I am using Tempe pressure cells to generate a soil water characteristic curve (SWCC) by weighing the cells at successive matric equilibrium intervals. I say “theoretical” because according to Hopmans and others, one should slightly de-saturate the soil at the start to achieve even drainage. So for the sake of argument let’s say one starts at approximately 1 kPa. The standard procedure is to dry out the core at the end to obtain the weight of residual moisture and add this to the total outflow weight during the experiment. But what about the moisture lost between 0kPa and 1 kPa? And even if this were separately captured, judging the exact point of saturation is highly subjective. An alternative approach suggested in the literature is to use total porosity (calculated from bulk density, volume and particle density) as a proxy for saturation volume. Would this be the best approach?
The first limit to get a sample both saturated and equilbrated at 0 kPa water potential is related to the the fact that your sample has a height. If this height is 5 cm, when the pressure water potential is 0 kPa (atmospheric pressure, no pressure applied in the cell) at the bottom of the sample (pressure potential of 0 kPa + gravimetric potential taken at 0 kPa by reference at the bottom), the pressure water potential is -0.5 kPa (if we approximate -5 cm to -0.5 kPa) at the top (pressure potential of -0.5 kPa due to the water column height + gravimetric potential of +0.5 kPa because the height). Thus the total water potential is 0 kPa in the whole sample and there is no water movement. Because of the value of the pressure water potential which varies linearly with the height from 0 at the bottom to 0.5 kPa at the top of the sample. As a consequence, when there is no pressure applied in the cell, the water pressure is -0.5 kPa at the top and all the pores larger than the pore diameter given by the Jurin's law are desaturated (occupied by air) and this varies linearly from the top to the bottom of the sample where all the pores are saturated by water.
Dear Prof Bruand, merci pour la réponse détaillée! So in theory at least it seems like my start weight should equate to the 0 kPa origin point; good. However, if the sample core is dripping after being saturated, in order to determine the point at which to record the first weight, should I simply wait until all dripping ceases, and perhaps simultaneously begin weighing immediately and continue to do so every few hours until weight loss ceases under 0 kPa?
Probably is useful to follow van Genuchten et al., 1991 parametric models with many soil tables already published. The saturation teta is the inflection point of pressure volume curve, and as any other nonlinear regression depends on number of points and equilibria. Tempe cells are border line for saturation teta and the hanging column integrate the range to saturation. A reference of many is the chapter of Tuller and Or in Soil Water Characteristic.
I assume you use tempe cell to obtain the retention curve.
I this case you will apply different pressure or suction and wait for soil to be at equilibrium at that pressure(suction).
As you know, you will record the amount of water that is released from soil which was initially at saturation. And at the end you will weigh the soil and then you put in the oven to get the oven dried weight thereby obtaining the water content at the end of your experiment. Now you can go backward and calculate the water content at other pressures by adding the amount of water that was released at each pressure steps until you get to the initial water content which is the saturation point.
Does this sounds reasonable?
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