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Driplines are made of pipes and drippers (emitters). The correct intervals between drippers is essential for obtaining optimal results from any drip irrigation system.
Several variables may affect the selection of the optimal interval between drippers in a dripline:

Soil Texture for Drip Line Irrigation Systems

The pattern of the water distribution under the dripper depends mainly on the soil texture. The water distribution pattern is typically described as a “bulb” that is wetter in the center and drier on the exterior.

Short dripper intervals will form a sequence of “bulbs” that unite and form a “wetting strip” with a continuity of moisture in the soil along and under the dripline.
When the soil texture is coarse (sandy soil), the pattern of wetting is a narrow “bulb”: water penetrates the soil more quickly to the depth and more slowly to the sides.
When soil texture is finer, with more silt and clay in it, this “bulb” is wider and the wetting pattern will look like circles with a bigger diameter, compared to sandy soil.
Most plants consume the largest portion of water and nutrients from the upper layer of the soil.

When water and nutrients percolate deeper, the roots ability to consume water and nutrients decline, with reduced irrigation efficiency. Ideal dripper intervals will maintain the moisture in a way that the “bulbs” are connected and roots can develop along the dripline without dry barriers between wetting zones.

Plant size and spacing

Small, crowded crops benefit from shorter intervals between drippers. For example, plants such as lettuce or carrots are typically planted very close to each other. For such plants, soil moisture along the dripline needs to be highly uniform. Variability in moisture, especially at a young age, will produce undesirable variability in plant sizes and maturity time.  

When drippers are too far from the plants, rooting and development may slow down, and when drippers are too far from each other it is common to see an irregular growth pattern, in which plants close to the drippers are more developed. 

When nutrients are applied by fertigation, the growth difference will be even more pronounced because of the extra variability in nutrient distribution and uptake. 

For direct-seeded crops that are germinated by drippers, such as onion and carrots, continuous wetting is essential for uniform emergence of all plants. 

On the other hand, large plants, with deeper roots can adopt better to a non-sequential irrigation pattern.  

Disclaimer: Recommendations only… 

Root aeration requirements

Some plants are more sensitive than others to low soil aeration. On a fine textured (clay) soil, when a large plant is irrigated by a dripline, and a significant amount of water is emitted through few drippers, the soil near the drippers becomes saturated and the plant may temporarily suffer from lack of aeration. However, if the same amount of water, at the same irrigation intervals is delivered through more drippers, each dripper will emit less water per application. As a result, the ratio of water/air in the soil near the drippers will be more balanced and allow sufficient aeration, even during high irrigation demand. In orchards, more drippers between trees will enhance faster expansion of the root system, and faster and more sustainable growth of the trees.
More drippers encourage root growth and expansion in the soil. When there is a heavy nutrient demand, the extra roots, and the extra soil volume, serve as a buffer to the plants, and reduce nutritional fluctuations.
The downside of short spaces between drippers is the need for:
More drippers per area
A more massive conduction system to serve the increased flow rate

Root system geometry

A plant root system can be deep or shallow. The cause can be genetic, soil layering, or irrigation and crop management. For plants with deep roots, dripper distance can be bigger, however, when plants have shallow roots, irrigating with more drippers will increase the soil volume that participates in the irrigation, improving water and nutrient supply to the plants.
Weather considerations
Plant root growth is very adaptive to soil moisture gradient. When the water source is rain, uniformly covering the ground, the roots will spread wide. However, when the water is delivered from a point source, like drippers, then root density will increase close to the dripper, declining and diminishing far from the dripper.
During season transition: from rainy season to dry season or vice versa, the roots may need to adapt to drastic changes in the soil moisture pattern. When this change is fast, the plants may suffer from transient water stress. But with more drippers (more wetting points), the difference in soil moisture pattern will be more marginal, resulting in faster adaptation and minimal water stress during transition.

Clustering

In orchards with large trees there is a long distance between the trees, and in many cases, there is no reason to irrigate the space between these trees. For such orchards, like walnuts and palm trees, Metzer distributors offer an innovative dripline solution: Clustering.
The idea behind clustering is that each tree is irrigated by a group of drippers adjacent to the tree. This way the space between the trees is not irrigated and all water and nutrients are applied close to the tree.

Drip line irrigation system design and selection aids
For expert use, there are software tools that help optimize spacing of drippers and driplines
A good example of such a tool is DIDAS* (Drip Irrigation Design and Scheduling), a software package developed by ARO in the Volcani Research Center in Israel. The software optimizes drip irrigation layouts based on few parameters of crop, soil and climate. It also indicates irrigation water demand and predicts water use efficiency.

The software package is available for download after a short registration in the ARO website at https://app.agri.gov.il/didas/

* DIDAS – User-friendly software package for assisting drip irrigation design and scheduling, 2016   

S. Friedman, G. Communar, A. Gamliel : 

Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel

 
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