I think the critical areas for new research on water repellent soils have to be (i) development of modelling approaches, prefereably as components to existing dual permeanbility models, in which water repellence can be switched on and off for certain soil layers. This task is complicated by both the spatial and temporal nature of water repellence and the development of prefered flow paths.

(ii) The second big issue which has already been mentioned is scale, at what scale does water repellence matter, does water repellence effect catchment runoff, reforestation after wild fires, crop establishment, agrochemical mobilisation. I think the answer to these questions is yes water repellence is important but we lack the really sound understanding to relate and predict water repellence at the molecular level to the aggregate scale and larger to be able to predict spatially explicit flow paths both on the soil surface and the vadose zone in order to predict these outcomes. (iii) we obviously lack adequate understanding of how water repellence is influenced by antecedent soil moisture (not as simple as previously thought), temperature, land management and scale. (iv) I also suspect the issue of sub-critical water repellence hasnt been adequatly adressed. I suspect if a large range of soils were tested for intrensic permeability with water as opposed to ethanol solutions we would discover subcritical water repellence influces flow through a greater number of soils than we currently suspect.

Some of the work we have done in Tasmania has revealled a few surprises, firstly water repellence (potential - air dried) was seasonal being leached or denatured from the soil in winter and returning with new input of water repellent compounds in summer. This was not simply the effect of antecedent soil moisture, but a true loss and replacement process. Also we improved methods for understanding the effects of water repellence on water flow and found that contry to conventional understanding macropore flow was greatly impacted by water repellence, as well as flow through the smaller pores and soil matric. And thirdly by restricting air flow through the soil profile, we were able to induce similar levels of wetting front instability to that observed in water repellent soils. What impact these observations have on crop production, agrochemical leaching and catchment runoff are compleatly unkown.

Cheers marcus

If you are interested I have 2 papers avaiabe for download from my researchgate profile.

In my opinion, there are intriguing questions to be answered on any scale. Still the reasons for hydrophobicity are little understood on the molecular scale. Also, I see some lack of knowledge in linking processes occuring on the mm to dm scale (say aggregates, crusts) with remote sensing data, which do not resolve these structures, and of course, I also see some lack of knowledge on the landscape scale, where occurrence of flash floods or erosion may directly be linked to infiltration/runoff generation. Here I see perspectives of bringing things together to improve hydrological models. We also should not forget about the temporal scale. For example, hydrophobicity may change over time, say during drying, after moistening etc.

I agree with Thomas scales are variable and understanding the process is important. Marcus Hardie has researched this area on our Tasmanian soils and has found seasonal affects also. I will ask him to add a comment too. Rich

A good point would be to investigate the relationship between soil thermal properties and soil hydrophobivity behaviour. In some experiments I carried out, these relations are very interesting and have a crucial importance, because the heat flux transfer is an essential way and presenting a good agreement with the matric potential into the soil. Therefore, at this point, the effect of the hydrophobicity could be reduced.

I think the critical areas for new research on water repellent soils have to be (i) development of modelling approaches, prefereably as components to existing dual permeanbility models, in which water repellence can be switched on and off for certain soil layers. This task is complicated by both the spatial and temporal nature of water repellence and the development of prefered flow paths.

(ii) The second big issue which has already been mentioned is scale, at what scale does water repellence matter, does water repellence effect catchment runoff, reforestation after wild fires, crop establishment, agrochemical mobilisation. I think the answer to these questions is yes water repellence is important but we lack the really sound understanding to relate and predict water repellence at the molecular level to the aggregate scale and larger to be able to predict spatially explicit flow paths both on the soil surface and the vadose zone in order to predict these outcomes. (iii) we obviously lack adequate understanding of how water repellence is influenced by antecedent soil moisture (not as simple as previously thought), temperature, land management and scale. (iv) I also suspect the issue of sub-critical water repellence hasnt been adequatly adressed. I suspect if a large range of soils were tested for intrensic permeability with water as opposed to ethanol solutions we would discover subcritical water repellence influces flow through a greater number of soils than we currently suspect.

