Comparative evaporation measurements above commercial forestry and sugarcane canopies in the KwaZulu-Natal Midlands.
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Date
1999
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Abstract
An understanding of the water use of different crops commonly grown in an area is
essential for the implementation of integrated catchment management in South Africa.
With increasing pressure on water resources, mainly due to the recent changes in the
Water Act, it has become important to determine the actual water demands of agricultural
and other crops. Policy makers require knowledge of whether forestry canopies use more
water than grassland and other agricultural crops.
The Bowen ratio and Penman-Monteith methods were used in a comparative study of the
evaporation from Saccharum, Acacia and Eucalyptus. All of the research was conducted
at marginal sites located in the KwaZulu-Natal Midlands of South Africa over a period of
two years.
The Bowen ratio energy balance (BREB) technique combines the Bowen ratio (J3) (the
ratio between the sensible, H and latent heat flux density, XE), with the net irradiance (Rn)
and soil heat flux densities (G) to calculate evaporation. A comparative study of the sitespecific
energy balance components (Rn, G, H and AE), general climatic conditions
(rainfall, solar irradiance and air temperature) and other site-specific parameters (leaf area
index and average canopy height) was conducted on Saccharum and young commercial
forests consisting of Acacia and Eucalyptus. The energy balance highlighted important
differences in the energy balance components between the different canopies. The
differences between the reflection coefficients at the three sites contributed mainly to the
differences in the evaporation rates. The low reflection coefficients of the forest canopies
(Acacia and Eucalyptus) (0.1 and 0.08 respectively) were smaller than of the sugarcane
canopy (0.2). This resulted in more energy available (« 6 %) for partitioning between the
sensible and latent heat flux densities and higher evaporation rates for the forestry
canopies. Where low leaf area indices existed (Acacia and Eucalyptus sites) (LAI < 2),
the soil heat flux density contributed up to 40 % of the net irradiance (G = 0.4 Rn). The evaporation rates for Saccharum, Acacia and Eucalyptus averaged 2 mm day"1 in
winter and 5 mm day"1 in summer. The slightly higher summer evaporation rate for
Eucalyptus (5.6 mm day"1), compared to Acacia (4.9 mm day"1), resulted from the lower
reflection coefficients and canopy resistance (rc) for Eucalyptus (ocs = 0.08, rc = 35 s m"1)
compared to Acacia (ocs = 0.1, rc = 45 s m"1).
Automatic weather station data (solar irradiance, air temperature, water vapour pressure
and windspeed) were applied to site-specific Penman-Monteith equations to predict
evaporation for all three sites. Statistically significant relationships (slope, m « 1,
r2 > 0.8) were found between the measured (Bowen ratio) and simulated (site-specific
Penman-Monteith) evaporation estimates. The current study has demonstrated the
effectiveness of applying the Penman-Monteith equation to forest and sugarcane canopies
to predict evaporation, provided accurate net irradiance, soil heat flux densities and
canopy resistances are used.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.
Keywords
Evaporation (Meteorology)--KwaZulu-Natal.--Measurement, Sugarcane--Water Requirements--KwaZulu-Natal., Eucalyptus--Water requirements--KwaZulu-Natal., Acacia--Water requirements--KwaZulu-Natal., Theses--Agrometeorology.