JWSS - Journal of Water and Soil Science

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To obtain soil-moisture characteristic curve experimentally is time-consuming and usually subject to considerable errors. So, many investigators have tried to predict soil-moisture characteristic curve by different models. One of these models predicts soil moisture characteristic curve based on soil particle size distribution and bulk density. In this model, soil particle size distribution curve is divided into a number of segments, each with a specific particle radius and cumulative particle mass greater than that of the radius. Using these data, soil-moisture characteristic curve was estimated. In this model, a scale factor, α, is used which may be considered as a constant, or obtained by logistic or linear procedures. The average values of α for clay, silty clay, sandy loam, two loam soils, and two silty clay loam soils were 1.159, 1.229, 1.494, 1.391, 1.393, 1.253 and 1.254, respectively. For most conditions, soil particle size distribution curve is not available, but only the percentages of clay, silt, and sand could be obtained using soil textural data, which is not enough to draw a precise soil particle size distribution curve. In this situation, a precise soil particle size distribution curve must be initially developed on the basis of which the soil moisture characteristic curve can be predicted. In this study, using soil textural data of seven different soils, soil moisture characteristic curve of each was estimated. In these estimations, logistic and linear methods were used to obtain the α value. Then, the results were compared with those of measured soil moisture characteristic curve. For estimation of soil particle size distribution curve, two extreme values for soil particle radius, 125 and 999 m, were used. The results indicated that using particle radius of 999 µm is more appropriate. On the other hand, it was found that for clay, silty clay, and sitly clay loam texture, it is more appropriate to employ a linear equation to determine for estimating soil-moisture characteristic curve while the logistic equation can be more appropriately used for loam and sand loam textures.

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A research was conducted in Fars province Agricultural Research Center in Zarghan area from 1999 to 2002 to determine the water requirement and crop coefficient of wheat, applying lysimeter. The results indicated that the water requirements of wheat were 720, 712 and 674 mm in the years of 1999-2000, 2000-2001 and 2001-2002, respectively. Using Penman-Monteith method for estimating reference crop potential evapotranspiration, the crop coefficients for wheat at a four-stage crop growth were 0.37, 0.64, 1.10 and 0.51, respectively. Due to the inaccessibility of the whole weather data, we tried to figure out a solution to determine wheat water requirement to schedule irrigation planning for future. In this respect, we made use of a ten-day class A pan mean evaporation and crop coefficient.

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To investigate the effects of different irrigation levels on yield and oil content of rapeseed, and to determine the irrigation requirement and irrigation scheduling, an experiment with Randomized Complete Block Design consisting of four irrigation treatments replicated 3 times, was conducted in Zarghan Agric. Expt. Station during the years of 2000-2003. The treatments were based on the cumulative evaporation values of 50, 75,100 and 125 mm from class A pan (T₅₀, T₇₅, T₁₀₀ and T₁₂₅). The depth of water for each treatment was determined according to the deficit of field capacity and soil moisture content before irrigation. In the three years of experiment, the cultivars: Okapi, Orient and Likord were cultivated and the annual data related to yield and seed oil contents of each cultivar was analyzed separately. In the first year of experiment, the effect of different treatments on yield of Okapi cultivar was not significant, but the maximum and minimum yields were obtained at the T₇₅ and T₁₀₀ treatments equal to 2678 and 2050 kg ha^-1, respectively. The effect of different treatments on seed oil content was significant at the level of 5 %, and the maximum and minimum seed oil contents were obtained at the T₁₀₀ and T₇₅ treatments equal to 42.50 and 41.66 %, respectively. In the second year of experiment, the effect of different treatments on the yield of Orient cultivar was significant at the level of 5 %, and the maximum and minimum yields were obtained at the T₅₀ and T₁₂₅ treatments equal to 3133 and 2133 kg ha^-1, respectively. The effect of different treatments on seed oil content was significant at the level of 5 %, and the maximum and minimum seed oil contents were obtained at the T₇₅ and T₅₀ treatments equal to 46.38 and 44.82 %, respectively. In the third year of experiment, the effect of different treatments on the yield of Likord cultivar was significant at the level of 5 %, and the maximum and minimum yields were obtained at the T₅₀ and T₁₂₅ treatments equal to 3667 and 2250 kg ha^-1, respectively. The effect of different treatments on seed oil content was significant at the level of 1 %, and the maximum and minimum seed oil contents were obtained at the T₅₀ and T₁₂₅ treatments equal to 47.63 and 44.60 %, respectively. Also, the best irrigation frequency for the three rapeseed cultivars in the Zarghan area was obtained equal to 10 to 12 days.

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  Soil particle size distribution and bulk density are used for estimating soil-moisture characteristic curve. In this model, soil particle size distribution curve is divided into a number of segments, each with a specific particle radius and cumulative percentage of the particles greater than that radius. Using these data, soil-moisture characteristic curve is estimated. In the model a scale factor, a , is used which may be considered as a constant, or obtained by logistic or linear procedures. F or most conditions, soil particle size distribution curve is not available, but only the percentages of clay, silt and sand could be obtained using soil textural data. In this situation, at first a precise soil particle size distribution must be developed, based on which the soil-moisture characteristic curve can be predicted. According to the previous studies, using particle radius of 999 µ m is more appropriate than radius 125 µ m. Also, adjusted coefficients for estimating soil particle size distribution curve for radii 1 to 20  µ m was obtained. In this study, using the soil textural data of 19 different soils from UNSODA database, soil-moisture characteristic curve of each was estimated with logistic and linear methods based on initial and adjusted soil particle size distribution estimation. The estimated values were compared with the measured data. The results indicated that for most soils, using the combination of logistic and adjusted particle size distribution estimation procedures is more appropriate than the previous methods.

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Soil water characteristic curve shows the relationship between soil water content and matric suction, which has an important role in water movement in the soil. The measurement of this curve is expensive and time-consuming in laboratory therefore, many methods have been proposed for its estimation including pedotransfer functions. By using the pedotransfer functions, soil water characteristic curve can be estimated based on other easily measured soil physicochemical properties. Parametric pedotransfer functions have been offered for parameters of the existing soil water characteristic curve models. In this study, 12 internal and external parametric pedotransfer functions of Brooks and Corey, Campbell and van Genuchten models were used and evaluated for 30 top soil samples in Fars province. To this end, the soil water characteristic curve and other necessary soil properties were measured, and then all soils according to the texture were divided into three groups of fine, medium and course textures. The results showed that the parametric pedotransfer functions of van Genuchten model were better than the other models, beacause of the better fit of this model to the measured data. Also, the results demonstrated that the parametric pedotransfer functions of Wosten et al. were the most appropriate method for estimating the soil water characteristic curve for the selected soils in Fars province, and that internal pedotransfer functions were not appropriate