C. Przybyla, Agric. Eng. Department of Land Reclamation and Environment Shaping, Agricultural University of Poznan, Poznan, Poland
The paper presents the results of investigations performed since 1986 on seven large-scale sprinkler irrigation systems in the Wielkopolska region of Poland. The prediction of irrigation requirements is based on the use of meteorological data, hydraulic properties of soil and applied irrigation. Problems concerning the use of this scheduling system in practice are also described. The influence of the predicted irrigation needs on the use of the sprinkler systems and their effectiveness has been evaluated. The effect of implementing the irrigation scheduling was to increase the use of the irrigation equipment and, hence, the area over which sprinkling was performed.
Determining the dates and depths of irrigation is often beyond the user of large sprinkler irrigation systems. This problem may well be solved by an irrigation scheduling based on predictions of the irrigation needs of a given area that take account of the particular cultivation, soil conditions, type of irrigation equipment, the amount and quality of water available for irrigation, and weather conditions.
The results presented here have been collected over a period of five years (1986-1990), and they refer to seven sprinkling irrigation systems covering areas from 236 ha to 522 ha in the region of Wielkopolska. On two selected areas, apart from the studies concerning the use of the sprinkler systems, an investigation was carried out into the water economy of the soils irrigated by the sprinkling machines. Within the studies on the use of the sprinkling irrigation system, the condition of the devices was analysed and the maintenance costs were estimated. Both the organization and technology of sprinkling irrigation and the degree of sprinkling system use were evaluated, and the yields obtained from the investigated areas were analysed against the background of fertilization and sprinkler irrigation. The method of irrigation scheduling was based on the computer analysis of the climate, soil and plant data (Jensen, 1970; Jensen and Wright, 1976; Schefke, 1987; Przybyla and Fiedler, 1992; Przybyla, 1994). The main outcome of this method is the possibility to supply irrigation system users with information about successive dates of irrigation of particular crops. This method can be used for single fields, agricultural complex farms or a region.
CHARACTERISTICS OF THE REGION, FARMING AND IRRIGATION SYSTEMS
The productivity of agriculture in the Wielkopolska region depends heavily on soil type and local climate conditions, which in this part of Poland are not favourable. The area is characterized by soils with low water storage capacities, deep groundwater levels and unfavourably distributed precipitation during the growing period (Figure 1).
These are the main natural factors affecting the effectiveness of irrigation in the Wielkopolska region. Most of the region has a long-term average yearly precipitation value of about 500 mm. In the vegetation period (April to September) the total precipitation varies from 240 mm to 290 mm, and in dry years is barely half of this value.
In the 1970s, the planning of irrigation works concentrated on large-scale irrigation systems that brought about a significant increase in agricultural production. This was associated with the agricultural policy of those times which favoured the development of state farms and restricted individual farming. The policy resulted in large sprinkler irrigation systems covering areas of several hundred hectares. Correct use of such systems creates many problems whose solution is frequently beyond the users in the Wielkopolska region.
FIGURE 1 - Zone with maximum precipitation deficiency, area of Wielkopolska region and area investigated
The size of fields included in the irrigation scheduling programme ranged from 230 to 250 ha. The irrigated area accounted for 20 to 95% of the total area of these farms. The structure of vegetation in the area under irrigation was: 37% grain crops, 33% fodder plants, 14% root crops, 10% rape, and 5% legumes.
The water for irrigation was taken from lakes through water intakes and pumping stations with a total capacity of from 210 to 500 ls-1. The underground pipelines with diameters ranging from 150 to 400 mm were installed according to the rules for semi-stable sprinklers and their siting was chosen to meet the particular needs of a given area. The network of underground pipelines was equipped with hydrants cooperating with motor-move systems. A motor-move system is a wheel-mounted sprinkler BK-10 type boom with additional sprinklers on trail lines to irrigate a block area. The unit remains stationary whilst irrigating and on completion of the cycle is moved to the next operating position by a built-in power unit. The operation of this system requires approximately 20 to 30 minutes of labour every 2 to 4 hours to move it to a new position. The working widths of the motor-move systems used ranged from 180 to 380 metres. Each pipeline was equipped with from a few to a few dozen sprinklers with a sprinkling range of from 15 to 30 metres and an intensity of from 12 to 24 mmh-1. In the vegetation period one wheeled sprinkling unit could supply the needs of an area of from 15 to 25 ha depending on the working width (Przybyla and Fiedler, 1992).
THE IRRIGATION SCHEDULING SYSTEM
The irrigation scheduling is prepared on the basis of data describing the climate, soil and vegetation conditions with the use of a special computer program (Jensen, 1973). In the scheduling of a sprinkler system the essential part is the calculation of the water requirement for irrigation, including the magnitude of actual evapotranspiration (Zhi, 1994). It is a central problem in the control of irrigation since the actual evapotranspiration (ETa) is closely connected with the phases of plant vegetation and the availability of water in the soil. The first problem has already been adequately solved, but the essential difficulty still lies in determining the effect of the degree of soil moisture on the actual magnitude of evapotranspiration. An attempt to solve this problem was undertaken in the BILANS program (Przybyla, 1995). The BILANS program is designed to calculate the water stored in the top layer of the soil irrigated by sprinkling irrigation. The water reserves are determined by the calculation of 24-hour water use based on the Penman formula (Przybyla, 1995). First the reference evapotranspiration (ET0), is calculated, then the actual evapotranspiration (ETa, mm d-1), taking into account the crop coefficient (Kc) and the crop stress factor (Ks) which introduces a correction relative to the availability of soil water:
Eta = Kc Ks ET0 [mm d-1]
In the correct use of the sprinkling machine, the coefficient Ks = 1.0, since the water management refers only to the water readily available in the root zone of the plant. The actual evapotranspiration (ETa) must be reduced for conditions of water deficit, when the amount of water stored in the plant root zone is smaller than that corresponding to the threshold of readily available soil water (WK). The Ks coefficient depending on the actual water stored in the target layer of irrigation was calculated according to the formula:
, (when: Reserve < WK)
actual water reserve in the target layer of irrigation in mm,
threshold of readily available soil water (pF = 2.8 - 3.0)
soil moisture at wilting point equal to double hygroscopicity in mm, (pF = 4.2),
an exponent equal to 1.5 and 2.5 for heavy soils (clay) and light soils (sandy) respectively.
