To measure the soil water retention curve (SWRC) in the laboratory, four intact specimens were taken with two replicates from the soil at layers with the depths of 10–15 cm, and four from the soil at layers with the depths of 30–35 cm by using sampling cylinders with the capacities of 100 cm3 and 50 cm3. After saturation of the samples, 0, 20, 30, 100, 330, 1000, 3000, 8000 and 15,000 hPa matric suction were applied to the samples using sand box and pressure plate devices. After equilibrium, their average weighted water content was measured33,34. The pressure plate consists of a chamber, such as a pressure-cooker, whosepressure could be increased by a compressor. When the pressure reaches the desired potential point, it is relieved through the drain valve and the lid is removed. Soil samples are removed quickly and their mass water content is measured. The specimens are again inserted into the device and the lid is put on and the pressure is increased to the next potential point. The mass moisture is again measured as above at this point.

The average soil porosity was calculated using the equation 1-(BD/2.65) and is considered as saturated moisture content. The SWRC model was fitted to the measured soil water retention data using RETC software. The equation of van Genuchten35 is as follows:

where θ(h) denotes soil volumetric water content (cm3 cm−3), h is the matric suction of the soil (hPa), θr is the residual water content (cm3 cm−3), θs represents saturated SW content (cm3 cm−3), α is the inverse of suction at the turning point (hPa-1) and n is the pore size distribution index (–)36.

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