HYDROPHOBICITY - A CONCERN FOR HYDROLOGICAL MODELLING?
Müller, K.,1 Mason, K.,1 Simpson, R.,1 Van den Dijssel, C.,1 Robertson, C.,1 Clothier, B.1
1
The New Zealand Plant & Food Research Institute Ltd.
Introduction and aims
Hydrophobicity or soil water repellency (SWR) is a surface condition that reduces the affinity of
soil to water. It causes water infiltration rates to decrease, rain or irrigation water to pond on soil
surfaces, time to the start of runoff to decrease, and runoff rates and volumes to increase. It is a
transient soil property and its prediction is challenging mainly because we do not understand its
ecological significance and root source. Some research showed that the degree of SWR was
positively correlated with soil water content, and the concept of a site- and soil-specific critical
water content was introduced (Dekker et al. 2001). But other authors have reported that SWR
varied non-linearly with soil water contents (de Jonge et al. 2007). In addition, the expression of
SWR is also highly variable at the micro-scale (Hallett et al. 2004) as well as at larger scales
(Regalado and Ritter 2006). A lot of research has been directed at investigating potential causal
relationships between SWR and other soil properties including texture, pH, bulk density,
mineralogy of the clay fraction, organic carbon content and quality, and microbiological
community composition (Doerr et al. 2007). In addition, the unambiguous isolation of the effects
of SWR on water dynamics in soils from other hydrological parameters is extremely difficult. In
particular, its effects on runoff and solute loss have been only poorly quantified. Our aims were
thus to directly quantify the effects of SWR on runoff and phosphorus (P) losses at a hillslope site,
and to measure how SWR changes sorptivity and infiltration dynamics over seasons.
Method
For this purpose, we conducted runoff experiments with our runoff measurement apparatus
(Jeyakumar et al. 2014) on a severely water-repellent pastoral Pumice Soil in the laboratory and
field (Fig. 1). Prior to all experiments, superphosphate was applied at 45 kg P/ha to the pasture to
quantify the potential risk of fertilizer loss. To isolate the effect of SWR from those of other
potential hydrological parameters, we sequentially simulated run-on with two liquids (i) water and
(ii) aqueous ethanol (30% ethanol, v/v, as a reference fully wetting liquid), to the same intact soil
slab in the laboratory, with air-drying to the initial water content between the two events. In the
field, we compared results of runoff experiments conducted in parallel on adjacent fields with the
same two liquids. We analyzed the runoff response to 60-min long runoff events (60 mm/h) from
(i) air-dried undisturbed slabs in the laboratory (0.08 m 2, Fig. 1a), and (ii) field plots of 0.5 m 2 (Fig.
1b). We used the same principle to isolate the effect of repellency from other soil parameters on
sorptivity and infiltration by measuring water and 30% ethanol infiltration into intact soil cores with
our fully automated solute transport apparatus (Fig. 1c). A tension disc infiltrometer applied the
liquids to the top of a core of 10-cm diameter at -50 kPa tension for the water experiments. The
tension was adjusted for the experiments with ethanol taking into consideration the solution’s
specific density and surface tension. We determined the degree and persistence of SWR prior to
the start of all experiments using the molarity of ethanol droplet (MED) and the water drop
penetration time test (WDPT), respectively (King 1981).
Figure 1: Our runoff measurement apparatus (ROMA) operates with intact soil slabs of 0.08 m2 in the
laboratory (a) and at the 0.5 m2-plot scale in the field (b). To determine sorptivity and hydraulic conductivities
of repellent soils we use our fully automated solute transport apparatus (SOLO) (c).
