抄録
In alluvial fan and riverside area (in Japan) there are a lot of thin soil type paddy fields, . underbeneath which there lies gravel stratum. These paddy fields are used to miss flooded irrigation water, so lots of water are often lost during irrigation season. In Tazawa District, Akita Prefecture, we have carried out some physical surveys in the field of this type to study the method to prevent seepage loss of water.
The main results are;
1. In the land, which has 40-60cm thick surface soil with about 10-3cm/sec of Darcy's coefficient of permeability and contains gravel layer in subsoil, it takes 8-10 years at least to get the settled condition in which infiltration rates show about 20mm per day, so far as soils are dealed by human hands only to develop the land to a paddy field.
2. Even when the surface soil is only 15-30cm thick, if suitable earth moving, such as surface soil pre-moving and gravel layer smoothing and surface soil re-moving back, is done for about 3 hours by buldozering operation, we can expect to get pretty good fields, whose infiltration rate is 20mm per day in 3-4 years after.
3. The more buldozering time increases, the less seepage loss becomes. Compaction by catapiller belts in case of smoothing gravel layer will form impervious layer in the subsoils.
4. Local variance of infiltration rate in a field is higher as the seepage loss in it is greater.
5. As the time elaspes after reclaiming field, high leaking spots gradually vanish, and total leakage from a paddy field decreases.
6. It seems in buldozering that gravel cutting as well as banking sites show less permeability than untouched ones.
7. Say percolation depth near along the upper-ridge, d1', that of along lower ridge, d2', that of inside of the field, d, there was found a relation d2>d< d1 which seems to suggest that the seepage water from the upper side fields do not make way into neighbouring field, but percolate down into deeper layers.
