流域面積高度積分 (Hypsometric Integral, HI) 為一種以計量方式描述流域地形的參數。近年有部分學者以面積高度積分作為推論臺灣地區構造相對活動性的指標。然而面積高度積分值的大小受流域大小的影響，此種積分值的面積依賴(Area dependence) 特性，可能會影響其對構造活動的解釋。此外，當討論的流域跨越山區、丘陵到平原等不同的地形區時，亦即以上游和下游次集水盆地為不同計測標的時，因上、下游之主要河流作用不同 (上游以侵蝕為主，下游以堆積為主)，其面積高度積分是否也會表現出不同的特徵，而有空間依賴 (Space dependence) 的現象，以致影響到上、下游地區面積高度積分意義的解讀？在計算面積高度積分的過程中，數位高程資料 (DEM) 的解析度，是否會影響流域面積高度積分的計算結果？這些可能因觀察或計測尺度的差異而造成的面積高度積分意義差異，是本文所要討論的重點。 本文選擇流經臺灣西部麓山帶並從西部海岸平原入海的朴子溪、三疊溪、急水溪和二仁溪等四個流域為測試區，測試之後發現，以不同DEM解析度計測同一個次集水盆地時，平原型次集水盆地及面積小於1km2的次集水盆地，其面積高度積分差異較大，而高差較大的山地型和丘陵型次集水盆地，其面積高度積分值則對DEM解析度較不敏感。各研究流域的下游平原型次集水盆地面積高度積分都普遍偏高，而中、上游丘陵型次集水盆地面積高度積分則明顯偏低，顯示面積高度積分的確存在空間依賴的現象。而各次集水盆地高差與面積高度積分呈一對數負相關的關係，利用此一關係並經由統計方式可修正面積高度積分的空間依賴現象，如此便能進行上、下游次集水盆地面積高度積分的比較分析。而在同一流域中，以不同集流閾值所萃取出之不同面積大小的次集水盆地組合與各年代地層以及活動構造作疊圖分析，可以發現當集流閾值小於1km2時，面積高度積分同時受到岩性與構造的影響，而當集流閾值大於2km2時，則主要反應構造活動的影響。因此以面積高度積分推論構造活動性時，亦需注意面積高度積分值的面積依賴現象。本研究顯示，在進行臺灣西南部麓山帶前緣流域面積高度積分的構造活動性探討時，次集水盆地集流閾值必須大於2km2，才能避免岩性的干擾，呈現出該地的構造活動性。

The hypsometric integral (HI) of drainage basin is a morphometric parameter for describing basin geomorphology. Lately the HI has become an index to infer activities of active tectonics in some orogenic belts. This paper examines three issues related to the HI. First, the HI of different drainage basins may be attributed to different geomorphic factors. Thus the “area dependence” of the HI may effect the interpretation of active tectonics. Second, if we divide a drainage basin into several subbasins, the HIs of subbasins located at upper stream and lower stream may have different geomorphic meanings when a study area covers mountain, hill and plain. This represents the “space dependence” of the HI. Third, calculation of the HI can be affected by DEM resolution. Our study area covers the SanTieh River, PuTzu River, ChiShui River and ErJen River with drainage through the mountain front and coastal plain of western Taiwan into the sea. The results show variations of HIs for plain-type subbasins and small subbasins (area<1km2) measured by different DEM resolutions. And the influence of DEM resolution on HIs of subbasins in mountain-type and hill-type that have higher elevation drop is trivial. Moreover, there are higher HIs on plain-type subbasins and lower HIs on hill-type subbasins, thus confirming the space dependence of the HI. Elevation drop and the HI of every drainage subbasin has a logarithmic relation. Using the relation, one can correct the space dependence of HIs of subbasins before comparing the HIs of hill-type and plain-type subbasins. In this study we controlled the drainage area of subbasin by changing the threshold of accumulation. By overlaying lithologic and tectonic maps on different drainage areas of subbasin groups, we found that the HIs of subbasins showed the effect of lithology and tectonic using a threshold smaller than 1km2. However, the HIs showed almost entirely the effect of tectonic elements using a threshold larger than 2km2. We therefore recommend a threshold larger than 2km2 as the suitable scale for discussing tectonic activities in the mountain front of western Taiwan. At this scale, the HIs of subbasins can exclude the effect of lithology.