有機栽培對茶園水文收支與通量的影響

Abstract

氣候變遷導致極端乾旱事件頻率與強度上升，坪林茶園仰賴天然降雨補給田間水分，了解茶園水文收支平衡，為提升茶園面對氣候風險韌性的關鍵。本研究目標為量化有機（ORG）與慣行（CONV）栽培對茶園水文收支的影響。以蒸發散通量與多層土壤含水量的觀測數據，搭配對應的水文模式模擬茶園水文收支平衡。模式除了包含蒸發散與降雨逕流歷程外，特別加入降雨截留模組模擬冠層差異對於水文收支的影響。模式參數除了來自現地土壤的物理分析外，進一步採用Optuna進行參數最佳化。土壤分析結果發現，有機茶園土壤具有較低的總體密度與顯著更高的土壤有效水分（AWC）以及有機質含量；相較於CONV茶園，模式模擬結果顯示ORG茶園較高的蒸發散量主要由其較高的樹冠截留量與非飽和層的根系吸收量所提供。二氧化碳收支方面，有機茶園白天 CO2 淨吸收量顯著優於慣行茶園，除了與其擁有較高的LAI值（Leaf Area Index）因而具備較大的二氧化碳吸收能力之外，推測也與其擁有顯著較低的平均VPD，且與在冬季出現溫度與相對濕度（T/RH）的遲滯現象（相位滯後約 1 小時）有關。本研究使用 CCM方法分析各參數間之因果關係， ORG 的蒸發散主要與中、深層土壤水連結，二氧化碳通量受土壤含水量驅動的影響較慣行茶園弱，映證其茶樹高效的水分利用效率，以及其二氧化碳通量有更高比例來自植被光合作用與呼吸作用，而非受土壤層支配。總體而言，有機栽培使茶園擁有更良好的土壤結構和冠層特性，使其在水分流失與動態補充之間達到有效的平衡，同時提升了微氣候調節能力與碳匯潛力，為氣候變遷下茶園的永續農法實踐提供了關鍵的科學依據。

Climate change has led to an increase in the frequency and intensity of extreme drought events. Since Pinglin tea plantations rely on natural rainfall for water supply, understanding their hydrological balance is key to enhancing resilience against climate risks. This study aims to quantify the impacts of organic (ORG) and conventional (CONV) cultivation on the water balance of tea plantations. Observational data of evapotranspiration fluxes and multi-layer soil water content were integrated with the hydrological model to simulate the water balance. In addition to evapotranspiration and rainfall-runoff processes, the model incorporates a rainfall interception module to simulate the effects of canopy differences on the hydrological balance. Model parameters were derived from in-situ soil physical analyses and further optimized using Optuna. Soil analysis results indicated that soils in the organic tea plantation had lower bulk density and significantly higher available water content (AWC) and organic matter content. Compared to the CONV plantation, model simulations showed that the higher evapotranspiration in ORG was primarily contributed by its greater canopy interception and root uptake from the unsaturated zone. regarding the carbon dioxide balance, the organic plantation exhibited significantly superior net CO2 uptake during the daytime compared to the conventional plantation. This is attributed not only to its higher Leaf Area Index (LAI), which provides greater CO2 absorption ability, but also likely to its significantly lower average Vapor Pressure Deficit (VPD) and the occurrence of a temperature-relative humidity (T/RH) hysteresis phenomenon during winter, with a phase lag of approximately 1 hour. Using Convergent Cross Mapping (CCM) to analyze the causal relationships among parameters, this study found that evapotranspiration in ORG was mainly linked to soil water in the middle and deep layers. The influence of soil water content on CO2 fluxes was weaker in ORG than in CONV, confirming the high water use efficiency of the organic tea trees and indicating that CO2 fluxes were driven more by vegetation photosynthesis and respiration than by the soil layer respiration. Overall, organic cultivation resulted in superior soil structure and canopy characteristics, achieving an effective balance between water loss and dynamic replenishment. It also enhanced microclimate regulation and carbon sequestration potential, providing a critical scientific basis for sustainable farming practices in tea plantations under climate change.