Seasonal variability of daily evapotranspiration and energy fluxes in the Central Andes of Peru using eddy covariance techniques and empirical methods
Introduction
Of the component of the hydrological cycle, evapotranspiration (ET) is the less documented compared to precipitation or runoff essentially due to the difficulties of its measurement. In the surface energy balance, usually ET is the largest term, indicating its extraordinary climatic relevance. The involved mechanisms include the physical aspect of the water change of phase in the surface-atmosphere interface and the plant physiological processes when vegetation is present. Indeed, the latter is still crudely represented in the conceptual models of ET. For a recent review of the state-of-the-art, see Cuxart and Boone (2020).
On a more local impact, for accurate estimations of water balance in a catchment, monitoring ET is crucial for planning adequate crop irrigation programs, (Qiu et al., 2011; Smith, 2000), ensure conservation and management of water resources (Cleugh et al., 2007), and proper planning of hydraulic works (Jhorar et al., 2011). In Peru, where these issues are of extraordinary importance, no direct measurements of ET were available and decisions are usually based on hydrological modeling.
Before the progressive implementation of eddy-covariance (EC) systems, still scarce, the experimental estimation of the actual values of ET was made traditionally with evaporation tanks in meteorological stations, essentially abandoned nowadays, and with weighing lysimeters for agronomical purposes in some research-oriented locations (Girona et al., 2011). Therefore, empirical approaches were needed for the estimation of ET using standard meteorological data. The most popular ones, providing a value for the potential Evapotranspiration in well-watered surface conditions (ETp), are currently the Food and Agriculture Organization (FAO) adaptation (Allen et al., 2006) of the Penman-Monteith equation (Penman, 1948; Monteith, 1965) and a simplification of it, the Priestley-Taylor equation (Priestley and Taylor, 1972), both of which allow the estimation of ETp at the daily and subdaily time step. An important obstacle in using the mentioned equations to calculate daily value of ETp is the partial absent of some meteorological information, and in this case the FAO recommends the use of the Hargreaves equation (Hargreaves and Samani, 1985).
The EC technique has been used to estimate real ET with a high degree of accuracy, as shown in many studies (Matsumoto et al., 2011; Kosugi et al., 2007), and the values that it provides are taken as reference to compare with estimation from equations and satellite measurements (Liu et al., 2013). However, the method presents challenges which may result in some loss of the total flux essentially related to the presence of hectometric circulations (Mauder et al., 2020), and the values show discrepancies with those from lysimeters, especially at the subdaily scale (Hirschi et al., 2017). Therefore, the comparison of the result of empirical methods with EC values should take the uncertainties of the EC systems into account.
In South America the EC technique has been used essentially in the Amazon basin (Shuttleworth, 1988; Costa et al., 2010; Hasler and Avissar, 2007). Juárez et al. (2007) showed that the ET in the southern Amazon region is mainly related to soil water content, while Souza-Filho et al. (2005) found that the ET in the northern Amazon region is strongly associated with variations in net radiation. The use of the technique at high altitudes is relatively recent (Coners et al., 2016), including the equatorial Andes (Carrillo-Rojas et al., 2019; Ochoa-Sánchez et al., 2019, Ochoa-Sánchez et al., 2020).
The Andes extend along western South America from the Patagonia region (55°S) to Venezuela (11°N). In the tropical zone, they are surrounded to the east by the warm and moist Amazon region, while the western slopes are influenced by dry, cool air coming from the eastern Pacific Ocean (Houston and Hartley, 2003). Seasonal precipitation in the southern tropical Andes (Central and southern Peru, Bolivia, and northern Chile) is marked by a wet season during the December–March period and a dry season during the June–August period (Vuille and Keimig, 2004; Garreaud, 2009; Segura et al., 2019; Espinoza et al., 2020, Kumar et al., 2020) This region distinguishes from the area closer to the Equator, characterized by the absence of a marked dry season (Espinoza et al., 2009, Laraque et al., 2007, Segura et al., 2019).
