Saturated Flow and Soil Structure

A decisive portion of many of the cycles of energy and matter in our natural environment passes through the soil. To possess knowledge of what happens in soils, formerly nearly exclusively an attribute of farmers and foresters, is today indispensable for everyone who is interested in the interactions between human life and the environment, or who has to intervene actively in environmental cycles in the frame of his duties. To carry out measures which are supposed to have a positive effect on the environment without knowledge on the dynamics of water in and below the rootzone can be compared to the operation of mines without the knowledge of geology. For the water balance and for the air balance in the soil, for the movement of soil solutes, for soil formation, for soil erosion, as well as for many other processes in the soil, the morphological properties of the upper decimeters of the soil cover are of special relevance. For just this zone, however, important laws of classical soil physics, with which one tries to quantify the movement and the persistance of water and solutes, are valid only to a reduced extent, because just here the structure of the soil is very heterogeneous. The same is true for aquifers with large voids.

An open question in soil physics and hydrology, namely the quantification of water and solute movement in larger voids, something one could live with reasonably well until now, is rapidly becoming urgent and calls for an answer. What has brought this development about? In short, it is mankind immediate need for more nutrition and more life space. Officials in adminstrative entities and financing organizations worldwide know that by the year 2000, it will be very difficult to close the gap between growing world population and food supply. The “green revolution”, which was based on successful high-yielding varieties of some important food crops, can probably not be repeated based on new varieties or crops developed by successes in gene technology. Thus, the increase in production which one can expect realistically from production areas under irrigation in arid zones by improving water management becomes one of our great hopes. In humid zones, landscape management to preserve water resources — always intimately linked to water management — becomes increasingly necessary to preserve quality and quantity of life space, protection of the ecology not being a separate aspect of this. In semi-arid and semi-humid zones, loss of food production potential and environmental quality shows itself in various forms. Nearly always, water is involved. Thus, more precision in the analysis of the water balance and in the active control of water quantities has become a topic that is of relevance to everybody everywhere. One million cubic meters of water in many situations are not a “rounding quantity” in calculations any more. Willingly or not, in many cases society will be prepared to pay as much for the preservation of adequate quantities and qualities of components of water balances as they do presently for medical care. As in medicine, there will be competition between solutions of different price, and good solutions will have a market even if they are expensive.

The complex of equations and parameters suggested in this work serves to determine and evaluate the hydromechanical effect of given structures in laboratory and field experiments. In field trials one would possibly choose other spacings and orientations for the cuts through the soil and set some parameters in a manner other than described here. The characterization of the morphology of inter-aggregate voids and of the flow of water through a particular soil or medium having such voids and/or impermeable inclusions refers directly to the column experiments using an artificially induced structure. Readers who immediately prefer to explore the details and are not acquainted with the so-called “particle size counter” (PSC) would be advised to read Chapter 4.1.1 to 4.1.3 in advance. The method of obtaining the structure images, from the soil cuts and of interpreting the images the knowledge of which may be helpful for the understanding of this section, is explained there.

The procedure for setting up the 45 soil columns was as follows:

The columns could be thought of as large cylinder samples taken from a soil layer near the surface. The “extraction region” could also be part of a formation in a groundwater catchment zone. The structure of this imaginary layer subjectively appears to be a very homogenous crack structure. Twenty five cylinder samples can be classified according to selected hydromechnical and morphometric parameters. At the “extraction points” where PV_er is greater, it is found that Md and Rh are also greater, following the relationships shown in the Appendix. The columns can also be arranged so that in addition to the mentioned relationships there is also a connection of the parameters with Fa: in four sequences Fa increases with PV_er, Md and Rh. The data also allow the setting-up of four sequences with increasing or decreasing values of Diff, Devq and Sk.

The hydrodynamic laws for the flow of water through those soil voids, the properties of which are determined by the size distribution and the deposition pattern of the mineral particles, no longer apply to already very small inter-aggregate voids arising from processes of soil structure formation. The significance of differences in soil structure in the field and in structural fluctuations with time for the movement of water can, however, only be quantified if knowledge about the effect of the morphometry of inter-aggregate voids on the flow processes is available.