Evaluation of qualitative and technical performance of subsurface drainage in phaseolus vulgaris l. as a second crop of paddy fields (physical model scale)

Subsurface drainage system is required to eliminate water logging and establish planting condition for the second crops in the autumn and winter seasons in the large part of paddy fields in northern Iran. Proper depth and distance in a drainage system minimizes the negative effects of drainage effluents on the environment. This research was carried out in a physical model scale of paddy fields in agricultural faculty of Gillan University during 1396 to evaluate the qualitative and quantitative performance of the subsurface drainage in the second crop cultivation. The phaseolus vulgaris l. crop was planted in the physical model after filling the box with the soil of paddy fields and creating the hard pan layer at the depth of 15 cm. Drainage water and soil solution samples were collected during the occurrence of precipitation and their electrical conductivity, acidity, sodium, calcium, magnesium, nitrate and orthophosphate parameters were measured. The pressure head and drainage discharge were measured before and after precipitation events. The trend of electrical conductivity of the drainage water was decreased 60% as compared to the initial value and the amounts of sodium adsorption ratio (SAR) were oscillated due to precipitation and calcium and magnesium adsorption in the soil. The maximum amount of nitrate in the drainage water was 46.9 mg/l at the beginning of the crop growth period, which exceeded the permissible level of drainage discharge into the environment and indicates the importance of fertilizer management. The concentration of nitrate in the soil was affected by the distance from the drain, while the concentration of orthophosphate was not affected. The mean values of pressure head (7-17 cm) with respect to the root depth, showed an efficient drainage performance regarding to the drainage discharge and no water logging at the root depth during the growth period.