Patterns of surface energy exchange and evapotranspiration in relation
to water availability in an oasis-desert ecotone
A knowledge of the exchanges of energy and water over the terrestrial
surface is the first step to understand the ecohydrological mechanisms,
particularly in water-limited ecosystems in the dryland environments.
However, patterns of energy exchange and evapotranspiration (ET) are not
well understood in the oasis-desert ecotone, which plays an important
role in protecting oasis against the threat of desertification in
northwestern China’s arid regions. Here the continuous measurements of
surface energy fluxes were made using eddy covariance in conjunction
with auxiliary measurements for two years (2014-2015) at a shrubland
within an oasis-desert ecotone in the arid regions, northwestern China.
Statistical analysis on 30-min time scale indicates that about 50% of
daytime net radiation (Rn) over the shrubland is dissipated as H on
average, which peaks in spring; one third Rn is consumed by soil heat
flux (G). Only 9% of Rn was consumed for latent heat flux (λE), which
peaks in summer (21% in 2014 and 16% in 2015), corresponding to the
season with highest rainfall among all seasons. Daily mean ET is about 1
mm·d−1 during growing season of the shrub species. The rapid and
transient increase in ET occurs following a rainfall event. A switch in
surface soil moisture from 0.04 to 0.11 m3·m−3 causes an increase in Rn
by about 11% and λE by 151% at the shrubland, respectively.
Accumulated annual ET were 195 and 181 mm in 2014 and 2015,
respectively, exceeding the corresponding P by about 87 and 77 mm,
indicating that groundwater may be another important source of water for
ET over the shrubland aside from P. These results provide valuable
insight into the mechanisms of sustaining energy and water balance at
the ecotone, and then produce some management guidelines for allocating
water resources and protecting vegetation.