Case study for estimating the potential evaporation in tunnel farming in Central Japan

Abstract

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.