Once precipitation fall into surface it affects surface soil moisture and after infiltration occurs, subsurface SM. Thus, I assume that both layers are always correlated, but with some shifts in time.
I calculated cross-correlation between two time series of surface and subsurface SM in R. Now I have information about average number of days (lag time) which when used to shift one time series, gives me the greatest linear correlation coefficient value between them.
However I wonder if:
http://stackoverflow.com/questions/10369109/finding-lag-at-which-cross-correlation-is-maximum-ccf
My observations : answer NO !
Why ? because the relation between 2 layers is not a statistical but a physical one.
ie. In the heat transfer in the soil the lag and the thermic amplitude between 2 layers
is used to calculate the thermal soil diffusivity.
That means a lag in a transfer gives information on the sediment (or rock, or soil ...)
but cannot be used as prevision tool.
Dear Mateusz,
a moist soil that gets into drying conditions gets a very small dry layer as soon as all surface water has evaporated. That layer then will slowly increase when no water is added to the surface. This is called "self mulching" of the soil.
The water vapour above such a soil, so the air moisture content, will then depend on the air movement in the atmosphere above it, wherever it comes from, and not on the sub-soil moisture content (either near the surface or at any distance from that drying surface).
Best wishes, Kees
It's correct, the relationship between surface and root-zone soil moisture is changing over time. Specifically, if you apply physical relationships (e.g. Richards equation), you can expect a different relationship during wetting and drying periods.
However, if you look to the actual observations, through simplified relationships you are able to estimate root-zone soil moisture from surface measurements. I suggest you reading our paper on this topic:
https://www.researchgate.net/publication/233946180_A_physically_based_approach_for_the_estimation_of_root-zone_soil_moisture_from_surface_measurements
Article A physically based approach for the estimation of root-zone ...
Yes you can apply the Richards equation applying additional field observations. By simplified relationships you can estimate the root-zone soil moisture from surface measurements.
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