Groundwater enhances ET in dry season by acting as an additional source of root zone soil moisture. The increase in ET due to GW-supplied moisture has been estimated to be 7-21% in the Sand Hills of Nebraska [Chen and Hu, 2004], 5-20% in northeastern Kansas [York et al., 2002], and 4-16% at global scale [Niu et al., 2007]. 

Owing to large variations of climate, vegetation, and soil properties, the influence of water table dynamics on global-scale hydrological simulations is also variable. The control of these variables on groundwater-supplied ET is briefly explained here.

Two model experimental scenarios are considered; one considering capillary flux from groundwater reservoir (WC) and the other ignoring it (NC).

Global Influence of Capillary Flux:

Fig. 1: Difference between runoff (R), evapotranspiration (ET), and degree of saturation (DS) of root zone soil moisture in NC and WC simulations. (a), (e), and (h): global map of RWC-RNC, ETWC-ETNC, and DSWC-DSNC, respectively. (b), (f), and (j): latitudinal mean of RWC and RNC, ETWC and ETNC, and DSWC and DSNC, respectively. (c), (g), and (k): latitudinal mean of fractional change of R, ET, and DS, respectively, in which dashed line indicates mean. (d), and (h): latitudinal mean of precipitation, and net radiation, respectively.

ET increases by ~9% globally in WC.  The largest increase can be seen in 15oS-30oN and 40oN-50oN regions (Figure 1f) characterized by relatively low precipitation (Figure 1d) and sufficient net radiation (Figure 1h).  In humid regions, the effect of capillary flux on ET is marginal due to sufficient moisture availability from precipitation.  Globally, root zone saturation increases by ~11.4% in WC with a significant increase across the latitudinal profile (Figure 1j) except in the wet regions.  In high latitudes, although the root zone saturation degree in WC is ~30% wetter than that in NC (Figure 1k), the incoming radiation is limited to increase ET (Figure 1h).  To the contrary, in arid or semi-arid regions (e.g., the Sahara and much of Australia), there is a negligible increase in root zone soil moisture as the WTD (capillary flux) is in general deep (weak).

Components of Increase in ET:

Fig. 2: Components of increase in ET due to GW-capillary flux

Most of the increase in ET is driven by increase in soil evaporation. Transpiration also plays significant role and is more pronounced in dry season. In high latitudes of both hemisphere, the increase in ET has large seasonal variation and is higher in dry season with sufficient radiation. Since, the contribution of ET components and timing are both variable in different spatial regions, controls of climate, soil, and vegetation are investigated.

Change in ET vs Precipitation and Climate:

Fig. 3: Fractional change in ET vs precipitation climatology.

The larger the precipitation, smaller is the increase of ET. If the precipitation is larger, the region with stronger seasonal variation of precipitation (larger CoV) has larger increase in ET. For regions with similar precipitation characteristic, the regions with low-extremely humid, (and high-extremely arid) Budyko dryness index have small increase in ET. For regions with dryness index (1-2.5) have the largest increase but it also depicts large spread.

Soil Resistance to ET:

Fig. 4: Soil resistance for (a) WC simulation and (b) NC simulation.

Soil resistance is dependent upon the degree of saturation of soil, which is a function of soil property as well as climate (actual moisture condition). The regions with largest resitance to soil evaporation have the smallest increase in ET. Soil resistance decreases significantly for WC in high latitude and semi-arid regions compared to NC. Semi-arid regions have the largest increase in ET as abundant radiation energy is available.

Vegetation Resistance to ET:

This section of the research is being carried out now and will be updated in near future.

  Top   Home