Abstract
Recently the additional water demand for paddy field irrigation, named the “lot management water requirement”, has been receiving attention in Japan. This comprises the runoff water from outlets, and is made up of the intended surface drainage and the spillage of furnished water. It can be thought of as another form of demand in addition to that for percolation and evapotranspiration, the net water requirement in fields. However, the actual state of its spread is still not clear.
Initially, in the present study, the circumstances in which the lot management water requirement was brought to the surface were briefly and generally outlined. Secondly, its actual mechanism and form was discussed mainly by close analysis of the records of water management and water budget ing in all 27 test field lots in Shiga prefecture. The results revealed the actual states existing as follows;
1) Paddy lots are kept submerged in the ifrst half of the growing season. In contrast, in the second half, intermittent irrigation without sumbmergence is widely spread, thus changing the rate and total quantity of percolation.
2) Intended surface drainage mainly breaks out just before transplanting and mid-summer drainage. It is established in correspondence with other cultivation techniques, and can be recognized as being representative of the recent intensive form of water management practice with the control of ponding depth.
3) Spillage with flow-through varies according to the lot, the year and the growing stage. It is determined by the water management practice of the farmer, and is influenced by intake rate, the shape and size of the lot, and by the irrigation system used.
