BFD50: usable field capacity in rootable soil

The usable field capacity of a soil is the part of the field capacity available for vegetation. It contains the amount of water that a site far from groundwater can retain against gravity in natural storage after sufficient saturation and corresponds to a suction voltage of pF 1.8 to 4.2 according to the convention.土壤有效田间持水量（usable field capacity）是指可供植被利用的田间持水量（field capacity）部分。其指代远离地下水位的地块在充分饱和后，可在自然储存状态下抗衡重力留存的水量；按照行业惯例，该指标对应的吸力标度为pF 1.8至4.2。