間歇灌溉（AWD）對有機水稻田甲烷排放與地下水補注之影響

Abstract

本研究以桃園區農業改良場有機水稻田作為研究場域，分別採用傳統慣行湛水灌溉（Continuous Flooding, CF）與間歇灌溉（Alternate Wetting and Drying, AWD）管理，探討不同灌溉管理對有機水稻田甲烷排放及地下水補注的影響。研究結果顯示，間歇灌溉可顯著降低甲烷排放，輪作田區全年排放量明顯低於連作田區。然而在一期作中，間歇灌溉並未呈現出節水的效果。透過簡易水平衡計算指出，輪作田區具有較高的滲漏量，可能導致灌溉需求增加。不過整體而言，由於該田區於二期作改種植旱作，大幅減少灌溉用水，使其全年灌溉總量仍低於連作田區。在地下水補注方面，雖然輪作田區於一期作滲漏量較高，但連作田區因全年維持湛水，其年度總滲漏量仍高於輪作田區，推估輪作田區對於整體地下水補注的貢獻可能少於連作田區。另一方面，根據地下水位監測結果可發現，一期作灌溉開始後，田區觀測地下水位明顯上升，並維持至二期作結束才逐漸下降，顯示區域性的灌溉活動對於地下水變化具有即時且顯著的影響。本研究亦使用THMC（Thermal Hydrology Geo-Mechanics Reactive Model）模式模擬該區域灌溉與降雨對於地下水位的影響，結果顯示降雨與灌溉行為對於地下水位有顯著的影響，僅考慮降雨的地下水位雖然會受到降雨產生短期抬升，但在補注灌溉與降雨同時存在之情境可使地下水位呈現緩慢上升的趨勢。本研究結合實際觀測資料與地下水位模擬結果，分析降雨與水稻田灌溉對桃園沿海地下水位的影響，將有助於未來了解執行間歇灌溉對於地下水補注的衝擊。

This study was conducted in organic paddy fields at the Taoyuan District Agricultural Research and Extension Station, where traditional continuous flooding (CF) and alternate wetting and drying (AWD) irrigation practices were implemented to investigate the effects of different irrigation management strategies on methane emissions and groundwater recharge. The results indicate that AWD significantly reduced methane emissions, with the annual emissions from the rotation field being markedly lower than those from the continuously cultivated field. However, during the first rice cropping season, AWD did not demonstrate a water-saving effect. Based on a simplified water balance analysis, the rotation field exhibited higher percolation losses, which likely increased irrigation water demand. Nevertheless, because the rotation field was converted to an upland crop during the second cropping season, irrigation water use was substantially reduced, resulting in a lower annual irrigation volume compared to the continuously cultivated field. With respect to groundwater recharge, although the rotation field showed higher percolation during the first cropping season, the continuously cultivated field maintained flooded conditions throughout the year, leading to a higher annual cumulative percolation. This suggests that the overall contribution of the rotation field to groundwater recharge may be lower than that of the continuously cultivated field. In addition, groundwater level observations revealed a clear rise following the onset of irrigation during the first cropping season, which persisted until the end of the second cropping season before gradually declining. This pattern indicates that regional irrigation activities exert an immediate and pronounced influence on groundwater dynamics. Furthermore, the Thermal Hydrology Geo-Mechanics Reactive Model (THMC) was employed to simulate the impacts of irrigation and rainfall on groundwater levels in the study area. The simulation results demonstrate that both rainfall and irrigation significantly affect groundwater levels. While groundwater levels respond to rainfall events with short-term rises when rainfall alone is considered, scenarios incorporating both irrigation and rainfall lead to a gradual and sustained increase in groundwater levels. By integrating field observations with groundwater modeling, this study provides insights into the combined effects of rainfall and paddy field irrigation on groundwater levels in the coastal area of Taoyuan, contributing to a better understanding of the potential impacts of implementing AWD irrigation on groundwater recharge.