In order to provide water security in the twenty-first century, there is universal agreement that a continuation of current policies and extrapolation of trends is not an option. Also clear is that from both water supply and development perspectives, the world's arid and semi-arid regions are those currently and potentially experiencing the highest water stresses. One third of the world's land surface is classified as arid or semi-arid, and about half of all countries are directly affected in some way by problems of aridity. The hydrology of arid and semi-arid areas is also known to be substantially different from that in more humid regions. It is therefore essential that investigation methods appropriate to the former are developed and applied, and that strategies for arid and semi-arid region water resources development recognise the principal characteristics of in-situ hydrological processes.

Shallow groundwater systems are important as a source of water, for sustenance of stream baseflow, and for wetland and riparian ecosystems. They are also central to waterlogging, and dryland and irrigation salinity problems. Response time to hydrologic change and pollutant loadings is fast among shallow aquifiers, and it is important that hydrogeologists and natural resource managers understand the unsaturated zone processes which links human activity at the soil surface and the underlying groundwater, and vice versa. This volume of papers explores practical aspects of soil and surface water interactions with groundwater, including modelling of flow and contaminant transport in the unsaturated and saturated zones.

Groundwater use is of fundamental importance to meet the rapidly expanding urban, industrial and agricultural water requirements in (semi) arid areas. Quantifying the current rate of groundwater recharge and define its variability in space and time are thus prerequesites for efficient groundwater resource managment in these regions, where such resources are often the key to economic development. Attention focuses on recharge of phreatic aquifers, often the most readily-available and affordable source of water in (semi) arid regions. These aquifers are also the most susceptible to contamination, with the recharge rate determining their level of vulnerability. (Semi) arid zone recharge can be highly variable, the greater the aridity, the smaller and potentially more variable the natural flux. Its determination is an iterative process, involving progressive data collection and resource evaluation; there is also a need to use more than one technique to verify results. Direct, localised and indirect recharge mechanisms from a spectrum of known sources are addressed in the framework of recharge from precipitation, intermittant flow and permanent water bodies. The approach taken for each of these reflects the nature and current understanding of the processes involved. The volume also reviews current recharge estimation challenges, outlines recent developments and offers guidance for potential solutions.