On Evapotranspiration from Developed Farm Land** Using Water Balance Method:Studies on the hydrologic cycle in developed farm land (I)

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It is very important to understand the changes occurring in the hydrologic cycle caused by the development of farm land for appropriate planning and evaluation of forests. We have been carrying out observations of rainfall and discharge in developed farm land and investigations of soil in order to grasp the resulting changes in hydrologic components. In this paper, the water balance method was used to estimate actual evapotranspiration from an area of developed farm land and the characteristics of the evapotranspiration were discussed. The results were as follows. i) Annual evapotranspiration was in the range between 630mm and 820mm, depending on annual precipitation. The evapotranspiration ratio, which is defined as the ratio of actual evapotranspiration (Et) to pan evaporation (Epan), was also dependent on annual precipitation and the range of fluctuation was between 0.6 and 0.9 (Table 1). ii) The value of Et in summer was approximately 3.8 mm/d, while that in winter was about 0.8 mm/d (Fig. 7). The ratios, Et/Epan, in summer and in winter are approximately 0.7 and 0.9, respectively. It was considered that the ratio should be influenced by the conditions of soil wetness and the intensity of potential evaporation.
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