| Introduction:The purpose of this paper is to analyze and discuss the following questions via studying the relationship among water moving laws in soil-plant-atmosphere continuum (SPAC) and water and forest matter production in water-collecting afforestation :The suitable soil water conditions and allowing soil water deficit degrees for forest growth in semi-arid areas on the loess plateau in North Shaanxi; Grade classifications of soil water productivity and the effective dynamic evaluation of soil water upon forest production; Available use efficiency of rainfall water resources for water-collecting afforestation and water production efficiency of forest individual and community; Rainfall water resources environmental capacity for black locust (Robinia pseudoacacia.L) and cypress (platycladus orinentalis) in semiarid areas on the loess plateau in North Shaanxi; i.e. the water nutrient area and unit area required by the numbers of forest trees can be sustained by rainfall water resources during the maturity of forest trees; and the main research approaches are as follows: SLI-Type siphon self-recording rain gage and E601-Type water surface evaporator are adopted to observe and measure the meteorological data or elements. LNW-50A neutron probing method is adopted to monitor soil dynamic variations . TTN-500 Type original soil column soil evaporator is used to observe and determine soil evaporation; and ZIZ-4 Type plant water meter is used to monitor and determine tree water vigor.The concept of water environmental capacity, SPAC theory, soil physics, plant drought-resistance physiology and ecology, and forest hydrology are the principal theoretical bases for the experimental design and result analysis of this research project. Pick diffusion laws, Farquhar stoma constraint theory, runoffproduction theory on the slope surface and water balance equation are the calculation bases for data processing and relevant parameters. The major research results are as follows: The management norms for suitable soil water conditions and forest land soil water are determined with black locust and cypresses to improve water use efficiency as the core in the semiarid areas.Black locusts are suitable for SWS threshold values of 11.0% - 15% ( approximately to the field holding capacity of 52.0% - 74.o% ) ; the most suitable for SWC of 13% (approximately to the field holding capacity of 62.0% ) or so; the allowed maximum soil water deficit SWC is 10% (approximately to the field holding capacity of 48% ) or so; and the soil water aggregate compensation point is 4.5% (approximately to the field holding capacity of 21.5%) or so.Cypresses are suitable for SWS threshold values of 9.5% - 13.5% (approximately to the field holding capacity of 45.0% - 64.0%) in between; the most suitable for SWC of 10% or 10.5% (approximately to the field holding capacity of 48.0% or 50.5%) ; the allowed maximum soil water deficit SWC is 8.5% or so (approximately to the field holding capacity of 40.5% ); and the soil water aggregate compensation point is 3.8% (approximately to the field holding capacity of 18.0%) or so. The functions of rainfall and water collection make soil water increase and promote the growth of forest trees. Soil water environment in forest lands or woodlands is in the state of continuous changes. Accordingly, monitoring its dynamic process can play an important role in managing soil water in woodland, evaluating water-collecting benefits, predicting forest tree growth and determining water environment capacity.The instant value of soil water obtained by fixed time and fixed position observation and measurement can, in fact, reflect the intermittent results of interaction of each environmental factor in SPAC. Research results indicate that soil water contents in woodlands treated with different water-collecting methods show no significant difference in spring and autumn except that soil water contents in woodlands are improved greatly in the rainy season, whereby indicating...
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