Measuring Method - UMS HYPROP Operation Manual

Measurements considering the shrinkage characteristics of the soil sample
are described in Schindler et al., 2015. Comparing the measuring results of
HYPROP and classical methods (sand box, kaolin box, pressure pot) demons-
trated good congruence (Schelle et al., 2010, 2011, 2013a, b; Schindler et al.,
2012). Systematical differences could not be found.

Measuring method

HYPROP measures the water tension/water content relation ("retention curve",
®
"pF/WC curve") of a soil sample. It also measures how the unsaturated
hydraulic conductivity depends on the water tension/content ("Ku curve").
This is based on the evaporation method according to Wind (1968) in Schindler's
model (1980).
With this method two tensiometers are positioned in two depths of a soil samp-
le sitting in a sample ring. The plane in the middle between the two tensiome-
ters is identical with the horizontal symmetry plane of the column. The sample
is saturated with water, basally closed and set on a balance. The soil surface
is open to the ambient atmosphere so that the soil water can evaporate.
HYPROP measures the water tension in two horizons of the soil sample over
the evaporation process by means of two vertical tensio shafts (similar to T5
tensiometers) . The change of the sample mass over time is determined by
weighing. The medial pF value of the sample is calculated based on the aver-
age value of the two tensions. The medial water content is calculated based
on the mass change. This results in one measuring value per point in time for
the pF/WC curve.
The evaporation rate results from the mass differences, based on this the vo-
lume flow is calculated at each point in time. The values of the hydraulic
conductivity with increasing desiccation are a result of how well the soil can
transport this water to the top where it evaporates. If the conductivity is poor
the soil on top dries out whereas it remains wet on the bottom. So the upper
tensio shaft indicates "drier" than the lower tensio shaft which is still "wet". If
the conductivity is good the whole water of the sample is evaporated and
bother tensio shafts indicate almost the same values. The detailed calculation
basics of the method as well as the check of its validity are explained in Peters
and Durner (2008) and Peters et al. (2015).
77 | Theory