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.
Highlights
Investigating the effect of shallow groundwater on land surface temperature and surface energy balance has two advantages
Through its effect on thermal conductivity and volumetric heat capacity of the soil profile, shallow groundwater alters the propagation of heat in the subsurface and thereby affects soil temperature and ground heat flux
While the change in thermal conductivity affects the intensity of ground heat flux and the depths of both diurnal and annual heat penetration, the change in volumetric heat capacity alters the amount of heat that can be preserved in the soil layers
Summary
Investigating the effect of shallow groundwater on land surface temperature and surface energy balance has two advantages. The majority of investigations that used remote sensing for detecting shallow groundwater effect on surface temperature was conducted between the late 1960’s (Chase, 1969) and the early 1980’s (Heilman and Moore, 1982) These studies were accompanied with relevant in-situ measurements and modeling efforts; Quiel (1975) measured the radiant temperature of gravel with varying depth of the groundwater table. The measured subsurface soil temperatures and the depths of groundwater brought forth poor correlation His model was not advanced enough to simulate the inter-connection among energy balance components at the land surface. It did not consider the effect of groundwater on soil moisture and the thermal properties of the vadose zone. Afterwards, we describe the numerical modeling experiments that had been implemented to depict the expected pattern and magnitude of this effect
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