Estimating the Annual Changes in Soil Moisture Content and Drought Frequency in Fields Using a Tank Model

— The case of pasture lands in Okunakayama Highland, Iwate Prefecture —

Abstract

In order to obtain a reliable appraisal data for irrigation projects, we estimated annual changes in soil moisture content within a 40-cm surface layer for the past 33 years (1978 to 2010) from April to September as well as drought frequency using the soil tank model. The investigation was conducted on pasture lands of Okunakayama Highland in Iwate Prefecture. Our results showed that decreasing of volumetric water content ratio which had lower amounts of the soil moisture than depletion of moisture content for normal growth mostly occurred between 1978 and 1999. It suggests that the serious drought had apparently caused damage on a comparatively large scale in this area when the decrease in volumetric water content ratio which had lower amounts of the soil moisture than depletion of moisture content for normal growth continued for more than 8 days, and had occurred about 6 years within the 33-year period. However, the drought was unevenly distributed over the period, and all these serious damages were occurred during the 22-year period from 1978 to 1999 and that no drought had occurred for 11 years since the year 2000.