| 摘要: | 蒸發散與降雨入滲是地表與大氣間水氣交換的主要通量，了解蒸發散與入滲現象是如何改變土壤水分變化，並且進一步影響到地下水位改變，這是本研究主要目的。透過建置在中央大學氣象坪之持續水文與微氣象觀測，推估潛勢能蒸發散量、實際蒸發散量以及入滲量。針對降雨濕潤過程，利用土壤張力計與含水量計的觀測數據，推估入滲量與土壤保水曲線參數，透過分析含水量與地下水位的變化，解析含水量上升與地下水位上升之間的關係。而針對乾燥過程則是以土壤水分損失推估實際蒸發散量，並比較在乾燥過程中含水量與地下水位的變化。
分析資料為從2016/12/9至2018/9/31期間挑選乾燥與濕潤事件，結果顯示研究區域內的地下水呈現穩定的梯度，長期來看，側向的地下水流入與流出量相差並大。乾燥事件中平均側向入流量為0.102mm/hr，而平均流出量為0.117mm/hr；濕潤事件中的平均側向入流量為0.1mm/hr，而平均流出量為0.09mm/hr；所有長期平均(包含降雨剛停止後，或是斷斷續續的降雨事件等，無法判斷是否為乾燥或濕潤過程的情況)的側向入流量為0.041mm/hr，而平均流出量為0.042mm/hr，因此推論側向地下水流動對整體的地下水位的增減並沒有顯著影響。
降雨事件時，地下水位的上升大致可以分成兩個階段，首先未飽和層在地表下400cm處之含水量尚未上升，也就是垂直入滲尚未抵達含水層，但有地下水位上升紀錄；第二個階段才是持續入滲抵達含水層。而且濕潤事件中，在孔隙水壓上升階段時，地下水側向流動並未有明顯改變，因此推論降雨初期地下水位的上升是因為近地表土壤含水量增加所形成地表靜水壓力上升的向下傳遞。整合2016/12/9至2018/9/31的事件中，可以發現地下水位上升的時間要比-400cm含水量上升時間平均早約9.5小時，而在雨水入滲至地下水位後，側向的入流、出流量才會開始有明顯的改變。挑選許多降雨事件的統整與歸納後，對中大氣象坪地區而言，累積降雨量要達到約40mm時，水分才有入滲至地下水位的現象(即-400cm含水量有上升的現象)。乾燥過程中也發現類似現象，由於地下水位在未降雨時側向的流入與流出差異不大，但是-400cm處含水量尚未顯著降低時，地下水位就下降，因此推論蒸發散使近地表孔隙水壓下降，近地表土壤含水量的下降也會因靜水壓力下降出現地下水位下降的現象。
;Evapotranspiration (ET) and rainfall infiltration (RI) are primary fluxes of water exchanges between land surface and atmosphere. The objectives of this study is to investigate how ET and RI affect soil moisture and groundwater variations. Integrated hydrometeorology measurements were established at the meteorology station inside the campus of National Central University to provide continued observations for calculations of potential ET, real ET, and RI values. During rainfall wetting period, tensiometers and soil moisture sensors provide estimations of RI and parameters of soil characteristic curves. During non-rainfall drying periods, amounts of real ET were estimated by losses of soil moisture. Changes of soil moisture and groundwater levels were analyzed to investigate connections of soil moisture and groundwater during wetting and drying processes.
Data selected from 2016/12/9 to 2018/9/31 were separated into wet events and dry events. Persistent groundwater flow gradients were observed. For wet events, averaged groundwater inflow and outflow velocities are 0.102 mm/hr and 0.117 mm/hr, respectively. For dry events, averaged groundwater inflow and outflow velocities are 0.1 mm/hr and 0.09 mm/hr, respectively. Long-term averaged (for example: after the rain has just stopped, or intermittent rain events, that is, it is impossible to determine whether it is a dry or wet process event.) groundwater inflow and outflow velocities are 0.041 mm/hr and 0.042 mm/hr, respectively. Regional groundwater inflow and outflow do not have significant effects on changes of groundwater levels.
During rainfall events, the rise of groundwater levels were caused by two different effects. The first one is due to the increase of near surface hydrostatic pressures as the increase groundwater levels were early than that of soil moisture at -400 cm. This hypothesis was supported by insignificant differences between groundwater inflow and outflow velocities. The later second effect is the recharge of infiltrated water. Rainfall events observed between 2016/12/9 and 2018/9/31, the rise of groundwater levels are 9.5 hours earlier than the increase of soil moisture at -400 cm. After the infiltrated water reach saturated zones, significant differences between groundwater inflow and outflow velocities were observed. Based on rainfall events analyzed in this study, 40 mm of accumulated rainfall is capable to induce sufficient infiltration to cause the increase of soil moisture at -400 cm and reach saturated zone subsequently. For dry events, declines of groundwater levels were earlier than decreases of soil moisture at -400 cm. It is suspected that the declines of groundwater levels were induced by the decrease of soil moisture due to reduces of near surface (i.e., above -400 cm) hydrostatic pressure. This hypothesis was supported by differences between groundwater inflow and outflow velocities were insignificant during dry events. Drying or wetting in near surface soil moisture will induce changes in groundwater levels to quickly reflect changes of near surface hydrostatic pressures caused by variations of soil moistures due to ET and RI. |
