Abstract
The Nganjuk District experiences a tropical monsoon regime characterized by annual rainy and dry seasons, and the area receives approximately 80% of annual precipitation within the 5 to 6 months of the rainy season. The district has three planting seasons, the wet season (November-February), first dry season (March-June), and second dry season (July-October). A main factor of agricultural production sustainability is managing for climatic variability. The main cause of variability is the El Niño Southern Oscillation, with El Niño years having below-average rainfall and La Niña years having above-average rainfall. Water shortages in the dry season are a problem, especially in El Niño years, whereas flooding occurs in La Niña years. We analyzed trends in agricultural production and the impact of El Niño and La Niña events in a tropical monsoon region by performing a case study of Nganjuk District in eastern Java Island. Groundwater use for conjunctive irrigation in the dry season increased agricultural production by about 0.5 crops yr-1. In El Niño years, groundwater was required for irrigation in the dry season, whereas in La Niña years surface water was almost sufficient or exceeded the demand for irrigation water in the dry season. The harvest index of rice was relatively stable, with a small increasing tendency from 1982 to 2010 (1.22 to 1.82 crops yr-1), and was not influenced by El Niño or La Niña events. Secondary crop production fluctuated, and the failed-harvest index increased in La Niña years. Agricultural production was more secure when farmers applied conjunctive irrigation. Because secondary crops are vulnerable to waterlogging under excess water conditions, early warning of a La Niña year is important so farmers can adopt a farming strategy that will avoid harvest failure in the dry season.
