2004 Volume 17 Issue 5 Pages 468-481
This paper aims to find out changes with temporal and spatial distribution in pan-evaporation in the Chao Phraya River in Kingdom of Thailand. The monthly pan-evaporation data during the 1982-2000 period was obtained from all-available observatories and analyzed. The following results were obtained: (1) the pan-evaporation value is the highest in the lower Chao Phraya River basin which is closer to the ocean, and its value is gradually degreases in upper basin; (2) the March-May period sees much evaporation than other months of the year; (3) and trend tests show significant decrease in pan-evaporation in 24 out of 27 observatories.
The trend tests on annual precipitation over the Chao Phraya River Basin in the corresponding period show that no significant decreasing or increasing trends at any stations. This result does not match a finding that precipitation increase decreases pan-evaporation in the U.S.A. (Lawrimore and Peterson; 2000), which finding can be derived from the complementary relationship (Brutsaert and Parlange; 1998). The reason for decrease in pan-evaporation without any precipitation change could be degrease in the solar radiation, which cannot be confirmed because of lack of data.