Effect of Conjunctive Use of Water for Paddy Field Irrigation onGroundwater Budget in an Alluvial Fan

摘要

Conjunctive use of surface water and groundwater for irrigation plays an important role inthe hydrology of alluvial fans, both as a source of recharge to the groundwater and as a cause ofdischarge by pumping. Therefore, reliable estimates of distributed groundwater recharge anddischarge are critical in analyzing groundwater budgets in alluvial fans. Nasunogahara basin, TochigiPrefecture, Japan, is an alluvial fan with abundant shallow groundwater, which is extracted and used forirrigation. In this study an integrated surface water- groundwater model, which combined a modifiedtank model with a finite difference model, was used to evaluate quantitatively groundwater recharge anddischarge capacities of each recharge source and discharge route and to assess the effect of conjunctiveuse of surface water and groundwater for paddy field irrigation on the water balance of the shallow aquiferof Nasunogahara. The tank model calculated recharge by different recharge sources to the shallowaquifer and also calculated the amount of groundwater pumped for irrigation. Distributed groundwaterrecharge and discharge were estimated at each node in the groundwater flow model based on thepaddy field area around the node. Results of the model indicate that the calculated water tableelevations were able to describe the actual behavior of the shallow aquifer adequately. From the modelsimulation runs, it was found that although paddy fields are big users of groundwater by pumping, theirrigation water applied is used in a cyclic manner to recharge the shallow aquifer. The percentage of therecharge from paddy fields to the total net recharge is 21 %, which is mainly provided from paddy fieldsirrigated by canal water. These facts indicate the importance of the conjunctive use of surface water andgroundwater for irrigation in maintaining the hydrologic system of the basin.