在流域的概念下，水通常沿著流域的地形特徵流動不會跨越流域，但實際模擬中下游土地開發區域，不易掌握實際流域邊界位置，會有相鄰流域綜合模擬時出現跨流域逕流的現象。本研究區域為臺灣西南部的高屏溪及東港溪，採用地文性淹排水模式模擬，藉由雨量分析Horner公式取得研究區域雨量站各重現期的雨量進行淹水模擬，探討是否會產生跨流域逕流，模擬結果透過ArcGIS呈現。
研究區域中流域交界線的斷面兩側，平均高程皆為高屏溪流域高於東港溪流域。模擬結果顯示研究區域跨域逕流初期並不明顯，5年重現期開始產生現象，25年重現期以前跨域逕流為局部不易辨識方向，25年重現期開始跨域逕流現象明顯可辨識為高屏溪流域流向東港溪流域，50年重現期以後東港溪除負擔自身流域降雨產生逕流外，還需承受部分高屏溪跨域逕流，表示隨著降雨量及淹水深度的增加，相鄰流域間水流方向決定因素從地形高程改為水位較高或上升速度較高。而東港溪下游右岸在50年重現期模擬的時候，最大水位幾乎已達計畫堤頂高，隨時可能有溢堤之風險，在考慮極端氣候影響下，可能有再檢討改進的需求，而在流域下游處改善區域進行淹水改善，道路加高的方案明顯優於在地滯洪。
跨域逕流伴隨土地發展程度日益明顯，土地開發活動改變地表高程或土地坡度，使跨域逕流易發生在包含建築、農業及交通用地的已開發土地，尤其建築及農業用地，最不易產生跨域逕流的則是森林的位置。因此在進行綜合防洪管理時，須考慮相鄰流域產生跨域逕流，並常態性滾動檢討劃設的流域邊界線，以符合真實存在的自然現象。

The concept of watersheds, also known as catchments, is essential for both flood man-agement and water resource administration, as it significantly enhances the assessment of overland flow characteristics. Watershed boundaries are generally defined by elevation, since water naturally follows the geological traits of watersheds and does not transcend these boundaries. However, changes in elevation and land usage due to human develop-ment have led to ambiguous and unstable boundaries between adjacent watersheds in the flat terrain downstream.
This study focuses on the Kaoping and Donggang River watersheds to examine the cross-watershed runoff phenomenon during rainfall events of varying return periods. Ac-cording to land use surveys in the area, the region near the boundary of the two water-sheds is heavily developed, primarily for agricultural, residential, and transportation purposes. As the study area was extensively developed, cross-watershed runoff was noted even for the numerical simulation of 2-year return period rainfall.
The size and depth of the areas experiencing cross-watershed runoff stabilize in numeri-cal simulations of 25-year return period or more, due to the obstruction of further surface runoff by surrounding high-elevation terrain. Therefore, as planning for flood mitigation, it is essential to consider cross-watershed runoff from neighboring watersheds in addition to the conventional surface runoff.

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