Investigating the potential of fixed and draped netting technologies for increasing water productivity and water savings in full bearing apple orchards under micro-irrigation

BACKGROUND
The installation of protective netting over fruit orchards is a promising adaptation response to
stressful climatic conditions and climate change. Through reductions in direct radiation, air
temperature and wind speed, and increases in air relative humidity, the microclimate under protective netting is milder. Daily and seasonal whole tree and orchard water use are potentially reduced. The extent of the response depends on changes occurring in the soil, in the tree (including regulation of transpiration through the stomata) and in the atmosphere under the net, compared to open trees. Thus, all aspects of the soil-plant-atmosphere continuum under netting must be better understood to guide the necessary adjustments in irrigation management.

Other significant benefits of netting include reductions in sunburn, wind damage and hail
damage. The effects on yield, fruit size and other fruit quality parameters are variable according to the available literature and may depend on the regional climate, the type of netting, and the cultivar. The relationships between water use and fruit yield and quality (and thus water productivity) under nets are not well understood. Research on protective netting for apple (Malus domestica Borkh.) orchards in South Africa is limited and has not quantified the water-related benefits of production, quality and profitability at tree and orchard levels. Uptake of protective netting technology will be strengthened if multiple benefits and overall increased profitability over the orchard lifetime can be demonstrated. Since both fixed (covering the whole orchard) and draped (covering the tree row only) netting systems are currently available to farmers, research is needed to determine whether the water use and water savings differ between these systems.

RATIONALE
Increasing pressure on water resources in South Africa is a serious threat to the sustainability
of the deciduous fruit industry. Water resources used by the industry for irrigation are coming under increasing strain due to: i) increasing competition from residential and industrial users; ii) the policy goals relating to greater equity of allocations; iii) the impacts of climate change with possible reductions in water supply to irrigation farmers, and; iv) the increase in demand of irrigated orchards under rising temperatures.
Water insecurity has been identified as a major risk by the deciduous fruit industry where
production is totally reliant on irrigation. In turn, the importance of the deciduous fruit industry to national agricultural exports, foreign exchange earnings and employment cannot be underestimated. It is essential that the industry remains competitive and provides growth opportunities. This means that the water-related risks, but also the options to manage them, must receive attention through robust practical research, capacity building, and knowledge dissemination. Given the pressures in both supply and cost, growers will have to increase the water productivity of irrigated apple orchards. The use of protective netting over apple orchards is one of the promising emerging technologies that can be implemented.

AIMS AND OBJECTIVES
General aim
To compare water use of a high producing open and netted (fixed and draped) full bearing
apple orchards under optimal management and unstressed water use conditions, in order to
determine water savings per ha and per ton.

Specific objectives
1. To measure and model water use (expressed as evapotranspiration (ET)) of open and netted (fixed and draped) full bearing apple orchards under micro-irrigation.
2. To determine apple yield and quality of open and netted (fixed and draped) full bearing apple
orchards.
3. To quantify water use efficiency and water productivity per ha and per ton of open and netted
(fixed and draped) full bearing apple orchards.
4. To quantify water savings per ha and ton with micro-irrigation of netted (fixed and draped) full bearing apple orchards.
5. To explain the components of reduction of water use through transpiration and evaporation.
6. To budget and evaluate the reduction of water costs and increase of income from apple
production, in comparison with the increased capital and maintenance costs of fixed and draped netting discounted over time

The study was conducted in the Koue Bokkeveld (KBV) and Elgin-Grabouw-VyeboomVilliersdorp (EGVV) production regions in the Western Cape, South Africa. Both regions have a Mediterranean-type climate with high levels of solar radiation in the growing season. The cultivars studied were ‘Rosy Glow’ (a high-coloured spontaneous single-limb mutation of ‘Cripps Pink’), ‘Golden Delicious’, and ‘Golden Delicious Reinders’®, a mutant of ‘Golden Delicious’. High-colouring red and bi-colour apple cultivars are seen to be financially viable under costly fixed netting systems, but are prone to poor colour development in lower light conditions. Green and yellow cultivars are sensitive to sunburn damage and changes in peel pigmentation. Draped netting is regarded as a better option for less profitable, but widely grown cultivars, in existing orchards.

Data were collected over three growing seasons, 2018-2019, 2019-2020 and 2020-2021. Three trials were conducted, as summarised in Table I. All orchards were high-yielding with good fruit quality. In the ‘Rosy Glow’ orchard, a white 20% knitted netting was installed in 2014 above the trees (4 m) in one section of the orchard, which was split into a control and a netted section. A fixed (permanent) flat structure was used. The trial was conducted for two consecutive seasons. In ‘Golden Delicious’ and ‘Golden Delicious Reinders’®, black draped netting with a 24% shade factor was installed after final fruit thinning in early summer, and removed at harvest. All orchards were irrigated using a micro-sprinkler irrigation system, and the soils were predominantly sandy.

Two treatments were established: 1) an open control, and 2) a netted section. Four rows of
trees were marked and trees in the central two rows were used for measurements. Ten single-tree replications were marked in these two rows. Sap flow (transpiration, T), soil water content, and the microclimate in each orchard were monitored from full bloom in October until full leaf drop in July of each season. Four trees were instrumented for sap flow monitoring using the Heat Ratio Method (HRM), two trees were instrumented with time domain reflectometer soil water content sensors, and water flow meters were used to record irrigation volumes. Automatic weather stations were installed in the orchard using calibrated sensors. Orchard evapotranspiration (ET) was estimated using the soil water balance approach, and modelled using an adapted version of the dual source Shuttleworth and Wallace model. Trees were harvested and yield, fruit maturity and fruit quality recorded. Quality control data, pack out percentages into classes, and prices obtained for each class were provided by the pack house for the first two trials. Other measurements included soil physical properties, cover crop transpiration, and orchard floor evaporation, shoot and fruit growth rates, leaf area index, leaf
gas exchange, and leaf and stem water potential. Physical and economic water productivity were calculated from seasonal T, ET, yield and orchard income.

