Frequently Asked Questions
FAQs
What is the difference between soil moisture and water potential?
- These two articles will explain the differences:
And this article explains why most people should measure both variables:
Does field capacity vary depending on whether the soil has previously been in a dry or wet state? If so, what error margin might that cause if I plan irrigation scheduling according to FC?
- This is true. What you are looking at is the effect of hysteresis, which is generally not a big concern. Depending on the soil type and how big the hysteresis effect is, it can actually shift the field capacity point slightly. If you are concerned about this, you may want to use water potential to schedule irrigation, for example with the TEROS 21 or a tensiometer. If you’d like more information about this, contact customer support.
How can you measure capillary water potential?
- Capillary water potential is tied to matric potential. So if you are measuring matric potential with a tensiometer or a TEROS 21, you are essentially measuring the effect of the capillaries or those different pore sizes. You can also use the HYPROP. The WP4C will also work assuming the soil has a negligible osmotic potential.
We monitor soil moisture using water content. How can we integrate this into a soil moisture release curve?
- One of the best ways to do this is to take some samples and measure the soil moisture release curve for that soil, generating a functional relationship. Then you can take that curve and use your water content values to set your irrigation points through your release curve function. Another option is modeling it. If you know some information about the soil type and pedology, there are pedotransfer functions you can use by inputting those variables, and it will predict a soil moisture release curve. This method is not as accurate, but it’s a possible option.
Which depths should I consider for active roots in maize (or other crops) for irrigation management?
- You can refer to the literature for rooting depths of maize or other types of crops. As for the sensors, we recommend a combination of TEROS 12 soil moisture sensors and TEROS 21 matric potential sensors to get the whole picture.
What modeling programs can you use to model soil moisture release curves?
- There are a few different models out there to model soil moisture release curves. ROSETTA is a program from the US Salinity lab that has been around for a long time. Hydrus is another tool that can be used to model soil moisture release curves. One thing to remember is that these models don’t take into account all of the factors that can change a soil moisture release curve. So if you decide to model your soil moisture release curve, remember they aren’t perfect.
Now VWC trends are used to determine field capacity and stress onset. Is this a more accurate method than water potential?
- This is one approach to take. The issue with using water content measurements is you have to wait until you observe stress occurring to make this type of set point. We recommend a physical water potential measurement as a better way to determine a stress set point. As for field capacity, you can still use the physical measurements to set your field capacity point. The most important thing to understand is that the traditional -33 kPa point for field capacity is not a good rule of thumb to follow. Read why here:
How do you develop a soil moisture release curve in highly variable soils?
- If you have a site with highly variable soils you will need to generate a curve for each individual soil type. One approach would be to map out the site and select the soil types of most importance and then create soil moisture release curves for those soils.
What is the relationship between soil profile and hydraulic conductivity in explaining the overall hydrology of the terrain?
- Hydraulic conductivity through a profile is a small component used to explain the overall hydrology of the terrain. You need to understand hydraulic conductivity in order to understand how the water is going to move through the soil. But, if you want to understand this across a large landscape, you will need to know how the soil varies and how the hydraulic properties change across the site.
How could one determine the change in hydraulic conductivity at a soil-spoil interface?
- There are two approaches you could take. You could measure the two materials independently and see which one is going to be most limiting. Typically, hydraulic conductivity is governed by the most limiting layer. You could also just make the measurement in the field and see the interaction between the two layers. Again this rate will be governed by the most limiting layer.
Lab Ks measurements move from the bottom to the top. Is Ks similar if water goes from the top to the bottom of a sample?
- As long as the calculations are done correctly there shouldn't be a difference if water is percolated from the bottom or the top of the sample.
How would you measure horizontal hydraulic conductivity in field conditions? How would you disaggregate the horizontal component of hydraulic conductivity from the vertical one?
- This depends on whether you are measuring saturated or unsaturated hydraulic conductivity. It can be difficult to try and do this for saturated hydraulic conductivity in the field. In theory, if you took a sample from the field and measured this in the lab, horizontal vs. vertical should be the same in terms of hydraulic conductivity. Hydraulic conductivity is independent of whether the flows are vertical or horizontal because these components are corrected for. If you want to measure unsaturated hydraulic conductivity in the field and look at how it changes in the horizontal vs. vertical position, this could potentially be done with tensiometers and water content sensors positioned in a grid to look at the movement of water and change in water potential. Find more information on how to measure hydraulic conductivity here:
How does soil structure affect K values? How are they related?
- Soil structure and aggregate stability will have a large impact on the hydraulic conductivity values. Depending on the type of soil structure that is formed and the strength of that structure you will see a larger development of macropores within the soil which will have the ability to transmit more water than if the soil had poor structure or no structure. Learn more about this here:
What parameters are the most important to consider a soil as "healthy"?
- There are many parameters to consider, and each is important. Typically, researchers look at parameters that relate to a more stable soil: improved aggregate stability, improved nutrient levels, and more biological activity. There are several groups working in this area, I would recommend looking at the work being done by the Soil Health Institute as starting point.
What is the importance of hydraulic conductivity at plot level, for example, 5x5 plots with the same land use (e.g., a corn field)?
- Hydraulic conductivity can be important, even at the plot level. It depends on the variability of the soils within the area. Even if the site is under the same land use, we can see variability in the soils across a small area which can result in a difference in hydraulic properties.
Is it appropriate to measure hydraulic conductivity at plot level season after season?
- Season-to-season variability will depend a lot on management. From a land management/treatment perspective, if there are things being done that will ultimately lead to improved soil structure and hydraulic properties, you may need to look at seasonal differences. If the land is always being managed the same, you may not see much of a difference in soil hydraulic properties season to season.
What is the difference between infiltration and unsaturated hydraulic conductivity?
- Infiltration and hydraulic conductivity are related. Infiltration is a measure of a soil's ability to infiltrate water from the surface, and it will change as the soil's moisture conditions change. Infiltration rate is not typically corrected for three-dimensional flow and is not referenced to specific moisture condition. Hydraulic conductivity is a one-dimensional value that is specified to a specific moisture condition and can be used when trying to model water movement in soil. The equation that shows the relationship between infiltration and hydraulic conductivity is shown on slides 10 and 11 of this webinar:
Can you refer me to a reliable measurement method to estimate and predict unsaturated hydraulic conductivity?
- See this article:
What is more significant to measure for irrigation: saturated or unsaturated hydraulic conductivity?
- Saturated hydraulic conductivity is more significant in this application because it's important to understand what will be the most limiting factor. As the soil is wetting up, there are matric forces that help pull water into the soil. But as we approach saturation near the surface, there may be ponding of water which could ultimately lead to runoff, and that will affect what your irrigation rates can be.
Episode 5: Soil water content measurement methods
Episode 4: Soil moisture release curves
Episode: 2 Soil hydraulic properties
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