Treated municipal wastewater for irrigation (Part 5): Soil hydraulic conductivity

Abstract

A long-term trial was conducted in commercial vineyards in the Coastal region of South Africa to assess the impact of irrigation with treated municipal wastewater (TMW) on Cabernet Sauvignon and Sauvignon blanc vineyards. Grapevines were irrigated using TMW for 11 years and were either rainfed (RF), irrigated with TMW via a single dripper line (SLD) or received twice the volume of wastewater via a double dripper line (DLD). The near-saturation hydraulic conductivity (K_ns) for the DLD treatments was lowest. The K_ns at the surface of the soil could also be related to the electrical conductivity of the saturated soil extract (EC_e) in the topsoil. 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.

Introduction

The effect of irrigation using wastewater on soil chemical properties is well documented, but its effect on soil physical properties, e.g. soil hydraulic conductivity and infiltration rate, is largely unknown particularly when used for vineyard irrigation.¹ This could be due to the fact that changes in soil physical properties are difficult to quantify, because they tend to occur only over the long term and are greatly variable.² Furthermore, most of the studies were conducted in laboratories using artificial solutions rather than in situ. Results of a laboratory study investigating the effect of sodium adsorption ratio (SAR) and potassium adsorption ratio (PAR) on soil hydraulic conductivity showed that it was considerably reduced when the SAR or the PAR exceeded 20.^3,4 In another study, a combination of solutions with known SAR and PAR was used to investigate the binding of Na⁺ and K⁺, and it was concluded that exchangeable sodium percentage (ESP) corresponding to a given SAR was increasingly lowered at higher K⁺ concentrations.⁵ Using three soils of contrasting mineralogy packed in soil columns, it was found that soil mineralogy and Na⁺ and K⁺ concentrations in solutions were key factors influencing the soil hydraulic conductivity.¹

Taking the above-mentioned into consideration, the objective of the study was to assess the long-term effects of irrigation with TMW on soil hydraulic conductivity in commercial vineyards in the Coastal region of the Western Cape.

Methods

The field trial was carried out in full-bearing, commercial vineyards on a farm near Philadelphia in the Coastal region of the Western Cape from the 2006/07 until 2017/18 seasons. Three experiment sites were selected in different landscape positions. The first site was in a Sauvignon blanc vineyard located on the shoulder of a hill. The second and third sites were in two Cabernet Sauvignon vineyards situated on a back- and a footslope, respectively. Details of the characteristics of the vineyards, irrigation treatments and application, as well as an assessment of the water quality and nutrient load was reported previously.^6,7,8 Grapevine water status, vegetative growth and yield, as well as 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.

FIGURE 1. Using a mini disk infiltrometer to measure near-saturation hydraulic conductivity in a vineyard near Philadelphia.

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 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

FIGURE 2. Effect of rainfed conditions (RF) and irrigation with treated municipal wastewater via single (SLD) and double dripper line (DLD) on the near-saturation hydraulic conductivity (K_ns) (suction = 2 cm) of (A) a shoulder, (B) a backslope, and (C) a footslope during the 2017/18 season.

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

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.

Conclusions

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.

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.

References

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7. Howell, C., Hoogendijk, K., Myburgh, P., Lategan, E. & Hoffman, E., 2024. Treated municipal wastewater for irrigation (Part 1): Irrigation application and water quality. Wineland. Submitted for publication.
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For more information, contact Carolyn Howell at howellc@arc.agric.za.

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