Groundwater Evapotranspiration -Underestimated Role of Tree Transpiration and Bare Soil Evaporation in Groundwater Balances of Dry Lands

Maciek W. Lubczynski

Abstract This paper analyzes and emphasizes the importance of groundwater évapotranspiration (ETg) in groundwater balances. The ETg diminishes the net groundwater recharge that constrains groundwater flow and replenishment of groundwater resources. The ETg consists of two different components, groundwater transpiration (Tg) and groundwater evaporation (Eg), both not yet well identified in hydrogeology. The ETg values are the largest in dry locations with shallow groundwater table. The significance of the ETg however is the largest when its relative contribution to groundwater balance is high i.e. when its rate is comparable with groundwater recharge.

Keywords Groundwater balance • Transpiration • Evaporation • Dry lands • Modeling

21.1 Introduction

Groundwater represents more than 95% of fresh water resources of the earth, so it is the most reliable source of usable water. The abstraction of groundwater however is not the easiest and the cheapest but its quality is usually by far the best. As such, the management of this precious resource should be done carefully. The management of groundwater resources is typically realized by groundwater models which rely on groundwater balances. It is therefore imperative that the groundwater balance

Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, The Netherlands e-mail: [email protected]

components (groundwater fluxes) are defined in reliable way so that groundwater balance can be closed accurately. For the elementary unit of an aquifer, groundwater balance equation is:

where: QGin : groundwater inflow (in case of 2-D solution this would be lateral inflow), R : groundwater recharge, QGout : groundwater outflow (in case of 2-D solution this would be lateral outflow), ETg : groundwater evapotranspiration, DS - change of groundwater storage; Qxt : external groundwater sink and sources (for example well abstraction).

It is widely assumed that the main and the most common driving force of natural groundwater flow is groundwater recharge. However, in many places of the world, particularly in dry, water limited environments (WLE) defined by Abrahams and Parsons [1] as areas where yearly P/PET < 0.75, the recharge is reduced in dry seasons by the influence of groundwater evapotranspiration (ET ). The ETg represents in situ water loss from saturated zone or its capillary fringe hydraulically linked with an aquifer [2]. When ETg is present and significant, the driving force of groundwater flow is the net recharge (Rn). The Rn is defined as:

It is the Rn, and not the R, that is responsible for the replenishment of groundwater resources. Therefore quantifying ETg is important particularly in groundwater management of dry lands. This paper aims at emphasizing this importance by presenting the ETg and discussing its role in groundwater balances.

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