The Role of DEM Resolution and Evapotranspiration Assessment in Modeling Groundwater Resources Estimation: A Case Study in Sicily

Abstract

The reliability of hydrological response simulated by distributed hydrological models in river basins with complex topographies strictly relies on the adopted digital elevation model (DEM) resolution. Furthermore, when the objective is to investigate hydrologic processes over a longer period, including both wet and dry conditions, the choice of a proper model for estimating actual evapotranspiration can play a key role in water resources assessment. When dealing with groundwater-fed catchment, these aspects directly reflect on water balance simulations and consequentially on groundwater resource quantification, which is fundamental for effective water resources planning and management at the river basin scale. In the present study, a DEM-based inverse hydrogeological balance method is applied to estimate the active mean annual recharge of the northern Etna groundwater system within the upstream part of the Alcantara river basin in Sicily region (Italy). Despite this area representing a biodiversity hot-spot, as well as the main water source for a population of about 35,000 inhabitants, so far little attention has been paid to groundwater estimation, mainly due to lack of data. In this context, this work aims to improve knowledge on groundwater recharge at the annual scale in this case-study area. In particular, the main objectives of this study are: (1) to quantify the influence of the DEM resolution on groundwater resource estimation and (2) to investigate the influence of the method used for evapotranspiration assessment on the model’s results. More specifically, groundwater and surface flows are evaluated by considering different DEM resolutions (i.e., 20, 60, 100, 300, 500 m) and three different theoretical approaches for evapotranspiration calculation (i.e., the Turc method, a modified-Turc method, and the Budyko model).

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“…As a general rule of this methodology, to achieve the closure of the water balance, it is verified that the difference between the calculated annual average recharge and the total water outflows from the hydrogeological domain, represented by both natural effluxes and withdrawals, is less than 10% of the calculated average annual recharge [38,39,45].…”

“…The results achieved with the application of this methodology are reported below. The method proposed for the assessment of groundwater recharge is a simple parsimonious methodology based on the topography of the territory and the water balance equation [29,30,38,39]. In particular, the mean annual potential recharge, or effective infiltration, I of small and medium groundwater systems can be calculated, through the inverse hydrogeological balance technique, from the effective rainfall P e (i.e., gross rainfall P minus evapotranspiration ET) and the hydrogeological conditions.…”

“…In what follows the empirical Turc formula for assessing actual ET at the annual scale has been replaced by the Budyko-Schreiber formulation [32,38]. The methodology for ET estimation based on Budyko curves describes the theoretical energy and water limits to the basin water balance.…”

“…The value of effective infiltration I ei was obtained through the potential infiltration coefficients, X R . Based on the values reported in the literature and knowledge gained directly in the area, a potential infiltration coefficient X R equal to 0.25 was assigned to both the metamorphic and the sedimentary aquifer systems, while a value of 0.85 was assigned to alluvial deposits [38,39]. Then, the monthly average effective infiltration rate expressed in mm for each grid cell of the study area was determined by Equation (5).…”

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“…The potential infiltration index was used in the SINTACS method to estimate net recharge of the area, through the inverse water budget [44][45][46][47]. The potential infiltration coefficient X R (that can assume values between 0 and 1) was estimated on the surface lithology of the hydrogeological complex, and other parameters that depend on different lithology characteristics [48]. Each lithological formation represented by era and period in Figure 11 has been divided by type of complex and a potential infiltration coefficient has been assigned to each one.…”