Results
In our laboratory runoff experiments with ethanol no runoff occurred. The final drainage coefficient
was around 80% (Fig. 2a). These measurements demonstrate the soil’s runoff behavior under
hydrophilic conditions. In the water experiments, the high persistence of SWR was reflected in a
runoff coefficient of about 80% throughout the 1-h runoff experiments. Drainage started after
approximately 30 min, and stayed below 10% of the water rate applied (Fig. 2b). Dissolved
reactive phosphorus (DRP) concentrations in runoff samples were highest in the first sample and
decreased exponentially with time (Fig. 2c). In total, 18 ±6.9% and 1.5 ±0.1% of the applied P
were lost as DRP in runoff and drainage, respectively. The field experiments confirmed that SWR
increased runoff and P losses (Fig. 3). About 19.3 ±5% of applied P was lost as DRP in runoff.
The field runoff coefficients were generally lower than those measured in the laboratory reflecting
the increased likelihood for water to re-infiltrate into the soil along the longer plots. The infiltration
experiments are on-going. We conclude that SWR is an important factor in hydrological modelling
and should be included in models to address appropriately this increased risk of surface water
contamination by solutes exogenously applied to water-repellent soils.
Figure 2: (a) Runoff and drainage coefficients in lab experiments with 30% ethanol. Initial average soil water
repellency (SWR) characteristic of the slabs: Persistence of SWR measured by the water drop penetration
time test: WDPTact = 6267 ± 2135 s; WDPTpot = 9800 ± 1470 s; Degree of SWR measured by the molarity of
the ethanol droplet test MED: 102.5 ± 0.6° (b) Runoff and drainage coefficients in preceding experiments on
the same slab with water. Initial conditions: Persistence of SWR: WDPTact = 7867 ±3137 s; WDPTpot =
10333 ±4382 s; Degree of SWR: 102 ±1.2° (c) Dissolved reactive phosphorus (DRP) concentrations in
runoff. All results are averaged over three slabs. Bars represent standard deviations of the means.
Figure 3: (a) Runoff and drainage coefficients in field experiments with 30% ethanol (initial average SWR
characteristic of the plots: Persistence of SWR: WDPTact = 4950 ±5224 s; WDPTpot = 9300 ±2235 s; Degree
of SWR: 102.8 ±1.6°) and water (initial average SWR characteristic of the plots: Persistence of SWR:
WDPTact = 1117 ±723 s; WDPTpot = 6750 ±995 s; Degree of SWR: 102.7 ±1°. (b) Dissolved reactive
phosphorus (DRP) concentrations in runoff. All results are averaged over three plots. Bars represent
standard deviations of the means.
References
de Jonge, L. W., Moldrup, P. & Jacobsen, O. H. (2007) Soil-water content dependency of water repellency in
soils. Soil Science, 172(8), 577-588.
Dekker, L. W., Doerr, S. H., Oostindie, K., Ziogas, A. K. & Ritsema, C. J. (2001) Water repellency and critical
soil water content in a dune sand. Soil Science Society of America Journal, 65, 1667-1675.
Doerr, S. H., Ritsema, C. J., Dekker, L. W., Scott, D. F. & Carter, D. (2007) Preface: Water repellence of
soils: new insights and emerging research needs. Hydrological Processes, 21(17), 2223-2228.
Hallett, P. D., Nunan, N., Douglas, J. T. & Young, I. M. (2004) Millimeter-Scale Spatial Variability in Soil
Water Sorptivity: Scale, Surface Elevation, and Subcritical Repellency Effects. Soil Sci Soc Am J, 68(2),
352-358.
Jeyakumar, P., Müller, K., Deurer, M., van den Dijssel, C., Mason, K., Le Mire, G. & Clothier, B. (2014) A
novel approach to quantify the impact of soil water repellency on run-off and solute loss. Geoderma, 221222, 121-130.
King, P. M. (1981) Comparison of methods for measuring severity of water repellence of sandy soils and
assessment of some factors that affect its measurement. Australian Journal of Soil Research, 19, 275-285.
Regalado, C. M. & Ritter, A. (2006) Geostatistical tools for characterizing the spatial variability of soil water
repellency parameters in a laurel forest watershed. Soil Science Society of America Journal, 70, 1071-1081.