新型称重式大型蒸渗仪为我国首创、国际先进，能够直接测定蒸散量和潜水蒸发量。本文利用此蒸渗仪和相关配套仪器测定的资料，在初步分析蒸渗仪对周围农田水、热及蒸散规律代表性的基础上，估算了潜水对农田蒸散的贡献份额，并建立了一定水位变幅内潜水蒸发与蒸散量的比值和地下水位的经验关系式。

A new lysimeter, built in 1991 in Yucheng Field Experimental Station of Chinese Academy of Sciences (CAS) and started operation since then, is 5 m in depth and 3.14 m ² in area. The weight of the lysimeter is about 3.4×10⁴ kg, and the measurement resolution is about 60 g, e.g. 0.02 mm in water depth. The ground water table inside the lysimeter can be adjusted to be the same as that in the adjacent area. The new lysimeter measures evapotranspiration and evaporation from ground water surface at the same time following the concept of water balance:P+I+E_g=P_a+ET+R±ΔSwhere P, I, E_g, P_a, ET, R, ΔS is precipitation, irrigation, evaporation from ground water, deep percolation, evapotranspiration, runoff and soil moisture variation of the whole column respectively. This paper examines firstly the representability of evapotranspiration measurement of this lysimeter, which shows that there is a good consistency between the variations of soil moisture and heat condition inside lysimeter and that outside. The variation of soil moisture storage of 0 cm -150 cm inside lysimeter ( ΔW ₁ , in mm/d) is well related to that outside lysimeter ( ΔW ₂ , in mm/d) with correlation coefficient of 0.93 in the form: ΔW ₂ =0.77* ΔW ₁ -0.0178, which indicates that ΔW ₂ is roughly 77% of ΔW₁ . The evapotranspiration measured by the lysimeter can represent well that of the adjacent area even though one coefficient of approximately 0 75-0 85, according to the comparison of several methods, is needed in order to get real evapotranspiration of that adjacent area. The figure of scatter points of measured E_g and ET show that their relationship is not either simple linear or non linear, and both of them may exist. The average ratio of E g to ET during the period of Dec. 11,1995 to May 30,1996 is about 17.1%, when winter wheat grew and the ground water table was between 1.89 m and 2.65 m. The empirical relationship of the average ratio PRE under one certain water table and this water table has been obtained with the coefficient of about 80%:PRE=-0.559*D+1.358where D is ground water table in m. PRE is about 30% when D =1.89 m, and 17.1% when D =2.12 m according to the above relationship.