n Southern African Forestry Journal - Modelling maximum canopy conductance and transpiration in stands not subjected to soil water deficits : scientific paper




There is much current interest in predicting the maximum amount of water that can be transpired by &lt;I&gt;Eucalyptus&lt;/I&gt; trees. It is possible that industrial waste water may be applied as irrigation water to eucalypts and it is important to predict the maximum transpiration rates of these plantations in an attempt to dispose of this contaminated water. It is also useful to have a reference against which to compare evapotranspiration at any &lt;I&gt;Eucalyptus&lt;/I&gt; site to estimate the degree of reduction in potential water use caused by soil water deficits. This paper proposes a simple model that can be used to predict maximum rates of daily transpiration by short-rotation <I>Eucalyptus grandis&lt;/I&gt; plantations experiencing no significant soil water deficits or fertility limitation. Daily sap flow data recorded in a single average tree in an irrigation and fertilisation trial at Came plantation in the KwaZulu-Natal Midlands were used to estimate mean daily canopy conductance. Analysis of daily and seasonal variation in conductance confirmed that solar radiation and vapour pressure deficit are the dominant factors reducing canopy conductance below potential values, when soil water availability is high. A simple canopy conductance model based on these data was then used with the Penman-Monteith equation to predict daily transpiration rates by <I>E. grandis&lt;/I&gt; trees at a second site situated on Frankfort plantation in the vicinity of Sabie, Mpumalanga. These model predictions were compared to a mean daily sap flow rate recorded in four sample trees over a full year. Transpiration estimated using the Penman-Monteith equation and canopy conductance model agreed well with the observed daily sap flow (R&lt;sup&gt;2&lt;/sup&gt; = 0.79). The total observed annual sap flow at Frankfort was equivalent to 1320 mm, with a mean daily rate of 46 &lt;I&gt;&#8467;&lt;/I&gt; tree<sup>-1</sup>. The corresponding modelled annual sap flow was 1226 mm. A seasonal change in the relationship between canopy conductance and vapour pressure deficit, observed in a previous study on E. grandis, was again apparent from the Came plantation data.


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