To show advantage and precaution of subsurface irrigation, we studied the effects of water table levels and their changes on shoot and root growth of onion. Onion plants were grown in light-colored andosol-filled stainless steel frame controlled water table at -30cm and -40cm with fewer amounts of surface watering (25% of normal precipitation). Control was irrigated with similar amount of normal precipitation. The -30cm water table treatment had higher water contents of surface soil (10cm) than those in the -40cm. In both water table treatments, water potential of each soil layer located from -15 to -35cm hovered higher than that in the control. Plants in higher water table had shallower root zone and smaller root length. More leaf fresh weight and maximum leaf length at harvest time was, -40cm > control > -30cm, and the difference in maximum leaf length seemed to arise during the bulb enlargement stage. On the other hand, no difference observed in those bulb yields among treatments. The -30cm treatment had higher bulb dry matter percentage than other treatments. At leaf development stage with elevated water table in pot slightly promoted leaf, bulb and root growth, while the treatment at the beginning of bulb formation stage decreased leaf and root growth. At bulb enlargement stage, the elevation of water level in root box began to necrotize root tip. These results showed that onion crops with lower water table than root zone could get bulb as much as weight of those with normal amount of precipitation. The elevation of water table especially after the beginning of bulb formation stage would inhibit root growth and result in smaller shoot.

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Pine trees (red and black pines) at Takata-matsubara, which is scenic coast in Rikuzen takata city (*1), were completely broken down due to logging or breaking by huge tsunami at Great East Japan Earthquake. Although just one big red pine tree was survived, it was submerged at high tide or ground water level was increased due to declining of ground level by 84 cm, inhibiting root growth and its function. sheet pile of 6 m long was driven surrounding the tree (15m×15m) to prevent the flow of ground water towards the tree from the body of sea water. However, ground water level inside the sheet pile was increased by the spout of ground water, due to quick sand phenomena, which has been known to occur when sheet pile driving was conducted at sand area in sea coast. We tried to drain water using drain pump, but could not suppress the increase of ground water level. Roots system at surface soil layer lost their vitality and died due to flooding and salt damage. Leaves were green-colored and kept their vitality in middle of March 2011, immediately after the tsunami disaster, however they became to be brown gradually and almost died in August 2011, as a result of the increase of transpiration by increasing temperature then. Thus, utilizing of sheet pile driving technique at sand area in sea coast has high risk extremely with consideration of spout of ground water due to quick sand phenomena. In such a condition, lifting root system technique using root clump for transplanting can be considered. If lifting and transplanting using heavy machine was hard to conduct, alternative methods of making slope and pulling the tree on the rollers (*2) by winch or jacking-up can be considered. Early transplantation is required because extending new roots occurs in early spring. Tsunami went through along spur, reaching to 25 m high, and cedar plantation was lost by salt damage. Precious giant cedar in the Imaizumi-Tenmangu shrine was also died by the salt damage. Here we discussed the tree damage due to flooding and salt damages to roots and effects of flooding on the sandy soil, as a result of the increase of ground water level by sheet pile driving technique at sand area in sea coast.

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