The Mekong River Basin (MRB) is the biggest basin in Monsoon Asia. About 80% of the agricultural lands, which occupy about 40% of the basin are rain-fed paddy rice area. Therefore, it is assumed that changes in rain-fed paddy rice production affect the total agricultural production to a great degree in the Mekong River Basin. While there are many factors affecting the productivity of rain-fed paddies, such as climate, water use, rice varieties, applications of manure, fertilizer and agro-chemicals, sowing date and other agronomic practices, this paper focuses on the relation between rainfall and yields of rain-fed paddies. Agricultural statistics and rainfall data were collected and analyzed for all 24 provinces in Cambodia for the years 2001 and 2002. Factors such as soil fertility and other natural conditions were removed by comparing the yield and rainfall in one province for different years. Special attention was given to the relation between yields of paddy in the wet season and rainfall, considering factors such as rice varieties, soil fertility, irrigation ratio and the ratio of area damaged by flood, drought, and insect. Although it is not easy to assess those impact factors on yields because they are organically interactive, the following results were obtained: (1) The ratio of high yielding varieties (HYV), soil fertility, and irrigation ratio among many factors that affect yields individually, especially if they are combined, (2) Total rainfall did not have a significant influence on rice yields even for the rain-fed paddies if it was over 700 mm in wet season, and one of the reasons for this would be that there exist supplementary water uses through small ponds and water ponding in local land depressions in and around paddies.
Stability of river banks almost 10 m or more in their heights and movement of their failed soil mass are analyzed by a simplified Junbu method and the Newtonian law of motion. In analysis the bed near the bank slope was degraded drastically to make relative bank heights increase and water level was supposed in two cases: during and after flooding time. Then, calculation was carried out in cases of several river bank heights of 12-19 m. The outputs of analysis are values of the safety factor for bank slope, while acceleration, velocity, and movement distance for failed soil mass. Results show that bed degradation increases in relative banks height which have strong influence on the stability of river bank and the higher the river bank is, the farther the failed soil mass moves.
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