Some of the work we have done in Tasmania has revealled a few surprises, firstly water repellence (potential - air dried) was seasonal being leached or denatured from the soil in winter and returning with new input of water repellent compounds in summer. This was not simply the effect of antecedent soil moisture, but a true loss and replacement process. Also we improved methods for understanding the effects of water repellence on water flow and found that contry to conventional understanding macropore flow was greatly impacted by water repellence, as well as flow through the smaller pores and soil matric. And thirdly by restricting air flow through the soil profile, we were able to induce similar levels of wetting front instability to that observed in water repellent soils. What impact these observations have on crop production, agrochemical leaching and catchment runoff are compleatly unkown.

Cheers marcus

If you are interested I have 2 papers avaiabe for download from my researchgate profile.

I would like to share some of future research needed in this area

Study the relationship between aggregation and hydro -thermo regime

Study the relationship between texture and hydro -thermo regime by using advance techniques

Study the mulch and soil profile moisture relation

Study the influence of biochar on soil hydro and thermo regime

Many good points were made already so I may only add a small hint here: Research done by Marc-Oliver Goebel, Joeg Bachmann and Susanne Woche dealt with the impact of the wetting fluid. For most of our tests we use demineralised or even destilled water and compare it to ethanol, if we compare at all. My suggestions is to also think about the kind of wetting fluid and how it does change the contact angle or wettability (which is a result of three interface tensions). A good remark was made by Andrea Carminati who wondered why plants act in such a way that on the one hand they form an advantageous environment for water uptake and on the other hand prevent the soil from taking up new water: Mucilage allows higher water uptake, but dried once it may cause hydrophobicity. His idea was that the plants prevent the root from drying as the surrounding soil is hydrophobic (he may correct me if I am wrong, I have shortened the explanation also).

So one of the challenges would be to use exactly that fluid that is flowing in the soil to really understand the routes and paths the "water" is taking (or at least to know how the differences are compared to pure water or any other laboratory fluid).

Dorthe

these are really interesting points you make. In relation to the wetting fluid I total agree we need to think about the chemistry of the fluid moving through pores, more specifically the chemistry of the water flowing through different sized pores, the diffierence between infiltration of new water ie rainfall which is almost pure, and displacement of existing soil water which contains different solutes, and yes infiltration of pure wetting fluids.

In the following paper, we further devbelop and demonstrate an approach for investigating the influence of different solute / wettability fluids for infiltration into different pore size ranges in a water repellent soil in which we correct for differences in the density and visocosity of the different fluids such that true pore contribution to flow can be assessed. Note infiltration of water and infiltration of ethanol at the same supply tension involves flow through different sized pores such that direct compariaon of infiltration rate or hydraulic conductivity is not valid between two different infiltrating fluids.

Hardie M, Deurer M, Doyle R, Lisson S, Cotching WE, Mattern K (2012) Development of unstable flow and reduced hydraulic conductivity due to water repellence and restricted drainage. Vadose Zone 11: doi:10.2136/vzj2011.0099. (avaiable on reserch gate)

Dear Marcus, I verified recently that for some soils, there is a large variation in soil volume and porosity when we start from a condition of dry soil to saturation condition. I believe this should also be considered in assessments of the infiltration of a liquid in a porous medium, especially in soils with characteristics of expansion and contraction.

Los estudios del carácter hidrófobo-hidrófilo de los suelos es un tema con muchas imlicaciones tecnológicas que justifican los esfuerzos que se ha hecho, se hacen y se harán en el futuro.

La justificación se este carácter se encuentra en las propiedades superficiales del suelo .

El estudio de estas propiedades es microscópico aunque su manifestación es macroscópica.

Hay un problema en el estudio de los suelos, es su diversidad y heterogeneidad.

La simulación de un suelo en laboratorio es complicado, podemos hacer estudios de componentes mayoritarios del suelo, e interpretar los resultados macroscópico observados, pero los suelos están compuestos por contaminantes, materia organica… que afectan a estas propiedades superficiales (carga superficial, energía libre superficial) y por ende modificar el comportamiento del suelo simulado.

En cualquier caso el estudio y la justificación del carácter hidrófobo e hidrófilo de los suelos es microscópico, aunque la heterogeneidad de los suelos hacen estos estudios complejos aunque abordables por investigadores con pericia.

Nice discussion. I would highlight the note of Marcus regarding comparative tests with different liquids ... e.g. infiltration under the same tension. The fact that different pore sizes are involved in the process is sometimes ignorred. It seems that there will be more questionable points in using such technics.