Early work on water management was with traditional tall wheat crops. Singh (1945) reported that first irrigation to wheat should be at 65-75 per cent of field capacity by applying 7.6 cm of water per irrigation to saturate 122 cm soil depth. In India, wheat is cultivated as post-rainy season crop and hence responds to irrigation.
The relationship between delta and depth of irrigation water on medium loam soils of Punjab suggested that 25 to 30 cm of water gave the same yield irrespective of number of irrigations ranging from two to six.
The pioneer work (1909-1965) by several researchers laid foundation to scientific water management for dwarf wheats. After the introduction of dwarf wheats, agronomists identified six well defined phonological growth stages: crown root initiation (CRI), tillering, jointing, anthesis, milk formation and dough ripe stages, which were considered important for irrigation scheduling.
Critical Stages for Irrigation:
Optimum soil moisture for normal growth and development vary with the stage of plant growth. In wheat, crown root is formed near the soil surface irrespective of the depth of sowing. Crown roots start developing at the node and successive underground nodes, which are closely spaced. Lateral buds at the node give rise to tillers.
The roots, which help in establishment of tillers, can be formed only in the moist soil. Tillering is directly related to soil moisture availability at crown root initiation. Number of grains per ear and the grain size are drastically reduced leading to low grain yield due to inadequate soil moisture at flowering and milk formation stages.
Experiments on critical stages for soil moisture stress although the country indicated the necessity for irrigation at crown root initiation, flowering and milk developments. The effect of timing of first irrigation on dwarf wheat was studied at 13 locations in various wheat zones of the country for a period of four years. First irrigation at CRI consistently gave the highest grain yield.
The average grain yield for treatments, which received first irrigation at 14,21, (CRI), 28, 37, 42 and 49 days after sowing were 4.0, 4.3, 3.8, 3.6, 3.3 and 3.0 t ha-1 respectively. In general, delay in irrigation at CRI resulted in about 27 per cent reduction in grain yield.
Irrigation Schewling:
Experiments for four years at various locations in the country under AICWIP indicated necessity for irrigation at CRI, CRI and late tillering and flowering to milk stage. Average yield reduction from withholding the first irrigation was 1.171 ha-1 and when the first two irrigations were withheld, yield was 1.91 t ha-1. In comparing treatments 2 to 7, where one irrigation was withheld at various stages, milk stage appeared to be the most critical after the CRI stage.
In general, 4 to 6 irrigations are needed for optimum yield under different wheat environments. First irrigation should be given at CRI stage. Other irrigations should be given at late tillering, late jointing, flowering, milk and dough stages (Table 2.5).
Two to three more irrigations appear to be ideal for wheat on sandy soils.
Under limited irrigation water supply, the following irrigation schedule maybe followed:
1. If water is available for only one irrigation, it should be applied at CRI to tillering.
2. If available for two irrigations, first irrigation at CRI to tillering and the second 7 or 8 weeks after first irrigation (flowering).
3. If water is available for three irrigations, the first at CRI and the second and third at intervals of 6 to 7 weeks.
Thus, the three irrigations should be given at CRI, flowering and milk stages for high wheat yield. Since, higher number of irrigations may not have significant effect on grain yield on timely sown wheat, the last three irrigations may be unnecessary at several instances.
Approaches of Scheduling Irrigation:
Among the several approaches for scheduling irrigation to crops, soil moisture deficit and climatological approach are the two commonly used approaches for irrigating the wheat crop. Scheduling irrigation at 50 per cent depletion in available soil moisture (DASM) is optimum for crop growth and yield.
If irrigation water is a limiting factor, irrigations may be scheduled at 50 per cent DASM at the three critical stages and at 75 per cent DASM at other stages. When tensiometers are used for scheduling irrigation, soil suction of 0.4 to 0.6 bars appears to the optimum.
Relatively more practical climatological approach of IW/CPE or ID/CPE (ratio between fixed depth of irrigation water and cumulative pan evaporation) as the basis for scheduling irrigation water has been suggested by Prihar (1978). Highest wheat yield and high water use efficiency (WUE) were with IW/CPE of I..0.
Later work established that the optimum irrigation regime could be IW/CPE of 0.9 with 4.5 to 7.5 cm water per irrigation, which amounted to 300-400 mm water in Indo-Gangetic region of Punjab. However, in non-traditional wheat areas, Bandyopadhyay (1997) reported highest grain yield with irrigation depth of 5 cm at IW/CPE ratio of 1.2 at Memari (WB). A ratio of 0.5 was ideal on black soils of Indore (MP), Rahuri (Maharashtra) and Siruguppa (Karnataka).