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I'm trying to understand the Richards equation that can be used to compute the flow of water in soil. In a textbook I found the following:
Unfortunately, the textbook doesn't really explain what the pressure gradient actually is. Is it the matrix potential? Are the soil pores actually pulling water from the surface into the soil?
Or is it maybe the groundwater? The large the difference between surface and groundwatertable the higher the "pressure gradient"?
The pressure as measured at a point z+dz (dz infinitesimally small) and z.
At the surface, pressure is the atmospheric pressure. Just below the surface, let's say at 1 cm depth, pressure is atmospheric pressure + whatever weight of the water column (taking into account saturation and so on).
If the pressure gradient is in equilibrium with the gravity, you will have no flow (water in a glass do not mix, although at the top of the glass the pressure is lower than at the bottom, the water at the bottom feels a little push upwards equal and contrary to the push downward it receives from the weight of the water just above)
Think about this: you are at the bottom of a water reservoir (bottom not sealed). Pressure is the atmospheric pressure + depth of the reservoir. This is enough to induce a flow of water outwards from the reservoir, seeping below/through the dam.
Figure is not my own work, it is from a publication of Olsen and Stephens (2016) that can be found at this URL https://www.researchgate.net/publication/319206472_RELEARNING_HOW_TO_LOOK_AT_PIEZOMETRIC_DATA_FOR_SEEPAGE_EVALUATION
