Comparison of field methods for estimating evaporation from bare soil using lysimeters in a semi-arid area
Author(s)
Wang, Wenke
Zhang, Zaiyong
Wang, Hao
Luo, Jie
Date issued
August 1, 2020
In
Journal of Hydrology
Vol
November 2020
No
590
From page
125334
To page
125348
Reviewed by peer
1
Subjects
Bare soil evaporation FAO-56 skin method Groundwater level fluctuation method Maximum entropy production Phreatic evaporation Semi-arid
Abstract
Evaporation from bare soil is an important component of a catchment water balance. However, it is arguably one
of the most challenging hydrological processes to estimate and measure accurately. Several approaches to estimate
soil evaporation exist, but their performance for specific water table conditions remains unclear. This
study investigated the performance of four commonly used approaches and several ways on how to implement
them: the energy-balanced based FAO-56 method with the skin evaporation enhancement (FAO-56 skin), hydraulic
methods based on groundwater level fluctuation (GLF), Darcy’s law, and the maximum entropy production
(MEP) method based on non-equilibrium thermodynamics theory. Three lysimeters with different water
table depths were used at a research site in the Guanzhong Basin of China. The lysimeters were equipped with
soil moisture probes. Water table fluctuations were also measured. The data allow us to accurately estimate
evaporation rates using a water balance approach and are used to assess the performance of the analysed
methods. The results show that: (1) The MEP method performed best for all water table conditions, but tends to
overestimate evaporation if the water table is below the extinction depth. The extinction depth is the depth of
the water table were there the contribution of groundwater to bare-soil evaporation is zero. In our case, the
extinction depth was 78 cm. (2) The FAO-56 skin method underestimated evaporation where the water table was
above the extinction depth, and vice versa. (3) The groundwater level fluctuation method significantly overestimated
the evaporation if the specific yield was estimated using hydraulic methods. The groundwater level
fluctuation method should be combined with a soil water balance, independent of water table conditions. The
method can only be applied if the water table is above the extinction depth. (4) Conceptually, Darcy’s law was
suitable for estimating evaporation. However, the estimation of the required parameters is challenging. A good
fit could only be obtained through calibration to measured evaporation rates.
of the most challenging hydrological processes to estimate and measure accurately. Several approaches to estimate
soil evaporation exist, but their performance for specific water table conditions remains unclear. This
study investigated the performance of four commonly used approaches and several ways on how to implement
them: the energy-balanced based FAO-56 method with the skin evaporation enhancement (FAO-56 skin), hydraulic
methods based on groundwater level fluctuation (GLF), Darcy’s law, and the maximum entropy production
(MEP) method based on non-equilibrium thermodynamics theory. Three lysimeters with different water
table depths were used at a research site in the Guanzhong Basin of China. The lysimeters were equipped with
soil moisture probes. Water table fluctuations were also measured. The data allow us to accurately estimate
evaporation rates using a water balance approach and are used to assess the performance of the analysed
methods. The results show that: (1) The MEP method performed best for all water table conditions, but tends to
overestimate evaporation if the water table is below the extinction depth. The extinction depth is the depth of
the water table were there the contribution of groundwater to bare-soil evaporation is zero. In our case, the
extinction depth was 78 cm. (2) The FAO-56 skin method underestimated evaporation where the water table was
above the extinction depth, and vice versa. (3) The groundwater level fluctuation method significantly overestimated
the evaporation if the specific yield was estimated using hydraulic methods. The groundwater level
fluctuation method should be combined with a soil water balance, independent of water table conditions. The
method can only be applied if the water table is above the extinction depth. (4) Conceptually, Darcy’s law was
suitable for estimating evaporation. However, the estimation of the required parameters is challenging. A good
fit could only be obtained through calibration to measured evaporation rates.
Publication type
journal article
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