Creating a full moisture release curve is challenging. There is no single instrument capable of measuring the range of soil water potentials needed for a full curve, and historically, even those instruments capable of measuring part of the curve had limitations that affected the final results.
Measuring water potential in the wet range with the HYPROP and into the dry range with the WP4C requires care and skill. But with good measurement techniques, it is finally possible to get a complete, high-resolution soil moisture release curve. This application guide will illustrate the techniques needed to perform these measurements.
Factors to be considered when combining HYPROP and WP4C data
Osmotic vs. matric potential
In soils, the total water potential is the sum of four different components (Ψt): matric potential (Ψm), osmotic potential (Ψo), gravitational potential (Ψg), and pressure potential (Ψp). In a soil moisture release curve, two main components are active: matric and osmotic potential. The HYPROP measures matric potential only, and the WP4C measures matric potential and osmotic potential.
In soils with a significant salt concentration, the water potential component must be considered when combining HYPROP and WP4C data. The osmotic potential will need to be determined and subtracted from the WP4C readings. Most soils have a lower salt concentration, however, and won’t need this adjustment.
It is possible for one water content to have multiple water potentials. The progress of the retention curve depends on whether the measurement is based on a dry soil which is saturating (wetting curve) or a saturated soil which is drying (drying curve). At the same water content, the soil moisture tension on a drying curve will always be higher than on a wetting curve.
One reason for this phenomenon is the heterogeneity, or irregular sequence, of coarse pores and fine pores. Coarse pores drain faster than fine pores, but fine pores refill faster than coarse pores. Another factor that affects the progress of a soil moisture release curve is that air is trapped differently depending on the method and speed of pore saturation (Hartge, Horn 1999, 148ss).
Because of hysteresis, the method used for determining a water retention curve must always be considered. When using the more time-consuming adsorption method, oven-dry material is saturated to the desired water content. In contrast, the more common desorption method (HYPROP) is based on the measurement of a drying soil. In order to replicate and combine measurement values generated by two different instruments, it is essential to choose the same method (Hartge, Horn, 1999, 148ss).
What does this mean for combining HYPROP and WP4C measurements?
The HYPROP measurement is based on the evaporation method, where a saturated soil sample is placed on a scale and exposed to natural evaporation, which is on the drying portion of the curve (see HYPROP manual). The WP4C measurement can be made by using either the wetting method or the drying method.
Because of hysteresis, combining wetting and drying curves results in curves that do not match. This case is illustrated by the two curves in Figure 1. At certain water contents, the retention data measured with HYPROP on the drying curve have higher water potentials than the WP4C-measured data on the wetting curve. To prevent this problem, use a drying method with the WP4C. This places the samples on the same drying curve as the HYPROP, as illustrated in Figure 2.
WP4C measurement on a desorption curve
There are two options for making a WP4C measurement on a desorption (drying) curve:
The water content of the desired tension reading is defined, and the weight of the sample at that water content must be calculated. A disturbed sample is saturated beyond the defined water content, placed on a scale, and evaporated until reaching the calculated weight.
Disadvantage: This method is extremely time consuming, and if the calculated weight is missed, the sample must be re-saturated.
Option 2 (recommended)
At the end of the HYPROP measurement, WP4C samples are taken from the same soil sample.
Advantage: The measurement is continued on the same drying curve as the HYPROP measurement, allowing for better pairing of data.
Option 2 is described step by step in the following section.
Collecting WP4C samples
- Perform the HYPROP measurement until the second tensiometer shows the air entry, as indicated in Figure 3 (within the red-marked area). Waiting longer makes the WP4C samples too dry. Stopping significantly earlier may have the opposite effect. Thus, it is up to the user to determine at which suction (within the acceptable red-marked area) to obtain the data.
- After stopping the HYPROP measurement, remove the sampling ring (including the soil core) from the HYPROP device (see Figure 4). Perform all operations in a large drying pan to avoid losing any soil. In some cases, as with clay soils, it is not possible to separate the sample from the tensiometer shafts without wetting it before dismounting. If this happens, stop the HYPROP measurement after cavitation of the top tensiometer. Or, a second option for clay samples is to use pieces of a drinking straw around the tensiometer shafts that can slide off when removing the sample.
NOTE: For sandy soil samples, leave the sample on the HYPROP and collect the WP4C samples while removing the soil sample in layers. Collect samples following similar methods to step 4.
- Prepare six WP4C sample cups to be filled with soil from the HYPROP core. Note the tare weight of the sample cups (Figure 5). Label each cup, and make note of which cup is used for each sample.
- Take two WP4C samples from the top of the soil core. Use a spoon or other suitable instrument to get enough soil material to fill the first two WP4C cups half full (Figure 6). Seal the sample by placing the lid on the sample cup. If WP4C measurements won’t be made soon after sampling, seal around the cup with tape to avoid evaporation.
- Turn the soil core around, and repeat step 4 for the next two cups.
- Use a suitable cylinder, and press the soil column until half of the sample is out of the sampling ring. Cut the sample in half with a long, sharp knife (Figure 7). Take two more WP4C samples out of the middle following the procedure described in step 4.
NOTE: When storing the samples, leave them at room temperature to avoid condensation.
Taking WP4C readings and post-processing
- For option 1, it is necessary to leave the WP4C samples in closed cups for approximately 24 hours equilibration time. For option 2, additional equilibration time is not necessary.
- Carefully clean the HYPROP measuring head, and place all remaining soil together into one drying pan. Place the drying pan in the oven at 105 °C for 24 hours to determine the dry soil weight.
- Place the first of the six WP4C samples into the WP4C, and determine the water potential as described in the WP4C manual.
- Immediately(!) after finishing the WP4C measurement, remove the sample and place it on a precision scale (accuracy of +/-0.001 g) to determine the gross weight (WP4C + cup) of the “moist” sample.
- Repeat step 3-4 for all remaining samples.
- After measuring all six samples in the WP4C, dry them in the drying oven, at 105 °C for 24 hours.
- After drying, remove the samples from the oven, let them cool down in a desiccator, and determine the gross weight (i.e., sample plus cups or drying pan) and tare weights (if not noted before).
Data evaluation with HYPROP Fit
- The weights for the dry soil core plus the six WP4C samples need to be combined and added to HYPROP FIT. Insert total dry soil mass into the respective box of the HYPROP FIT software (tab Evaluation, see Figure 8).
- To add the WP4C data into the HYPROP FIT software, insert the weights and MPa values (or optional pF values) for each WP4C measurement into the “Add WP4 data points” menu (see Figure 9). This requires software version HYPROP FIT 3.0 and up. The weighting factor is ´1´ by default. The water contents will be calculated automatically considering the bulk density of the respective HYPROP sample.
- The resulting data will be illustrated in the retention data window of HYPROP FIT and can be fitted with a suitable parametric model.
- Durner, W. (2015): Recommendation for an optimal combination of HYPROP and WP4 measurements to get a full range retention curve. Braunschweig.
- Hartge, H.; Horn, R. (1999): Einführung in die Bodenphysik. Stuttgart.
- METER Group Inc. (n.d.): Tools and Tips for measuring the full soil moisture release curve. Pullman.