Articles
Assessment of eddy covariance energy balance closure over a drip irrigated moringa plantation in a semi-arid region of South Africa
Article number
1409_7
Pages
45 – 50
Language
English
Abstract
Eddy covariance (EC) is one of the most common micrometeorological methods used for determining field-scale crop evapotranspiration (ET), which is a key parameter needed to fine-tune irrigation, and optimize water use efficiency and nutritional water productivity.
The theory of EC specifies that measurements be performed over flat, homogeneous vegetation, with a sufficient fetch.
However, in reality, these conditions are seldom met due to mixed crop management practices employed by farmers.
Additionally, EC flux observations represent a dynamic spatial area “denoted flux footprint”, which is influenced by the prevailing wind and atmospheric stability.
Such atmospheric conditions tend to be highly unstable in semi-arid areas.
Moreover, the EC theory assumes negligible advective fluxes, but such conditions may occur particularly where there is a mix of irrigated and dryland areas.
These factors accumulate, resulting in the sum of the energy balance components not being zero, and this imbalance, also called “energy balance closure error”, can make up to 30% of the net radiation.
An average lack of closure above 30% is usually regarded as unacceptable.
This study therefore assessed the EC energy balance closure error over a drip irrigated moringa (Moringa oleifera) stand established at a farmer’s site in the Limpopo Province in South Africa.
Turbulent flux measurements were conducted on a half-hourly basis for two consecutive months during the 2021/22 growing season (December to January). The stand was 1.0 ha in size, planted with 9-year-old trees, spaced at 2×2 m with a maximum of 49% canopy cover.
The resultant uncorrected closure error was 19%, suggesting that the ET data can be used for relatively long time-step (daily or weekly) agricultural water management and planning applications.
For shorter time-step applications in irrigation scheduling and crop modelling that require half-hourly or hourly ET data, it is suggested that the EC data be corrected using the Bowen ratio forced closure method for more accurate determination of moringa ET.
The theory of EC specifies that measurements be performed over flat, homogeneous vegetation, with a sufficient fetch.
However, in reality, these conditions are seldom met due to mixed crop management practices employed by farmers.
Additionally, EC flux observations represent a dynamic spatial area “denoted flux footprint”, which is influenced by the prevailing wind and atmospheric stability.
Such atmospheric conditions tend to be highly unstable in semi-arid areas.
Moreover, the EC theory assumes negligible advective fluxes, but such conditions may occur particularly where there is a mix of irrigated and dryland areas.
These factors accumulate, resulting in the sum of the energy balance components not being zero, and this imbalance, also called “energy balance closure error”, can make up to 30% of the net radiation.
An average lack of closure above 30% is usually regarded as unacceptable.
This study therefore assessed the EC energy balance closure error over a drip irrigated moringa (Moringa oleifera) stand established at a farmer’s site in the Limpopo Province in South Africa.
Turbulent flux measurements were conducted on a half-hourly basis for two consecutive months during the 2021/22 growing season (December to January). The stand was 1.0 ha in size, planted with 9-year-old trees, spaced at 2×2 m with a maximum of 49% canopy cover.
The resultant uncorrected closure error was 19%, suggesting that the ET data can be used for relatively long time-step (daily or weekly) agricultural water management and planning applications.
For shorter time-step applications in irrigation scheduling and crop modelling that require half-hourly or hourly ET data, it is suggested that the EC data be corrected using the Bowen ratio forced closure method for more accurate determination of moringa ET.
Authors
N.A. Araya, N. Mulovhedzi, S. Amoo, C.P. du Plooy, S. Gokool, A. Clulow
Keywords
Bowen ratio, closure error, evapotranspiration, irrigation scheduling
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