14.7: Management

The success of any irrigation system, particularly microirrigation, depends on management. Irrigating by small quantities frequently is quite different from sprinkler and surface irrigation methods where larger, less frequent applications are normal. With microirrigation, precise information on crop water requirements is required to determine the appropriate irrigation amount. Feedback information on soil water or plant water status is frequently used to schedule irrigations for microirrigation systems.

Wetted Area

A major difference among irrigation systems for agronomic crops is the portion of the soil surface wetted. Most microirrigation systems wet only a portion of the cross-sectional area of the soil profile, as depicted in Figure 14.2. The percent of the surface area wetted, P_w, by microirrigation systems compared to the entire cropped area, depends on the volume and rate of discharge at each application point, the spacing of water applicators, and soil type. No bestor minimum-wetted area percentage has been found, but systems having high P_w values provide more stored water, which is a valuable advantage in the event of system failure. A reasonable design objective for widely-spaced crops such as vines, bushes, and trees is to wet between one-third and two-thirds of the soil surface dedicated to each plant. In regions that receive considerable supplemental rainfall, values of P_w less than one-third are acceptable for fine-textured soils. Maintaining P_w below two-thirds for widely-spaced crops maintains dry strips for cultural practices. In closely-spaced row crops with the laterals in every or every other crop row, P_w approaches full coverage.

Spray emitters wet a larger surface area than drip emitters. They are often used on coarse textured soils where wetting a large surface area would require a large number of drip emitters. Figure 14.2 shows a comparison of wetting profiles for drip and spray emitters.

Salinity

Microirrigation has potential advantages where the soil or irrigation water is saline. The principal advantage is that with microirrigation the water content of the root zone is maintained high and nearly constant. As a result, the salt concentration of the soil solution is low and steady, and thereby not creating as much salt stress as a system where the soil dries between irrigations with congruent significant increases in salt concentration.

A potential disadvantage is the uneven salt distribution in the soil profile. Refer to Figure 14.2 to see comparable salt profiles for various irrigation methods. This uneven distribution can cause problems if the irrigation system fails and the crop roots begin extracting water from areas of high salt concentration, or if salts that are shallow in the profile are flushed into the root zone by rainfall. When used on annual crops, moving the plant row spacing geometry can cause problems if the salts have not been leached.

Water Requirements

The plant canopies of young or widely-spaced crops shade only a portion of the soil surface area and intercept only a portion of the incoming solar radiation. Conventional estimates of water requirements of young crops assume a portion of the applied water will be lost to nonbeneficial consumptive use. This loss is through evaporation from the wetted soil surface or through transpiration from undesirable vegetation. Most microirrigation systems reduce evaporation losses to a minimum, so transpiration by the crop accounts for practically all of the water consumed.

Assuming that evaporation during application is minimal and no upward flow from groundwater into the root zone, the gross irrigation requirement can be expressed as:

d_g = ET + d_p + d_r – P_e – ΔS (14.8)

where d_g = gross irrigation requirement,

ET = evapotranspiration,

d_p = deep percolation,

d_r = runoff Pe = effective precipitation, and

ΔS = the change in soil water storage.

All terms are normally expressed in units of depth. The volume equivalent for each term in Equation 14.8 is the product of the irrigated area and each term. Thus, if d_g was 2 inches and the irrigated area was 5 acres, the volume of water needed would be 10 acre-inch or 271,540 gallons.

One of the objectives of microirrigation is to maintain soil water content constant. If this is achieved, ΔS in Equation 14.8 is zero. For well managed microirrigation systems, runoff should be zero. If salinity is not a hazard, then the irrigation requirement does not need to include water for drainage (deep percolation).

Irrigation scheduling involves two primary decisions: when to irrigate (timing) and how much to apply (amount). Microirrigation inherently implies frequent irrigations. Depending on the system and the sophistication of the controls, irrigation frequency can be from once in several days to multiple times every day. Many commercial systems operate daily or every other day. The operational time for a microirrigation system should not exceed 20 hours per day. In case of repair or maintenance requirements, time is required to catch up. This is particularly critical during periods of peak crop water use.

One of the primary design considerations in microirrigation is determining how many emitters are required to meet the irrigation requirement. The number required can be determined by:

\(n=\dfrac{d_gA}{q_e \times T} \) (14.9)

where: n = number of emitters,

d_g = applied (gross) depth of irrigation required,

A = irrigated area,

q_e = emitter discharge, and

t = application time.

For surface and subsurface drip systems the spacing between emitters can be specified to the manufacturer. Typical drip emitter spacings are from one to several feet. The spacing between emitters will depend upon the spacing between laterals, the irrigation requirement, and water availability. For example, if you wish to drip irrigate a field of tomatoes and the rows are 3 feet apart, you might place a lateral along each crop row or midway between adjacent rows. If you placed a lateral in every row and the irrigation requirement was 0.27 inches per day, you wanted to irrigate 1 hour per day, and you chose an emitter with a discharge of 2 gal/hr, then Equation 14.9 could be used. To determine the area to be irrigated by each emitter solve Equation 14.9 for A as illustrated in Example 14.10.

Example 14.10

Determine the area each emitter would irrigate in a trickle irrigated tomato field with one drip lateral serving every row and one lateral between two rows.

Given: d_g = 0.27 in

t = 24 hr

q_e = 2 gal/hr

Find: Area irrigated by each emitter with one drip lateral in each row

Solution

Rearranging Equation 14.9 with n = 1:

\(A=\dfrac{q_e \times t}{nd_g}=\dfrac{2\text{ gal/hr}/7.48\text{ gal/ft}^3\times 1\text{ hr}}{1\times 0.27\text{ in}/12\text{ in/ft}}=12\text{ ft}^2 \)

Thus for 1 lateral per row each emitter will irrigate 12 ft² . With 1 lateral per two rows that answer would be 6 ft² per emitter.