Accurate in situ determination of unsaturated soil hydraulic propertie
s is often not feasible because of natural variability of most field s
oils, and because of instrumental limitations. Therefore the soil hydr
aulic properties are often measured in the laboratory, or derived by c
omputer models using simple standard laboratory methods. This paper an
alyses problems in describing field hydraulic properties of a Ap horiz
on of a silty loam, basing on data from different laboratory methods:
(i) A standard pressure plate apparatus and (ii) a constant-head perme
ameter were used to measure the static retention characteristics and t
he saturated hydraulic conductivity independently. (iii) An instantane
ous profile method was applied to measure water retention and conducti
vity simultanously. Relatively new technics involving ''undisturbed''
soil samples instrumented with mini tensiometers and Time Domain Refle
ctometry (TDR) mini probes characterise the experiment. The models by
Mualem and van Genuchten (MvG) were used to describe the soil hydrauli
c functions. The different laboratory results were then compared with
the hydraulic field properties measured in instantaneous profile manne
r. The laboratory method allows a high spatial and temporal resolution
; this facilitates an investigation of some of the assumptions made, w
hen fitting the MVG models to hydraulic data. A reasonably good descri
ption of the hydraulic data was obtained when setting the residual wat
er content, theta(I), to 0 and the pore connectivity factor, l, to 0.5
because theta(I), and I were not sensitive. However, a poor fit resul
ted when the saturated water content, theta(s), was equated to the por
osity, and the saturated hydraulic conductivity, k(s), to its independ
ently measured value. Values for theta(s) and k(s) derived from field
measurements were somewhat higher than those obtained from laboratory
samples. To demonstrate the influence of the different input data on a
water balance, the cumulative drainage from an initially saturated so
il column was simulated with different sets of hydraulic parameters es
timated from field and laboratory data. Parameters derived from the la
boratory results consistently yielded lower predictions of cumulative
drainage compared to hydraulic parameters derived from field measureme
nts. The differences were relatively small when an initial water conte
nt corresponding to 60 cm suction (field capacity) was used.