Groundwater and unsaturated zone evaporation and transpiration in a semi-arid open woodland

E Balugani,M.W Lubczynski,L Reyes-Acosta,C Van Der Tol,A.P Francés,K Metselaar

doi:10.1016/j.jhydrol.2017.01.042

E Balugani, M.W Lubczynski + Show 4 more

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https://doi.org/10.1016/j.jhydrol.2017.01.042

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Abstract

Studies on evapotranspiration partitioning under eddy covariance (EC) towers rarely address the separate effects of transpiration and evaporation on groundwater resources. Such partitioning is important to accurately assess groundwater resources, especially in arid and semi-arid areas.The main objective of this study was to partition (evaluate separately) the evaporation and transpiration components of evapotranspiration, originated either from saturated or unsaturated zone, and estimate their contributions in a semi-arid area characterized by relatively shallow groundwater Table (0–10m deep).Evapotranspiration, tree transpiration and subsurface evaporation were estimated with EC tower, using sap flow methods and HYDRUS1D model, respectively. To set up the HYDRUS1D model, soil material properties, soil moisture, soil temperature, soil matric potential and water table depth were measured in the area. The tree transpiration was sourced into groundwater and unsaturated zone components (∼0.017mmd−1 for both) and accounted for only ∼6% of the evapotranspiration measured by the EC tower (∼0.565mmd−1), due to the low canopy coverage in the study area (7%). The subsurface evaporation fluxes were also sourced into groundwater and unsaturated zone components using the SOURCE package, and their relative relevance in total evapotranspiration was assessed.Subsurface evaporation was the main flux year-round (∼0.526mmd−1). During late autumn, winter and early spring time, the unsaturated zone evaporation was dominant, while in dry summer the relevance of groundwater evaporation increased, reaching one third of evapotranspiration, although errors in the water balance closure point still at its possible underestimation. The results show that, in arid and semi-arid areas with sparse vegetation, the often neglected groundwater evaporation is a relevant contribution to evapotranspiration, and that water vapor flow should be taken into account in the calculation of extinction depth.

Highlights

In arid and semi-arid areas the scarce water resources are usually stored as groundwater
To study partitioning and sourcing of ET in semi-arid conditions we selected a 2 Â 2 km study area, further referred as the maximum footprint area (MF, Fig. 1), enclosing the maximum extent of all the footprints of the eddy covariance (EC) tower installed in the center of that area
The tree transpiration had the same magnitude in the two dry summers of 2009 and 2010 (Fig. 5, Reyes-Acosta and Lubczynski, 2013) despite of differences in rainfall and water table depth (WTD)

Summary

Introduction

In arid and semi-arid areas the scarce water resources are usually stored as groundwater. While the precipitation in the Mediterranean region is decreasing (Gualdi et al, 2013; Mariotti et al, 2015), the demand for the limited groundwater resources is increasing (Scanlon et al, 2006). In such conditions it is imperative to quantify the main water input constraining groundwater resources, i.e. the net groundwater recharge (Chenini and Ben Mammou, 2010). The evapotranspiration of groundwater resources is often underestimated, both because evaporation processes are. The underestimation of groundwater evapotranspiration often results in the overestimation of the net recharge. It is critical to define accurately groundwater evapotranspiration which may represent a small but relevant reference percentage of total evapotranspiration

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