Method of Measuring Macropore Volume Contributed to Underdrain Discharge in Clayey Multi-purpose Paddy Field

Abstract

Improvement of drainage is one of the most important problems in converting a paddy field into a multi-purpose one in Hokuriku Area where clay-type soils are widely distributed. Macropores, whether cracks or those made by roots, play an important role in the underdrainage of a field with clay-type soil. Underdrainage in a field is usually estimated by measuring the permeability of soils using 100 cm3 core samples. However, it is difficult to estimate changes in volume of macropores caused by seasonal wet-dry cycles or cropping system in a field. This report deals with a method of estimating the vertical distribution and seasonal changes of volume of macropores such as cracks in a field by operating the outlet of the underdrain pipe. The procedure of this method is summarized as follows: 1) Close the outlet of underdrain when rainfall is respected. 2) Measure the water level in the underdrain pipe after the rainfall ceases. 3) Open the outlet of underdrain and measure the total discharge of water from the underdrain pipe. The total discharge of water corresponds to the volume of macropores between the water level and the depth of underdrain pipe. The vertical distribution of macropores can be estimated from the results of experiments for several independent rainfall. Changes in macropores can also be estimated by comparing the results of the different sets of experiments conducted in the concerning seasons or years.