Soil Hydrological Characteristics And Evapotranspiration Of Apple Orchards On The Gully Region Of Loess Plateau

Soil hydrological characteristics, evapotranspiration and microclimate monitor system were monitored in this study to investigate water budget at the different spatial and temporal scales. The study site is located in Changwu tableland which is typical representative of the gully region of Loess Plateau. On the basis of fixed-point monitoring, we analysed the characteristics of apple trees canopy interception, evaluated the evapotranspiration of apple orchard, changes of soil moisture, and evaluated the water budget status of apple orchard. The main results were as follows:Forest canopy is one of the important factors affecting atmospheric rainfall redistribution. The result indicated that in the process of rainfall redistribution for grown and young apple orchard, throughfall accounted for the maximum share followed by canopy interception and the stemflow was the lowest; throughfall rate of grown apple orchard was lower than that of young apple orchard while the canopy interception rate of grown apple orchard was higher, so the grown apple orchard tended to intercept the rainfall while the young apple orchard tended to form throughfall. Different rainfall levels had different influences on the redistribution of rainfall in canopy layer: the throughfall, penetration rate, stemflow, stemflow rate and canopy interception will increase along with the increasing of rainfall level while the canopy interception rate decreases; for the same rainfall level, the phenological characteristic of apple orchard had a significant influence on the rainfall redistribution process: the throughfall rate first decreases and then increases along with the time, the canopy interception rate first increased and then decreased along with the time, in the whole growing season, the throughfall rate and canopy interception rate was in negative correlation relationship.Evapotranspiration of the apple orchard showed an obvious bimodal curve, with first peak was observed in the second half of July or the first half of August and the second one, in the first half of September. In the years from 2012 to 2014, the evapotranspiration accounted for 103%, 104% and 103% of the total rainfall during the growing season in 2012-2014, respectively. In addition, the evapotranspiration of the young apple orchard was 12.1 mm greater than rainfall in 2012, 18.2 mm greater than rainfall in 2013, but 1.2 mm lower than rainfall in 2014. The variation coefficient of evapotranspiration within growth cycle was about 1.0, indicating a large degree of pulsation. The Loess Tableland is a typical rainfed agricultural area, where the water used for evapotranspiration mainly comes from natural rainfall. Thus, the amount of rainfall could directly affect the quality and quantity of the production of apple orchard. In the dry(2012) and dried-normal(2013) years, as the evapotranspiration was greater than rainfall, the rainfall could not meet the requirements for water consumption of apple orchard, resulting in water shortage according to the change of soil water content. However, in the normal year(2014), precipitation could meet the requirement for water consumption of young apple orchard. Meanwhile, the soil water and evapotranspiration of apple orchard had a rapid hydrological response to rainfall in the Loess Tableland, and rain water fallen into apple orchard can be rapidly transformed in the form of soil evaporation and vegetation transpiration.Soil water content in 9-year-old apple orchard, 19-year-old apple orchard, corn field and wheat field was investigated in the Changwu Tableland to define the soil hydrological characteristics and soil drying state of main cropland and orchard on the Loess Plateau. Results showed that, the 9-year-old apple orchard had the largest soil water storage(186.5 cm) in 0-600 cm soil profile, followed by the corn field, wheat field and 19-year-old apple orchard in which soil water storage averaged 183.6, 158.6 and 132.8 cm, respectively. There was no significant difference between the corn field and 9-year-old apple orchard, but significant differences existed between other fields. Soil water content in 0-200 cm for the four fields showed medium variation(10%<CV<100%), while soil water content in deep soil layer was relatively stable(CV<10%). The depth of soil water depletion for the 19-year-old apple was 500 cm, while the depths for the 9-year-old apple, corn field and wheat field were all 300 cm. The rain replenishment depth of the 19-year-old apple orchard was 250 cm, while the depths for the 9-year-old apple orchard, corn field and wheat field were all deeper than 600 cm. Compared with other fields, soil desiccation in the 19-year-old apple orchard occurred most severely. Specifically, the 0-200 cm soil layer dried seasonally, the 200-250 and 250-320 cm soil layers dried severely, and the 320-600 cm soil layer dried extremely. A persistent soil dry layer formed. Soil desiccation in the corn field and 9-year-old apple orchard occurred seasonally, and a temporary soil dry layer formed in the shallow soil layer in the case of water shortage.Water balance is one of the most important indicators to describe the function and characteristic of ecosystem. The water balance closure was in good situation according to the comprehensive analysis on apple orchard; but in the case of small or large rainfall, these neglected factors and test operation deviation leaded a relatively poor water balance closure of orchard system. During the experiment, the evapotranspiration contribution in sequence, from small to large, was canopy interception, soil evaporation and water evapotranspiration. For 9-year-old apple orchard, the percentage of these three components to evapotranspiration was 10.1%, 34.8% and 55.1%, respectively; for 19-year-old apple orchard, it was 8.1%, 30.3% and 61.6%, respectively; for 10-year-old apple orchard, the percentage of these three components to evapotranspiration was 7.9%, 27.0% and 65.1% respectively; for 20-year-old apple orchard, it was respectively 12.9%, 25.3% and 61.8%; at the same time, soil evaporation of young apple orchard was larger than that of order apple orchard, while transpiration of order apple orchard was larger than that of young apple orchard; and during observation period, soil evaporation and evapotranspiration was subject to first-increase-then-reduce tendency, soil evaporation reached to the maximum in June and the plant transpiration reached to the maximum in July, such change law had a close relationship with the environment and orchard growth state.