Recently, understanding has grown that surface water and groundwater can only be completely comprehended by an integrated study of the two. Many sketches of the hydrological cycle indicate rain falling, flow through the unsaturated zone and movement via groundwater before re-emergence as springs and baseflow. The concept is understood, but practical application is frequently lacking usually because there is a profound lack of the required data.
The development of groundwater resources will normally have an impact on surface water occurrence with, in the extreme case, a complete drying up of the surface water. To avoid this, most groundwater management plans try to restrict abstraction to some percentage (<100%) of annual recharge. Yet, knowledge of the absolute quantity of recharge is seriously lacking.
Recharge can be estimated as the balance between rainfall, runoff and evapotranspiration. Chapter 2 provides an extensive review of these topics with new data sets and examples from temperate and semi-arid environments.