Field swelling, shrinking, and water content change in a heavy clay soil

Abstract

We measured the volumetric water content (using neutron moisture meters) and height changes (using rods fixed in the soil at different depths) of several soil layers during swelling or shrinking of a heavy clay soil in the Liverpool Plains of New South Wales under 3 treatments: (1) a sorghum–fallow–lucerne, (2) continuous lucerne, and (3) continuous fallow. Treatment 1 resulted in weak drying and shrinking (in the sorghum phase), followed by weak wetting and swelling (fallow), and then strong drying (lucerne) accompanied by shrinkage of up to about 140 mm in the top 3 m of the profile. Treatment 2 resulted in repeated wetting and drying, with repeated swelling and shrinking. The continuous fallow treatment (3) had an initially dry profile (following a lucerne crop) and we roughened (chisel ploughed) the surface to promote infiltration. This resulted in a strong wetting event with swelling of up to about 200 mm in the top 3 m of the profile. There were clear trends between water storage change and height change for the first and third treatments, but not for the continuous lucerne treatment.

The relationship between water storage change and height change was consistent with a constant of proportionality (α) of 0.33, unchanging with depth. The data were also consistent with α increasing slightly in deeper soil layers. There was no evidence of α approaching 1 (which is sometimes reported in the literature for wet soil) or zero (which is often reported in drier soil and equated with residual shrinkage), nor of differences in behaviour on wetting and drying.

The height change measurements permitted estimation of the true water storage changes with swelling or shrinkage taken into account. Failure to take volume change into account (i.e. of treating the neutron moisture readings as correct) led to errors in the estimated profile water storage of up to 80 mm. The apparent (uncorrected for volume change) profile moisture storages were readily corrected using a simple analysis based on the assumption of proportionality between water storage change and volume change. The analysis leads to a correction 1/(1 – αθ) (where θ is the volumetric water content). For our data the correction was about 15–20%.