HE Igbadun and BA Salim
Maize (Zea mays.L) farmers in the traditional irrigation schemes in middle Mkoji sub-catchment, Tanzania observes three irrigation scheduling practices. This paper presents a simulation study of the impacts of these scheduling practices on yield and soil water balance of the maize crop. The three scheduling practices include irrigating at 5 days and 7 days intervals throughout the crop growing season, respectively; and irrigating at 7 days interval from planting to vegetative growth stage and 5 days interval from flowering to crop maturity stages. ISIAMOD, a crop growth cum irrigation simulation model, was used to simulate grain yield, soil water balance components and crop water productivity responses for the three scheduling practices over a range of water application depths. Simulated grain yield varied from 1338 to 4023 kg/ha. Seasonal water applied and deep percolation varied from 425 to 1800 mm, and 50 to 113 mm, respectively. The crop water productivity in terms of water applied varied from 0.22 to 0.57 kg/m3. These values closely agree with field measured values reported by some researchers for the study area. Irrigating maize fields at 5 days interval throughout the crop growing season or at flowering to crop maturity gave higher water productivity output only when application depths per irrigation did not exceed 30 mm. Water application beyond this depth only led to very high deep percolation losses without appreciable difference in crop yield compared to irrigating at 7 days interval throughout the crop growing season. Moreover, the productivity of water applied dropped by about 30 and 50 %. This implies that farmers who irrigate at 5 days interval because of they have access to water do not have any advantage (in terms of yield and water productivity) over those who irrigate at 7 days interval except they minimize water applied to their fields. Water application depth for higher productivity under the 7 days irrigation interval for the maize crop in the study area was 40 to 45 mm depth. Beyond this depth, there was no appreciable increase in grain yield but a fall in productivity of applied water and a buildup of deep percolation. To avoid over irrigation and the consequences associated with it, maize farmers at any sector of the irrigation scheme in the study area are advised to observe 7-day irrigation interval and keep water application depth within 40-45 mm per irrigation.
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