Agriculture has great economic importance in Peru, and several studies have analyzed the water balance to determine the partition of precipitation between runoff and ET (Lavado et al., 2009; Zubieta et al., 2015). The complex topography of the southern tropical Andes induces a great spatial variability of precipitation (Garreaud, 1999; Houston and Hartley, 2003; Garreaud, 2009; Segura et al., 2019; Espinoza et al., 2020) and the low density of meteorological stations makes challenging the analysis of the hydrological cycle in this Andean region. A similar characterization for ET in the region is still pending and this work intends to be a contribution to it. There is a previous study of ET in the Mantaro valley but only in a dry month (Callañaupa, 2016).
So far, only empirical or hydrological model estimates of ET have been provided for the Peruvian Andes (Lavado et al., 2009; Zubieta et al., 2015). Hellström and Mark (2006) calculated ET in the Llanganuco Valley (Central Peruvian Andes) for the wet (December) and dry (June–July) seasons using the BROOK90 model, finding a strong seasonality, with mean ET values of 2.63 mm/day during the wet season and 0.03 mm/day during the dry season.
To tackle the obvious need of ET reference measurements in the central Andes, the Geophysical Institute of Peru (IGP) installed in 2015 an Eddy-Covariance (EC) system at the Huancayo Observatory. This system of EC consisting of a krypton hygrometer and a sonic anemometer, recording temperature, humidity and wind at a frequency of 10 Hz to compute turbulent sensible and latent heat fluxes. Measurements of the radiation budget at the surface and ground heat flux (obtained from upper soil temperature and moisture) are also available. This equipment shall allow to document the surface energy balance in a point representative of the area, including the actual ET.
We describe the area, data and methods in Section 2, the observed annual cycle of ET using the experimental estimations in Section 3, in which the adequacy of the empirical approaches for the site are also discussed. Furthermore, conclusions from the result section are stated in Section 4.
Section snippets
Study area
The Huancayo Observatory of the Geophysical Institute of Peru (12.04° S, 75.32°, 3330 masl) is located in the Mantaro River Basin, in the Junín region of the Central Peruvian Andes (Fig. 1). The main valley is oriented in the NW-SE direction, having a gentle slope of 0.2° between the cities of Jauja and Huancayo, with mountain ranges at both sides reaching altitudes above 5500 masl, and the Cunas tributary to the west of the Mantaro River. The Observatory is located on the western side of the
Wet and dry season at the Huancayo Observatory
The daily precipitation time series for the July 2016–June 2017 period is shown in Fig. 2a. Following previous studies (see introduction), our results show a strong seasonality in the annual cycle of precipitation. Rain events are observed from September to May, with a dry spell in November. The September–November season is catalogued as the onset of the rainy season with precipitation values of 66 mm, while the core of the wet season is observed during the January–March season with
Discussion and conclusion
In this study, we used the eddy covariance technique to analyze seasonal evapotranspiration and energy fluxes in the high central Peruvian Andes for the first time, at the tower installed in the Mantaro Valley. The estimated annual amount of ET for the period July 2016–June 2017 is approximately of 800 mm/year (from Section 3.4), very similar to the amount of precipitation. However, it is important to remember that while precipitation is measured in a single point, the eddy covariance technique
Author statement
Individual contributions for the present work, the contributions were distributed as follows: Conceptualization: Stephany Callañaupa Gutierrez and Hans Segura Cajachagua; methodology: Stephany CallañaupaGutierrez and José Flores Rojas; software: Stephany Callañaupa Gutierrezand Miguel Saavedra Huanca; validation: Stephany Callañaupa Gutierrez, Hans Segura Cajachaguaand Joan Cuxart; formal analysis: Stephany Callañaupa Gutierrezand Miguel Saavedra Huanca; investigation: Stephany Callañaupa
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the project, “Study of the physical processes that control the superficial fluxes of energy and water for the modeling of frosts, intense rains and evapotranspiration in the central Andes of Peru” (400-PNICP-PIBA-2014). The authors thank NASA for the NDVI (MOD13Q1) product and the evapotranspiration (MOD16A2) product. JC thanks IGP for the support to his visit to the Huancayo Observatory. Also, the authors appreciate the work of Luis Suarez and Lucy Giraldez,
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