Finally, where the results allowed, the water use savings of these orchards under netting and
in the open were quantified, and the components of reduction of water use through transpiration and evaporation estimated. A farm enterprise model was adapted and used to model the increased income from production under netting over the orchard’s lifetime, taking into account the water savings and other benefits, as well as the increased costs discounted over time.

Both types of netting altered the orchard microclimate, but to varying extents. Results under fixed netting are summarised in a recent publication by Lulane et al. (2022) entitled: “Quantifying water saving benefits of fixed white protective netting in irrigated apple orchards under Mediterranean-type climate conditions in South Africa” published in Scientia Horticulturae. Averaged over two seasons, the nets reduced daily total solar radiation by ~ 12%, wind speed by more than 36%, and reference evapotranspiration by ~ 12%. Seasonal T was ~ 11% lower under nets, while ET was only ~ 4% lower (Table II). The differences in air temperature and relative humidity between the two treatments were very small, likely because of the small size of the fixed netted area. While the lower transpiration rates under the nets were expected, and they mirrored the changes in the atmospheric evaporative demand, the small difference in ET between netted and open orchards was rather surprising. Given
that microlysimeter measurements of soil evaporation showed significantly lower soil evaporation under the fixed nets, the small ET difference can only be explained by a much more active vegetation cover on the orchard floor under the nets than in the open. This result suggests that while the fixed nets reduced tree level water use rates, careful management of the orchard floor is essential to maximize the water saving benefits of fixed nets. There is a higher likelihood of more vigorous weed and cover crop growth under the fixed nets than in the open, possibly due to more favourable growth conditions, i.e. milder microclimate and relatively wetter soils. This observation is further supported by the performance of the modified Shuttleworth and Wallace model (Dzikiti et al., 2018) in which the transpiration component under both treatments was accurately predicted, but the model significantly under-estimated the orchard floor evaporation, and hence ET under the nets. Another confounding factor in this study could be the substantially higher irrigation levels that were applied under nets than the control treatment, creating a wetter micro-environment. However, the possibility of much more growth of weeds and cover crops under the fixed nets should be investigated in future studies as these tend to diminish the water saving benefits of the fixed nets. Based on transpiration measurements, the physical water productivity (kg of fruit per m3 of water transpired) was 14-15% higher under the nets, while this benefit was much smaller (~ 1%) when calculated on an ET basis. The economic water productivity (Rand per m3 of water transpired) ranged between 20 and 45%, while no meaningful treatment difference was found when the calculations were based on ET. Observations on yield and fruit quality under fixed nets confirmed findings from earlier studies.

The draped nets reduced the solar radiation within the tree canopies by an even larger proportion (30-35%), presumably because of the higher shade factor (~ 24%). The air temperature was on average 1-2°C cooler while the relative humidity remained 5-10% higher under the nets. The higher relative humidity can be explained by the poor air circulation under the nets and transpiration of water vapour from the trees which gets trapped under the nets. The effect of this was a decrease in the vapour pressure deficit of air under the nets by between 0.1 and 0.2 kPa which reduced the atmospheric evaporative demand. Transpiration declined by ~ 9% under the draped nets (Table II). This figure is an average over two seasons, but also from two different sites. The difference in water use based on ET data were mixed between the two seasons likely because of methodological limitations. It was difficult to accurately predict the deep drainage component in the soil water balance calculation which may explain some of the differences, especially given the very high irrigation levels in some orchards. With much more precise needs-driven irrigation practices (i.e. avoidance of over-irrigation and deep drainage) of drape netted orchards it is likely that changes in ET would support a greater water use savings. Yield was higher under the draped nets, varying between 6% (farm data, 2020, linked to smaller fruit size) and 14% (trial data, 2021, not significantly different) between the years and sites.

Protective netting installed over apple orchards in the Western Cape of South Africa has very clear benefits for production and marketable yield, and thus farm income, even when considering the costs of installation and maintenance. This study has confirmed that a saving in water use per hectare and per ton of fruit in high-yielding irrigated apple orchards is possible, adding to the other benefits of this technology. While the results were influenced by net type (fixed white, black draped), cultivars, tree age/size, production region, and season, a reduction in orchard level transpiration of between 3% and 15% was found under netting compared to the open control across three orchards. Orchard evapotranspiration differed more widely, between a reduction of 19% and an increase of 16%. Thus, absolute water savings varied. Physical water productivity (kg m-3) based on transpiration was consistently increased (10-34%); but the values based on evapotranspiration ranged from no effect to a 30% increase. Economic water productivity (R m-3) showed clear benefits of netting, with increases of 3-30% based on evapotranspiration.

Challenges with complex measurement techniques and other factors, such as optimised irrigation scheduling for the two treatments, lead us to conclude that these results should be regarded as an initial indication of potential water use savings under nets. The moderate savings achieved are likely partially explained by the microclimatic dynamics under the nets used. A relatively small area of fixed netting with open sides, and draped netting that only covers the canopy, result in no other changes in microclimate except a reduction in solar radiation and wind speed. Thus, evapotranspiration is not clearly reduced, especially where applied irrigation may be more than required. Very precise irrigation under nets is likely to yield greater water savings benefits. There was also some evidence to suggest that more vigorous growth of the cover crop under the fixed net contributed to a higher orchard evapotranspiration, and that a greater water savings may be achieved with adjusted cover crop management.