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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.
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main canal due to its higher capacity (43.21 l s-1) and proximity to the riverbank as compared to the other canal types. Expressed as percentage of the water flow capacity of the canals, the canal loss was highest for the field canals, since these were destroyed during tillage, so that the canal banks were not stabilized like the main and secondary canals. The negative impact of seepage on production was more pronounced in the case of field canals, as it led to more water logging than the seepage from other canals.
In addition, the field canal network covered the largest area in the scheme.
Farmers preferred field canals because they allowed them to keep their plot sizes.
Apart from high seepage losses, field canals dried and cracked before the next irrigation event. Field canals overtopping during night irrigation was common and increased water logging and unmeasured canal water loss. These factors resulted in high irrigation water loss, water logging and production losses. Therefore, large farmlands within 8-15 m from the field canals were out of production. Although farmers thought secondary canals occupied more land than field canals, in practice field canals rendered more land unproductive and resulted in higher water losses than secondary canals. The total volume of water lost from the 3077-m long canal system (comprising all canal types) could have irrigated 9 ha of land at 50 mm irrigation depth per day for the irrigation season.
The increasing water demand due to the extension of tail and pump irrigation have made management of the irrigation water more complicated. The duration of the irrigation intervals increased, which resulted in crop water stress and cracks in the vertic soils. In addition, due to decreased canal flow capacity, the time needed for sufficient irrigation increased, and farmers were forced to conduct nighttime irrigation, resulting in large losses due to inefficiency and unpredicted canal flow rate during the night.
Water was a more constraining factor than land around the scheme during the irrigation season. Ample downstream plain land was out of production six months a year (December to June). On the other hand, water from night stream flow, springs and shallow groundwater was still not used properly. Night water storage will increase
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water use efficiency. It is possible to use the stream bank for this temporary storage. A simple profile leveling survey of the Guanta stream bank showed great potential for night water storage (up to 20 000 to 45 000 m3), which could be used during the day.
Farmers invested about 50% of their produce in kind for pump irrigation, with an increasing trend of pumping activities.
However, because of higher production costs, pump irrigation typically resulted in lower financial water productivity than downstream gravity irrigation. This means that there is a greater possibility to improve water productivity in downstream gravity irrigation than in upstream pump irrigation.
The variation in RWS indicates that more water was lost for tef and under gravity irrigation. Values ranged from 0.8 to 4.0, where 0.8 indicates deficit irrigation to maximize water productivity (Molden et al. 1998). In a public surface irrigation scheme in Mexico, RWS was higher than 2.0 and showed differences with respect to water access and water cost (Kloezen and Garcés-Restrepo 1998). Compared to the above study that used RWS at scheme level, RWS for tef was extremely high.
4.5.2 Production and productivity
Area and water productivity of selected crops was comparable with findings of other studies around the study area. Haileslassie et al. (2009b) reported yields of 892-972 kg ha-1 for wheat and 981-1312 kg ha-1 for tef produced in rain-fed conditions in the same watershed. The EWP values were 0.21-0.23 kg m-3 for wheat and 0.24- 0.33 kg m-3 for tef in the study of Haileslassie et al. (2009b), which used the improved approach of water productivity calculation. In this study, land productivity was relatively lower while the water productivity was three times higher as compared to values in the study of Haileslassie et al. (2009b). The difference observed between these rain-fed and irrigation values arose from differences in crop varieties and water management practices. On the one hand, short-maturing tef and wheat varieties with lower ETc values than for rain-fed production were used for irrigation. On the other hand, irrigation intervals were too long to create acceptable soil water stress conditions whereby the water stress coefficient was reduced to 0.3 during some irrigation
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intervals. Bekele and Tilahun (2007), using experimental deficit irrigation at Sekota, Ethiopia obtained onion yields of 5500-25000 kg ha-1 with 9-10 kg m-3 water, which is about eight times higher than the observations in this study. This shows that opportunities exist to increase onion production and productivity in the area. Flood irrigation using a very low flow rate (less than 1 l s-1 during the night or day) and irrigating deep cracks after prolonged irrigation intervals decreased the water productivity of tef and wheat as compared to the field observations on water application. For example, tef irrigation water was almost half as productive as ET water compared to wheat and onion productivity due to higher drainage water losses.
The conventional way of quantifying water productivity underestimated water productivity values, since the total water transpired was used to produce total biomass while the estimation considered either grain or straw. This approach has more a practical application in mixed crop-livestock systems where the straw biomass is a very important livestock feed.
4.5.3 Implications for livestock production
As the importance of crop production for livestock is worth considering, it is also important to stress the importance of crop residue as livestock feed during the irrigation season. About 11428 kg grass, 18490 kg wheat straw and 12884 kg tef straw (42884 kg DM in total) were produced from 18 ha (20%) land of the scheme during the irrigation season. Based on 8.5 kg DM per day maintenance need for one TLU, 84 TLU can be fed for 60 days. This can cover TLU from 26 households or TLUs on 37 ha according to the livestock holding and stocking rate of the area studied determined by Descheemaeker (personal communication, 2010). Dry matter production from the relay maize cropping and other minor crops in the scheme was not considered in this calculation, and the potential of the scheme to support livestock feed is expected to exceed the above indicated figures. Therefore, increasing the biomass productivity of each drop of irrigation water and on each plot of land within the scheme has strong implications for livestock water productivity of the mixed crop-livestock systems.
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