桃園地區休耕地變遷對局地水文氣候的影響

人類活動是影響氣候的一個重要因子，而土地利用的變化可以透過改變地表能量通量來影響當地的水文循環。在過去，有不少的研究討論不同地表型態對台灣局部氣候的影響，但針對水田轉為休耕地的下邊界地表條件之變遷卻很少被著墨。本研究主要聚焦於休耕地對區域氣候的影響，選定近年來由於經濟政策，水稻田面積明顯減少的桃園市做為研究區域。在此研究中，除了進行實地觀測，討論水稻田與休耕地對於局地溫度效應，我們同時利用天氣研究與預報模式（Weather Research and Forecasting model, WRF）進行數值模擬實驗，探討休耕下陸地大氣交互作用與大氣物理過程的改變，並在模式中修改水稻田的物理設定，使模擬接近現實狀況。在數值模式中，使用1995年的土地利用資料為實驗的控制組，模擬桃園第一期稻作插秧期（2月中旬至4月中旬）和第二期稻作插秧期（7月中旬至9月中旬），弱縱觀條件下的晴朗與降水個案。結果顯示，在晴朗條件下，可感熱通量和近地表溫度增加；潛熱通量和近地表比濕降低。再者，由於季節性太陽輻射能量和季風風向與強度上的不同，在降雨個案中，第二期插秧期的降水在休耕後發生顯著變化，而在第一期差異不明顯。與水稻田相比較，實地觀測結果顯示，休耕地的每日地面最高溫度更高。透過這項研究，可以說明經濟政策改變地貌，進而可能影響當地的氣候，我們透過理想化數值模式的結果發現，當發生大規模休耕情況時，桃園當地的水文氣候將有可能會被影響。

關鍵字

陸氣交互作用；水稻田；休耕；土地利用變遷； WRF模式

並列摘要

Human activities are one of the main factors affecting the climate. Changes in land use can affect the local hydrological cycle by changing the surface energy flux. Many studies have discussed the influence of different land surface conditions on the local climate in Taiwan. However, the changes in land boundary conditions for the conversion of paddy fields to fallow land were rarely investigated. This study focuses on how the paddy field's transformation can impact the regional climate and takes Taoyuan City as the research area, where the number of rice paddies has decreased significantly in recent years because of the local economic policy. This study used in-situ observations and the numerical model, named Weather Research and Forecasting (WRF) model, to explore how the anthropogenic land-use changes affect the local hydroclimates. We modified the physical setting of paddy land in the WRF settings to mimic the land's paddy and fallow conditions in the simulations during both the sunny days and rainy days under weak synoptic conditions in mid-February to mid-April (FMA) and mid-July to mid-September (JAS). Results show increased sensible heat fluxes and temperature at 2m, and decreased latent heat fluxes and specific humidity at 2m in the dry cases. Furthermore, the rainy cases show significant changes in precipitation in JAS but not in FMA, due to the different seasonal net energy and wind direction. On-site field results also reveal larger surface daily maximum temperature over the fallow land compared to the paddy. This study demonstrates that the economic policy may significantly affect the local hydroclimate, and modeling results suggest that the occurrence of a large-scale fallow situation can impact the local hydro-climate in Taoyuan.