Water retention relationships

Some samples from the Toce River basin and from the Mella River basin were investigated to attempt a classification of the water retention parameters. The experimental water retention relationships are interpolated using the Gardner and the Brooks and Corey theoretical relationship. In Figure 9.10, the Brooks and Corey relationships of two different soils of the Toce River basin are plotted together with the experimental points. The variation of the pore-size distribution index X is also superimposed. As it can be seen, the pore-size distribution index X decreases with finer soil texture, such that, at the same saturation degree, more energy is needed to extract a unitary weight of water.

The standard practice to estimate the water retention parameters for hydrological and climatological models (PILPS 1994) on the basis of the texture classification seems not to be completely reliable, at least for these mountain soils. In Figure 9.11, in fact, average values for the Brooks and Corey's parameters, with the standard deviation as error bar, are represented versus the ASTM grain size classification: a clear trend of the parameters does not seem to be recognisable with increasing characteristic soil grain size. Some sandy soil samples from the Toce River basin (Eccel et al. 2001) and the samples from the Mella River basin were investigated to observe the sensitivity of the water retention parameters to the organic matter xo.

In Figure 9.12, the Brooks and Corey's parameters of the water retention relationship, the pore-size distribution index and the bubbling pressure are plotted against the organic matter for three soils characterised by dominant sand. The pore-size distribution index X seems to be more sensitive than the bubbling pressure to the organic matter xo: in particular, at increasing values of xo a decrease of X can be observed. So, applying Equation (9.5), the same effective saturation se needs a great energy to be extracted as the organic matter




to ro







- b-

u o u o Qo n n n


0 o

! o ! Ie? ! 9 !

o 0





Figure 9.9 Dependency on the altitude of the saturated conductivity of the soils of the Toce River basin (laboratory analysis)

Capacitor Frequency
Figure 9.10 Soil water retention relationships of a loamy sand and of a sandy loam sample from the Toce River basin: experimental data and Brooks and Corey theoretical relationship




















iz si




□ Pore size distribution index

□ Bubbling pressure


GP(1) GM(4) SP(1) SM(41) ASTM grain size classification

Figure 9.11 Average values of the Brooks and Corey's parameters of some soils of the Toce River basin plotted versus the ASTM grain size classification. The standard deviation is represented as error bar. In brackets, the number of the experienced soils is represented

Bubbling pressure

Toce River basin

Pore size distribution index l = 0.71e-0022xo R2 = 0.8506

p g ling 6

Bubbling pressure

C.C1 C.C2 C.C3 C.C4 C.C5 C.C6 xo Organic matter content (-)



Figure 9.12 Brooks and Corey's parameters plotted against the organic matter content (xo) for some sandy soils of the Toce River basin

C.C1 C.C2 C.C3 C.C4 C.C5 C.C6 xo Organic matter content (-)



Figure 9.12 Brooks and Corey's parameters plotted against the organic matter content (xo) for some sandy soils of the Toce River basin

Organic Fertilizer Water Retention
Figure 9.13 Water retention relationships of three sandy soils of the Toce River basin characterised by different organic matter content (xo)

Mella River basin

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

Mella River basin




: ■

: ■

■ C



. O




■ ■


R> D

1 = 0.11 xo-0340


□ ♦ 1


R2 = 0.2



♦ ♦♦ ■







xo Organic matter content (-)

xo Organic matter content (-)

Figure 9.14 Sensitivity of the pore-size distribution index of some soils of the Mella River basin to the organic matter content and to grain size distribution

Mella River basin

g 14

Mella River basin

g 14




- ■

■ □


_ o □ & ♦


O -






□ < ■

' ■ ■

0*> ♦ —1—1—

■ 1 <



xo Organic matter content (-)

xo Organic matter content (-)

Figure 9.15 Sensitivity of the bubbling pressure of some soils of the Mella River basin to the organic matter content and to the grain size distribution

increases, and the effect is more evident at lower soil moisture contents. This fact is kept in evidence in Figure 9.13, which represents three water retention relationships of the same soils, normalised in regard to the bubbling pressure.

Also for the samples from the Mella River basin, a similar behaviour for different soil classes was observed, both on the basis of a texture classification (Figures 9.14 and 9.15) and on the soil saturated conductivity classes (not presented in the figures). The pore-size distribution index of different soils of the same class seems to be quite sensitive to the organic matter, while a trend is hard to recognise for the bubbling pressure. Considering particularly Figure 9.14, the organic matter seems to significantly affect the variation of the pore-size distribution index particularly for sand- and gravel-soils, with a decreasing X with increasing xo. Observing the high spread of the data of the same textural class, the organic matter content seems to have almost the same importance in the determination of the pore-size distribution index as the grain size distribution.

