The Thermodynamic Effect of Shallow Groundwater on Temperature and Energy Balance at Bare Land Surface

Abstract

Within the foregoing half century, several studies debated over the effect that shallow groundwater has on land surface temperature (Myers & Moore, 1972; Huntley, 1978; Quiel, 1975). As land surface temperature is a key factor when the process of energy and water exchange between land surface and atmosphere occurs, we can presume that shallow groundwater naturally affects the entire surface energy balance system. Shallow groundwater affects thermal properties of the region below its water table. Further on, it alters soil moisture of the zone above its water table which results in affecting its thermal properties, the magnitude of evaporation, albedo and emissivity. Hence shallow groundwater affects land surface temperature and the surface energy balance in two different ways; direct and indirect (Figure 1). The direct way (henceforth referred to as thermodynamic effect) is through its distinctive thermal properties which make groundwater acts as a heat sink in summer and a heat source in winter, and affects heat propagation within soil profile. The indirect way is through its effect on soil moisture above water table and its related effects (i.e. evaporation, soil thermal properties of vadose zone, land surface emissivity and albedo). Studies that investigated the thermodynamic effect commenced by the work of Kappelmeyer (1957), who could successfully use temperature measurements conducted at shallow depth (1.5m) to locate fissures carrying hot water from deep groundwater. Birman (1969) also found a direct relationship between shallow ground temperature and depth to groundwater. Works by Cartwright (1968, 1974), Bense & Kooi, 2004, Furuya et al. (2006) and also works by Takeuchi (1980, 1981, 1996) and Yuhara (1998) cited by Furuya et al. (2006) showed that soil temperature measurements at some depth (0.5-2 m) depth were useful for locating shallow aquifers in summer and winter and also for determining the depth of shallow groundwater and the velocity and direction of its flow. On the other hand, a number of studies considered the indirect effect of shallow groundwater in terms of its effect on soil moisture of the vadose zone and at land surface (York et al., 2002; Liang & Xie, 2003; Chen & Hu, 2004; Yeh et al., 2005; Fan et al., 2007;