Abstract
Retrospective reasoning was discussed for obtaining the time series of variable source area (VSA) in a terraced sorgo field, which was made by cutting the clay layer andbanking.
At the upside cutting area in the field, it is presumed that most of the groundwater flows slowly and affecting the base component of the runoff discharge or loss component, because the slope of the water head is small. On the other hand, at the downside banking area in the field, most of water must flow quickly in the banking soil on the steep slope on the clay layer. As the area of the big discharge under the ground seem to change according to the water level under the ground around the downside in the field, this area is expressed as VSA in this paper.
It is possible to hypothesis that the vertical flux from/into VSA in this field isthe measured precipitation only. Using this kind of hypothesis for getting vertical flux from/intoVSA easily, it becomes practical to introduce common equations for the characteristic curve method of the kinematic wave model to the analysis of VSA. For this analysis, the precipitation and runoff were measured.
The time series of VSA is shown as a group of the characteristic curves on the distance-time plane. The main procedure for arriving at the time series of VSA is by calculatingthe water depth from the lower end of the field to upside along the characteristic curve. It is expected that the water depth on characteristic curve converge to zero in the field. This point of conversion is the upper boundry of VSA.
For obtaining the reasonable time series of VSA, selecting the direct component of runoff or exactly determining the parameters are needed.
In the example, one of the time series of VSA given under the proper conditions was reasonable. Even though the rainfall intensity was weaker than 1.5mm/5min, VSA got larger so long as it was rain. The area of VSA to which the rainfall intensity was strongest was not largest.
