Planning a Pecan Irrigation System

Irrigation

In South Africa rainfall often is not sufficient, and fails to satisfy the water requirements of pecan-nut trees for optimal production. During the critical growth stage, additional irrigation is usually necessary.

Despite the fact that the pecan-nut tree has a deep-tap root system, it is adequate to supply the top 1m with water.

Recommended wetting area based on tree age

Age (years)
Diameter (m) of wetting zone
Wetting area (m2)
1 – 2

2,0

3
3 – ­5
3,0
7
6 – ­10
4,5
16
11 – ­15
6,5
33
15+
8,0
50

The water requirements of a pecan tree is higher than any other nut fruit.  For the survival of the pecan tree during its first year, additional irrigation is usually necessary after approx. every 2 weeks, during a dry period.The watering should be given fairly deep so as to soak the whole area around the roots. Mulching the ground around the tree with dry leaves or straw helps to economise irrigation water, this also helps when establishing trees. Occasional irrigation should be applied in the years to follow, as and when needed. Flooding, furrows, sprinklers and drip irrigation are common methods of irrigation. Drip irrigation can be effective for establishing an orchard, but beware of over watering the pecan tree as excessive moisture will damage it.

The Pecan nut tree naturally loves water and for this reason South Africa’s rainfall is not sufficient to meet the water needs. It is usually during the critical growth stages that irrigation has an impact on production. For this reason it is very important to manage irrigation water optimally.

 To use irrigation water optimally is much easier said than done. As producer, one must look holistically at the whole principle of irrigation. It does not help to install the best irrigation system if poor land preparation is done or poor plant material is used.
All aspects relating to the cultivation of Pecans should be taken into account during the planning of an irrigation system. There should be started well in advance to obtain the necessary information. The proper planning and design of an irrigation system is critical for the optimal management of irrigation and requires a careful consideration of various aspects. The planning of an irrigation system must include the following:

Management – current management style of producer, management inputs required for successful scheduling, infrastructure, operation and maintenance
of system.
Water – source of water, water availability (when and how), quality of water, what is the delivery of the source, filtration.
Soil – potential and / or effective soil depth, texture (sand, clay and silt) stone rate, water holding capacity, infiltration rate of the soil, chemical and / or physical limitations.
Crop – what and when is the peak water demand and what is the seasonal water needs (do I have enough water), what is the expected effective root depth, when is the critical times when the crop is sensitive to a shortage of water, at what abstraction levels do we wish to irrigate against (how empty must the tank be before irrigated).
Irrigation system – is the planned system suitable for the soil type, what is the hours per day and days per week available for irrigation, is there a power supply, what must the system’s output be to meet the peak demand, must fertilizers or other chemicals be administered through the system (central dose vs the block).

The planning of an irrigation system begins with the producer. It is not expected from the producer to be an expert on irrigation, but he must be willing to cooperate with the necessary expertise in order to bring the best plan to the table. The designer and other experts must keep the management style and infrastructure of the producer into consideration when recommendations regarding the irrigation system are made.
The planning of an irrigation system is a multi-disciplinary cooperation between many different experts – not just the producer and / or designer.

We will now focus on the irrigation system itself, and specifically the choice between drip or micro irrigation systems.  All irrigation systems are watering the ground from where the tree can absorb water. Each system has its own advantages and disadvantages and the choice of system therefore depends on the producer’s own abilities and preferences.

Netafim proposed drip irrigation to the Pecan Nut Industry more than 10 years ago. Initially the principles of drip irrigation were amended from Mexico. Due to discussions and visits to producers, meetings with advisers and own experience acquired over the past 10 years, Netafim suggests that the following adjustments must be made regarding drip irrigation:

Quantifying Water use of Pecans:

Introduction:
There is currently a gap in knowledge in water use of fruit tree orchards in South Africa, which often results in poor irrigation water management. Consequently, the Water Research Commission of South Africa solicited a research project in 2006 to quantify water use of fruit tree species. Water use of fruit tree orchards varies depending on several factors and making measurements under a wide range of field conditions is time consuming and expensive. Crop water use modelling is therefore on effective way to estimate water use of fruit tree orchards under a wide range of conditions.

Materials and Methods:
Transpiration of mature pecans was monitored for three consecutive growing seasons (from 2009 to 2012) at Cullinan, South Africa and soil evaporation was estimated for the experimental period using the dual crop coefficient approach of the FA056 model. These estimates, together with transpiration values, were used to obtain water use from the orchard. A simple equation developed in New Mexico, USA, by Somani et 0/. (2011), which uses fractional ground cover (fc) to predict crop coefficients and water use of pecans, was tested for seasons with differing fc and weather conditions, by comparing actual to predicted water use.

Results and Discussion:
Seasonal water use of mature pecans at Cullinan varied from 943 to 1035 mm and fc varied from 82-98 %. Average crop coefficients throughout the season varied from 0.6 (at the beginning and end of the season) to 1.35 (when the canopy cover was at a maximum). The Somani et 0/. (2011) equation predicted water use of pecans fairly well on a monthly and seasonal basis, using reference crop coefficients from New Mexico, but foiled to perform well for shorter intervals. An adjustment of reference crop coefficients for the climatic conditions in South Africa provided better estimates of water use with the Somani et 0/. (2011) equation on a weekly and monthly basis.

Conclusions:
The Somani et 0/. (2011) equation can therefore be used to schedule irrigation of pecans in South Africa, with adjusted reference crop coefficients for the specific climate and measurements of fractional ground cover, with the final goal of achieving maximum orchard productivity and profitability.

