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variance, which compares groups sample means with one factor at a time (SPSS Inc.
2007), for the 3 crop types, and t-test for gravity and pump irrigation types. Means were compared using the least significant difference test at significance level p ≤ 0.05.
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Table 4-2 Canal water losses due to water surface evaporation and seepage from Guanta small-scale irrigation scheme
Canal type N†
Average flow rate
(l/s)
Std.
Error
Loss (l/s/100m)
Std.
Error
% loss (100 m−1)‡
Std.
Error
% loss/
100m/30l/s
Main canal 121 43.2a 0.4 2.4a 0.4 6.5a 1.0 4.5b
Secondary canal
57 33.0b 0.7 1.6b 0.6 4.4b 2.2 4.0b
Field canal 49 2.9c 0.3 0.4c 0.3 2.5c 12.9 25.9a
†Number of observations
‡Percentage with respect to the seasonal average flow rate within each canal type
Values indicated by different superscript letter (a, b, and c) are significantly different at p ≤ 0.05
Table 4-3 Seasonal water productivity and land productivity of grasslands along earthen canals, in drainage basins, and in wetlands in Guanta irrigation scheme for the 2009 irrigation season
Seasonal water productivity (kg m-3) Seasonal land productivity (kg ha-1)
Canal boundaries†
Drainage
basin‡ Wetland§
Canal boundaries†
Drainage
basin‡ Wetland§
N† 32 38 14 32 38 14
Total area (ha) 1.19 0.34 0.28 1.19 0.34 0.28
Mean 0.8b 1.2a 0.4b 6225.9b 9207.4a 3174.2b
Std. Error 0.1 0.1 0.1 641.9 1031.8 654.8
Literature values
(Haileslassie et al. 2009b) 0.5-0.65 0.61-0.79 1835-2386 3326-3866
†Grassland along canals; ‡wetland due to drainage water; •wetland due to overflow of irrigation water; †number of biomass samples. Values indicated by the different superscript letter (a, b, and c) are significantly different at p ≤ 0.05.
4.4.2 Comparative performance
Relative water supply, reflecting the availability of water in relation to crop demand, was 1.04 and 1.18 for wheat and onion, respectively (Table 4-4), indicating that the total water applied was similar to the crop needs. With a significantly different RWS value of 4.1, the average water applied for tef was four times (up to seven times in some plots) higher than the requirements. Tef had a much higher variation in RWS as compared to wheat and onion (p ≤ 0.05.)
Relative water supply was significantly lower (p ≤ 0.05) for motor pump than for gravity irrigation. Values were 0.5 and 1.35 under pump and gravity irrigation,
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respectively, for onion. Wheat and tef were not planted in either gravity or pump irrigation types, thus a comparison was not possible. This indicates that farmers under-irrigated their farms when using pumps and over-under-irrigated when using gravity irrigation.
Table 4-4 Relative water supply for different crops and irrigation types
Parameter Type N† Mean Std. Error
Relative Water Supply (-) Wheat 4 1.0b 0.1
Onion 3 1.2b 0.1
Tef 4 4.1a 0.6
Relative Water Supply (-) Pump 5 0.8b 0.1
Gravity 6 1.9a 0.2
†Number of observations.
Values indicated by different superscript letter (a and b) are significantly different at p ≤ 0.05
4.4.3 Crop production and productivity
The productivity analysis revealed that grain biomass yield for tef and wheat was very similar at 770 and 759 kg ha-1, respectively, whereas straw yield was slightly higher for tef at 2048 kg ha-1 compared to 1864 kg ha-1 for wheat (Table 4-5). The onion yield was 5903 kg ha-1. Water productivity was higher for onion than for the cereals. On the other hand, irrigation water productivity (IWP) of crops was lower than evapotranspired water productivity (EWP) due to irrigation water application losses for both water productivity approaches for all crops, and for grain/bulb and crop residues.
Due to high application losses, onion and tef had statistically similar EWP but statistically different IWP. Conventional IWP ranged from 0.18 to 1.39 kg m-3, while improved IWP ranged from 0.68 to 1.78 kg m-1 (Table 4-5).
In addition to RWS, a comparison of the amount of irrigation water applied to the amount of crop water needed showed that the total amount of irrigated water did not match that needed by the crops, especially for tef. The water input was much higher than the evapotranspiration water requirement. The irrigation water requirement and water application varied greatly among the selected crops. The irrigation water requirements of the crops were significantly different (p ≤ 0.05), but farmers applied almost equal amounts of water for wheat and onion. As a result, a strong variation (p ≤ 0.05) in irrigation water losses (ranging from 0 to 78% of the
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required water) due to over-irrigation was observed between the crop types (Table 4-6). Wheat was not irrigated beyond field capacity, and no irrigation loss was observed. Farmers irrigated wheat and tef two to three times in the growing season, while onion was irrigated seven to eight times. The short-maturing tef variety can produce grain and straw within 47 days, requiring the lowest irrigation water amount.
