We have a nice new study just published in Hydrology and Earth System Sciences today, based on the DPhil work of Kathryn Clark. In this study we describe the seasonal and annual water budget of the Kosñipata valley in the Peruvian Andes, the valley that has been the focus of our ecological studies for the past ten years in the ABERG consortium. It is very challenging to describe the complete water budget of a valley, and this has rarely been done, especially in the tropics. Kathryn conducted a careful evaluation of all our rainfall data, an used TRMM (Tropical Rainfall Measuring Mission) satellite data to interpolate it over the valley. She measured river run-off throughout the year from two hydrogauges (no easy task in the intensive river flows after tropical rainstorms in this catchment). Evapotranspiration was estimated by applying former postdoc Josh Fisher's model (now at Nasa Jet Propulsion Laboratory). And finally to quantify cloud water inputs (the hardest component to get) we worked with the University of Southern California (Josh West's group) to apply a novel technique using the deuterium isotope signal in the river water. Overall the annual water budget was closed to within 1.6±13.7%, an amazing achievement. Cloud water interception accounted for only about 10% of this. However, we found that in the wet season there was less river run-off than expected, whereas in the early dry season there was more run-off than expected. This could only be explained by persistent groundwater storage in the fractured bedrock (the soils are not deep enough to store much water), which acts as a transient reservoir that sustains river flow (and quite possibly plant water supply) in the dry season.
Overall, this paper represented a huge amount of work by Kathryn, and is an amazing analysis across very diverse strands of data. One of the reviewers suggested this paper might be a "potential future classic" and I am tempted to agree...
The paper can be downloaded here
Clark, K. E., Torres, M. A., West, A. J., Hilton, R. G., New, M., Horwath, A. B., Fisher, J. B., Rapp, J. M., Robles Caceres, A., and Malhi, Y.: The hydrological regime of a forested tropical Andean catchment, Hydrol. Earth Syst. Sci., 18, 5377-5397, doi:10.5194/hess-18-5377-2014, 2014. Supplementary material.
The abstract is below:
Abstract. 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 Kosñipata catchment (basin area: 164.4 km2) 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 δD of waters. Field streamflow was measured in 2010 by recording height and calibrating to discharge. River run-off was estimated to be 2796 ± 126 mm yr−1. Actual evapotranspiration (AET) was 688 ± 138 mm 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 streamflow throughout the year. These results demonstrate the importance of transient groundwater storage in stabilising the annual hydrograph in this region of the Andes.
Yadvinder Malhi is an ecosytem ecologist and Professor of Ecosystem Science at Oxford University