2023 Volume 14 Issue 1 Pages 9-16
It is crucial to establish appropriate water management in agriculture. Due to financial difficulties, it is also necessary for farmers to work together with government agencies in water management. Existing water allocation models, however, require much effort to collect data. They are also black boxes difficult for farmers to understand, which may discourage farmers from participating in irrigation planning. In this study, we attempted to construct a farm-block-level water balance model with a simple tank model concept and easily collectible data. The model consists of two parts that reproduce the water distribution process from the farm block intake to each field (Canal Tank Model; CTM) and the water balance process in the paddy field (Paddy Tank Model; PTM), respectively. The model coefficients were identified using data from 2002 to 2003, and the model validity was evaluated using data from 2004 and 2005. The CTM coefficients were identified for 2002 and 2003 because of the different land use patterns. The CTM runoff coefficients and hole heights were changed on days when the farmers were considered to have operated diversion ratios significantly. The PTM coefficients were identified using the SDFP method and trial and error referring to the groundwater level. However, the PTM seepage holes were fixed to the corresponding values that the vertical seepage was approximately 5 mm/d based on the on-site survey. As a result, despite the arbitrary water manipulation by farmers, the same CTM coefficients for 2002, 2003, 2004 and 2005, when the land use pattern was similar, were able to reproduce the water allocation well. This water allocation model can be used to estimate the optimal delivery water management rate and to quantitatively evaluate the excess water withdrawal resulting from the labor reduction in water management.
