Abstract
The rice plants in the deltaic area of the Chao Phraya are classified into eight types, from I to VIII, based on the plant height.
Type I occurs in a geological depression in the middle of the delta, called Trough, and is distinctively characterized by an extremely wide height range, varying from 150cm to 400cm (Figs. 2&3). Type II covers the Old Delta and has a reasonably wide height range and a rapid lateral change of plant height (Figs. 2&4). Type III grows all along the marginal zones of the deltaic area, and has a pattern of plant height distribution similar to Type II, though the average height is slightly shorter than Type II. Type IV to VIII are found in the Young Delta and are characterized by a narrow range and a smooth lateral change of plant height distribution (Figs. 2&5).
The subdivision of the Young Delta divided according to Types IV to VIII demonstrates that the further inland one goes the taller the plants are (Fig. 2). This phenomenon is explained by the help of a simplified delta model in which a scarp, characteristic of all deltas, separates the Old and Young Deltas. During a large and prolonged flood, the water inundates the deltaic area, with the flood depth increasing as one goes inland (Fig. 7). The greatest depth occurs at the foot of the scarp. The existing distribution of the plant height in the Young Delta seems to be a reflection of the topo-hydrographic conditions mentioned above.
