Water transfer through market mechanisms has gained increasing attention as a means to achieve higher efficiency in water resource management. However, dealing with the transfer of irrigation water, which is characteristically different from resources of other kinds in the sense that it forms a part of a region’s water cycle,presents a severe challenge. This paper presents two case studies illustrating how the special features of irrigation water constrain the free trade of irrigation water resources. One case involves the water transfer within an irrigation project on the South Island of New Zealand. The other is related to the redistribution of water between regions within the Drought Water Bank in California, USA. Both are cases in which simple market mechanisms have not been applied. Public authorities responsible for the management of regional water resources handle the entire process of individual cases of water transfer. In such cases, public authorities conduct investigations related to the technological possibilities presented by the transfer and then carry out subsequent reviews of the project’s impact on regional economies and ecosystems. Social receptiveness is the strongest constraining factor of the free trade of irrigation water, which invariably creates relations among surrounding actors, irrespective of the transfer size.

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 This study evaluated the applicability of tank models calibrated to an extremely large storm event using ordinary storm events. Rainfall and runoff were observed at three granitic watersheds having different forest recovery conditions: we studied a poor-vegetation watershed, a vegetation-recovered watershed, and a forested watershed. Parameters of the tank models were calibrated using rainfall and runoff data observed for ordinary storm events having total precipitation of less than 150 mm. Then those models were applied to simulate hydrographs for an extremely large storm event (Tokai heavy rain) having total precipitation of 457 mm. In the poor-vegetation watershed and vegetation-recovered watershed,which were characterized by thin soil layers, contributions of surface runoff to the total discharge were large even under ordinary storm events. In such watersheds, the tank models calibrated using the ordinary storm events reproduced hydrographs of the extremely large storm event. However, for the forested watershed, where surface runoff was rarely observed because of thick soil layers, the tank model was inaccurate for the extremely large storm event. The simulated hydrograph produced smaller peaks and gentler responses than the observations. We concluded that a large contribution of surface runoff generated only during an extremely large storm event caused underestimation of peak flows for the forested watershed.

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 Quantitative assessment of groundwater-level influencing factors such as land use and pumping schemes is important to achieve sustainable groundwater use. This study examined the Tedori River Alluvial Fan in Ishikawa Prefecture,Japan, where groundwater levels have long been declining. Observations and numerical simulations using MODFLOW reveal some remarkable results. Comparisons of groundwater level observations during the irrigation and the non-irrigation periods revealed that groundwater levels during the irrigation period are about 5 m higher than during the non-irrigation period, which implies that paddy irrigation water contributes significantly to groundwater recharge.Furthermore, groundwater levels during the non-irrigation period have declined 5 m during 1993-2009. Discharge observations of the Tedori River confirm that much river water infiltrates into shallow groundwater. A groundwater simulation model was developed considering the observed data. Steady state groundwater flows in the irrigation period for several scenarios of land use and pumping schemes were set based on past, present, and predicted future human activities. Simulated results indicate that groundwater flows in the upper and middle parts of the fan are affected considerably by paddy area and pumping discharge. Especially, in cases where the paddy field area is smaller by more than 30 % from the present condition, the groundwater levels were not maintainable by pumping regulations. Drawdown of the water level of the Tedori River also causes a slight groundwater level decline along the river.

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