Revised Project proposals of SOIL 6010—Soil Physics Theory
Jenny Hung
The name of the proposed tool is ETSMD, standing for Evapotranspiration (ET)
estimation from Soil Moisture Dynamics. The ETSMD tool will estimate
evapotranspiration for the days without precipitation, and partition evapotranspiration
into its two components including evaporation from soil and transpiration from
vegetation. Evapotranspiration is the largest component in the water budget on
rangelands, accounting for more than 90% of the precipitation in arid and semiarid
regions (Wilcox & Thurow 2006). An improved understanding of ET and its temporal
dynamics hold the key to mechanistically understand and predict the potential change of
other smaller hydrological components such as streamflow of a given watershed under
global climate change.
Moran et al. (1996) combined the Penman-Monteith equation with measurements
of surface temperature and spectral reflectance to estimate evaporation rates of
semiarid grassland. This approach does not require estimation of the excess resistance,
and evaporation (E) and transpiration (F) is assumed to be linearly associated with
temperature difference variations (Moran et al. 1996):
Tc - Ta = a (T) ………… Eq (1)
To - Ta = b (E) ………… Eq (2)
where Ta = air temperature (°C); Tc = crop foliage temperature (°C); To = soil surface
temperature (°C); a and b are semiempirical coefficients.
ET = E + T = ET (1 - Vc) + ET (Vc) ………… Eq (3)
where Vc = fractional vegetation cover of a given watershed
Eq (1) / Eq (2) => T / E = (b) (Tc - Ta) / (a) (To - Ta) ………… Eq (4)
Also, from Eq (3), T / E = Vc / (1 - Vc) ………… Eq (5)
Combine Eq (4) & Eq (5), T / E = Vc / (1 - Vc) = (b) (Tc - Ta) / (a) (To - Ta) …… Eq (6)
The application of the Eq (6) is to get the semiempirical coefficients (a and b).
Because the T / E would not be the same under different vegetation species with the
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same fractional vegetation cover (Vc), the semiempirical coefficients (a and b) reflect the
different vegetation species.
Actual ET of semiarid grassland can be calculated from soil water balance (Rana &
Katerji 2000):
ET = P - ∆S ………… Eq (7)
where P is precipitation, and ∆S is soil water storage.
However, because the ETSMD tool will estimate evapotranspiration for the days
without precipitation, P = 0 in Eq (7). Therefore,
ET = - ∆S ………… Eq (8)
But after a precipitation event, there might be amount of soil moisture (S) as deep
drainage (D) if the rainfall amount reaches the threshold. Thus,
ET = - ∆S = S - D………… Eq (9)
The ETSMD tool requires the gravimetric soil water content at different depths (5,
20, 45 and 80 cm) of three sites, precipitation (mm), air temperature (°C), crop foliage
temperature (°C), soil surface temperature (°C) and fractional vegetation cover of a
given watershed (%). The drainage will be accounted for the soil water content
increased at the deepest depth (80 cm). Rangeland managers and scientists who are
interested in the ET on rangelands are the expected end-users for the ETSMD tool. The
ETSMD tool will improve upon not only separating ET into evaporation from soil and
transpiration from vegetation reflecting different vegetation species, but also consider
the deep drainage of soil moisture after a significant precipitation event.
References:
1. Moran MS, Rahman AF, Washburne JC, Goodrich DC, Weltz MA, Kustas WP.
1996. Combining the Penman-Monteith equation with measurements of surface
temperature and reflectance to estimate evaporation rates of semiarid grassland.
Agricultural and Forest Meteorology 80: 87-109.
2. Rana G, Katerji N. 2000. Measurement and estimation of actual
evapotranspiration in the field under Mediterranean climate: a review. European
Journal of Agronomy 13: 125-153.
3. Wilcox BP, Thurow TL. 2006. Emerging issues in rangeland ecohydrology.
Hydrological Processes 20: 3155-3157.
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