Evaluation of Modern Irrigation Techniques with Brackish Water
Sammanfattning: Modern irrigation techniques are becoming increasingly important in water-scarce countries especially in arid and semiarid regions. Higher crop production and better water use efficiency are usually achieved by drip irrigation as compared to other irrigation methods. Furthermore, by using drip irrigation simultaneously with brackish irrigation water, some of the water stress due to shortage of fresh water resources can be managed. The objective of the current study was to investigate the influence of geometric design, soil type, irrigation regime and amount, and salinity of irrigation water on soil water and salinity distribution as well as irrigation efficiency using drip irrigation techniques in Egypt and Tunisia. Field and laboratory experiments as well as numerical simulations were used to achieve these objectives. Two field experiments were conducted at two different sites and soil types in Tunisia. The first experiment was conducted to explore the effect of different drip irrigation treatment (i.e., surface drip irrigation with and without plastic mulch and subsurface drip irrigation) and regime (i.e., daily and bi-weekly) on soil water and salinity distribution as well as contaminant transport for sandy loam soil. The second experiment was carried out to investigate the mobility of different tracers (bromide and dye) under surface drip irrigation in loamy sand soil. Lab experiments using soil samples collected in Egypt and Tunisia were made to estimate soil hydraulic properties, soil texture, and soil moisture content. Numerical simulations for surface and subsurface drip irrigation and alternate partial root-zone surface and subsurface drip irrigation with brackish irrigation water were executed to investigate the effect of geometric design, irrigation regime and amount, and salinity of irrigation water on soil water and salinity distribution as well as irrigation efficiency for different soil types in El-Salam Canal project region, Egypt. Field experiments showed that maximum dye penetration depth during daily and bi-weekly irrigation occurred for subsurface drip irrigation. Also, during the bi-weekly irrigation, dye depth was not only larger but also occupied a larger soil volume than for the daily irrigation. Thus, bi-weekly irrigation increases the risk for groundwater contamination. Also, higher soil moisture content within the flow domain occurred with mulching treatment and daily irrigation. Simulations displayed a very close agreement with observed soil wetting. Multiple tracer experiments revealed that the bromide moved faster than dye. Therefore, fertilizers transport deeper than organic pollutants under surface drip irrigation in initially dry loamy sand soil. Numerical simulations verified this. On the other hand, numerical simulations for surface drip irrigation in El-Salam Canal cultivate land showed that soil hydraulic properties govern the shape of the wetted zone. The wetted depth was larger in sand while the wetted radius was lower as compared to loamy sand and sandy loam. Simulation results for subsurface drip irrigation (SDI) showed that deeper emitter depth increases the potential groundwater contamination risk and fertilizer leaching especially in sandy soil and shallow rooted plants. Also, it is preferable to control the wetted volume of any soil type by regulating the amount of irrigation water according to soil hydraulic properties. Simulation results also revealed that higher moisture content values within the flow domain and higher root water uptake rates occurred in case of short inter-plant emitter distances (IPED) under alternate partial root-zone surface and subsurface drip irrigation (APRDI and APRSDI, respectively). Therefore, Short IPED is preferable especially for root systems with limited lateral extension. Salinity results showed that as the salinity of irrigation water increased, the salinity levels at the soil surface at the location of the plant trunk under APRSDI increased. Therefore, APRSDI is more suitable with non-saline irrigation water, especially for shallow rooted plants. However, in case of using brackish irrigation water, short IPED and shallow emitter depth are recommended for reducing soil salinity below the plant trunk.
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