Characteristics of soil evaporation at two stages of growth in apple orchards with different ages in a semi-humid region

Soil evaporation (E) accounts for a significant portion of the hydrological cycle especially under hot and dry conditions. The Loess Plateau of China contains the world’s largest area of apple orchards cultivated in a rather extensive way (720 trees ha−1), while few studies were conducted to explore the characteristics of E at different stages of growth in apple orchards with different ages in water-limited regions. In this work, we measured E, sap flow, throughfall, stemflow and the leaf area index (LAI) in 7- and 17-year-old rainfed apple orchards in Changwu County on the Loess Plateau from May to September 2012–2015. We divided the stages of growth of the apple trees into stage I (rapid foliar growth) and stage II (rapid fruit enlargement and maturation) based on the seasonal variation of LAI and constructed empirical models for predicting E in each orchard at different growth stages. Tree age clearly affected the dynamics of daily E and the ratio of E to evapotranspiration (E:ET) for apple orchards. Daily E and E:ET during the four growing seasons were significantly higher for the 7-year-old orchard (1.3±0.5 mm d−1 and 55.1±10.4%, respectively) than the 17-year-old orchard (1.1±0.5 mm d−1 and 44.1±10.9%, respectively) (p < 0.05). Daily E and E:ET for both orchards also differed between the two growth stages and were significantly higher at growth stage I (p < 0.05) according with the higher solar radiation reaching the soil level. The stage of growth influenced the performance of empirical E models for apple orchards. Compared with measured daily E, simulated daily E was underestimated for the young orchard and overestimated for the old orchard at growth stage II. The relationships between daily E for each orchard and biotic/abiotic factors differed between the two growth stages. Daily E for both orchards was more likely to be affected by meteorological factors and soil temperature in the 0–20 cm layers at growth stage II. This study highlights the significant effects of tree age and growth stage on the characteristics of daily E and E:ET and the performance of empirical E models for apple orchards in a semi-humid region.