Groundwater Evaporation

Groundwater evaporation represents groundwater loss by direct evaporation from water table. This process takes place in bare soil environments and it is the most distinct in dry lands with shallow water table and coarse unsaturated zone material. Groundwater evaporation (E ) is part of the subsurface water evaporation (EJ, often also called bare soil evaporation.

During wet seasons (Fig. 21.2) the unsaturated zone contains large volume of soil moisture usually above field capacity. This moisture evaporates (EJ whenever appropriate PET and VPD conditions occur. Towards dry season, the same aquifer looses unsaturated zone moisture, after some time to start evaporating groundwater (Eg). With large PET and large VPD, this process changes from capillary transport into vapor transport once unsaturated zone moisture is removed. In locations with deep water table, during dry seasons, the unsaturated zone moisture cannot be entirely dewatered, so the evaporation process involves always both, capillary and vapor transport. The depth at which soil abruptly changes its moisture status delineates evaporative front above which water transport occurs mainly in the vapor form.

The Es can be accessed through various methods such as eddy flux towers or lysimeters. However, its partitioning into Eg and Eu and the extraction of Eg is difficult. The partitioning of Es is relatively easier if Eu=0, i.e. when the unsaturated zone is dry so that the assumption Eg=Eu can be made. This happens only in dry seasons in

Shallow water table

Deep water table &

Wet season

Dry season

dry season

E„ E„ E„

Et Et Eg

Eg E„ Eg E„ Eg

r f r

f \ 1

Î i I I I

L ? 1 1 f

fw

■ |1 HI 2 3 4 f 5 $ 6 17

Fig. 21.2 Groundwater evaporation in dry lands: 1 - aquifer; 2 - capilary fringe; 3 - unsaturated zone with prevailing capillary flow; 4 - unsaturated zone with prevailing vapor flow; 5 - capillary flow; 6 - vapor flow; 7 - water table

Fig. 21.2 Groundwater evaporation in dry lands: 1 - aquifer; 2 - capilary fringe; 3 - unsaturated zone with prevailing capillary flow; 4 - unsaturated zone with prevailing vapor flow; 5 - capillary flow; 6 - vapor flow; 7 - water table dry climatic zones. Otherwise the procedure of separating Eg is complex because the evaporation process takes place either through liquid or vapor transport. Furthermore, as pointed out earlier, vapor is not directly measurable. Because of these difficulties, the partitioning of subsurface evaporation has not been developed yet.

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    Is ground water evaporated?
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    How does groundwater evaporate?
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