说明光合作用对土壤水分存在一个“阈值”反应，此值为土壤田间持水量的6 5%左右。
This indicates that photosynthetic rate has the threshold response to soil moisture and the threshold value is about 65% of field capacity.
而当降水强度较大，以致使土壤水分大于田间持水量时，超过田间持水量部分的土壤水会很快流往较低的台阶。
However, when precipitation rate is large enough and soil water goes beyond the field capacity, the excessive soil water will flow laterally to the lower levels through noncapillary pores.
当过量的水从饱和土壤中排出后，余下的水量被称为田间持水量。
The amount of water that remains after the excess has drained away from the saturated soil is known as the field capacity.
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