溪頭試驗林2005年泰利颱風期間雨水與溪水化學變化研究

Abstract

本研究目的主要在探討2005年泰利颱風 (Typhoon Talim, 8/31 ~ 9/5）期間，溪頭試驗林雨水與溪水水文及水化學變化。泰利颱風期間溪頭累積雨量為382 mm，分別由2次尖峰降雨 (184 mm及198 mm) 所組成。颱風期間，溪頭雨水偏酸性 (pH=4.21~5.40)，電導度偏低 (1.2~22.6 μS/cm)，各項離子濃度亦較平日降雨為低，僅於前期降雨尖峰時Na+、Cl－及SO42－ 濃度略為升高，可能顯示颱風降雨初期受海水鹽分的影響。泰利颱風降水使溪頭巨石坑溪與神木坑溪流中形成2次流量尖峰。溪頭溪水化學變化可歸納為下列2種形式：（1）濃度稀釋現象：即物質濃度隨著水位升高而下降，包括電導度、pH、Ca2+、Mg2+、Na+、Cl－、NO3－、SO42－等等，其中NO3－稀釋現象極為特殊，可能為泰利颱風前多次大雨淋洗，以致NO3－來源庫消耗，不及恢復，但仍需未來研究證實支持。（2）濃度增高現象：即物質濃度隨著水位的上升而升高，包括懸浮質與K+，其中神木坑溪流中懸浮質濃度於2次尖峰流量遠高於巨石坑溪，主要來自先前921地震及2001年桃芝土石流及泰利颱風三重干擾的影響。此些水質變化顯示2005年泰利颱風可為溪頭試驗林帶來大量潔淨雨水輸入，然巨石坑與神木坑溪流可能因過去干擾受害程度不同，而呈現不同水化學變化。

Rainfall and streamwater collecting system was set up to study chemical changes in precipitation and streamwater in Chitou Experimental Forest during 2005 Typhoon Talim event. Total rainfall of 382 mm (comprised of two distinctive 184 mm and 198 mm peak rainfall) was recorded in Chitou experimental watershed from 2005/08/31~09/05. Bulk precipitation was acidic (pH4.21~5.40) with low conductivity (1.2~22.6 μS/cm), and with low concentration for most anions and cations, yet higher concentrations of Na+, Cl－ and SO42－. In early part of peak rainfall could be from the sea salt spray. Heavy rainfall from typhoon also induced twin-peak stormflow in both Giant-Rock Creek and the Big-Tree Creek. Conductivity, pH, Ca2+, Mg2+, Na+, Cl－, NO3－, and SO42－ in both creek exhibited decreases in concentrations with increasing streamflow. Diluting NO3－ in this study showed totally different trend to other studies in Taiwan. Whether or not the NO3－ pools in both watersheds were adequately replenished from previous leaching caused by antecedent precipitation, needs further study. While suspended solids, and K+ showed increases with increasing streamflow. Extreme increase in suspended solids in the Big-Tree Creek was caused by the combined effects from Talim typhoon storm flow and the last debris torrent from typhoon Toraji in 2001 and the 921 earthquake in 1999. These solute behaviors suggested that typhoon Talim could bring heavy input of clean rainfall to the Chitou watersheds, yet stormwater chemistry could be controlled by the distinctive disturbance history for Giant-Rock Creek and Big-Tree Creek respectively.