SPATIAL VARIABILITY OF THE SOIL WATER AVAILABILITY FOR PLANTS AT THE FIELD
SCALE
Tomáš Orfánus, Viliam Novák, Vlasta Štekauerová, Viliam Nagy
1Institute of Hydrology, Slovak Academy of Sciences, Racianska 75, 831 02 Bratislava,
orfanus@uh.savba.sk, novak@uh.savba.sk, stekauer@uh.savba.sk, nagy@uh.savba.sk
Abstract: The critical soil water content of limited availability for plants la characterizes an average
soil water content of the upper soil root zone layer at which transpiration rate is starting to decrease
followed by the biomass production decrease (Novák, Havrila, 2006). It is not constant value, but it
moves in the relatively wide range of soil water content of soil root zone, in which transpiration is not
limited by lack of water. The bottom boundary of this range can be characterized by the value of the
“critical water content of limited availability” la, corresponding to the minimum transpiration rate. Thus,
estimation of the range of soil water available for plants requires knowing of values of the wilting point,
the field capacity and the transpiration rate. The wilting point defines the soil water content at which
plants irreversibly suffer from water stress while the field capacity defines the soil water content at the
transition between gravitational and capillary water. These critical water contents are not
unambiguously defined in terms of their physical sense. However, as an approximation, the wilting
point and the field capacity can be identified with particular soil water potential values on soil water
retention curve (WRC), assuming that they are only a function of the hydraulic properties of the soil
profile. On the other hand, the critical soil water content of limited availability for plants la is according
Novak et al. (2000) influenced not only by soil hydraulic properties but also by intensity of transpiration
(Eqs 1–3).
la =  k 1 
1

k2
(1)
k2 = 0.67 . v
(2)
 = - 2.27 Etp + 17.5
(3)
where la is the “critical water content of limited availability” [–], k1 , k2 [–] are the so called „critical“
SWC indicating the beginning and the end of the transpiration decrease rate range, v [–] is the SWC
of permanent wilting point (Kutílek, Nielsen, 1994). Coefficient  depends on the potential transpiration
value Etp [mm]. The parameter v represents the influence of hydraulic properties of soil on calculation
of la and the parameter Etp the influence of the Soil – Plant – Atmosphere Continuum (SPAC).
Detailed hydrophysical research was performed on the plot of 150 x 300 m near Moravský Svätý Ján
village on Záhorská nížina Lowland (south-western Slovakia). The sampling was realized at 10th April
2002 in a regular network (spacing was 20 m) from horizons of 10–15 cm (128 samples) and 30–35
cm (28 samples). The volumetric soil water content, saturated soil water content and saturated
hydraulic conductivity were estimated in a laboratory for all samples. On the research plot there were
identified two textural classes: loamy sand and clay loam.
The actual soil water content is highly variable in space. It was tested whether (in flat field-scale areas)
this variability can be satisfactorily reproduced by the variability of soil characteristics, when using onedimensional mathematical model HYDRUS – ET (Fig. 1).
Fig. 1: The comparison of measured and simulated
distributions of actual soil water content at 10th April
2002 on the research plot.
12
frequency
10
PDFtazke
merane
8
PDF lahke
The
turquoise and yellow curves are the distributions of
merane
PDFtazke
simulated
values for loamy sand and clay loam,
modelovane
PDF lahke
respectively
and the rose and blue curves are the
modelovane
corresponding to distributions of measured values.
6
4
2
0
0
0.2
0.4
soil water content [-]
0.6
The influence of soil texture on soil water content is
strong, as it was expected, but their variability is close.
The simulated values of the soil water content are
overestimated by about 6 % of volume if compared with
measured values. This bias is comparable with measurement errors of some methods and therefore
acceptable. We assume that the overestimation of simulated values is due to hysteresis effect and
(considering the shallow groundwater table) using the wetting branch of the WRC would provide even
better results. Both measured and simulated distributions of the soil water content values keep
approximately the gauss-curve shape and the standard deviation moves between 4 and 6% of
volume.
Sampled soils were used to measure 43 water retention curves. The critical water contents; field
capacity (pF = 2.0 for loamy sand and 2.7 for clay loam) and wilting point (pF = 4.18) were determined
for these samples.
Then, the soil water movement was simulated by mathematical model HYDRUS – ET (Šimunek et al.,
1987) for a whole vegetation season (beginning form 10th April 2002). Maize was the modeled crop.
From the outputs of the model we choose the maximum and minimum values of potential
evapotranspiration Etp to calculate the range of possible values of critical soil water content of limited
availability for plants depending on transpiration rate too and to determine the influence of individual
textural classes on its spatial variability (Fig. 2). Values of the critical soil water content of limited
availability for the clay loam moves between 0.19 and 0.34 if the transpiration is the highest calculated
(5 mm / day) and in the range from 0.09 to 0.25 when the transpiration is relatively low (1 mm/day). In
case of loamy sand the corresponding intervals of the critical soil water content of limited availability
for the same transpiration rates are (0.16–0.25) and (0.066–0.15).
Fig. 2: The spatial variability of the critical soil
water content of limited availability for plants.
water content of limited availability
18
16
frequency
14
12
10
8
6
4
2
0
0
0.1
0.2
soil water content [-]
0.3
0.4
The turquoise curve represents the variability
of la in loamy sand soil when the daily
evapotranspiration is low (1 mm/day). The
average value is 0.09 and the standard
deviation is 0.026 % vol. The yellow curve
represents the same for highest value of daily
evapotranspiration (5 mm/day). The average
value is 0.19 and the standard deviation is
0.026. The rose curve is the variability of la in
clay loam soil by low evapotranspiration rates
(1 mm/day) and the blue one is the same for
Etp = 5 mm/day. The average values are 0.15
and 0.25, respectively and the standard
deviation is 0.042 for both evapotranspiration
rates.
The spatial variability of the soil water content in the root zone in our study is significant and the
modeling showed that it is influenced mostly by the spatial variability of the hydrophysical
characteristics of the soil (soil hydraulic conductivity, soil water retention curves). The spatial variability
of the soil water content can be characterized by the Gauss-type distribution. The critical soil water
content of limited availability for plants la spatial variability is close to the variability of the soil water
content. The la values for soils studied strongly depend on both, the transpiration rate and the textural
heterogeneity. The difference between them was estimated Δla = 0.1 which represents up to 100 mm
of water layer in one meter thick soil layer.
Key words:
capacity
Spatial variability, soil water content, water availability for plants, wilting pint, field
References:
KUTÍLEK, M. & NIELSEN, D.R. 1994. Soil hydrology. Catena Verlag, Cremlingen - Destedt, Germany,
370 pp.
NOVÁK, V. 1990. Estimation of critical soil water contents to evapotranspiration (in Russian).
Počvovedenie, (Soviet Soil Sci.), No. 2, 137–141.
NOVÁK V., HAVRILA J. 2006. Method to estimate the critical soil water content of limited availability
for plants. Biologia, Bratislava, (In press).
ŠIMŮNEK, J., K. HUANG., M. ŠEJNA, TH. M. VAN GENUCHTEN, J. MAJERČÁK, V. NOVÁK, J.
ŠÚTOR. 1997. The HYDRUS - ET software package for simulating the one-dimensional
movement of water, heat and multiple solutes in variably-saturated media. Version 1.1. Institute of
Hydrology, Slovak Academy of Sciences, Bratislava.