Having investigated the creek water quality in the Shiroishi Plain in comparison with the Saga Plain, the author now concludes that there were several historical and regional backgrounds which affected the quality of creek water as follows: 1. Pollution of the creek water in the Shiroishi Plain appears to be strikingly worse in comparison with the Saga Plain particularly during non-irrigation seasons. 2. A number of reservoirs were constructed extensively within and outside the catchment area of the Shiroishi Plain after the Edo period as farmland reclamation progressed in order to cater for increasingly emerging demands for agricultural water. 3. Although creeks have a function to circulate the reservoir water to the irrigated farmlands in the Shiroishi Plain, the density of creek is lower in comparison with the Saga Plain because of the smaller drainage basin. 4. Emerging water demands after mid 1950s were attributable to readjustment of paddy fields and reformation into well-drained paddy field. Excessive consumption of groundwater has given rise to the land subsidence caused by depletion of ground water reserves. 5. In the Shiroishi Plain where a chronic shortage of irrigation water is prevailing, creek water is repeatedly used under normal irrigation conditions. A long stagnation of irrigation water within a limited boundary is considered one of negative factors exacerbating creek water quality. 6. Half of the farmlands in the Shiroishi Plain grow onion as a secondary crop, and creek water has been somehow polluted especially during non-irrigation seasons whilst farmlands in the Saga Plain grow wheat as a secondary crop in the same non-irrigation seasons with lesser contamination of irrigation water. 7. The differences in the water quality between the Shiroishi Plain and the Saga Plain reflect the types of crop being grown and the circumstances how the agricultural water is circulated in the farmlands.
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