Long-term field observation was clarified over ten years for the water budget of Cambodia’s forest watershed, one of the world’s most data-sparce areas. The La Niña years had large amounts of annual rainfall exceeding 1,800 mm, often marked by monsoon breaks during the rainy season. The El Niño years had smaller amounts of annual rainfall between 1,100 mm and 1,200 mm, aside from that in 2016. The strong El Niño during 2015-2016 caused large drops in both river and groundwater levels in the Cambodian forest catchment. However, the former levels of groundwater and river water were restored by 953.4 mm of rain received during the second half of the rainy season in 2016. Thus, the monsoon breaks may be correlated with the increased amount of rainfall in the second half of the rainy season.