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农田土壤水分散失是在二个表面上进行的，一是土壤表面称之为蒸发，另外是作物表面称之为蒸腾。两者均与土壤水分呈线性相关，即随着土壤水分增大而增大。但土壤水分与光合速率关系研究表明，当土壤水分低于６５～６９％田持时，随着土壤水分增大光合速率增大，若土壤水分高于６５～６９％田持时，随着土壤水分增大光合速率降低。光合作用对土壤水分有一阈值反应。这一阈值可考虑为节水灌溉的田间土壤水分控制标准
Farmland soil moisture loss is carried out on two surfaces, one is called the soil surface evaporation, the other is the crop surface called transpiration. Both are linearly related to soil moisture, ie, increase with increasing soil moisture. However, the relationship between soil moisture and photosynthetic rate shows that when the soil moisture is lower than 65-69%, the photosynthetic rate increases with the increase of soil moisture. If the soil moisture is higher than 65-69% Moisture increased photosynthetic rate decreased. Photosynthesis has a threshold response to soil moisture. This threshold can be considered as a field water control standard for water-saving irrigation