Dynamic simulation of apple yield and dynamic response of deep soil moisture under rain-fed apple orchards of different planting densities at Baoji

More apples have been planted in recent years at Baoji, developing into a major apple cultivation area in the Loess Plateau where apple industry was critical for local rural economic development. Baoji is in a semi-humid climate zone where apple orchards are mainly rain-fed without artificial irrigation. Because apples need plenty water for growth, there has been deep soil desiccation in apple orchard fields which has in turn caused severe fluctuations in apple yield. Thus apple production bases in the region have faced a serious threat to sustainable, long-term development. To determine long-term variations in yield and soil water of rain-fed apple orchards of different planting densities in semi-humid climate tableland and gully-land regions of the Loess Plateau, apple yield and the 0 15 m layer soil moisture dynamics in apple orchards of 6 different planting densities at Baoji in 1965-2009 were quantitatively simulated and analyzed using the WinEPIC model. From planting densities of D1 (2 m × 3 m), D2 (2 m × 4 m), D3 (2.5 m × 4 m), D4 (3 m × 4 m), D5 (4 m × 4 m) and D6 (4 m × 5 m), and planting years of 1 45 years, the study determined the optimum planting density and cultivation period of rain-fed apple orchards. The results showed that for the 42 years (1968-2009) of apple production, yields of apple orchards of different planting densities initially increased gradually and then declined with severe fluctuations. The average apple yield of the first 21 years was significantly higher than that of the second 21 years in the 42 years of apple production. Apple yield improved with increasing planting density from D1 to D3 and some times D4, which was 833 1 000 plants per hectare. Apple yields of different planting densities changed positively with precipitation trend. With increasing planting density, soil available water capacity in the 0 15 m soil layer in apple orchards decreased, while formation rate of dried deep soil layer increased. Based on the trend and amplitude of variations in yield during 1968-2009, the number of drought stress day, available soil water capacity and soil moisture distribution, the optimum planting density was D3 or D4, and the optimum cultivation period of rain-fed apple orchards was 30 years at Baoji.