Richard W. Healy 2010 <p>The flow of heat in the subsurface is closely linked to the movement of water (Ingebritsen <span class="italic">et al</span>., 2006). As such, heat has been used as a tracer in groundwater studies for more than 100 years (Anderson, 2005). As with chemical and isotopic tracers (Chapter 7), spatial or temporal trends in surface and subsurface temperatures can be used to infer rates of water movement. Temperature can be measured accurately, economically, at high frequencies, and without the need to obtain water samples, facts that make heat an attractive tracer. Temperature measurements made over space and time can be used to infer rates of recharge from a stream or other surface water body (Lapham, 1989; Stonestrom and Constantz, 2003); measurements can also be used to estimate rates of steady drainage through depth intervals within thick unsaturated zones (Constantz <span class="italic">et al</span>., 2003; Shan and Bodvarsson, 2004). Several thorough reviews of heat as a tracer in hydrologic studies have recently been published (Constantz <span class="italic">et al</span>., 2003; Stonestrom and Constantz, 2003; Anderson, 2005; Blasch <span class="italic">et al</span>., 2007; Constantz <span class="italic">et al</span>., 2008). This chapter summarizes heat-tracer approaches that have been used to estimate recharge.</p><p>Some clarification in terminology is presented here to avoid confusion in descriptions of the various approaches that follow. <span class="italic">Diffuse recharge</span> is that which occurs more or less uniformly across large areas in response to precipitation, infiltration, and drainage through the unsaturated zone. Estimates of diffuse recharge determined using measured temperatures in the unsaturated zone are referred to as <span class="italic">potential recharge</span> because it is possible that not all of the water moving through the unsaturated zone will recharge the aquifer; some may be lost to the atmosphere by evaporation or plant transpiration. Estimated fluxes across confining units in the saturated zone are referred to as <span class="italic">interaquifer flow</span> (Chapter 1). <span class="italic">Focused recharge</span> is that which occurs directly from a point or line source, such as a stream, on land surface. Focused recharge may vary widely in space and time. If the water table intersects a stream channel, estimates of stream loss are called actual recharge, or just recharge. If the water table lies below the stream channel, estimates are referred to as potential recharge. For simplicity, all vertical water fluxes are referred to as <span class="italic">drainage</span> throughout this chapter. Whether the estimated quantity represents actual or potential recharge or drainage depends on the circumstances of each individual study.</p> application/pdf 10.1017/CBO9780511780745.009 en Cambridge University Press Heat tracer methods chapter