LAUSR

Abstract Annual, seasonal, and monthly rainfall trends in the Baram River basin (BRB) in the equatorial tropical rainfall region of Borneo for 1970–2014 were statistically determined with Mann–Kendall (MK) and Spearman’s Rho (SR) test using data from 27 rain gauges. The mean monthly rainfall in the BRB varied from 252 to 381 mm with the southwest monsoon (SWM), northeast monsoons (NEM), and annual rainfall having shown comparatively high mean values (1371 mm, 1632 mm, and 3654 mm, respectively). The lag-1 autocorrelation test revealed that the rainfall data were homogenous and noncorrelated. The MK and SR tests identified varying trends (significant or nonsignificant, increasing or decreasing) in annual, seasonal, and monthly rainfall. Over the 45 year period, decreasing trends were observed in annual, the SWM, and monthly rainfalls whereas increasing trends were observed for the NEM season. The spatial distribution of rainfall trends in the BRB showed an oscillation of increasing and decreasing trends in monthly rainfall. At the same time, annual and seasonal trends showed a consolidated spread of increasing trends in the middle and lower parts and decreasing trends in the upper part of the river basin. A comparison of terrain elevation with the spatial distribution of rainfall trends did not reveal a significant relationship in the BRB; however, it was noted that rainfall distribution in the region was generally controlled by elongated mountain chains and isolated mountains of higher elevation. With the exception of a few stations, rainfall in the BRB showed a nonsignificant decreasing trend over the period from 1970 to 2014 of approximately −0.04 mm/year at the rate of −0.001 percent per year. The results and findings of the present research can be used for better organization of agriculture, especially rain-fed agricultural activities, and other developmental projects including river basin management for hydroelectric projects.