English abstract
Water resources augmentation together with conservation have been adopted by the Sultanate of Oman to combat water scarcity. Use of Reclaimed Water (RW) from sewage treatment for irrigation contributes to this strategy. Over 55 sewage treatment plants in Oman's different governorates produce more than 100 million m (Mm) of RW per year, which can be used for irrigation. Agriculture in Oman, contributes to partial food sovereignty and forms an important part of rural livelihoods. It accounts for 3.2% of the GDP in Oman in which fruits occupy an area of 53% followed by perennial forages (30%), vegetables (11%) and field crops (6%). If Oman decides not to use expensive water from desalination plants or fresh groundwater for irrigation, then the maximum area that can be irrigated solely with RW. will be determined by the day when demand for evaporatranspiration is maximum, During other days, there will be surplus of RW, unless a conjunctive water manag strategy, by supplementing RW with groundwater is formulated. If a conjunctive water management strategy is adopted, then the area under crops may be increased, and ter withdrawal will be reduced. Hence, the objective of this study was mainly to maximize the use of RW, supplemented by groundwater, by identifying short season crops, and changing the area under cultivation of such crops. Field studies were conducted to assess yield components of wheat, cowpea and maize crops rotation grown with RW for irrigation. Furthermore, the impact of RW irrigation on soil physical and chemical properties prior/after wheat, cowpea and maize cultivation were also assessed. Results showed that by using RW conjunctively with groundwater (assuming irrigation salinity of 1 ds m-' and RW availability of 38,267 m2 day!) cropping area can be increased from 695 ha to 2245 ha of wheat, 313 ha to 782 ha (250% increase) of cowpea and 346 ha to 754 ha (218% increase) of maize. Of the total irrigation requirement 24.24 Mm', 57.6% was to be met with RW and 42.4% was to be met with groundwater. The results on the effect of reclaimed water irrigation on soil physico-chemical characteristics indicated that the soil salinity in all soil depths decreased with time. It was 1.79 dS m' after wheat crop, 1.01 d m after cowpea, and 0.76 dS m-after maize. The soil pH was not affected by the irrigation with reclaimed water. The organic carbon did not show significant difference between the wheat and cowpea harvest timing. OC% was increasing with time in soil irrigated with reclaimed water. The saturated hydraulic conductivity, Ksat didn't show significant difference among the water types and the interaction between water types and soil depths. The soil nitrogen, phosphorus and potassium were not affected by any of the three water irrigation types. Total N was highest after cowpea harvest in RW irrigation, The highest concentration of P and K was found to be in the upper layers. P and K decrease with the increase in soil depth. Although RW didn't significantly show any effect with respect to sodium and magnesium concentration, it increased with time of harvesting. Calcium concentration was affected by the interaction of irrigation water type with soil depth. The highest Ca was observed in soil irrigated with desalinized water at 60 and 90 cm soil depth, though the RW contained higher Ca. RW didn't affect the soil trace elements concentration like Iron (Fe), Sulphur (S) and Manganese (Mn), Silicon (Si), Barium (Ba), Boron (B), Cobalt (Co), Lead (Pb) and Zinc (Zn). Fe concentration increased with soil depth and decreased with harvesting time. Mn decreased with time of harvesting. Si and B concentrations in the soil were found to be higher in the middle stage (after cowpea harvesting) for all soil depths. Ba decreased with time. Zn was increasing with time and decreasing with soil depth. The effect of RW was obvious on wheat agronomic characters. The RW irrigation increased wheat plant height (cm), chlorophyll, leaf area (cm), leaf length (cm), grain yield (t ha'') and the WUE (kg grain m). The RW irrigation improved all cowpea growth characters under study except the dry forage yield. The RW irrigation affected the growth parameters of maize. Nitrogen concentration was also higher under RW irrigation in respect to wheat tissue, cowpea pods and maize cobs. RW had no effect on the element concentrations in wheat straw, cowpea plants and maize plants except for Ni concentration in wheat plants, and Na and Mn concentrations ini maize plants. Based on the findings of this study, in order to fully utilize RW, it can be concluded that decision makers should consider use of the RW conjunctively with GW in areas where groundwater of relatively good quality is available for irrigating wheat, cowpea and maize (on a rotational basis). All trace elements were in a very low concentration in the soil after growing wheat, cowpea and maize crops and as such are not likely to pose any environmental concerns. There were no adverse impact of reclaimed water irrigation on yield and chemical characteristics of plants and grains of crops under study. The positive impacts of yield and WUE are significant.
