Abstract
In a series of agricultural land uses over a toposequence comprised of upland crop fields and lowland paddy fields, hydrological aspects are essential to an understanding of impacts of agricultural managements on groundwater environment. The objective of this study was to evaluate amounts and flow paths of shallow groundwater discharging from an agricultural catchment using a physical model. About 61% of the shallow groundwater percolated into the deeper aquifer and exited the catchment without passing through the paddy fields. Only the rest of the shallow groundwater, which was within 70 m of the paddy fields, reached the paddy fields after 14 years of travel. These results indicated that in a convergent-shaped catchment a large part of leached nutrients from upper crop fields can leave the catchment without being consumed in lowland paddy field area. We defined a risk of excess nutrient discharge from the catchment as a probability that 95% of the recharged groundwater bypasses the paddy fields, assuming that the most influential parameter falls within an uncertainty range of two orders of magnitude around the value giving the modeFs deterministic solution. The evaluated risk came to 44.8% even when the uncertainty range would include 99% of the whole population of the parameter.
