepared.
Note that the stations will “deteriorate” over the week as the water is pulled through the filter paper; the filter paper will begin to clog with sediment. The last day of labs is the most likely to encounter issues.
Set up 8 sets and have 2 spare in case there is a problem. Note that it is most useful to have both “spare” be the coarse sand because students moving from a station to the spare station will have limited amount of time. Confirm with instructor as to how many coarse vs. fine they would like to have.
Place watch glasses on top of samples to minimize evaporation (this can be an issue even after a short amount of time).
Reset the seals on the units and

Recording Data

Once all the equipment has been set-up for use, the following steps can be followed while proceeding with the experiment.

Increase the soil water tension in a step-wise manner by lowering the outflow unit. A schedule of recommended soil water tensions and appropriate equilibration times is found above (Table 2):
Keep a running account of the cumulative outflow by measuring the volume of water released in response to each tension increase. Use a graduated cylinder (note that 1 cm³ = 1 mL). Report these data in the “cumulative outflow volume” column on the data collection sheet provided in the appendix.
Measure the height and inside diameter of the soil cylinder. Report these data on the data collection sheet. They will be used to calculate total soil volume.
Perform the following calculations:

Calculations

For each soil sample:

Calculate the total volume of soil used (Vt)
Calculate the soil porosity (f)
Calculate the pore volume (Vf)
Record the volume of soil water (Vw) at saturation (T = 0)
Calculate the volume of water (Vw) left in soil at each tension. The volume of soil water should decrease as tension increases
Calculate the volumetric water content (q) corresponding to each tension
Plot the partial water retention curve for each sample on graph paper.
Identify and mark the air entry value (AEV) and air intrusion value (AIV). Enter these values on line h and i of the data collection sheet.
Using the capillary rise equation, calculate the pore diameters corresponding to the AEV and AIV of each sample after rearranging to solve for r (radius).

References:

Nilesh, Khera, G., GnY, & Gny. (2018, May 19). Water Retention in Soil: Characteristics and Treatment. Retrieved August 25, 2020, from https://geographyandyou.com/water-retention-soil/
Hillel, D. 1971. Soil and water, physical principles and processes. Chapter no. 3: State of water in soils. Academic press. New York. USA.

Appendix

Table A1. Data Collection
Data for MEDUIM SAND		Data for FINE SAND
a) Cylinder height (h)		a) Cylinder height (h)
b) Cylinder inside diameter (d)		b) Cylinder inside diameter (d)
c) Total volume (V_t)		c) Total volume (V_t)
d) Bulk density ( $\rho$ _b)	1.61 g/cm³	d) Bulk density ( $\rho$ _b)	1.49 g/cm³
e) Particle density ( $\rho$ _s)	2.65 g/cm³	e) Particle density ( $\rho$ _s)	2.65 g/cm³
f) Soil porosity (f)		f) Soil porosity (f)
g) Pore volume (V_f)		g) Pore volume (V_f)

Table A1.1 Data Collection Sheet Cont.
MEDIUM SAND				FINE SAND
Tension (cm)	Cumulative outflow Vol. (cm³)	Soil water V_w (cm³)	$\theta$ (cm³/cm³) V_w/V_t	Tension (cm)	Cumulative outflow Vol. (cm³)	Soil water V_w (cm³)	$\theta$ (cm³/cm³) V_w/V_t
0	0.0			0	0.0
10				20
20				40
25				50
30				60
40				70
60				80
-	-	-	-	90

h) Air Entry Value (AEV) (m)				h) Air Entry Value (AEV) (m)
i) Air Intrusion Value (AIV) (m)				i) Air Intrusion Value (AIV) (m)
j) Radius (mm) of pores corresponding to AEV				j) Radius (mm) of pores corresponding to AEV
k_ Radius (mm) of pores corresponding to AIV				k_ Radius (mm) of pores corresponding to AIV