Abstract
In Japan, more and more paddy fields, left fallow mainly by nationwide rice production control policy, have recently been changed to upland crop fields or artificial wetlands. Since such change or diversification of land use probably has an impact on the water quality environment of the whole neighborhood, it must strategically be implemented in the context of land use arrangement for environmentally sound watershed management. Of great importance is then to make effective use of self-purification functions fulfilled by paddy fields or wetlands. The present study is associated with development of a simulation-optimization model for designing an optimal land use on a watershed scale. The model is composed of a hydro-environmental subsurface watershed simulation model, represented by coupled 3-D Richards and advection-dispersion equations, and a GA (Genetic Algorithm)-aided optimization part. An optimization problem is formulated which, with the most use of self-purification, minimizes the objective function expressed in terms of a weighted total of discharged nutrient from a watershed and residual nutrient within the watershed, taking the location of paddy fields in fallow, changed to upland crop fields or wetlands, as a decision variable. An example problem for a small watershed of an irrigation tank is solved that requires deciding the optimal locations of paddy fields that can be changed to other land use categories. The results show that the model developed could be a powerful tool for making tactics to combat the environmental impact resulting from land use diversification.
