Efficacy of small-farm reservoirs (SFRs) was evaluated experimentally as supplemental irrigation of rainfed agriculture under a tropical monsoon climate. The experiment was conducted during two crop seasons from November 2013 to February 2014 in the rainy season and from March to June 2014 partly in the dry season. Five SFRs were constructed in Gondangrejo, Central Java, Indonesia, with the volumes designed to store runoff from the catchment area during the maximum daily rainfall in 2012 and 2013. SFRs harvested 10.4%-30.6% of total rainfall during the experiment, which was strongly influenced by the size and slope of the catchment area. In the first crop season, SFR1 irrigated an onion farm and other four SFRs irrigated paddy fields sufficiently for rice cultivation albeit four short droughts. In the second crop season, SFR1 was not used due to the labor shortage, and SFR2, SFR3, and SFR4 irrigated paddy fields sufficiently for rice cultivation. However, SFR5 could not irrigate the paddy field without a supplemental use of groundwater due to the larger irrigation area than the catchment area. The results indicated potential of SFRs for enabling cultivation of valuable crops such as rice during dry seasons under a tropical monsoon climate.