Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Estimating groundwater recharge uncertainty for a carbonate aquifer in a semi-arid region using the Kessler's method Nuno Barreiras * , Luís Ribeiro CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001, Lisboa, Portugal ARTICLE INFO Keywords: Karst Semi-arid region Groundwater recharge Linear regression Variance Abstract: Groundwater recharge is an important hydrological parameter for the quantification of water budgets in order to better achieve a sustainable groundwater management. Such management is paramount in the southern region of Portugal because groundwater represents more than 75% of the public water supply, the water demand for irrigated agriculture and tourism industries is rapidly growing, and the region is severely threatened by droughts and extreme dryness. Nevertheless, the estimation of recharge to carbonate aquifers in semi-arid regions remains challenging. Uncertainty analyses are still scarce and the natural variability of re- charge is unknown in most cases. The present work aims at estimating the recharge rate in carbonate aquifers of southern Portugal based on the Kessler method, identifying the confidence interval with known variability and reduced uncertainty. The outcome is a cost-free graphical tool that calculates an annual recharge rate per year, proving to be a reliable tool for annual recharge estimation and allowing to overcome the limitations of the chronological variability and data acquisition associated with other current methods. Acknowledging the con- fidence intervals of the annual recharge estimations can improve greatly the decision making process for the regional sustainable groundwater resources management. 1. Introduction Under normal circumstances in temperate and semi-arid regions, groundwater recharge occurs mostly in direct response to the infiltra- tion of rainfall (Flint et al., 2004; Alcalá et al., 2011). The infiltrating precipitation and surface water that enters an underlying phreatic aquifer through the water table - sometimes after the infiltration event - is called the aquifer recharge. The fraction of infiltration that remains is transpired by plants, evaporated from the soil or returned to the surface as interflow or shallow ephemeral perched aquifers discharge (Scanlon et al., 2006; Custódio et al., 1997). The total recharge (R T ), which is referred to the water table aquifers, may be transferred vertically to deeper aquifers through aquitards or by horizontal groundwater flow. This recharge has to be separated from what is the net recharge, which refers to what remains in the water table aquifer after discounting what is used by phreatophytes (de Vries and Simmers, 2002). Net recharge is then added to aquifer storage and moves toward the aquifer system discharge points and areas. Groundwater recharge to shallow, un- confined, fractured and carbonate aquifers in semi-arid/arid regions is mainly a concentrated recharge (R C ). This recharge occurs by rainfall infiltration, from watercourses, near-surface bedrock fractures and sinkholes (high bedrock permeability), affecting a small fraction of the territory and producing local hydrological and hydrogeochemical re- sponses in groundwater storage and quality (Flint et al., 2004; Alcalá et al., 2011). When groundwater recharge occurs in porous media through the rainfall infiltration then is called diffuse recharge (R D ). This type of mechanism gains importance to R T when it takes place on well- developed soils in the plain regions with temperate to humid climates (Keese et al., 2005; Maréchal et al., 2009; Healy, 2010). In well-developed karstic systems the response of the precipitation can be perceived in discharge points very rapidly, which is a clear in- dication of karstic aquifers with conduit flow behavior (Kessler, 1965; Andreo et al., 2008; Li et al., 2008). In this case, or in fissured and bare bedrock areas in arid regions, R C represents a very significant fraction of R T (Wood et al., 1997; Simmers et al., 1997). Depending on variations in climate, lithology, soil type, fractura- tion, vegetation and land use, slope, etc., most large semi-arid carbo- nate regions and in particular in karstic aquifer systems located in a temperate climate, conditions are established to the combined and variable contribution to R T of both diffuse and concentrated recharge mechanisms (Lerner et al., 1990; Wood et al., 1997; de Vries and Simmers, 2002; Alcalá et al., 2011). Even if the quantification of re- charge by each mechanism is still difficult to calculate separately and accepting that R C is a very important fraction of R T , at these conditions https://doi.org/10.1016/j.jaridenv.2019.04.005 Received 14 June 2018; Received in revised form 15 January 2019; Accepted 2 April 2019 * Corresponding author. E-mail address: nuno.barreiras@tecnico.ulisboa.pt (N. Barreiras). Journal of Arid Environments xxx (xxxx) xxx–xxx 0140-1963/ © 2019 Elsevier Ltd. All rights reserved. Please cite this article as: Nuno Barreiras and Luís Ribeiro, Journal of Arid Environments, https://doi.org/10.1016/j.jaridenv.2019.04.005