Acknowledgements:
Funding support is provided by the Water Research Commission and the Notional Deportment of Agriculture, Forestry and Fisheries.

References:
Somani Z, Bawazir S, Skaggs R, Lomgworth J, Pinon A, Tran V. 2011. A simple irrigation scheduling approach for pecans. Journal of Agricultural Water Management 98: 661-664.

Keywords: fractional ground cover, crop coefficients, transpiration.

Drip Irrigation System:

Photo 2 & 3: The wet patches of an underground drip system that can be seen on top of the ground.

Design – we still feel very strongly that a drip system must be designed to provide for four drip lines.  The goal is to water approximately 20-30% of the soil volume. This could mean that if the line spacing becomes narrower, less drip lines (for good reason) are required to conform to the norm of 20-30%. Although the system is designed for four lines, this is not to say that all four lines are used. With establishment of the orchard, only one drip line will be installed and as the tree grows and obviously the water requirement as well, the other drip lines will be installed in phases as needed.

It is important that all the design guidelines of an underground system for example:
Sufficient air valves, coil taps, filtration, flow measurement, etc. are still complied with.

Installation – to ease orchard traffic and implementation actions, the underground drip (SDI = Sub-surface Drip Irrigation) is recommended. SDI means that the drip lines must be installed 20-40cm below the ground by using a ripper.  As already mentioned, installation can be done in phases. The initial guideline from Mexico was an installation depth of 30cm. With the past few years’ root investigations, it seems that 30cm was too deep and therefore they changed the guideline to an installation of 15-20cm depth. The mutual phases for a typical installation will look as follows:

Photo 4: The ripper that can be used to install the underground drip.

Phase 1 (Settlement):  Establish young trees with one surface drip line on the tree row.

Phase 2 (3-5 years old trees):  Install the second drip line under the ground about 1.8m from the trunk. The first drip line remains in his position above the ground. The initial guideline from Mexico was closer to the trunk (1.5m), but because of the drip pipe that can be pinched by the structural roots that grow from the roots, the distance is extended to 1.8m.

Phase 3 (6-year-old trees):  Move the first drip line 1.8m away from the trunk (on the
opposite side of the second drip line) and install it under the ground. The roots should be given sufficient time to adjust at the different watering zones and therefore the first drip line will be moved away from the tree at a later stage.

Phase 4 and 5 (10-year-old trees or depending on water requirement):  The
additional third and fourth drip line can be installed under the ground 1.0m from the first and second drip line as the tree’s water needs increase.

Adjustments:  During visits to Mexico in 2011, we observed that the trees should be established with two drip lines. The two drip lines are about 30-50cm from each other (above the ground) and in year three, the third line (approximately 1.5 – 1.8m from the trunk) will be installed under the ground. The following year, the fourth line on the opposite side, will be installed under the ground. Afterwards, the first two drip lines on the outside (about 1.0m away) from the second and third line are installed under the ground. The Mexicans have very good results with the initial two lines and it gives you a good alternative for faster growth. Scheduling is very important in this case to prevent over-irrigation and water logging.

Delivery and spacing of drippers – with the past few years experience, the recommendation is to reduce the delivery of drippers. 1.6 liters / hour and 2.3 liters / hour drippers give the best results, especially in sandy soils. Spacing can range from 0.5 – 0.75m between drippers. The lower output drippers lead to longer irrigation hours with a consequent improved hydraulic efficiency of the system and with the approaching spacing, they obtained strip watering sooner.  With good reason and a more extensive irrigation approach, the initial recommendation of 3.5 liters / hour with 1.0m between drippers can still be applied with success.

Micro Irrigation System:  
Design – although most Pecan Orchards started with drip irrigation, there are also a number of inquiries regarding micro systems. When a micro system is chosen, we must try to wet a maximum of 60% of the surface coverage with at least a 90% uniformity coefficient within the watered strip. The challenge again is to develop and install a system for young and mature trees. Just like the drip irrigation system the focus area for the system design and the installation thereof must be the mature trees itself.  However, filters and pumps can be adjusted as the trees grow, but main lines, branch lines, etc. should be ready for the adult system.

As with drip irrigation, the installation of the proposed micro systems can be done in phases:

Phase 1 (establishment): start with a small micro sprinkler (low output and small diameter) near the tree. The sprinkler may have a reflector on to water a smaller diameter at the beginning. As the tree grows, the reflector is broken down to produce larger diameter coverage.

Phase 2:  Add another sprinkler of the same delivery on the opposite side of the tree in order to enlarge the root zone as well as to meet the tree’s growing water demand. The sprinklers can be moved further away from the tree to make the harvesting process easier.

Phase 3 and 4 (Optional):  Replace one of the small micro sprinklers with a higher delivery sprinklers who can water larger diameter coverage. Replace the second small micro sprinklers at a later stage with the same high delivery sprinklers. The sprinklers can also be moved further from the trunk. It is important that the sprinklers.

Figure 2: The installation phases of a micro-irrigation system.

When a decision must be made regarding the type of irrigation systems, it is important to take other factors such as orchard floor management of the irrigation system interfering with harvesting, water and electricity charges, etc. into consideration.

It is very important to a make use of a professional irrigation consultant and / or company with a well known and traceable history. It is also important to visit different producers with different systems and talk about their experience. A good system starts with good and advanced planning. The land is the heart of the farm and the irrigation system is the “lifeline”!

Figure 1: Schematic representation of the installation phases of underground drip.

IRRIGATION PLANNING FOR A PECAN NUT ORCHARD
Willem Botha, Agriculturist – Netafim Suid Afrika.
Tel. 021 987 0477, e-mail: willemb@netafim.net

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