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 

HTML Comment Box is loading comments...