3 The Use Limit of Soil Water Resources by red plum apricot
The state of vertical distribution of soil water in the root soil zone space influence plant growth. Soil water stress influence root growth and root water uptake. If the soil water content in the MID is equal to wilting point and soil water content in the MID is equal to SWRULP, and there is not enough water supply from precipitation, most red plum apricot changes the colour of leaf from green to yellow or croci and leaves fall earlier than usual time in the growing season. Red plum apricot tree almost ceased growing, and red plum apricot fruit does not expanding, which influence the yield, quality and economic benefits of red plum apricot fruit even if the roots extended to a depth of more than MID and could absorb some water from the soil layers more than 290 cm deep, suggesting that the total amount water that red plum apricot roots absorbed from soil per unit time does not satisfy the need of plant transpiration and fruit development in the semiarid loess hilly region.
SWRULP is the limit of plants in using soil water resources. It can be defined as the soil water storage in the MID when all of the soil layers in the MID become DSLs. The soil depth, representative soil layer and Wilting coefficient see table1, and , the SWRLP in red plum apricot is 212.7mm, which is different from 252.8 in caragana shrubland at the middle and the top of the Heici mountain and 220.8 mm in alfafa grass, showed that the SWRULP changed with site condition and vegetation type.