利用大型入滲儀觀測動態土壤水力特性：動態毛細壓力、流速與舊水響應波速

Abstract

非飽和土壤層是一個複雜的多孔介質系統，土壤的水力特性極易受到其複雜結構與觀測尺度的影響。近年來，小型沙箱實驗已經證實動態效應對於保水曲線的影響，但尚未釐清是否受觀測尺度影響。本研究透過邊界壓力條件控制大型入滲儀內部土壤水分變化速率，並利用埋設的張力計與時域反射計(TDR)，量測在動態與靜態排水與汲水過程中的保水曲線。實驗結果顯示入滲儀內淺層土壤在相同張力下，動態排水過程可以保有比靜態過程較高的體積含水量，也就是所謂的毛細壓力的動態效應（dynamic effect）。但該效應隨土壤深度遞減。另一方面，非飽和層土壤水流速(velocity)與舊水響應波速(celerity)被認為會顯著影響新舊水(入滲水與殘餘水)互動、降雨-逕流機制與非飽和層的物質傳輸，但相關機制卻尚待釐清。搭配含水量與溫度量測，發現在入滲過程中，濕潤鋒的移動受供水機制與邊界條件的影響，含水量的資料顯示空氣侷限導致濕潤鋒面含水量降低。實驗的溫度資料顯示淺層時熱水(新水)大部分與含水量變化訊號重疊，但在中層以及深層熱水的訊號落後於冷水(舊水)的訊號，表示不同深度溼鋒面移動時包含的新水以及舊水比例有顯著的不同，同時舊水響應波速可達新水水分流速的兩倍以上。上述研究成果，將有助於量化觀測尺度對於動態毛細壓力的影響，並進一步釐清非飽和層土壤水的流速與舊水響應波速的不同。

The unsaturated soil is a complex porous media system, and the soil hydraulic properties are easily affected by its complex structure and observation scale. In recent years, small-scale sandbox experiments have confirmed the influence of dynamic effects on the water retention curve, but it has not yet been clarified whether it is affected by the observation scale. In this study, we control the soil moisture change rate inside the large lysimeter by the different boundary pressure conditions. And the tensiometer and time domain reflectometry (TDR) were used to measure the water retention curve during the dynamic and static drainage and imbibition processes. The experimental results show that the shallow soil layer during the dynamic drainage process can retain a higher volumetric water content than during the static process, which is so-called dynamic effect of capillary pressure. But this effect decreases with the soil depth. On the other hand, the soil water velocity and celerity of the unsaturated soil are considered to significantly affect the interaction between the new and old water (infiltration water and residual water), the rainfall-runoff mechanism and the solute transport in the unsaturated zone. The relevant mechanism has yet to be clarified. We used the changes in water content and soil temperature to detect the wetting front movement during the infiltration and the influence of boundary conditions. This study also observed that the water content of the wetting front decreases under the air-confined condition. The soil temperature data shows that the hot water (new water) signal observed from the upper sensors mostly overlaps with the water content change signal, but the hot water signal in the middle and deep area lags behind the cold water (old) signal. It implies that when the wetting front moves, the ratio of new and old water at different depths is significantly different. At the same time, the old water celerity can reach more than twice the velocity of the new water. The abovementioned findings will help quantify the observation scale influence on dynamic capillary pressure and further clarify the flow velocity and celerity of soil water in the unsaturated zone.