Special issue  

This is the last paper submitted by Peter Germann before he died in December 2020. Peter reviews the development of capillary flow theory since the work of Briggs (1897) and Richards (1931), who raised capillary flow to a soil hydrological dogma. Attempts to correct the dogma led to concepts of non-equilibrium flow, macropore flow, and preferential flow during infiltration. Viscous film flow is proposed as an alternative approach to capillarity-driven flow during unsaturated infiltration.

The Budyko framework which relies on the supply and demand concept could be effectively adapted and extended to quantify the role of drivers – both changing climate and local human disturbances – in altering the land-surface response. This framework is extended with a few illustrative examples for quantifying the variability in land-surface fluxes for natural and human-altered watersheds. Potential for using observed and remotely sensed datasets in capturing this variability is also discussed.

We are concerned about the sad state of affairs around groundwater in the developing countries of Asia and the obvious implications for sustainability. Groundwater production for irrigated agriculture has led to water-level declines that continue to worsen. Yet in the most populous countries, China, India, Pakistan, and Iran, there are only token efforts towards evidence-based sustainable management. It is unrealistic to expect evidence-based groundwater sustainability to develop any time soon.