(4) Rainfall: Rice yields in the rainfed paddy fields of
Northeastern Thailand are wholly dependent on the
amount and timing of rainfall. The amount of rainfall
tends to be higher in the provinces along the Mekong
River and lower in the southwestern parts of the
region (KKU-Ford 1982). There is also considerable
variability in the amount and timing of rainfall from
year to year and place to place with droughts
occurring more often in some parts of the region
than others. It has been suggested that trees in paddy
fields may represent an adaptation by the farmers to
drought risk. Although trees usually depress the yield
of rice plants growing under their canopies due to the
impact of shading (Vityakon et al. 1993) and the