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 over-estimated 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.

Automated summary

Soil evaporation (E) in apple orchards on the Loess Plateau of China was measured in 7-and 17-year-old rainfed orchards. The study found that tree age and growth stage significantly affected the daily E and E:ET characteristics, as well as the performance of empirical E models for apple orchards in a semi-humid region.