水梯田地勢對灌溉暗回歸水的影響與鵝尾山梯田現地調查

Abstract

有鑑於梯田的多功能性，近年來台灣水梯田的復育逐漸受到重視。由於水梯田受地形影響，灌溉與地下水流相較於平地水田有著更複雜的機制。然而過往研究對於水梯田地表下水流情形有不同的看法，尤其暗回歸水的存在與否尚未有共識。 因此，本研究透過COMSOL Mutiphysics®數值模擬的方式，探討水梯田暗回歸水的形成條件與機制，以及地勢對暗回歸水的影響。同時，本研究以台北市士林區平等里之鵝尾山水梯田作為研究場域，透過地電阻影像探測與鑽井岩心、觀測地下水位、量測水力傳導係數等現地調查的方式，觀測灌溉與降雨對水梯田地下水流的影響，並調查現地之暗回歸水的存在。此外，本研究以水平衡計算灌溉需水，針對鵝尾山水梯田現地提出灌溉建議。 模擬結果顯示，田間暗回歸水的形成條件為地下水位面抬升至地表，而當系統達到穩態而有暗回歸水產生時，平均約有53~55%的灌溉水成為暗回歸水，並且入滲率顯著減少；當坡度越緩、土層水力傳導係數越大、土層厚度越小及地下水位離地表越近時，越有利於暗回歸水的產生。現地調查結果顯示，灌溉可以維持地下水位至一定高度，然而難以抬升至地表形成暗回歸水。因此，不用考慮暗回歸水於鵝尾山水梯田灌溉水分規劃中，只需解決配水不均的問題。

Given the multifunctionality of terraces, the restoration of terraced paddy fields in Taiwan has gradually received attention in recent years. Due to the impact of topography, irrigation and groundwater flow of terraced paddy fields have a more complex mechanism than flat paddy fields. However, previous studies have different views on the subsurface flow in terraced paddy fields, and in particular, there is no consensus on the existence of subsurface return flow. Therefore, this study used numerical simulation by COMSOL Mutiphysics® to explore the formation conditions and mechanisms of subsurface return flow in terraced paddy fields, as well as the impact of relief on subsurface return flow. At the same time, this study took the Ewei Mountain Terraces in Pingdeng Vil, Shilin District, Taipei City as the research site, using electrical resistivity tomography (ERT) and on-site investigation, such as drilling cores, observing groundwater levels, and measuring hydraulic conductivity, to investigate the effect of irrigation and rainfall on the groundwater flow under terraced paddy field and the existence of on-site subsurface return flow. In addition, this study used water balance to calculate irrigation water requirement and gave irrigation suggestions for the Ewei Mountain Terraces. The simulation results show that the occurrence of subsurface return flow is conditional on the groundwater reaching the soil surface. Once the system reaches a steady state and the subsurface return flow occurs, an average of about 53~55% of the irrigation water becomes subsurface return flow, and the infiltration rate considerably reduces. The condition of subsurface return flow is prone to be achieved with a gentle relief, large-hydraulic-conductivity formation, thin soil layer, and high groundwater level. On-site investigation results show that irrigation maintained the groundwater level to a certain height. Nevertheless, the critical condition of the subsurface return flow was not found. Therefore, there is no need to consider the subsurface return flow in the irrigation water planning of the Ewei Mountain Terraces, and only need to solve the problem of the uneven water distribution.