Partitioning growing season water balance within a forested boreal catchment using sap flux, eddy covariance, and a process-based model

Nataliia Kozii,Samuli Launiainen,Kersti Haahti,Niles J Hasselquist,Pantana Tor-Ngern,Jörgen Wallerman,Eliza Maher Hasselquist,Ram Oren,Hjalmar Laudon,Jinshu Chi,Matthias Peichl

doi:10.5194/hess-24-2999-2020

Abstract

Abstract. Although it is well known that evapotranspiration (ET) represents an important water flux at local to global scales, few studies have quantified the magnitude and relative importance of ET and its individual flux components in high-latitude forests. In this study, we combined empirical sapflux, throughfall, and eddy-covariance measurements with estimates from a process-based model to partition the water balance in a northern boreal forested catchment. This study was conducted within the Krycklan catchment, which has a rich history of hydrological measurements, thereby providing us with the unique opportunity to compare the absolute and relative magnitudes of ET and its flux components to other water balance components. During the growing season, ET represented ca. 85 % of the incoming precipitation. Both empirical results and model estimates suggested that tree transpiration (T) and evaporation of intercepted water from the tree canopy (IC) represented 43 % and 31 % of ET, respectively, and together were equal to ca. 70 % of incoming precipitation during the growing season. Understory evapotranspiration (ETu) was less important than T and IC during most of the study period, except for late autumn, when ETu was the largest ET flux component. Overall, our study highlights the importance of trees in regulating the water cycle of boreal catchments, implying that forest management impacts on stand structure as well as climate change effects on tree growth are likely to have large cascading effects on the way water moves through these forested landscapes.

Highlights

In the hydrological cycle, water enters terrestrial ecosystems mainly through precipitation (P )
This study was conducted within the Krycklan catchment, which has a rich history of hydrological measurements, thereby providing us with the unique opportunity to compare different ET flux components to other water balance components as well as to directly assess the important role trees play in the boreal hydrological cycle
Vapor pressure deficit (VPD) ranged between 0 and 1.5 kPa, with a notable peak in the middle of July, which corresponded to a peak in air temperature

Summary

Introduction

Water enters terrestrial ecosystems mainly through precipitation (P ). 60 % of the annual terrestrial P (Oki and Kanae, 2006), yet the relative importance of ET varies considerably among different biomes, ranging between 55 % and 80 % of incoming P (Peel et al, 2010) Understanding this variation in ET is crucial, as the difference between incoming P and ET represents the available water in terrestrial ecosystems, which in turn has cascading effects on streamflow (Karlsen et al, 2016; Koster and Milly, 1997), groundwater recharge (Githui et al, 2012), and the ecosystem carbon cycle (Wang et al, 2002; Öquist et al, 2014). 12 million km of land area and represent the second largest biome behind tropical forests (Bonan, 2008) Given their large size, boreal forests regulate water and energy fluxes over a vast area and play an important role in global hydrology and climatology

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