Field Evaluation

Field evaluation of sprinkler systems in operation can play a major role in improving irrigation performance. Field evaluations of set systems include the following observations [10, 11, 19, 110]:

• sprinkler spacings;

• pressure at the sprinkler nozzles at different locations;

• pressure variation A P along a lateral and within the operating set;

• sprinkler discharges and respective variations along a lateral and within the operating set;

• applied depths in a sample area on both sides of a lateral or between two laterals (using a grid of containers);

• duration of the irrigation;

• SWD at the time of irrigation;

• wind speed and direction;

• irrigator management practices, including MAD and the target application depth; and

• pressure and flow rate at the inlet of the system.

These observations provide for the computation of the actual distribution uniformity and coefficient of uniformity of the system, the actual and potential application efficiences, effective depth applied, percentage of area adequately irrigated, and system evaporation and wind drift losses. This information is used to provide recommendations to the farmer relative to

• improvements in the irrigation scheduling practices;

• changes needed in the system (spacings, sprinkler nozzles, number of laterals or sprinklers operating simultaneously, pressure regulation);

• improvements concerning the pumping system; and

• maintenance of the system and components.

Field evaluations for traveling guns [104] mainly focus on the traveler velocity, applied depths perpendicular to the traveler direction, and pressure available at the gun sprinkler and upstream. They also provide for computation of performance and management parameters and allow recommendations relative to irrigation scheduling and to improve the management and maintenance of the system.

In the case of center-pivot laterals [111], applied water depths are observed along a radius and, when possible, along a travel path. Observations of the pressures and discharge rates of the sprinklers along the lateral and of the end gun are also desirable. The velocity of the lateral and the wetted width also should be observed, as well as the ocurrence of runoff and erosion. Computed performance and management indicators generally do not include the efficiency because depths applied are very small. Recommendations should pay particular attention to the rotation velocity because velocity determines the applied depths. They also should include any need for changing sprinklers along the lateral, and the working conditions of the end gun.

Field evaluations are extremely important for helping farmers improve operation and management of sprinkler systems, to achieve higher performance and decrease water losses. When sprinkler systems are used to apply liquid fertilizers and agrochemicals, field evaluations also help farmers to improve fertigation and chemigation [112].

Sprinkler Irrigation Management, Fertigation, and Chemigation

As discussed above, the uniformity of sprinkler water application [Eq. (5.148)] essentially depends on the conditions fixed at the design phase and how close actual conditions are to those assumed at design. The only exception is wind speed WS, an intermittant problem that is avoided by not irrigating during windy periods. Good maintenance is needed to maintain potential uniformities. Pumps must maintain their intended discharge and pressure, pipes and joints must be maintained leak-free, and nozzle wear and sprinkler operation must be monitored and problems corrected. When conditions deviate from those at design (i.e., pumping-depth decline or sprinkler-nozzle changes), the performance of the whole system should be reanalyzed.

The application efficiency can be influenced greatly by management. When neglecting the variables relative to uniformity, Eq. (5.149) simplifies to ea = f (Ic, ia, tt, SWD). (5.155)

The infiltration rate Ic may be influenced by soil management practices that avoid soil sealing and crusting or increase the intake rate of the soil. Among soil management practices are reduced tillage and direct seeding techniques [66]. High infiltration rates are particularly important in orchards and noncovering crops cultivated in soil where runoff and erosion can be critical, as is the case for tropical soils. The use of straw mulches and undertree vegetation are useful techniques.

The application rate ia is fixed at the design phase but can be modified somewhat after field evaluations. However, in case of center-pivot and linear-move systems, where ia is not constant but varies over time during the water application, the moving speed of the lateral can be adjusted to minimize the time during which ia exceeds Ic [113]. Field observations are required to appropriately decide on such adjustments. In row crops, where ia often exceeds Ic, furrow diking or reservoir tillage can be used to prevent runoff [86].

Variables ti and SWD in Eq. (5.155) depend on the irrigation scheduling practices. The time duration of irrigation ti is the variable controlling the irrigation depth. In set systems, the irrigation depth D (mm) is directly controlled by ti (h) when the application rate ia (mmh-1) [Eq. (5.142)] is known t = D/ia).

For mobile rainguns and mobile lateral systems, the irrigation depth is controlled by the system velocity [Eqs. (5.143) and (5.147)], and thus indirectly by the time duration of water application.

Sprinkler irrigation scheduling methods depend on two main factors: the frequency of irrigations and the size and technological level of the farm. Large farms are more often in control of pressure and discharge, timing of applications, and duration of irrigations and have better conditions to invest in both control systems and irrigation-scheduling sensing tools [114]. Small farms are also in control of irrigation timing and duration when delivery is made on demand or when they manage their own water source. However, they may have less control over pressure and discharge at the farm hydrant when the delivery system is not fully responsive to the demand during peak periods. Under these circumstances, both pressure and discharge drop, inducing low performances and requiring an adjustment of the irrigation time [115].

System controls [116] include timers and/or volumetric control valves, which enable automatic regulation of the application duration in each unit of the system; solenoid valves, which allow the selection of the portion of the area to be irrigated next; and pressure and/or discharge regulation devices, which permit maintainance of uniform flow in the operating system. Where farm pumps are used, control systems also are applied to the pumping system, mainly to automatically control the "on" and "off" conditions and to protect the motor against short circuits and overcurrent.

Control systems may be connected with irrigation sensing tools to automatically start irrigation when sensed variables reach a preset threshold value. Information from field sensors may be stored and handled at a control center and irrigation systems controlled remotely from there. This technology mainly applies to farms having several center-pivot systems where remote control is applied to irrigation, as well as to reduce energy costs. On the contrary, irrigation control systems are uncommon in small and medium-size farms, where irrigation valves are operated manually.

Because portable systems are labor consuming, they should be used to apply large depths, often 50-90 mm. Fixed systems may be used for variable frequency of irrigations, for D varying from 10 to 90 mm or more. Depths applied by mobile raingun systems are limited by the speed range of the system, with D commonly ranging from 15 to 50 mm. Mobile laterals are appropriate for frequent, daily up to 4-day applications (7-25 mm), and seldom are used for infrequent applications.

For set systems and traveling guns, when the MAD is large, the methods for irrigation scheduling are the same as for surface irrigation: water balance simulation models, monitoring the soil water status, or a combination of both. In areas with a large number of small farmers, the use of simple irrigation calendars may be more appropriate. The successful use of a simple simulation model through a videotel system for advising farmers within a large irrigation project has been reported by Giannerini [117]. Other examples of sprinkler irrigation scheduling practices are given by Smith et al. [78].

When frequent irrigations are applied, scheduling strategies should be based on the replacement of the volume of water consumed during the preceding irrigation interval. For center pivots, special irrigation scheduling models have been developed with modules for water application, fertigation, chemigation, and energy management. An illustration is model SCHED[118]. The estimation of ETc using the basal-crop-coefficient approach (see Section 5.1) is appropriate for performing the soil water balance for scheduling frequent irrigations.

Fertilizers and other chemicals can be applied with sprinkler systems and are commonly applied through center-pivot laterals. Systems must be equipped with a fertilizer and/or a chemigation tank and an injection pump. When nozzle diameters are very small (sprayers), a filter should be placed downstream of the injection point to prevent clogging by large fertilizer particles.

Fertigation and chemigation require high water-application uniformities [112]. Otherwise, the applied products are distributed unevenly in the field with negative impacts on crops where they are insufficient and, for the environment, where they are in excess. Details on application of fertilizers and agrochemicals with sprinkler irrigation are given in [72] and [119].

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