When the water remaining drops below the assumed water threshold corresponding to the threshold of readily available soil water (WK), the BILANS program signals the need for irrigation.
The periods of water depletion in the target layer of irrigation, below the level of critical moisture (WK), observed in the period of irrigation were related to a significant reduction of the actual evapotranspiration (ETa). The introduction of the Ks coefficient in accordance with the above-mentioned formula permits a closer determination of the real 24-hour values of ETa (Przybyla, 1994). In order to schedule irrigation for the next period, the meteorological conditions from the last day before the forecast are considered. The program permits changes in the depth of the targeted irrigation during the growing season, the field water volume (PPW), the threshold of readily available soil water (WK) and the soil moisture at wilting point (WW) by referring them to the depth of the target layer irrigation required in the period of the forecast. The scheduling begins in spring with the measurements of moisture in the target soil layer, whose values are taken as the reference values.
Systematic measurement of soil moisture at several characteristic sites using the neutron probe method permitted us to verify the model.
A comparison of the soil reserve values measured (Z1) and calculated using the BILANS program (Z2) for a pasture is shown in Figure 2.
SCHEDULING THE SPRINKLER IRRIGATION SYSTEM IN PRACTICE
During the application of irrigation scheduling to seven farms in Wielkopolska a database was used for each farm sprinkled. The database includes meteorological data from the meteorological station at Poznan-Lawica, the physical and hydraulic characteristics of soils and detailed information about crop rotations in the fields and irrigation sectors. For each field, the values of the following parameters are given: average size of the irrigated fields, optimum periods of irrigation, number of technical units working in a given field and their characteristics, time of sprinkling in the irrigation cycle, and time necessary for the change of the successive working localities of the sprinkling units.
FIGURE 2 - A comparison of the measured soil reserve (Z1) and the computed soil reserve in scheduling (Z2)
The blueprint of irrigation scheduling are yearly plans prepared at the irrigation scheduling centre. Every year, schedules and operation plans for irrigation are prepared which form an integral part of the control process. In addition to the data from the database, the plans include maps indicating the positions of the applied wheeled sprinkler system, directions of their displacement, the number of working and stand-by pipelines, and the reach of the irrigation units. The control information is transmitted by telephone lines. The data collected are input into the database and the calculation of the irrigation needs is relayed to the users. The scheduled irrigation system gives an organizational basis for the rational use of the irrigation systems.
THE EFFECT OF THE SPRINKLING SYSTEM USE ON YIELDS
The available irrigation system ensures the possibility of performing irrigation over the same area every year. The effect of implementing irrigation scheduling was an increased degree of use of the irrigation equipment and thus an increased area over which sprinkling was actually performed.
Figure 3 shows the values that characterize the degree of use of the irrigation systems in the period before introduction of scheduled irrigation, i.e., in the growing periods of 1986 and 1987, and in the periods of scheduled irrigation, i.e., in the years 1989 and 1990. The degree of the system's use shown by the percentage of the irrigated areas in relation to the total area covered by system (I), and the applied irrigation amount, are expressed as a percentage of the optimum amount (II). The total index of the irrigation system's utilization (III) is shown as the product of the two first values (I×II). The degree of use of the irrigation system was calculated only for periods of water deficiency. The optimum vegetation depth of water is calculated on the basis of precipitation deficiency in the growing period. The amount of water in a single sprinkling depends on the physical properties of the soil (field capacity) and on the depths of the target irrigation layers.
The results presented here indicate that the degree of use of the large-scale irrigation system in the period 1986-1990 was far from the optimum. It is evident that irrigation scheduling contributed to increasing the area irrigated and to improving the ratio of actual to optimal water applications.
The irrigated area increased on average from 18% to 31%, and the use of the optimum depth rose on average from 29% to 57%. As a result, the index which for the period before the scheduled irrigation (1986-1987) was on average only 5 %, increased in the scheduled irrigation period (1989-1990) to 18%, a more than threefold increase. Figure 4 shows the values of mean yield increase in the years 1989 and 1990, and the increments in percent.
Analysis of these values indicates that crop increments varied between 15% and 60% of the values without irrigation. As neither fertilization nor other agrotechnical processes were changed in the period analysed, the increased yield obtained can be attributed to irrigation scheduling.
FIGURE 3 - Weighted mean in degree of sprinkling device efficiency on investigated objects in the years 1986-1987 (without irrigation scheduling) and 1989-1990 (with scheduling)
FIGURE 4 - Mean percentage increase in selected yields on irrigated areas of the farms investigated in the years 1989 and 1990
This study has shown that the implementation of irrigation scheduling programmes can be successful, both at farm and field scale. In particular, the irrigation scheduling programme contributed to increasing the area irrigated and to adopting irrigation depths closer to optimal ones. In general, it also contributed to higher yields of the main crops.
However, the users of large-scale irrigation systems did not take full advantage of the potential offered by the irrigation systems.
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Jensen, M.E. and Wright, Y.L. 1976. The role of simulation models in irrigation scheduling. ASAE Paper No 76-2061.
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