Abstract Orchard evapotranspiration (ET) is a complex flux which has been the subject of many studies. It often includes transpiration from the trees, cover crops and weeds, evaporation from the soil, mulches and other orchard artefacts. In this study we investigated the contribution of the orchard floor evaporative fluxes to whole orchard ET focusing on the transpiration dynamics of understorey vegetation which is currently not well known. Data on the partitioning of ET into its constituent components were collected in apple (Malus Domestica Bork) orchards with varying fractional canopy cover. The study orchards were in the prime apple growing regions in South Africa. The orchards were planted to the Golden Delicious/Reinders and the red cultivars (i.e. Cripps’ Pink/ Royal Gala/Fuji). Tree transpiration was quantified using the heat ratio method and the thermal dissipation sap flow techniques. Understorey transpiration was measured at selected intervals using micro stem heat balance sap flow gauges calibrated against infrared gas analyser readings. Orchard ET was measured using an open path eddy covariance system while the microclimate, radiation interception, and soil evaporation were also monitored. Orchard floor evaporative fluxes accounted for as much as 80% of the measured ET in young orchards with dense understorey vegetation that covered most of the orchard floor. In these orchards the understorey transpiration was of the same order of magnitude as the bare moist soil evaporation suggesting that water use by the understorey vegetation was substantial. However, in mature orchards with a high canopy cover (>55% fractional cover), orchard floor water losses were less than 30% of the measured ET. Understorey transpiration rates were much lower, contributing less than 10% of the whole orchard ET. Significant volumes of water can be saved, especially in young orchards, by keeping the orchard floor vegetation short, reducing the area occupied by understorey vegetation, and by reducing the wetted ground surface area.

中文翻译：

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南非地中海气候条件下下层植被对苹果园蒸散分配的贡献

摘要 果园蒸散（ET）是一个复杂的通量，已成为许多研究的主题。它通常包括树木、覆盖作物和杂草的蒸腾作用、土壤、覆盖物和其他果园人工制品的蒸发作用。在这项研究中，我们调查了果园地面蒸发通量对整个果园 ET 的贡献，重点是目前尚不为人所知的下层植被的蒸腾动力学。在具有不同冠层覆盖率的苹果 (Malusdomesica Bork) 果园中收集了关于 ET 划分为其组成成分的数据。研究果园位于南非的主要苹果种植区。果园种植了金冠/驯鹿和红色品种（即克里普斯粉红/皇家晚会/富士）。使用热比法和热耗散树液流技术量化树木蒸腾作用。使用根据红外气体分析仪读数校准的微型茎热平衡液流计以选定的时间间隔测量下层蒸腾作用。果园 ET 使用开放路径涡流协方差系统测量，同时还监测小气候、辐射拦截和土壤蒸发。果园地面蒸发通量占果园测得的 ET 的 80%，在覆盖大部分果园地面的茂密林下植被的年轻果园中。在这些果园中，下层蒸腾作用与裸露的潮湿土壤蒸发量级相同，这表明下层植被的用水量很大。然而，在树冠高的成熟果园 (> 55% 的部分覆盖），果园地面失水量小于测得的 ET 的 30%。下层蒸腾速率要低得多，占整个果园蒸腾量的比例不到 10%。通过保持果园地面植被较短，减少下层植被所占的面积，以及减少湿地表面积，可以节省大量的水，尤其是在年轻的果园中。