Actual evapotranspiration (ET_a) is a key component of water balance. This study aimed to investigate the spatial variability and time stability of ET_a along a hillslope and to analyze the key factors that control the spatiotemporal variability of ET_a. The potential evaporation, surface runoff, and 0–480 cm soil water profile were measured along a 243 m long transect on a hillslope of the Loess Plateau during the normal (2015) and wet (2016) water years. ET_a was calculated using water balance equation. Results indicated that increasing precipitation during the wet water year did not alter the spatial pattern of ET_a along the hillslope; time stability analysis showed that a location with high time stability of ET_a could be used to estimate the mean ET_a of the hillslope. Time stability of ET_a was positively correlated with elevation (P<0.05), indicating that, on a hillslope in a semi-arid area, elevation was the primary factor influencing the time stability of ET_a.

实际蒸散量（ET _a）是水平衡的关键组成部分。本研究目的是研究ET _a沿山坡的空间变异和时间稳定性，并分析控制ET _a时空变异的关键因素。在正常（2015）和湿润（2016）水年期间，在黄土高原山坡上243m长的断面上测量了潜在的蒸发量，地表径流量和0-480 cm的土壤水分剖面。使用水平衡方程计算ET _a。结果表明，湿水年降水增加并没有改变ET _a的空间格局。沿着山坡；时间稳定性分析表明，ET _a具有较高时间稳定性的位置可用于估算山坡的平均ET _a。ET _a的时间稳定性与海拔高度呈正相关（P <0.05），表明在半干旱地区的山坡上，海拔是影响ET _a的时间稳定性的主要因素。