HYDROLOGICAL RESPONSES OF A WATERSHED TO VEGETATION CHANGES IN A TROPICAL SEMIARID REGION

ABSTRACT The objective of this work was to assess the effect of vegetation on the runoff coefficients of a Caatinga dry tropical forest before and after thinning. Thus, an experiment was conducted with evaluations in three hydrological years (2008, 2011 and 2013) in Iguatu, State of Ceará, Brazil. In 2008, the vegetation consisted of a 30-year regenerating Caatinga forest. The vegetation was subjected to thinning in 2009, 2011 and 2013, removing trees with less than 10-cm diameter at breast height. Hydrological responses were evaluated as a function of daily precipitation water depths, based on cumulative frequency distribution, by dividing precipitation events into three classes (CP) (CP≤30, 3050 mm). Significant differences between runoff coefficients before and after vegetation thinning were assessed through the Student's t-test (p<0.01). Before thinning (2008), CP≤30 mm showed the highest runoff coefficient, differing statistically (p<0.01) from the other years. The results of precipitation events of great magnitude (CP>50 mm) indicate that the runoff is greatly dependent on rainfall characteristics and soil moisture conditions. The greater development of herbaceous vegetation due to thinning reduced the surface runoff.摘要 本研究旨在评估疏伐前后植被对卡廷加热带干旱林（Caatinga dry tropical forest）径流系数（runoff coefficient）的影响。为此，在巴西塞阿拉州伊瓜图市开展了为期3个水文年（2008、2011及2013年）的定位试验。2008年时，试验样地的植被为已自然恢复30年的卡廷加热带干旱林。研究分别于2009、2011及2013年对植被实施疏伐，移除胸径小于10 cm的林木。本试验以日降水量为评估变量，基于累积频率分布将降水事件划分为3个等级（CP）：CP≤30 mm、30<CP≤50 mm及CP>50 mm，并据此分析水文响应特征。通过Student t检验（Student's t-test，p<0.01）评估植被疏伐前后径流系数的显著性差异。结果显示，疏伐前（2008年），CP≤30 mm等级的降水事件对应的径流系数最高，且与其余年份的对应值存在统计学显著性差异（p<0.01）。针对强降水事件（CP>50 mm）的分析结果表明，地表径流极大程度上依赖于降雨特征与土壤含水率条件。疏伐促进了草本植被的生长发育，最终降低了地表径流。