Design Principles

Emitter Placement

The first decision in designing a microirrigation system is selecting the type and placement of emitters. This will depend on the crop (annual or perennial, spacing, rooting extent, sensitivity to water and nutrient stress, economics), the soil (permeability and water-holding capacity), and the cultural practices (tillage and harvesting operations, availability and cost of labor). For example, for annual row crops, either portable surface drip, subsurface drip, or disposable thin-wall tubing may be used, depending on the economics and practices. For perennial crops, the microirrigation system can be above-or belowground and permanent. For trellised perennial crops, such as vineyards, the drip laterals can be hung from the trellising.

The amount of the soil that must be wetted is the most important factor in choosing emitter spacing. In general, less wetting of the soil surface reduces system costs and evaporation losses. However, experience with microirrigation systems increasing shows that, with some crops, a substantial portion of the potential rooting extent should be wetted to reduce risk and achieve maximum yields. This has motivated the recent trend from drip to microspray irrigation of tree crops. A reasonable objective of design for widely spaced crops, such as vines, bushes, and trees, is to wet between one-third and two-thirds of the cropped area. For water-stress-sensitive row crops, at least two-thirds of the area should be wetted, whereas less than half may be adequate for drought-tolerant crops, especially if significant rainfall is expected. If the soil or water contains significant salts, salinity management may be a critical factor in emitter placement. Salts cannot be allowed to build up to damaging levels in the active root zone.

It is difficult to accurately predict the horizontal water movement from a point drip source. Keller and Bliesner [11] propose the following empirical equation to compute the wetted width or diameter from a point source emitter:

where Dw is the subsurface wetted diameter (m), Vw is the volume of water applied (L), Ks is the saturated hydraulic conductivity of the soil (ms-1), q is the point-source emitter discharge (L h-1), and C2 is an empirical coefficient, 0.031, for these units.

Mechanistic models have been developed to simulate the soil water distribution around a point source. The two-dimensional and three-dimensional Richards equation often is utilized. However, these models require knowledge of the K(h) and h(d) curves (see Section 5.2), which are not available for most of applications. An analysis comparing several modeling approaches is presented by Angelakis et al. [132].

Simple field tests are the most reliable way to determine wetting diameter. They consist of selecting operating emitters at a few representative sites and then measuring the resulting wetting patterns.

The diameter of soil surface wetted by an emitter usually is about three-quarters as large as that measured at a depth of 30 cm. On sloping fields the wetted pattern may be

Table 5.12. Estimated wetted diameter Dw from a 4 L h 1 drip emitter by degree of soil stratification

Wetted Diameter (m)

Table 5.12. Estimated wetted diameter Dw from a 4 L h 1 drip emitter by degree of soil stratification

Wetted Diameter (m)

Soil or Root Depth and Soil Texturea

Degree

of Soil Stratification

Homogeneous

Stratifiedb

Layeredc

0.75-m Depth Coarse

0.5

0.8

1.1

Medium

0.9

1.2

1.5

Fine

1.1

1.5

1.8

1.5-m Depth Coarse

0.8

1.4

1.8

Medium

1.2

2.1

2.7

Fine

1.5

2.0

2.4

a Coarse includes coarse to medium sands; medium includes loamy sands to loams; fine includes sandy clay to loam to clays (if clays are cracked, treat as coarse-to-medium soils). b Soil of relatively uniform texture, but having some particle orientation or some compaction layering that gives higher horizontal than vertical permeability. c Soil that changes in texture with depth and in particle orientation as well as being moderately compacted. Source: Adopted from [11].

a Coarse includes coarse to medium sands; medium includes loamy sands to loams; fine includes sandy clay to loam to clays (if clays are cracked, treat as coarse-to-medium soils). b Soil of relatively uniform texture, but having some particle orientation or some compaction layering that gives higher horizontal than vertical permeability. c Soil that changes in texture with depth and in particle orientation as well as being moderately compacted. Source: Adopted from [11].

distorted in the downslope direction, but the actual wetted diameter will be similar to that on flat ground.

Table 5.12. gives estimates of the wetted diameters Dw of a standard 4 L h-1 emitter for different soil conditions and wetting depths. Values in Table 5.12 are based on daily or every-other-day irrigations that apply volumes of water sufficient to slightly exceed the crop's water-use rate. Almost all soils are either stratified or layered to some extent. However, assuming stratification or layering is risky. This must be determined by actual field checks.

Once the desired wetting pattern is established, the lateral spacing and placement and emitter spacing must be determined. In row crops, the lateral spacing decision generally will be between a few alternatives, for example, one or two laterals per bed or a lateral for each or alternate rows. In tree and vine crops, the decision usually will be between one or two lateral lines per crop row. Many growers have found that one drip lateral per vine row is adequate, but two drip laterals or one lateral with microsprays per tree row give better production. Emitter spacing then should provide the required wetting. The spacing of emitters (Se) along the lateral should be about 80% of the wetted diameter given in Table 5.12 (Se = 0.8Dw) to ensure full contact between adjacent wetting bulbs.

Lateral depth placement will depend on crop, soil, and cultural practices. Laterals often are suspended in vineyards and laid on the surface for trees. The disadvantage of on-ground or aboveground laterals is that they may be damaged by rodents or other animals, or by farming activities. The advantages are ease of placement, monitoring, and repair.

Permanent subsurface drip must placed below tillage depth. Deep placement may necessitate use of sprinklers to germinate the crop. In general, disposable thin-wall tubing is placed 40 to 100 mm below the surface, whereas permanent tubing is placed 200 to 500 mm deep.

Uniformity

Several parameters have been used to describe water distribution uniformity of microirrigation systems [see Eq. (5.119) to (5.125)]. Emission uniformity (EU, %), is used primarily to describe the predicted emitter flow variation along a lateral line:

where Cv is the coefficient of manufacturing variation [Eq. (5.160)], N is the number of emitters per plant, qn is the minimum emitter discharge rate (L h-1) computed [Eq. (5.158)] from the minimum pressure along a lateral in the subunit or in a system, and qa is the average or design discharge rate (L h-1). The ratio qn/qa expresses the relationship between the minimum and average emission rates resulting from pressure variations within the subunit or system. The factor (1 - 1.27Cv/VN) adjusts for the additional nonuniformity caused by manufacturing variation between individual emitters. The EU values recommended in ASAE Standard EP405.1 [19] for different site conditions are presented in Table 5.13.

Some authors [15] use the statistical uniformity Us [Eq. (5.125)], defining the coefficient of variation of emitter flow, Vq, as where qi are the observed or simulated emitter discharges and qa is the average discharge. It is assumed that this coefficient of variation results from different causes: manufacturing variation, pressure variation, and emitter plugging or clogging. The various sources of nonuniformity can be combined such that the relative importance of each on the overall

Table 5.13. Recommended ranges of design emission uniformities EU [19]

Emitter Type

Spacing (m) Topography

Slope EU Range

Point source on perennial crops

>4 Uniform

Steep or undulating <4 Uniform

Steep or undulating All Uniform

Steep or undulating

<2 90 to 95 >2 85 to 90 <2 85 to 90 >2 80 to 90 <2 80 to 90 >2 70 to 85

Point source on annual or semipermanent crops Line source on annual or perennial crops

variability can be determined:

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