Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial-temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface watergroundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the mid-summer drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong

The water balance dynamics and runoff components of a tropical forested catchment (46 km 2) on the southwestern Pacific coast of Nicaragua were studied combining hydrometry, geological characterization and hydrochemical and isotopic tracers (three-component hydrograph separation). The climatic water balance was estimated for 2010/11, 2011/12 and 2012/13 with net values of 811 mm year-1 , 782 mm year-1 and-447 mm year-1 , respectively. Runoff components were studied at different spatial and temporal scales, demonstrating that different sources and temporal contributions are controlled by dominant landscape elements and antecedent rainfall. In forested sub-catchments, permeable soils, stratigraphy and steep slopes favour subsurface stormflow generation contributing 50% and 53% to total discharge. At catchment scale, landscape elements such as smooth slopes, wide valleys, deeper soils and water table allow groundwater recharge during rainfall events. Groundwater dominates the hydrograph (50% of total discharge) under dry prior conditions. However, low soil infiltration capacity generates a larger surface runoff component (42%) under wet prior conditions which dominates total discharge. Our results show that forested areas are important to reduce surface runoff and thus soil degradation, which is relevant for the design of water management plans.

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