Developments in soil–water physics since the mid 1960s

The theory for movement of water in unsaturated soils published by Richards 70 years ago is still an important starting point for the analysis of most soil physical problems. Over the last quarter century, the interest in finding new solutions of the Richards equation by either analytical or numerical methods was intense, particularly with regard to field situations. Experimental methods became more diverse and sophisticated: electromagnetic methods for measuring water content and salinity are now reliable and widely available, inverse methods for inferring the soil physical properties have matured. But during this period, we see also a widening of the scope of soil physics beyond the classical theory of Richards with further studies of various multiphase aspects—particularly, simultaneous movement of water and air, simultaneous transport of heat and moisture, flow of water and transport of solutes in structured soils—, of water movement in soils subject to swelling and shrinkage, and of transport of solutes in unsaturated soils. The latter two subjects became manageable by replacing the traditional spatial descriptions by material descriptions in which, respectively, the solid phase serves as the reference continuum for the water and the water serves as the reference continuum for the solutes. Progress was driven not only by a healthy theoretical, computational, and experimental basis, but also by productive interaction with adjoining disciplines and by challenging societal problems.