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University of Pretoria (2018)

Quantifying the water use of apple orchards

Doko, Qamani

Titre : Quantifying the water use of apple orchards

Auteur : Doko, Qamani

Université de soutenance : University of Pretoria

Grade : M. Agric (Horticultural Science) 2017

Résumé
High yielding apple orchards (>100 t ha-1) are becoming the norm in South Africa and it is assumed that as yields increase, transpiration rates will also increase. This raises the need for improved knowledge on water use of these orchards in relation to water availability, given the increasing pressure on scarce water resources. Accurate estimates of transpiration are therefore necessary to assess the possible link between yield and water use. A project was therefore solicited, managed and funded by the Water Research Commission and the South African Apples and Pears Producers Association to quantify the water use and water relations of high yielding apple orchards. The heat pulse velocity sap flow technique was used to monitor transpiration rates in full bearing ‘Golden Delicious’ and ‘Cripps Pink’ orchards in the Koue Bokkeveld. Measurements of water relations included stomatal conductance and predawn and midday stem water potential. Estimates of canopy size were obtained by measuring the interception of photosynthetically active radiation (PAR) and leaf area index (LAI) of trees. Weather parameters were recorded by an automatic weather station and were used to calculate reference evapotranspiration (ET0). The transpiration rates for both ‘Golden Delicious’ and ‘Cripps Pink’ increased from spring to summer and decreased in autumn. The yield of the ‘Golden Delicious’ orchard was 98 t ha-1, with a total of 786 mm transpired throughout the season. The yield in the ‘Cripps Pink’ orchard was 85 t ha-1 with a seasonal transpiration total of 594 mm. The higher seasonal transpiration in the ‘Golden Delicious’ orchard was likely a result of a higher LAI (3.43 m2 m-2) of these trees as compared to the ‘Cripps Pink’ trees (2.82 m2 m-2). The results show that tree water use varies according to climatic conditions and canopy size. There was no clear relationship between transpiration rates and yield in the current study. ‘Golden Delicious’ trees transpired more water than ‘Cripps Pink’ trees throughout the season ; and this was a result of a bigger canopy size. These results were used as a basis for evaluating a modelling procedure of adjusting crop coefficients based on canopy height and size by Allen and Pereira (2009). Modifications to estimate transpiration crop coefficients (Kt) by Taylor et al. (2015) on citrus were used. The model overestimated the transpiration rates due to higher leaf resistance values.

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