On the Evaporation from the Soil Surface in the Southern Part of Kyushu

By using a lysimeter, a long term observation of evaporation from the well wetted soil surface was carried out at Miyazaki Agricultural Experimental Station in the southern part of Kyushu. The author compares this value with those estimated by heat balance and combination methods. The results may be summarized as follows: 1) Temperature difference between the soil surface and air at the height of 150cm is positive in the winter season. 2) Daily mean evaporation is 1-2mm/day in winter and 3mm/day in summer. The maximum value reaches 7-8mm/day, and the yearly one is about 900mm. 3) The ratio of evaporation measured by the lysimeter to that from small pan evaporimeter (20cm in diameter) is about 0.8. The ratio may be used for an approximate evaluation of evaporation from moist bare soil. 4) Sensible heat transfer coefficient shows the seasonal change with the range of 1-3×10-4ly/sec-1°C. Bowen's ratio obtained by heat balance method is somewhat different from that calculated from Eq. (8). The large discrepancy between the values in winter may be ascribed to overestimation of evaporation by the lysimeter. Bowen's ratio is shown as a function of the dryness factor of the soil surface and of net radiation (Eq. 9 and Fig. 2). 5) The relation between the sensible heat of evaporation lEs and net radiation Ss is obtained as lEs=0.78Ss+25. This relationship implies that more than 80% of net radiation is consumed by evaporation in all seasons. 6) The measured evaporation is compared with that by the heat balance method (Fig. 3). Fairly good-agreement is found between the two values. The combination method, which is incorporated with the dryness of the surface is also used for evaporation evaluation and yields a value in fair agreement with that by heat balance method. By the heat balance and the combination methods the evaporation from the bare soil can be evaluated by making use of climatic data.