Shallow groundwater effect on land surface temperature and surface energy balance under bare soil conditions: modeling and description

Abstract

Abstract. Understanding when and how groundwater affects surface temperature and energy fluxes is significant for utilizing remote sensing in groundwater studies and for integrating aquifers within land surface models. To investigate the shallow groundwater effect under bare soil conditions, we numerically exposed two soil profiles to identical metrological forcing. One of the profiles had shallow groundwater. The different responses that the two profiles manifested were inspected regarding soil moisture, temperature and energy balance at the land surface. The findings showed that the two profiles differed in three aspects: the absorbed and emitted amounts of energy, the portioning out of the available energy and the heat fluency in the soil. We concluded that due to their lower albedo, shallow groundwater areas reflect less shortwave radiation and consequently get a higher magnitude of net radiation. When potential evaporation demand is sufficiently high, a large portion of the energy received by these areas is consumed for evaporation. This increases the latent heat flux and reduces the energy that could have heated the soil. Consequently, lower magnitudes of both sensible and ground heat fluxes are caused to occur. The higher soil thermal conductivity in shallow groundwater areas facilitates heat transfer between the top soil and the subsurface, i.e. soil subsurface is more thermally connected to the atmosphere. For the reliability of remote sensors in detecting shallow groundwater effect, it was concluded that this effect can be sufficiently clear to be detected if at least one of the following conditions occurs: high potential evaporation and high contrast between day and night temperatures. Under these conditions, most day and night hours are suitable for shallow groundwater depth detection.

Reconnoitering the effect of shallow groundwater on land surface temperature and surface energy balance using MODIS and SEBS

“…10 show that both latent and sensible heat fluxes had a sharper trend to follow the water table depth than net radiation and ground heat flux had. In agreement with the findings and results of the companion paper (Alkhaier et al, 2012), net radiation, latent and ground heat fluxes decreased while sensible heat flux increased when the water table depth increased.…”

“…And, when the water table depth increased, net radiation, latent and ground heat fluxes, evaporative fraction and daily evaporation decreased, while sensible heat flux increased. This concords with the findings of a companion paper (Alkhaier et al, 2012). The observed clear relationships were the result of meeting both conditions that were concluded in the companion paper, i.e.…”

“…10a and d). Investigating the cause of this phenomenon reveals that some spots of these areas suffer soil salinity (Alkhaier, 2003). When accumulating at the land surface, the salt crust increases the albedo (Fujimaki et al, 2003).…”

“…In the companion paper (Alkhaier et al, 2012), we notice that the ground heat flux is computed in SHAW (and in many land surface models) by solving for a surface temperature that satisfies the complete surface energy balance. The balance equation (Eq.…”

“…In a companion paper (Alkhaier et al, 2012), we presented a detailed description of how shallow groundwater affects surface soil moisture, surface soil temperature and the various components of surface energy balance. We also discussed the optimum conditions under which this effect can be sufficiently clear to be detected by dint of satellite measurements.…”