Hi
Would you please let me know if the following is accurate as an answer to the question:
Suppose we have sensors which measure the volumetric soil water content of a soil layer for a long period (more than 6 months) and also high temporal resolution (half-hourly). Could I assign the maximum in this dataset to the saturation point?
I understand that we do need to have a lengthy rainfall event, how long the event should be so that the above proposal works?
And if there is any other way that I can get to the saturation point of a soil layer from the volumetric soil water content data/sensors, please let me know.
Mostly focused on the topsoil and preferably only using the dataset.
There are standard methods for testing soils to consider. Usually soil samples are taken and put into a container of water so the soil is saturated from the bottom, about 24 hours. Unless there is a water table to saturate soils in the field, ability in reaching saturation may vary with the conditions. I remember we had a dry period and wildfire, then a substantial rain, about 3-5 inches. Water was pounded on the surface the next day, but the soil scientist found the soil totally dry about 12-18 inches below the surface. I also learned that an upper clay layer needs to be saturated before the lower sandy layer can effectively gain water and this is related to the tension with which clay holds water as compared to sands. Some soil layers have difficulty becoming saturated, such as well developed forest soils seldom exhibit runoff due to high infiltration and macropores from roots, etc. other soils as hydric soils are saturated frequent enough to exhibit hydric soil indicators due to frequency of high water table. Sampling the soil depth or horizon is probably the most reliable using standard methods to saturate and measure.
I would not use just the high reading in 6 month period, unless at least I reviewed nearby stream gauging stations and the rainfall during the 6 month produced at least a bankfull streamflow event, which occurs generally about every year, and the stream channel is at a level where flooding begins for stable channels that have not aggraded or degraded. I would want to be pretty sure there was a reasonable likelihood of saturation based on evidence like the streams were near or at flooding. The other option might be to set up so the area with the data sensors can be irrigated for long enough so the detectors stabilize after reaching their approximate maximum. Saturating from the surface can apparently leave some air spaces unfilled, but unless the area periodically has a high water table, the air spaces may not be filled easily to saturation by rainfall anyway.
TO add to what William Hansen writes, you can pour water until saturation and see what wfv values are obtained from the soil sensor. Another approach is to take a soil core, bring it to the lab, measure weight, oven dry it, measure weight, and then saturate it keeping track of volume of water added. From that you can calulate whats the expected wfv of the soil core at saturation. Of course, the above method may not exactly represent the field conditions of porosity, rootgaps, etc, but can give an overall idea of the water holding cap of the soil in your region.
Depending on the moisture resolution of the soil sensor you are using and the datalogging frequency, you might be able to make such an inference if the reading 'pegs' at a high level--for example, if the sensor logs data once per hour, and after a heavy storm the output pegs at a maximum value for 5-15 readings (hours), you might be able to infer that the soil was saturated at that reading for that time period if the sensor has high resolution; since a high-resolution sensor would otherwise (if soil does not saturate) nearly always show at least small changes in moisture reading from hour-to-hour as the soil wets and then drains.
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