Analysis of Effective Rainfall in Paddy Field District by Applying CB Method in Rainy Days

Abstract

The authors examined how rainfall in CB, RB, and NB blocks affects the diversion requirement of paddy field district in which return flow is reused. Major conclusions obtained in this study are as follows.
1. The rainfall of NB block does not affect the diversion requirement.
2. The maximum effective rainfall of CB block which does not use return flow of upstream RB block is G_CB (water requirement of the CB block).
3. The maximum effective rainfall of CB block which uses return flow of upstream RB block is G_CB if G_RB (return flow of RB block) is greater than Q_CB (irrigation discharge of CB block).
4. The maximum effective rainfall of RB block is G_RB (water requirement of RB block) in case of G_RB≤Q_CB or ET (evapotranspiration) in case of D_RB>Q_CB.
By the forementioned method, estimating the effective rainfall of the paddy field district whose reuse ratio is 150%, the results are as follows.
1. The ratio of effective rainfall to rainfall is 9-15%. It is clear that the effective rainfall of paddy field district in which return flow is reused is very small.
2. The average ratio of observed effective rainfall to estimated effective rainfall is about 70%. This is standard value of paddy field district in which water delivery is controlled by central water management station.