In a canal command, maize crop is grown in an area of 30 ha. The crop evapotranspiration (ETᴄ) of maize is 840 mm per season and the effective rainfall during growing season is 20 mm. It is irrigated with water having salinity of 1.1 dS m⁻¹ by a surface irrigation method. If the leaching efficiency of the field soil is 0.8 and the average soil salinity tolerated by the maize crop for 100% yield is 1.7 dS m⁻¹, the depth of irrigation water in mm per season required to meet the seasonal ETᴄ and leaching requirement will be _____.
Official Answer Key:
1000 to 1013
Step-by-Step Solution & Explanation
Leaching requirement (fraction), using the standard steady-state formula for surface irrigation methods: LR = ECᴡ/(5×ECₑ − ECᴡ) = 1.1/(5×1.7 − 1.1) = 1.1/7.4 = 0.1486 (14.86%).
Adjusting for the field's leaching efficiency (0.8): LR_actual = LR/efficiency = 0.1486/0.8 = 0.1858 (18.58%).
Net irrigation requirement (ETᴄ − effective rainfall) = 840 − 20 = 820 mm.
Total (gross) depth of irrigation water, accounting for the leaching fraction: d = NIR/(1−LR_actual) = 820/(1−0.1858) = 820/0.8142 = 1007 mm. This lies within the accepted range 1000–1013. (The 30 ha crop area given is not required for this per-unit-depth computation.)
Section 6 — Agricultural Process Engineering
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