Groundwater Transpiration

Many studies documented already cases where plants called phreatophytes, mostly trees with deep roots, uptake groundwater. That amount of groundwater is known as groundwater transpiration (Tg). The estimate of Tg is not an easy task because standard methods of transpiration measurements can handle determination of total transpiration (T) which next to Tg includes also unknown water uptake by shallow roots from unsaturated zone moisture (T).

Water limited environments are characterized by long dry seasons that enforce various plant adaptations to overcome shortage of water resulting in water stress. Some of these adaptation strategies involve use of groundwater (Tg) when other sources of water are unavailable. The phreatophyte groundwater uptake is hydro-geologically relevant in locations, where phreatophytes are abundant and their Tg is significant as compared to other groundwater fluxes. The significance of Tg in water balances is the largest in very dry environments with shallow aquifers.

The exact mechanism of water uptake by plants is not known yet. There are two theories in that respect, cohesion-tension theory [5] and multi-force theory [6]. The common aspect of the two theories is the importance of plant water potential in upward (ascent) or downward (descent) bi-directional water movement, depending on the water potential gradient. The tree water management ability of the passive redistribution of water through the system of roots which act as conduits transferring water from moist to dry soils is called hydraulic redistribution (HR) [7]. HR ascent (also known as hydraulic lift) towards canopy occurs when soil water potential is larger than the leaf water potential and HR descent away from the canopy takes place when the leaf water potential is larger than the soil water potential [4].

Although HR is commonly attributed to phreatophytes, the hydraulic lift may also source deep soil moisture, not groundwater [8, 9]. In dry season (Fig. 21.1),

Hydraulic Redistribution
Fig. 21.1 Hydraulic redistribution: (a) during day time of the dry season; (b) during night time of the dry season; (c) during wet season (day and night) (After Lubczynski [4])

during day time, the water from underneath is lifted up through tap roots (sinkers) towards transpiring leaves thanks to large air VPD. Also during night time considerable quantities of water, amounting to an appreciable fraction of daily transpiration, are lifted up and that water is redistributed through shallow lateral roots into the soil dried up during preceding days [10]. In dry seasons, in case of fog [11] or substantial night moisture in the air [12], water can also be absorbed through leaf surfaces resulting in reverse flow direction in the stem. In wet seasons, during sunny days the direction of flow is typically upward but in rainy days, some tree species can send water downward to increase unsaturated zone water storage. That water is utilized later in water shortage periods, also indicating water stress adaptation mechanism.

The assessment of transpiration in hydrology and hydrogeology involves three main tasks: (i) measurements of total transpiration (T); (ii) extraction of groundwater transpiration (Tg) through partitioning of (T); (iii) upscalling of individual T t and Tg to plot or catchments scale.

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