The hydrology of tropical mountain catchments plays a central role in ecological function, geochemical and biogeochemical cycles, erosion and sediment production, and water supply in globally important environments. There have been few studies quantifying the seasonal and annual water budgets in the montane tropics, particularly in cloud forests. We investigated the water balance and hydrologic regime of the Kosnipata catchment (basin area: 164.4 km(2)) over the period 2010-2011. The catchment spans over 2500 m in elevation in the eastern Peruvian Andes and is dominated by tropical montane cloud forest with some high-elevation puna grasslands. Catchment-wide rainfall was 3112 +/- 414 mm yr(-1), calculated by calibrating Tropical Rainfall Measuring Mission (TRMM) 3B43 rainfall with rainfall data from nine meteorological stations in the catchment. Cloud water input to streamflow was 316 +/- 116 mm yr(-1) (9.2% of total inputs), calculated from an isotopic mixing model using deuterium excess (Dxs) and delta D of waters. Field streamflow was measured in 2010 by recording height and calibrating to discharge. River runoff was estimated to be 2796 +/- 126 mm yr(-1). Actual evapotranspiration (AET) was 688 +/- 138mm yr(-1), determined using the Priestley and Taylor-Jet Propulsion Laboratory (PT-JPL) model. The overall water budget was balanced within 1.6 +/- 13.7 %. Relationships between monthly rainfall and river run-off follow an anticlockwise hysteresis through the year, with a persistence of high run-off after the end of the wet season. The size of the soil and shallow groundwater reservoir is most likely insufficient to explain sustained dry-season flow. Thus, the observed hysteresis in rainfall-run-off relationships is best explained by sustained groundwater flow in the dry season, which is consistent with the water isotope results that suggest persistent wet-season sources to stream-flow throughout the year. These results demonstrate the importance of transient groundwater storage in stabilising the annual hydrograph in this region of the Andes.
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