HORTGRO’s irrigation seminar drew a large turnout of Western Cape fruit growers, indicative of the severity of the water situation.
“The future is dry,” says Prof Wiehann Steyn, crop production programme manager at HORTGRO Science. “Drought is the new norm, with the probability that, according to climate change forecasts, we’ll be getting 30% less rainfall in future.”
The Western Cape is green due to recent rains, but the winter rainfall is still below average
Warnings years ago already about the Western Cape water supply
Marinus van der Merwe of Agri Western Cape’s Natural Resources Committee painted a troubling picture of water access in the Western Cape, where agriculture has to compete with an ever growing urban population. “We’re in a perfect storm: there is a natural drought but we’re also experiencing a man-made drought. There have been projections that the Western Cape would run out of buffer water supply and since 2014 there have been troubling technical reports on a shortage of reservoir capacity. For all intents and purposes, the regions of the agricultural sector that also feed into this system is at 100% water restrictions, when we look at water budget projections at current levels. This will only be revised by 1 November, according to Department of Water and Sanitation sources.”
“It might take two normal rainfall seasons to refill Western Cape capacity but it looks worse for the Theewaterskloof Dam region and the situation in the Upper Berg River, Winelands and the EGVV [Elgin, Grabouw, Vyeboom, Villiersdorp, a major topfruit production area] is disastrous,” continued Van der Merwe, a topfuit (pomefruit) farmer himself. “Now we’re basically hoping for storms and floods to get the dams full because winter’s over. We drastically need aligned priorities to keep rural economies alive and we must create additional storage capacity – that is key to the survival of irrigation farming in the Western Cape.”
Irrigation design based on soil analyses
Planning irrigation under these circumstances necessitates all hands on deck. Daan Brink, technical advisor at the Two-a-Day Group pointed to the need for soil scientists to be brought in to analyse soil across a farm (the Western Cape has a very wide spectrum of soil types) in order to categorise blocks according to texture, restrictive layers and so forth. “The basis of irrigation is block design based on soil analyses. The current strategy is to irrigate less and in accordance with root distribution.”
As for the water usage differences between apple cultivars, he confirmed that this was a negligible factor in irrigation planning.
Louis Reynolds (Fruitful Crop Advice), Werner de Leeuw den Bouter (Dynamic Irrigation), Daan Brink (Two-a-Day) and André du Toit (Netafim)
Werner de Leeuw den Bouter of Dynamic Irrigation explained how to calculate economical block size depending on the annual irrigation requirement, evapotranspiration, crop factor as well as the application rate of the system. Then this design must be matched with the soil map. “You should spend a bit of time on your block layout to accommodate soil types, it’s highly recommended, because you’ll sit with your irrigation design for the next ten to twenty years.”
He suggested that rows in newer blocks should not exceed 100m in length, while keeping in mind annual inflation of about 7% in electricity tariffs. It may make financial sense to spend more money during the installation of an irrigation system with bigger pipes that will save electricity during its lifetime due to lower water flow resistance.
Agrimotion’s Mico Stander and Tiaan Snyman placed emphasis on soil moisture monitoring (through an array of methods, from the well-known tensiometer, to capacitance probes that provide continuous data sets, to phytomonitoring of sap flow, stem diameter, fruit growth) as well as on atmospheric monitoring of the vapour pressure deficit (VPD) and weather predictions. “This is the most integral part of your planning,” they said. “Capacitance allows you to set an upper line marking field capacity, which is the water held against gravity, while the lower line is the refill point of 50% available water when the plant is working harder to take up water. These lines may change throughout the season according to the plant's needs and the irrigation strategy. Irrigation scheduling should be done at least weekly and the more iterations of your irrigation cycle, the more accurate it will be.”
Irrigation according to phenological stages
A further refinement of irrigation scheduling, is irrigation according to phenological (fruit development) stages. Prof Stephanie Midgley, an independent plant production researcher associated with Stellenbosch University, set out the periods of top irrigation priority. The fruit cell division stage that lasts up to 40 to 50 days after full bloom is particularly sensitive to water stress because any shortcoming at this time can never be made up later, resulting in small fruit. However, tree water requirement during this stage is lower than during mid-summer and in the Western Cape the soil might still be relatively moist due to spring rains. Growers can save water during this stage by not irrigating unnecessarily, but care should be taken when soils are dry to not stress the tree.
Equal in importance is bud initiation in summer, when the foundation of the next season’s growth is laid down. Interestingly, during the period of fruit growth an apple tree is less water-sensitive and is able to extract moisture from the fruit to be used elsewhere in the plant, to be replenished at night.
However, severe water stress during this stage negatively affects fruit quality and the value of the crop by decreasing fruit size and increasing the incidence of sunburn. Hence, the decision of how much to irrigate at this stage may depend on whether tree's survival is the only consideration or whether fruit quality needs to be maintained.
Nelius Kapp (Soil2Root), Mico Stander (Agrimotion), Ruben Goudriaan (eleaf) and Tiaan Snyman (Agrimotion)
Mico Stander of Agrimotion elaborated on the principle of irrigation according to phenological stages. “To what degree can we stress the tree and within which dates? It’s difficult to answer but it also strongly depends on your soil type and root distribution within the soil.” Of equal importance is effective soil infiltration: runoff during irrigation is unacceptable, especially under current circumstances. Water budgeting will become a sine qua non for farmers, which should be done every four to six weeks to make sure you’re still on track with your water use.
Ranking orchards according to income per unit water
Spreading out the available water across all existing orchards merely results in equal misery for both farmer and tree, said HORTGRO’s Prof Steyn. Some hard choices will have to be made, explained Mico Stander. “You must use water where you’re most likely to make money, therefore you have to rank orchards according to income per unit water, Rand/m3. You’re going to see a lot of those ‘I’ll give it one more year’ trees being shaken out of the industry which isn’t necessarily a bad thing.”
Orchards should be divided between those that earn the highest dividends, and these receive 100% of their water needs; orchards that you want to keep alive receive 25% while those from which one wishes to obtain a reasonable yield and young orchards receive 50% of their water needs.
"It was a sad exercise but we had to remove orchards"
Lastly Jan Rossouw, estate manager of Lindeshof: Dutoit Agri, described how they had gone about fruit production during the 2016 season when they had less than half their usual water. “We kept a weekly record of every drop of water put down in our orchards. We reviewed our water budget every week, keeping evaporation in mind and distributing water timeously between the farm dams.”
They, too, had to reduce their orchards. “Every orchard was evaluated according to merit. It was a sad exercise but we removed 24ha. We determined the root depth of every orchard and I can’t overemphasise the importance of the phenological stages. And if you haven’t yet put down organic material in your orchards – now’s the time.”
They keep to a ratio of fruit to foliage of 1:25 and regularly measure fruit size for, as he pointed out, in this case, bigger is better. Foliage is pruned during summer, to keep down transpiration. Lastly, remember the post-harvest fertilisation programme when doing the water budgeting sums for the season.
“The tree looked after you throughout a difficult year and now you must look after the tree,” he concluded. “We have all the tools in our toolbox to get through this.”
