Response of crops on shallow water table soils irrigated with deteriorating water qualities

[摘要] This study was undertaken to investigate a number of issues regarding the effect of using salineirrigation water for crop production on soils with shallow water tables. The experiments wereconducted in large drainage lysimeters, filled with a yellow sandy soil and a red sandy loam soil inwhich shallow saline water tables were maintained at a constant depth of 1.2 m. Wheat, beans,peas and maize were grown under controlled conditions using irrigation water with salinities thatranged from 15 to 600 mS m-1. This facility was used to determine the effect of irrigation waterand water table salinity on crop yield and water uptake, as well as salt accumulation in the rootzone during growing seasons.The field experiments simulated conditions of adequate water supply to the crops throughirrigation in the presence of a shallow saline water table. Except for wheat that gave better yieldsin the more clayey soil, the growth of the other three crops was similar on both soils forcomparative irrigation water salinity treatments. The above-ground biomass of wheat, maize,peas and beans started to decline when irrigated with water of 600, 450, 300 and 150 mS m-1,respectively.The water use of all four crops, as indicated by evapotranspiration, declined with deterioratingirrigation water salinity. On a relative basis the evapotranspiration of peas, beans, maize andwheat decreased at rates of 0.0007, 0.0005, 0.0004 and 0.0001 mm per unit increase of soilwater salinity measured in mS m-1. A decrease in the osmotic potential of the soil water to -300kPa, which is equivalent to an electrical conductivity of 750 mS m-1, reduced evapotranspiration incomparison to the control by 7, 30, 38 and 53% for wheat, maize, beans and peas, respectively.The water use efficiency of the crops, expressed in above-ground biomass produced per unitmass water used, started to decline only when the threshold ECe-values were exceeded.Water uptake from the shallow water tables decreased with an increase in irrigation water salinityfor all four crops on both soils. The relative water uptake from the capillary zones above thewater tables declined linearly when the soil water salinity in these zones exceeded certainthreshold values. These values varied between 57 mS m-1 for beans to 279 mS m-1 for maize,with an average value of 136 mS m-1. The crops less affected by the increase in salinity, werewheat followed by maize, beans and peas.Salts accumulated at or just below the capillary fringe in both soils, with maximum accumulationat 700 mm from the soil surface or 500 mm above the water table. Equations were derived fromthe accumulation of salts in the root zone to calculate the salt accumulation in soils with restricted drainage during a crop growing season. These equations were incorporated in proposedprocedures for salinity management on irrigated soils. The procedures made provision for fivedifferent conditions: i) where added salts to the root zone accumulate without any possibility forleaching and the mean root zone salinity is lower than the crop ECe-threshold value; ii) whereadded salts to the root zone accumulate without any possibility for leaching and the mean rootzone salinity is higher than the crop ECe-threshold value; iii) where added salts can leachnaturally from the root zone, but with not enough irrigation water to supply in the crop waterdemand; iv) where the natural leaching of added salts can be accelerated by irrigating more thanthe required crop water demand; and v) to irrigate according to the crop water demand in orderto utilize rainfall for leaching.The different salinity management procedures were compared on the two soil types by means ofcomputer simulations for a range of irrigation water qualities and long-term climatic conditions.The simulated results indicated that under conditions with zero drainage, sustainable productioncould be maintained for only 25 to 40 years if good quality water was used for irrigation. Irrigationwater with an ECi > 50 mS m-1 resulted in severe soil salinisation and crop losses within 5 to 10years. On freely drained soils additional leaching was required within 5 years, even with the useof good quality irrigation water. It was clear from the simulated results that an increase in rootzone salinity in soils with shallow water tables, necessitate adaptations in the normal approachesto irrigation scheduling and irrigation water management.

[发布机构] University of the Free State