As previously mentioned, the water retention relationships of some soils were measured also over bulk and crumbled samples to verify the sensitivity of this technique to the sieving. Therefore, the moisture values, corrected with the skeletal fraction to homogenise them with the values measured over sieved samples, were compared with the corresponding water retention values of the same soils sieved at 2 mm. A good agreement (Figure 9.16) can be observed, so that the water retention relationships derived over sieved samples could be considered quite representative of the soil conserving its original structure. The agreement is better as the soil moisture decreases, that is, for higher values of the suction, as it has been pointed out in Salter and Williams (1965), but also at higher moisture values, the difference between moisture in disturbed and non-disturbed samples is low. The practice of sieving the samples if the skeletal fraction is high, as in the investigated soils, can be therefore considered quite reliable in determining the water retention relationships.


In the context of a long-term research programme, 146 and 80 sites of two mountain basins in the Italian Alps were investigated in order to produce realistic maps of the hydrological properties of surface soils. The two basins are dominated by metamorphic and sedimentary rocks, respectively, and soils have a small thickness so that the runoff and flood formation is heavily controlled by the upper soil layers. For their

characteristics, the investigated areas are representative of the typical geopedological conditions of the mountain basins in the Italian Alps. This research seems then to be of some interest because, at the date, there is only little experimental information about the hydraulic behaviour of the upper soil layers of experimental basis in this side of the Alps. On the other hand, a comparison can be done with the values of the upper layer saturated conductivity of the adjacent Ticino watershed (Eidg. Forschungsanstalt fur Landwirtschaftlichen Pflanzenbau 1980). The soil saturated conductivity expected for soils in the Ticino River basin is on average 1 order of magnitude higher than that expected for soils in the Toce River basin. From a hydrological point of view, the two basins seem to be characterised by a completely different mechanism of stormflow formation.

The single ring infiltrometer was used, as a standard reference method, to conduct field campaigns. The traditional way of interpreting the field infiltrometer data, based on an application of the Darcy's law, lead to values of the vertical soil saturated conductivity higher than those observed after laboratory analysis. Therefore, other methods, namely, Green and Ampt, modified with respect to the ponding depth in the infiltrometer; and Reynolds and Elrick (1991), to better take into account the ponding depth and the infiltrometer geometry, were applied to give a more realistic representation of the process. The application of these two methods resulted in soil saturated conductivities closer to those obtained in laboratory with a falling head permeameter without any evident bias that would be a priori expected because of the different size of soil volume. Moreover, a bias of about 1 order of magnitude with quite a good agreement was observed between the Darcy method and the Reynolds and Elrick method. Saturated conductivity ranging from 2.92 x 10-5 to 1.35 x 10-6m/s was observed for soils with different pedogenesis and land use in the Toce basin, while for the Mella river the vertical saturated conductivity ranges between 1.49 x 10-4 and 1.29 x 10-7 m/s.

Also the first layers of the soil were investigated and for several sites a less permeable layer was found just a few centimetres below the surface. In particular, for the soils of the Toce riverbasin, the exponential decay constant was estimated about 0.19 m. Therefore, considering the water path across the first soil layers, the infiltration should be governed by the lower layers of the soil and the runoff production as well, at least in a ''hortonian-infiltration excess'' theoretical framework.

The saturated conductivity of the soils of the Toce River basin was plotted versus altitude and a weak, but statistically significant, positive trend of the surface soil saturated conductivity can been observed as the altitude increases. Such a behaviour is probably due to the eluvium of the smaller particles in the higher soils and to the differential sedimentation as the slope gradient decreases downstream.

The water retention relationships of sieved and bulk or crumbled samples were measured finding that the traditional method of sieving the soil to prepare the samples does not seem to affect the results. The organic matter was found to have almost the same influence on the soil textural class in determining the pore-size distribution index and the behaviour of the water retention relationships especially at low degrees of saturation. In particular, a decreasing of the pore-size distribution index was found at the increasing of the organic matter content in the soil. The exponential law X = 0.71 exp(-0.022xo), and the power law X = 0.11x-0 340 were proposed respectively for the soils of the Toce River basin and of the Mella River basin.

This means that for hydrological and climatological applications, further research efforts are needed to provide methods to estimate first the spatial variability of organic matter, more than soil texture at the basin scale or land use. The wide spread of the data within each soil and land-use class, depending on the site location, poses serious limitations on the accuracy of the derived maps at the basin scale, even when derived after extensive investigations. Uncertainties up to 1 order of magnitude or even higher in the estimation of saturated conductivities of unknown soils can still remain after extensive measurements. How these uncertainties affect the reproduction of flood events in mountain basins will be the subject of future analyses.


This research was conducted in the context of the projects RAPHAEL (Contract ENV4-CT97-0552), CNR - GNDCI ''VAPI RIVERS'' and CNR- ''MAP''. The students who helped us for several months in the field and laboratory to collect most of the data we analysed are gratefully thanked.

Two anonymous reviewers are acknowledged for their suggestions and criticism to the first version of the manuscript.


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