However, farmers applied most water to tef fields resulting in the highest water loss (78% of the required water or 30% of the applied water was lost through drainage).
Thus, the lowest irrigation water productivity was observed here (table 4-6). The high water application for tef was due to the flood irrigation method and crack formation at each irrigation event.
Table 4-5 Yield and water productivity of main crops in Guanta irrigation scheme
Crop type N† Mean Std. Error
Grain/bulb yield (kg ha-1)
Wheat 15 758.7b 60.9
Onion 60 5903.0a 352.1
Tef 12 770.8b 56.7
Straw biomass yield (kg ha-1)
Wheat 15 1864.0a 210.4
Tef 12 2048.3a 170.0
Conventional straw EWP (kg m−3)‡
Wheat 15 0.8b 0.1
Tef 12 1.1a 0.1
Conventional grain/bulb EWP (kg m−3)
Wheat 15 0.3b 0.04
Onion 60 1.5a 0.09
Tef 12 0.4b 0.03
Conventional straw IWP (kg m-3 )§
Wheat 15 0.5a 0.05
Tef 12 0.5a 0.10
Conventional grain/bulb IWP (kg m-3)
Wheat 15 0.2b 0.02
Onion 60 1.4a 0.09
Tef 12 0.2b 0.03
Improved rain/bulb/straw EWP (kg m-3)
Wheat 15 1.2b 0.13
Onion 60 1.8a 0.10
Tef 12 1.5a 0.10
Improved grain/bulb/straw IWP (kg m-3)
Wheat 15 0.7b 0.07
Onion 60 1.6a 0.11
Tef 12 0.7b 0.12
†N: number of observations; ‡EWP: evapotranspired water productivity; •IWP: irrigation water productivity Values indicated by different superscript letter (a and b) are significantly different at p ≤ 0.05
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Table 4-6 Irrigation water application and requirement for different crops in Guanta irrigation scheme
Irrigation water applied (mm) Irrigation requirement (mm) Drainage loss (% of requirement)
Wheat Onion Tef Wheat Onion Tef Wheat Onion Tef
No. of
cases 4 3 4 4 3 4 4 3 4
Mean 388.9b 484.3b 541.2a 370.6b 452.0a 143.3c 0.0c 17.1b 77.8a
Std. Error 19.1 21.4 82.5 2.0 4.2 4.0 0.0 0.8 11.4
Values indicated by different superscript letter (a, b, and c) are significantly different at p ≤ 0.05
Farmers at the upstream side of the gravity-fed scheme invested in motor pumps, fuel and technicians to pump water to their fields. Farmers with land but without pumps shared half of their produce with those providing pumped water.
However, water from gravity irrigation was free. As a result, strong differences in amounts of applied water were observed for pump and gravity irrigation (Table 4-7).
Even though crops needed similar amounts of water independent of whether they were irrigated by pump or gravity (a small difference was observed due to difference in crop type and plantation and harvest time), farmers applied more water to gravity-irrigated crops than to pump-gravity-irrigated. Consequently, pump-gravity-irrigated crops were under-irrigated while gravity-irrigated crops were over-irrigated (Table 4-7). Even though there was overall under-irrigation during pump irrigation, in some cases excess water was applied above soil field capacity. As such, about 3.4% of the applied water or 2.5% of the required water was lost due to drainage.
Table 4-7 Irrigation water application and requirement for pump and gravity irrigation
Irrigation water applied (mm)
Irrigation water requirement (mm)
Drainage loss (%
of requirement)
Pump Gravity Pump Gravity Pump Gravity
No. of cases 5 6 5 6 5 6
Mean 309.2b 550.6a 429.1a 380.2a 2.5b 31.6a
Std. Error 20.4 19.8 12.9 15.7 0.6 3.8
Values indicated by different superscript letter (a and b) are significantly different at p ≤ 0.05
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Discussions with farmers revealed that there was an increasing irrigation water demand (data not presented). The diversion and main canal capacity was designed for 46 ha. However, about 20 ha of additional farmland were included by the farmers at the tail end of the scheme. In addition, farmers in the upstream irrigated 21 ha of land using motor pipes, which was not originally part of the scheme. Increasing water demand by both upstream and downstream communities aggravated the water shortage leading to prolonged irrigation intervals. As a result, the soil was deeply cracked in many locations, leading to high losses of irrigation water due to water percolation through the cracks.