There are two approaches: either a constant head approach or a falling head approach. I don't like falling head, as you never actually achieve a steady state, such that the mathematics for describing the flow have a number of assumptions which are hard to actually prove. the only time that falling head is appropriate is if the flow rate is very small ie less than about 1 mm per hour.

Constant head is a better approach, there are many techniques for doing it. I like approaches in which flow is forced upwards so as to displace air, rather than downwards water movement. head should be kept as low a manageable ie 1-3 cm.

Size of the core is really important, the bigger the better, I try to use 100 mm diameter cores, whilst my colleges at CSIRO use 300 mm diameter cores.

Interestingly even with biological control agents and ca cl to prevent dispersion, the second measurement of Ksat on the same core is ALWAYS lower than the first measurement, sometimes up to an order of magnitude lower.

Flow around the sides of cores due to slight movement or compaction during coring is also important. if you see and movement of soil away for core side walls you should use hot Vaseline and inject it into the crack using a hyper dermic needle.

Hope this is useful

The best discussion of these methods is in the Soil Physical Assessment for Land Evaluation, published by CSIRO ...known as the brown book.

1. Ks measurements in the lab are generally done with freshwater. Saline solutions are not used since the salt can precipitate out and alter the porosity (and hence, the conductivity) of the sample.

2. I agree with the answer above that the constant head method is better than the falling head. However, I have never come across the phenomenon of lower Ks value at second measurement. I am curious about the time interval between the measurements.

3. Flow from the sides is usually ignored since this would generally not be a significant volume, and any errors due to this easily falls within the range for other errors. That said, if you have chunks of the sample core falling off, or being displaced, then it may be best to stabilize it first.

thank you for your answers, i precise in my question saline solution, because my aim is  to evaluate the effect of saline solution on the structure of soil, and i am using the saturated hydraulic conductivity as a soil propriety which is related to the porosity and the agregate so we can use  to estimate the change of soil structure.

Hello,

You can find a theoretical and practical answer about the specific problem of hydraulic conductivity measurement of the soil at laboratory and field scale in : http://journal.frontiersin.org/Journal/10.3389/fenvs.2014.00024/abstract

You can also ask Amjad Assi ([email protected]), he made his Ph.D. on this subject (effect of saline solution on ... See http://journal.frontiersin.org/Journal/10.3389/fenvs.2014.00005/abstract: Assi AT, Accola J, Hovhannissian G, Mohtar RH and Braudeau E (2014) Physics of the soil medium organization part 2: pedostructure characterization through measurement and modeling of the soil moisture characteristic curves. Front. Environ. Sci. 2:5. doi: 10.3389/fenvs.2014.00005)

He used the HYPROP device for measuring the hydraulic conductivity of the pedostructure (not yet published).

Hello!

1. I suggest testing in a triaxial chamber. This would disable the flow around the sample. But, be careful with the applied hydraulic gradient and the chamber pressure. Too high pressures might cause a compression of the sample with resulting lower hydraulic conductivity.

2. I agree that water should move from the bottom to the top during the test.

3. The lower K at the second measurement in a constant-head permeameter can be explained: a) if swelling clay minerals are present that need certain time (sometimes very long) to swell to their full swelling capacity; they close the pores by swelling and reduce the hydraulic conductivity measured later; b) if the applied water pressure leads to the transport of the fine particles through the pores resulting in the pore colmatation.

4. Application of saline solutions is relevant only if it corresponds with the expected real situation that should be modelled by the permeameter test. I do not think that some special equipment is necessary for such tests.

Might be helpful:

https://www.researchgate.net/publication/27189450_Problems_of_Hydraulic_Conductivity_Estimation_in_Clayey_Karst_Soils

Article Problems of Hydraulic Conductivity Estimation in Clayey Karst Soils

If only Ks is measured (but at what scale?) the suggestions given above are pertinent. However, Ks is not the good parameter to choose to folow the impact of saline solutions (irrigation) on soil structure, why?

1) Ks is not well defined  in terms of parameter, characteristic of the soil medium and entering in the water transfer function of this soil medium.

2) The  existing methods currently used, in the laboratory or in the field, don't apply on the same object (size, environmental conditions of the sample in situ and in laboratory). They give different results that can only be statistical with great variation depending on the sample location.

3) Salinity has two kinds of impact on the soil medium: 1) one on the structure itself (morphology of primary aggregates and their arrangement  with the skeletum), and 2) the other on the hydrostructural  properties of the soil medium which are due to the swelling properties of the soil plasma (clay+silt + organic matter) contained in the structure:  the shrinkage curve and the water retention curve of the pedostructure.

Therefore, Ks measurement in laboratory will allow estimating the first type of impact only. As for the second type, it can be accurately assessed by measuring the two soil moisture characteristic curves cited above using the Typosoil device. The characteristic parameters of these curves can be easily extracted from these measured curves and are enough accurate for detect any change in the soil structure behavior due to the water quality used in irrigation practices.

Few information on TypoSoil are given at http://typosoil.com/

Salinity will affect greatly micro-porosity, to this end I would use core samples which have been saturated with saline water through capillary rise, then oven dry, measure infiltration to constant flux through the oven dried core using mini disc infiltrometer (Decagon design, 2 cm suction). This data will enable you calculate measure flux and hydraulic conductivity of soil cores exerted to different saline solutions...

Thank you all for your advice and your pertinent answers. so it's more accurate to measure an saturate my sample from bottom to top ( to avoide air entrapement ) , and also maybe i should also study to water retention curve and shrinkage curve to better highlight the swelling and dispersion phenomenon that happen when the soil structure is subjected to saline and sodic solution , and thank you Mr Erik braudeau for you link to a new equipement Typosoil , what about the reversibility of these phenomenon is there any link or study done nowadays that target the reverse of dregradation of soil structure ? all the study done are concerning the effect of gypsym ( the chemical effect upon soil structure) but how about the physical aspect of soil structure ? in other word , can we bring about a better soil structure after the soil has been degraded ( loss of porosity, compaction, dispersion, sweeling) ?