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Actual evapotranspiration (ETa) explains the exchange of water and energy between soil, land surface, and atmosphere. Despite its importance, it remains difficult to measure directly. Grasslands represent a widespread ecosystem for which further assessment of the measurement and estimation of ETa is needed. Thus, the objective of this study was to compare measurements and estimations of ETa in a mountain grassland ecosystem made using different approaches. The study was conducted in the Zhurucay Ecohydrological Observatory, located in the high Andes of Ecuador between 3,500 and 3,900 m a.s.l. The study area is a representative site of the páramo ecosystem, in which the vegetation mainly consists of tussock grasslands. ETa was measured or estimated using the following methods: eddy-covariance (EC), volumetric lysimeters (Lys), water balance (WB), energy balance (EB), the calibrated Penman-Monteith equation (PMCal), and two hydrological models [the Probability Distribution Model (PDM) and the Hydrologiska Byråns Vattenbalansavdelning model (HBV-light)]. During 1 year, precipitation (P) accumulated to 1,094 mm while ETa (measured with EC) accumulated to 622 mm (with ETa/P = 0.57). On a daily basis, the EC method measured average ETa rates of 1.7 mm/day. The best daily estimates according to percentage bias (pbias), normalized root mean square error (nRMSE), Pearson's correlation coefficient (r) and the volumetric coefficient (ve) came from the HBV-light model, followed by the PMCal and the PDM (pbias: −2 to −20%, nRMSE: 12-15%, r: 0.7-0.9, and ve: 0.7-0.8). On the other hand, the WB, EB, and Lys estimates showed a poor performance (pbias: −10 to −19%, nRMSE: 25-93%, r: −0.4 to 0.5, and ve: −0.5 to 0.7). As the methods used in this study are of different types (hydrological, micrometeorological, and analytical), their suitability and applications are discussed in terms of their costs, temporal resolution, and accuracy. This study identifies low-cost and easy-to-implement alternatives to EC measurements, such as hydrological models and the calibrated Penman-Monteith equation. This study also allows us to provide an increment of progress on the accurate closure of the water balance in grasslands.

Abstract:Understanding of evapotranspiration (ET) processes over Andean mountain environments is crucial, particularly due to the importance of these regions to deliver water-related ecosystem services. In this context, the detection of spatio-temporal changes in ET remains poorly investigated for specific Andean ecosystems, like the páramo. To overcome this lack of knowledge, we implemented the energy-balance model METRIC with Landsat 7 ETM+ and MODIS-Terra imagery for a páramo catchment. The implementation contemplated adjustments for complex terrain in order to obtain daily, monthly and annual ET maps (between 2013 and 2014). In addition, we compared our results to the global ET product MOD16. Finally, a rigorous validation of the outputs was conducted with residual ET from the water balance. ET retrievals from METRIC (Landsat-based) showed good agreement with the validation-related ET at monthly and annual steps (mean bias error <8 mm¨month´1 and annual deviation <17%). However, METRIC (MODIS-based) outputs and the MOD16 product were revealed to be unsuitable for our study due to the low spatial resolution. At last, the plausibility of METRIC to obtain spatial ET retrievals using higher resolution satellite data is demonstrated, which constitutes the first contribution to the understanding of spatially-explicit ET over an alpine catchment in the neo-tropical Andes.

The study of the environmental factors that control evapotranspiration and the components of evapotranspiration leads to a better understanding of the actual evapotranspiration (ET) process that links the functioning of the soil, water and atmosphere. It also improves local, regional and global ET modelling. Globally, few studies so far focussed on the controls and components of ET in alpine grasslands, especially in mountainous sites such as the tussock grasslands located in the páramo biome (above 3300 m a.s.l.). The páramo occupies 35 000 km 2 and provides water resources for many cities in the Andes. In this article, we unveiled the controls on ET and provided the first insights on the contribution of transpiration to ET. We found that the wet páramo is an energy-limited region and net radiation (Rn) is primarily controlling ET. ET was on average 1.7 mm/day. The monthly average evaporative fraction (ET/Rn) was 0.47 and it remained similar for wet and dry periods. The secondary controls on ET were wind speed, aerodynamic resistance and surface resistance that appeared more important for dry periods, where significantly higher ET rates were found (20% increase). During dry events, transpiration was on average 1.5 mm/day (range 0.7-2.7 mm/day), similar to other tussock grasslands in New Zealand (range 0.6-3.3 mm/day). Evidence showed interception contributes more to ET than transpiration. This study sets a precedent towards a better understanding of the evapotranspiration process and will ultimately lead to a better landatmosphere fluxes modelling in the tropics.

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