Super-efficient irrigation: tips for crop farmers

To ensure agricultural sustainability in South Africa, it is crucial that more efficient irrigation systems be implemented. Agricultural resource manager, Felicity Mitchell, explains how this can be achieved.

centre-pivot-irrigation-system
According to agricultural resource manager, Felicity Mitchell, crops under irrigation utilise 61% of South Africa’s freshwater resources.
Photo: FW Archive

South Africa’s average annual rainfall is 495mm, compared with the global mean of 860mm.

It is for this reason that South Africa is considered water-scarce and is classified as one of the world’s 30 driest countries.

Most of South Africa’s freshwater resources are supplied by summer rainfall that is often poorly distributed, which leads to regular, localised drought conditions.

According to Felicity Mitchell, agricultural resource manager at the KwaZulu-Natal Department of Agriculture and Rural Development (KZN DARD), there is, on average, a drought somewhere in the country every three years. “Our agricultural systems are highly reliant on climatic conditions.

While only 1,5% of our land surface area [of 1,22 million km2] is under irrigated agriculture, this generates 30% of South Africa’s total agricultural production. Most of the irrigation applied is supplementary to rainfall. Despite this, irrigated agriculture utilises 61% of our country’s stored freshwater resources.

This creates the perception that our agricultural sector is an extremely wasteful water user. In some cases, this perception is accurate, while in others it’s not,” she explains.

Threat to South Africa’s irrigation farmers
In recent years, many South African farmers have realised the necessity to improve water-use efficiencies, and have shifted away from flood irrigation to drip irrigation.

According to Mitchell, however, the negative perception of South African agriculture as a wasteful water user remains, and the Department of Water and Sanitation (DWS) is under public pressure to withdraw certain water allocations from the sector to transfer these allocations to the mining and manufacturing sectors, perceived to achieve higher returns per cubic metre of water utilised.

This is despite the fact that South Africa’s labour-intensive agriculture sector supports numerous jobs, and has the capacity to create even more.

“Irrespective of what the DWS decides to do with water allocations, the fact is that there is increasing pressure on our water systems from many quarters of South Africa.

Currently, 17 of the country’s 19 major water catchments are under demand stress and cannot support the growing needs from those systems. Our country as a whole, including the agricultural sector, is going to have to try to do more with less water.”

South Africa’s water services authorities have already implemented a 15% water-use reduction strategy for residential and business users, and a 50% water-use reduction strategy for the agricultural sector in many catchments.

According to Mitchell, irrigation farmers will thus either have to halve the area of irrigated land under production, or use only 50% of their normal water requirements for the whole area, and hope that sufficient rainfall arrives to provide the needed shortfall.

Both strategies are a threat to farming viability.

Adding to South Africa’s freshwater scarcity woes are forecasts that above-average temperatures will occur across the country from September 2016 to January 2017, resulting in increased evaporation and plant transpiration.

This year’s spring rain is expected to arrive later as a result of an extended neutral phase between the drought-causing El Niño and the rain-bringing La Niña weather phenomena.

“When rain eventually falls, there’ll be a high risk of soil erosion due to poor plant cover. No-till farmers will have a distinct advantage here. Again, even if above-average rain falls this spring and summer, the country’s water deficit is already so large that it will take time for surface and soil water reserves to recharge and for veld grazing to recover,” Mitchell says.

Lowering plant transpiration
Even with drip irrigation considered the most efficient water-saving system, crop transpiration can waste water and is a particularly difficult aspect of water use management to control.

Plants can move water from the roots to the apex at a speed of 2m/h to 6m/h.

The transpiration rate is affected by relative humidity, air temperature, wind speed, direct sunlight, the plant species, the stage of the plant’s growth, and the plant’s nutritional status.

“The better a plant’s nutritional status, the lower its transpiration rate. In irrigated agriculture, this can result in improved plant water use efficiency and more dry matter produced with less water.

A balanced crop nutrition programme results in the plants not having to continually draw water from the soil to try to find nutrients, and can reduce water losses through transpiration by 5% to 10%,” explains Mitchell.

Acidic soil environments (with a pH of less than 7) also lead to increased crop transpiration rates.

In these soil types, many plant nutrients are locked up, and plants growing here have to draw water from the soil continuously to find nutrients. Liming acidic soils to reduce acidity before planting is therefore essential for improved soil moisture savings.

In windier areas, establishing windbreaks around croplands and inter-cropping with taller and shorter crops can reduce transpiration.

Infiltration rate: a crucial figure
Some water-use efficiencies can be controlled more easily by a farmer. These include how much water to apply and how often.

The rate of irrigation water infiltration into a cropped soil can be controlled by first determining the terminal infiltration rate, and then irrigating according to this rate (mm/h), as well as knowing the volume of water each soil type can hold at field capacity, and not exceeding this amount (mm/ rooting depth).

Over-irrigating soil with a slower infiltration rate and low water-holding capacity will result in water collecting on the surface, which will be lost to evaporation or erosion.

“A farmer should also know what the hydraulic conductivity is within each irrigated land’s soil profile, and then only irrigate for the most limiting layer within the soil profile and not for the initial infiltration rate.

A limiting layer is a layer within the soil profile that is high in clay, strongly structured, hard rock or has a water table – all of which can significantly reduce water movement. Farmers should know the characteristics of their soils for the entire rooting depth.”

Irrigation water should fill only the crop’s root zone. Over-irrigating soils, even those with high terminal infiltration rates and high hydraulic conductivity, will result in water drainage below the root zone. Once this has occurred, capillary rise can move only about 10cm against gravity to recharge the root zone.

Mulching and organic matter in soil
Soil surface evaporation can be reduced by applying crop residues and other plant mulches over the entire land.

This will minimise surface runoff and soil erosion, enhance moisture infiltration into the soil, and reduce evaporation. It is also important to be aware that organic matter in soil can hold seven to 10 times its weight in water, thereby significantly increasing soil water-holding capacity.

“All farmers should know the soil properties of each land, and how these affect water-use efficiencies,” Mitchell says. “A farmer who practises no-till has already achieved much towards enhanced water savings and improved water-use efficiencies.”

Phone Felicity Mitchell on 033 355 9386, or email her at [email protected].

This presentation was given at the No-Till Club’s 2016 Conservation Agriculture Conference held at ATKV Drakensville, KwaZulu-Natal, from 6 to 8 September 2016.