Author(s): Masayuki Fujihara; Tomoki Izumi; Koichi Unami; Junichiro Takeuchi
Linked Author(s): Koichi Unami, Masayuki Fujihara, Tomoki Izumi
Keywords: Three-dimensional unconfined groundwater model; Paddy fields; Finite element method; Drought year
Abstract: It is well known that paddy irrigation contributes to groundwater recharge. Field researches have been carried out to identify which part of groundwater originates from irrigation water, by comparing the groundwater levels during the irrigation and non-irrigation periods. However, quantitative evaluation of water flux from paddy fields to an aquifer is very difficult in the context of Monsoon Asia, where the mean precipitation is higher during the irrigation periods while the annual precipitation differs year to year. Numerical analysis using a physically based model is a promising approach to clarify the effect of irrigation water on variation in groundwater levels, when comparisons are made between different irrigation regimes under the same climate conditions. This study focuses on the unconfined aquifer of Dogo Plain, Ehime Prefecture, Japan, with a total area of 63 km2, out of which the paddy fields cover about 26 km2. To numerically reproduce spatio-temporal distribution of groundwater in the aquifer, a quasi-three-dimensional unconfined groundwater finite element model is employed. Time series data of river water levels and pumping discharges from 47 wells for municipal and industrial purposes are used for setting inner boundary conditions. Two cases, where the paddy fields are irrigated and rainfed, are considered for a two years period from January 1st 1993 through December 31st 1994. These two years include both wet (1993) and dry (1994) extremes. In the irrigated case, infiltration at a rate of 2 mm/day is taken into account to represent groundwater recharge from the paddy fields during the 4 months long irrigation periods. The spatially averaged groundwater level is 120 mm and 176 mm higher during the irrigation periods in 1993 and 1994, respectively, when irrigated. The difference of groundwater levels is the largest in the south-western part of Dogo Plain, attaining to 900 mm in the drought year of 1994. This indicates the significant contribution of irrigation water to a rise in the groundwater table in this part of Dogo Plain, where paddy fields are the dominant land use and the river system is distant.
Year: 2013