著者

金子 良 上村 春美

出版者

The Japanese Society of Irrigation, Drainage and Rural Engineering

雑誌

農業土木研究 (ISSN:18847218)

巻号頁・発行日

vol.29, no.3, pp.113-118, 1961

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The value of evapotranspiration (E. T.) observed by a lysimeter does not correspond to that from a field having large area in natural condition.<BR>So it is worth making study to estimate E. T. from a field by the hydrological method without using a lysimeter.<BR>The presnt data were obtained on sandy soil at the meteorological observation place and the neighbering orchard of the National Institute of Agricultural Sciences in Hiratsuka.<BR>On the sandy land where surface run off does not occur, the following hydrological relations can be obtaind : <BR><I>P</I>=<I>E</I>+<I>M</I>+<I>G</I>… (1) <BR><I>G</I>= (<I>G</I><SUB>2</SUB>-<I>G</I><SUB>1</SUB>) +<I>H</I>·<I>pa</I>… (2) <BR>where P : precipitation, <I>E</I> : evapotranspiration, <BR>M : change of soil moisture, <BR>G : recharge of ground water, <BR>G1 : ground water flowing in to the locality from upper reach, <BR>G2 : ground water flowing away from the locality to downstream reaches, <BR>H : change of ground water level, pa. : air capacity of soil in the part of changing ground water level.<BR>A small rainfall on dried soil or no rainfall gives, G_??_0. In this case, the decrease of groundwater level corresponds to (<I>G</I><SUB>2</SUB>-<I>G</I><SUB>1</SUB>), and a certain relation can be obtained between the various ground water flows and the ground water levels.<BR>This is approximately applicable to the case where G≠0, making it possible to estimate the value of (<I>G</I><SUB>2</SUB>-<I>G</I><SUB>1</SUB>) in every month through the year.<BR>Eventually, the evapotranspiration, <I>E</I> on sandy land can be obtained from (1) and (2).<BR>Generally speaking, <I>E</I> of the sandy land is approximately from 600 to 700mm, and the ratio between <I>E</I> and the pan evaporation is almost equal to 0.7. This ratio is smaller in spring and larger in fall than other seasons.