As noted within the introduction to Chapter 5, most assessments of subsurface flow do not measure flows directly, but instead measure the variables or states of moisture content (Section 6.1) and pressure potential (Section 6.2) and estimate flow using the Darcy-Richards equation (see Chapter 14). This contrasts with the assessment of overland flow, which is normally measured directly using overland flow troughs (Section 6.5.2). Developments in methods of geo-electrical measurement (Kirsch, 2006; Section 6.1.7), water-quality sampling (Section 8.3) and water-quality analyses (Section 8.4) have, however, led to the increased use of water tracers for the direct measurement of subsurface flow (McGuire and McDonnell, 2007; Section 6.4).