Effect of different pre-sowing water application depths on wheat yield under spate irrigation in Dera Ismael Khan District of Pakistan

Spate irrigation is a method of flood water harvesting, practiced in Dera Ismael Khan (D.I. Khan), Pakistan for agricultural production for the last several hundred years in which during monsoon period flood water is used for irrigation before wheat sowing. A field study on the effect of different pre-sowing water application depths on the yield of wheat was conducted during 2006-2007. The spate irrigation command areas normally receive the flood water as a result of rainfall on the mountains during the months of July to September, which also carries a significant amount of sediment load. The flood water flows in different torrents and is diverted through earthen bunds to the fields for irrigation with depth of water application ranging from 21 to 73 cm and resulted in sediment deposition of 1.8-3.6 cm per irrigation. In this study, the effect on wheat yield of three different pre-sowing water application depths (D1 < 30 cm, D2 = 30-45 cm and D3 > 45 cm) were studied under field conditions. Fifteen fields with field sizes of about 2-3 ha were randomly selected, in each field five samples were collected for analysis of soil physical properties, yield and yield components. Five major soil texture classes (silty clay, clay loam, silty clay loam, silt loam and loam) were found in the area with water-holding capacity ranging from 23% to 36.3% (on a volume basis) and bulk density varied from 1.35 to 1.42 g cm⁻³. About 36% more grain yield was obtained from loam soil fields, followed by silt loam (24%) as compared to wheat grown on silty clay soil condition. The maximum wheat grain yield of 3448 kg ha⁻¹ was obtained from fields with water application depths of 30-45 cm and the lowest wheat yield was recorded in fields with water application depths greater than 45 cm. On-farm application efficiencies ranged from 22% to 93% with an overall average of about 49%. Due to large and uneven fields, a lot of water is lost. In general, the application efficiency decreased with increasing water application depth. Based on the results of this research, in arid to semi-arid environments, for optimum wheat yield under spate irrigation, the pre-sowing water application depth may be about 30-45 cm (September to July) and under or over irrigation should be avoided.