In this research, a method was proposed to estimate the potential evaporation in polytunnel fields. In order to calculate the potential evaporation by the Penman method, as part of an irrigation plan, the climatic conditions inside the tunnels needed to be accurately estimated by adjusting the climatic conditions outside the tunnels. Measurements taken in two polytunnels of watermelon plants from April to June of 2005 and 2006, respectively, revealed that the climatic conditions inside the tunnels differed from those in open fields outside the tunnels. The air temperature inside the tunnels was 2.0 to 4.5 °C higher than that in the open fields, the air saturation deficiency was equivalent to that in the open fields, and there was more or less no wind. The net radiation inside the tunnels was estimated by an equation that uses the transmission factor and shielding factor values. In this study, the crop coefficient of watermelon was assumed to be the ratio of the water consumption to the potential evaporation value estimated by the Penman method, which was almost identical to the design standard value used in irrigation planning in Japan. Furthermore, this study showed that rainfall outside the approximately 1.5-m-wide tunnels did not affect the soil moisture within the tunnels, because the soil moisture inside the tunnels did not increase in response to the 100 mm of rain that fell over two days. Thus, the potential evaporation inside a polytunnel can be estimated with the Penman method and then adjusted with respect to the climatic factor values from neighboring open fields. In irrigation planning, such tunnels should be reclassified as greenhouses, rather than open field sites, due to their lack of access to rainfall.

在这项研究中，提出了一种估计多隧道场中潜在蒸发的方法。为了通过Penman方法计算潜在的蒸发量，作为灌溉计划的一部分，需要通过调整隧道外部的气候条件来准确估算隧道内部的气候条件。2005年4月至6月和2006年分别在两个西瓜植物多隧道中进行的测量表明，隧道内部的气候条件与隧道外部的空旷地域有所不同。隧道内的空气温度比空旷地域高2.0至4.5°C，空气饱和度不足与空旷地域相同，并且或多或少没有风。隧道内部的净辐射是通过使用透射系数和屏蔽系数值的方程式估算的。在这项研究中，西瓜的作物系数被假定为用水量与通过Penman方法估算的潜在蒸发值之比，该值几乎与日本灌溉计划中使用的设计标准值相同。此外，这项研究表明，大约1.5米宽的隧道外部的降雨不会影响隧道内的土壤水分，因为隧道内的土壤水分并未响应两天的100毫米降雨而增加。因此，可以使用Penman方法估算多隧道内部的潜在蒸发量，然后相对于来自相邻空旷地域的气候因子值进行调整。