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Agricultural expansion and intensification is occurring in seasonally dry regions of Central America, while droughts are intensifying due to increasing water demand and climatic change. Empirical measurements of water consumption of major crops in this region are scarce but crucial to assess agricultural water use dynamics in the light of increasing regional water conflicts. We empirically quantify total crop water use (CWU) and water footprints (WFs) of rainfed upland rice (wet season) and groundwater-irrigated melons (dry season) grown sequentially as a double cropping system, one of the major cropping systems in the seasonally dry province of Guanacaste in northwestern Costa Rica. Data for this study cover 2years and were measured with a state-of-the-art eddy covariance water and carbon flux station. Upland rice only consumed green water (CWUgreen=383L/m(2)), while melons only consumed blue water (CWUblue=177L/m(2)). Irrigation was found to be 1.5 times larger than the actual melon water consumption, with better irrigation efficiencies than reported for melon farms in Brazil but slightly inferior to farms in Spain. Melon WFblue was 79m(3)/t, a much lower value than global and regional estimates reported but similar to values reported for melons produced in Brazil or Spain. Upland rice WFgreen (681m(3)/t) was reported for the first time and was proven to be much lower than flood irrigated-rice WFblue-green. Our results demonstrated lower overall water demand for upland rice-melon double crop compared to the two other major monocultures of the region (flood-irrigated rice and irrigated sugar cane).

Plain Language Summary Agricultural intensification is occurring in seasonally dry tropical areas. At the same time, droughts are becoming more severe due to climate change in many tropical regions. Empirical measurements of water usage in agriculture and water use efficiency are scarce in the tropics but are critical to achieving sustainable water management and resilient agricultural systems. We instrumented a production farm in the seasonally dry Guanacaste province of Costa Rica to measure agricultural water use for a regionally widespread double cropping system composed of irrigated melons and rainfed rice grown sequentially during the dry and wet seasons, respectively. For rice, 681m(3) of rainwater was used per ton of yield, which is much less water than the water used to produce rice in flooded fields. Seventy-nine cubic meters of irrigated water was used per ton of melons, which is similar to values reported for melons produced in Brazil or Spain. Irrigation of melons was found to be 1.5 times larger than crop water consumption during the dry season, and irrigation efficiencies were similar to Brazilian melon farms but slightly inferior to Spanish farms. We showed that the studied system used considerably less water than did two principal monocultures in the region (irrigated rice and irrigated sugar cane). This cropping system seems a competitive option for the Central American wet-dry tropics in terms of water use.