Response of crops on shallow water table soils irrigated with deteriorating water qualities
This study was undertaken to investigate a number of issues regarding the effect of using saline irrigation water for crop production on soils with shallow water tables. The experiments were conducted in large drainage lysimeters, filled with a yellow sandy soil and a red sandy loam soil in which 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 that ranged from 15 to 600 mS m-1. This facility was used to determine the effect of irrigation water and water table salinity on crop yield and water uptake, as well as salt accumulation in the root zone during growing seasons. The field experiments simulated conditions of adequate water supply to the crops through irrigation in the presence of a shallow saline water table. Except for wheat that gave better yields in the more clayey soil, the growth of the other three crops was similar on both soils for comparative 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 deteriorating irrigation water salinity. On a relative basis the evapotranspiration of peas, beans, maize and wheat decreased at rates of 0.0007, 0.0005, 0.0004 and 0.0001 mm per unit increase of soil water salinity measured in mS m-1. A decrease in the osmotic potential of the soil water to -300 kPa, which is equivalent to an electrical conductivity of 750 mS m-1, reduced evapotranspiration in comparison 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 unit mass 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 salinity for all four crops on both soils. The relative water uptake from the capillary zones above the water tables declined linearly when the soil water salinity in these zones exceeded certain threshold 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, were wheat followed by maize, beans and peas. Salts accumulated at or just below the capillary fringe in both soils, with maximum accumulation at 700 mm from the soil surface or 500 mm above the water table. Equations were derived from the 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 proposed procedures for salinity management on irrigated soils. The procedures made provision for five different conditions: i) where added salts to the root zone accumulate without any possibility for leaching and the mean root zone salinity is lower than the crop ECe-threshold value; ii) where added salts to the root zone accumulate without any possibility for leaching and the mean root zone salinity is higher than the crop ECe-threshold value; iii) where added salts can leach naturally from the root zone, but with not enough irrigation water to supply in the crop water demand; iv) where the natural leaching of added salts can be accelerated by irrigating more than the required crop water demand; and v) to irrigate according to the crop water demand in order to utilize rainfall for leaching. The different salinity management procedures were compared on the two soil types by means of computer simulations for a range of irrigation water qualities and long-term climatic conditions. The simulated results indicated that under conditions with zero drainage, sustainable production could be maintained for only 25 to 40 years if good quality water was used for irrigation. Irrigation water with an ECi > 50 mS m-1 resulted in severe soil salinisation and crop losses within 5 to 10 years. On freely drained soils additional leaching was required within 5 years, even with the use of good quality irrigation water. It was clear from the simulated results that an increase in root zone salinity in soils with shallow water tables, necessitate adaptations in the normal approaches to irrigation scheduling and irrigation water management.
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