Discussion and Conclusions

Groundwater evaporation is an important component of groundwater balance in locations with dry climate and shallow water table. In two such study areas in which the author was involved, i.e. in weathered and fractured granitic Sardon catchment in Spain [3] and weathered and fractured gabroic Pisoes catchment in Portugal characterized by thin 0-2 m top clay layer, [17], the ETg represented 30-40% of the recharge. In such study areas, disregarding ETg in groundwater balances carries the risk of mismanagement of groundwater resources.

Groundwater evapotranspiration consists of two different components, groundwater transpiration (Tg) and groundwater evaporation (Eg). The former refers to groundwater uptake by phreatophytes whereas the latter to direct evaporation of groundwater from water table or its hydraulically connected capillary fringe. Both components can be relatively easily assessed by the standard transpiration and evaporation measurements when moisture of unsaturated zone can be neglected. Such conditions however are rare because they only occur in shallow water table locations in dry seasons. In all other cases, transpiration and/or evaporation measured at the surface have to be partitioned.

Importance of groundwater transpiration at the catchment scale depends on the amount of phreatophytes, their species type and age and the hydrogeological and climatic conditions of the catchment. The importance of Tg is the highest in dry areas with shallow water table and large density of phreatophytes.

The extraction of Tg from Tt is possible by combination of sap flow measurements and stable isotope analysis, provided there is sufficient isotopic contrast between groundwater and unsaturated zone moisture. In case it is not, the groundwater isotopic content can be enriched although not in every country this is permitted. The partition of Tt is cumbersome, expensive and time consuming. It is also species-dependent, therefore the results of each individual experiment have to be well documented to create data base corresponding to different phreatophyte species. In parallel models optimizing this work should be developed.

The importance of subsurface evaporation in bare lands is the largest in dry climatic zones with shallow water table, coarse material of unsaturated zone and lack of vegetation. In Sardon study area [3] with savannah type of vegetation of sparsely distributed oak trees, the dry season Eg represented ~70% of ETg that varied from 0.55 to 0.70 mm/d over 3-4 months of different dry seasons. In other, Pisoes catchment in Portugal without phreatophytes (T = 0) but with top surface 0-2 m clay layer restricting evaporation, Eg was also significant and ranged from 0.28 to 0.70 mm/d over 4-5 months in different dry seasons.

When Eu cannot be neglected, the extraction of Eg from the subsurface evaporation (EJ measurements creates problem that does not have scientific solution as yet. The difficulty is because of: (i) continuous interaction between evaporation originated from groundwater (Eg) and the evaporation originated from soil moisture of unsaturated zone (Eu); (ii) interchange between liquid and vapor phases during water transport whereas the latter cannot be directly measured; (iii) difficulty of non-invasive and in-situ quantitative assessment of these processes in subsurface. Considering all these difficulties, the most realistic way to assess Eg seems to be modeling based on depth-wise profile measurements, characterizing not only capillary water transport but also vapor transport. This way for example Scanlon et al. [18] show that evaporation may originate from large depth. However, none of such studies provides explicit partitioning of Eg and Eu despite of large hydrogeological importance.

If E and T are defined then the ET can be calculated by Eq. 21.3. In dry condig g g tions and shallow water table, ETg is usually a significant component of groundwater balance. It is also a critical input in groundwater modeling and groundwater management. ETg is part of the net recharge which is a driving force of groundwater flow. Thus, its omission or underestimation in calibration of groundwater models leads to overestimation of aquifer transmissivities, overestimation of groundwater resources and mismanagement of groundwater resources.

Groundwater balance components are vulnerable to ongoing climatic and land cover changes. The majority of climatic models predict that the dry lands will become drier, dry seasons longer and rains more intense but less temporally distributed. With such changes, Eg will certainly increase and Tg possibly too. Land cover changes, such as expansion of urbanization, will reduce ETg whereas forestation impact on ground-water balance components is not that clear. Deforestation certainly will reduce transpiration and cloud formation but on the other hand it will also increase infiltration, runoff and subsurface evaporation. The available knowledge indicates that in dry lands, deforestation typically results in rise of water table which, in turn, results in the increase of subsurface evaporation causing the unwanted salt deposition.

With current tendency of climate change and land desertification, the importance of ETg in groundwater balances will increase, not only in arid and semiarid areas, but also in moderate climates, which experience nowadays increasingly long drought periods and frequent heat waves. Therefore the significance of ETg in these environments has to be evaluated as well.

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