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contrast, improved saturated hydraulic conductivity ( K s ) for a sandy soil (12% clay) irrigated with TMW for more than 12 years was reported. 16 As at the shoulder site, there were little difference between the treatments at the footslope in terms of K ns , despite the slightly lower K ns measured at the DLD plot (Figure 2C). These results were comparable to that of other researchers who observed no significant difference in K ns between wastewater irrigated and non-irrigated soils. 17,18 There was a strong relationship between the EC e of the 0 - 30 cm topsoil layer and the K ns (Figure 3). The relationship between EC e and K ns was best described using a reciprocal-Y logarithmic-X model with which K ns decreased significantly with an increase in EC e up to an EC e of 0.4 dS/m where after it was expected to plateau. Conversely, there was no correlation between K s and EC e of loam soils irrigated with TMW for over 50 years. 19 Acknowledgements • The project was funded by the Water Research Commission (WRC), Winetech and the Agricultural Research Council (ARC). • ARC for infrastructure and resources. • Staff of the Soil and Water Science division at ARC Infruitec Nietvoorbij for technical support. • Messrs Pierre Blake for permission to work in his vineyard, and Egbert Hanekom for managing the vineyard and technical assistance. 

RF SLD DLD

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100

80

60 K ns (mm/h)

40

20

K ns = 1/(0,061054 + 0,0261951*ln(EC)) (R

2 = 0,8449; p < 0,005; n = 9)

0

0

0.2

0.4

0.6

0.8

1

EC e (dS/m)

FIGURE 3. Effect of electrical conductivity of the saturated soil extract (EC e ) of the 0 - 30 cm topsoil layer on near-saturation hydraulic conductivity (K ns ) during the 2017/18 season.

Results and discussion No clear trend was observed that could explain the effect of TMW irrigation on the K ns at the shoulder site (Figure 2A). However, the results were similar to what was reported for summit landscape positions irrigated with TMW for over 40 years. 14 In contrast, at the backslope site K ns decreased with an increase in the amount of TMW applied (Figure 2B). The K ns at the backslope site was 103 mm/h, 66 mm/h and 38 mm/h for the RF, SLD and DLD plots, respectively. A significant decrease in the infiltration rate of a sandy soil containing 5.5% clay following four years of TMW irrigation was reported previously. 15 The decrease was also significant with respect to a rainfed control treatment and one irrigated with well water. In

soil chemical responses has also been reported. 6,9,10,11,12 Mini disk infiltrometers (Figure 1) were used to measure the near saturation hydraulic conductivity ( K ns ) of the soils in October 2017. Measurements were replicated five times in each treatment plot at each of the three landscape positions. Measurements were carried out on the grapevine row where a thin layer of fine sand was added to the soil surface to ensure a level surface and adequate contact between the base of the infiltrometer and the soil surface. 13 The TMW used for irrigation was also used for the K ns measurements. The electrical conductivity of the TMW irrigation water (EC w ) and SAR of the irrigation water were 1.3 dS/m and 3.8, respectively.

References https://www.wineland.co.za/treated-municipal-wastewater-for-irrigation-part-5/

Conclusion The K ns for the DLD treatments was the lowest. The reduction in K ns might be more pronounced in regions with lower rainfall. The K ns at the surface of the soil could be related to the EC e in the topsoil. Taking this into consideration, in the future it may be beneficial to quantify the formation of surface crusts that could form under wastewater irrigation. In addition, economically viable practices should be developed to alleviate such surface crusts. It should be noted that the results of this study represent specific in-field situations in three commercial vineyards under one set of climatic conditions.

For more information contact Carolyn Howell at howellc@arc.agric.za.

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TECHNICAL YEARBOOK 2024