Abstract In the tropical Okavango Delta, transpiration by trees is an important process partly responsible for maintaining the basin as a freshwater environment. Quantification of evapotranspiration from terrestrial landforms of the delta, fringed by riparian woodlands, is one of the main contributors to uncertainty in current hydrological modelling. We investigated sap flow of common trees in the distal, mid‐ and upper delta in July–August 2012, November–December 2012 and February–April 2013 using the compensation heat pulse velocity method. In the distal delta, four Diospyros mespiliformis individuals of different sizes were studied. Four trees of different species were studied in the mid‐ and upper delta. Sap flow density (SFD; flow per unit cross‐sectional area) was used as a common unit to facilitate comparison. Sap flow varied with tree size, species, season and location. It was positively correlated with tree size (r 2 = 0.67). Sap flow variation between seasons and across locations in all the species studied indicated two distinct groups. Group 1 transpired the least during the hottest season, November–December, and Group 2 the most. In Group 1, the highest average SFD was 1.17 l cm−2 day−1 during July–August; in Group 2, it was 1.07 l cm−2 day−1 during November–December. Changes in the hydrology of the delta would negatively affect the riparian woodland.

中文翻译：

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博茨瓦纳奥卡万戈三角洲选定河岸林地物种的树液流变化

摘要 在热带奥卡万戈三角洲，树木蒸腾作用是维持盆地淡水环境的重要过程。由河岸林地环绕的三角洲陆地地貌蒸发蒸腾量的量化是导致当前水文建模不确定性的主要因素之一。我们使用补偿热脉冲速度方法调查了 2012 年 7 月至 8 月、2012 年 11 月至 12 月和 2013 年 2 月至 4 月期间三角洲远端、中、上三角洲常见树木的树液流。在远端三角洲，研究了四个不同大小的 Diospyros mespiliformis 个体。在中三角洲和上三角洲研究了四种不同物种的树木。液流密度（SFD；每单位横截面积的流量）用作常用单位以方便比较。树液流量随树木大小而变化，种类、季节和地点。它与树木大小呈正相关（r 2 = 0.67）。在所有研究的物种中，季节之间和不同地点的树液流量变化表明两个不同的群体。第 1 组在最热的季节（11 月至 12 月）发生最少，第 2 组发生最多。在第 1 组中，7 月至 8 月期间的最高平均 SFD 为 1.17 l cm−2 day−1；在第 2 组中，11 月至 12 月期间为 1.07 l cm−2 day−1。三角洲水文的变化将对河岸林地产生负面影响。7 月至 8 月期间的最高平均 SFD 为 1.17 l cm−2 day−1；在第 2 组中，11 月至 12 月期间为 1.07 l cm−2 day−1。三角洲水文的变化将对河岸林地产生负面影响。7 月至 8 月期间的最高平均 SFD 为 1.17 l cm−2 day−1；在第 2 组中，11 月至 12 月期间为 1.07 l cm−2 day−1。三角洲水文的变化将对河岸林地产生负面影响。