A Procedure to Estimate Global Natural Recharge in Karst Aquifers

Abstract

Natural recharge in karst aquifers is a key component of global water resources, yet its estimation remains challenging due to the complexity of karst hydrogeological processes. The recharge assessment deserves special consideration, especially in the current global climate and sustainability challenges. This study poses a methodology to appraise natural recharge rates in karst aquifers worldwide, drawing on climatic and geological data. In this regard, this study applies a methodology previously developed by two of the authors, in which natural recharge over large areas is considered a fixed fraction of precipitation, which varies according to different lithologies of similar hydrogeological behavior (hydro-lithological units). Given that carbonate rocks are known to have the highest recharge rate relative to precipitation (34.3%), the method builds on existing karst and average precipitation maps to calculate worldwide recharge in karst aquifers. Recharge is appraised at 4,381,063.7 hm³/yr, which represents 34.5% of the global groundwater resources, a percentage that indicates the importance of karst in this regard. Based on maps of recharge values worldwide, this study highlights the importance of carbonate aquifers when compared with assessments of the world’s groundwater resources made by international institutions or other types of aquifers. The method is contrasted with other ways of assessing groundwater resources used in diverse regions of Europe. The impact of different climate change scenarios on the natural recharge of these karst aquifers has also been analyzed. Thus, under a climate change scenario in 2050, it is estimated that natural recharge will be reduced by about 10%.

1. Introduction

Knowledge of the sustainable recharge capacity of aquifers is of great theoretical and practical interest in terms of groundwater potential, especially considering that recharge values are not equivalent to available groundwater resources, given that a large part of the recharge is not exploitable due to technical and economic criteria. Other parts are not available either, as it is mandatory to preserve the base flow of rivers and the water levels of aquifer-dependent wetlands to prevent marine intrusion into coastal aquifers, among other reasons.

Groundwater can significantly contribute to climate change adaptation: its wide availability and generally large reserves, long retention time, and slow response or inertia make it naturally protected from seasonal and interannual variations in precipitation and temperature. Unlike surface reservoirs, aquifers lose negligible amounts of water through evaporation and transpiration. However, groundwater also has its limits of exploitation, and for sustainable groundwater management, overexploitation must be prevented. In this sense, Wada et al. (2010) [1] provide a global overview of groundwater depletion, which they define as extraction in excess of recharge. Other recent studies point in the same direction [2,3].

Recharge is also one of the least understood hydrogeological variables, as it varies in space and time and is difficult to measure directly. Natural recharge at a given location depends mainly on hydrometeorological factors (precipitation, temperature/evapotranspiration) and the nature of the terrain (permeability of the lithology of the aquifers). Still, it also depends on the existence and type of soil, relief, vegetation, land use, etc. [4]. Precipitation is the primary source of recharge in most real situations. Nevertheless, precipitation is a highly spatially and temporally variable phenomenon in the short term and the long term, fluctuating seasonally and interannually. This fact makes the assessment of water balance and recharge highly uncertain [5,6,7,8]. Moeck et al. (2020) studied many published works on recharge estimation in specific aquifers worldwide [9]. They concluded that precipitation rates and the seasonality of temperature and precipitation were the most important variables for predicting recharge. Therefore, multiple correlations between recharge/precipitation and temperature variables have been successfully tested for specific karst aquifers and short periods (weeks, months), with seasonal temperature variations being reflected [10,11,12]. Yet, for annual mean values, the temperature in a given region does not change significantly from one year to another. Conversely, precipitation has a significant interannual variability. Therefore, considering annual average values, there is a good correlation between precipitation and recharge for a given aquifer. In any case, recharge’s high dependence on climate indicates its sensitivity to climate change.

As mentioned above, one of the essential factors determining recharge is lithology, and among all types of ground, karst aquifers have the highest recharge rate relative to precipitation [9,13,14,15]. Carbonate rocks are the geological formations with the highest recharge rate relative to precipitation. Apart from the surface area occupied by karst in the world, this high recharge rate is why these regions have extensive groundwater resources compared to other land types. Karst regions offer a variety of natural resources (such as freshwater and biodiversity), cultural resources, and protected areas [16,17]. According to Goldscheider et al. (2020) [15], 15.2% of the global ice-free continental surface is characterized by the presence of karstifiable carbonate rocks; Europe boasts the highest percentage (21.8%), and the global importance of karst serves to justify future research and international water management strategies. Stevanović (2019) [18] first assessed the worldwide availability of water in karst aquifers, amounting to 3165 km³, with a recharge rate relative to precipitation of 20%. This study also estimated that 9.2% of the world’s population (about 678 million people) are karst water consumers and that 127 km³/yr of water is extracted from karst aquifers, about 13% of the total global groundwater abstraction. Hartmann et al. [19,20] applied the first large-scale simulation model to assess groundwater recharge in karst regions in Europe. The average values for four different climatic environments (humid, mountains, Mediterranean, desert) range from 35% to 40%.

This study focuses on global karst regions. The global distribution of karst was assessed departing from the map and database developed by the WOKAM (World Karst Aquifer Map) project (2012–2017). Based on this, Stevanović (2018) [21] and Goldscheider et al. (2020) [15] indicate that carbonate rock outcrops characterize 15.2% of the Earth’s ice-free land surface. The highest proportion of karst is found in Europe (21.8%), followed by North America (19.6%), Asia (18.6%), Africa (13.5%), Australia and Oceania (6.2%), and South America (4.3%). China has the largest absolute karst area (2.54 million km²) and the highest karst proportion (26.5%) among the ten world’s largest countries.

Regarding topographic distribution, 31.3% of all karst areas are in plains, 28.1% in moderately elevated terrain, and 40.8% in mountainous regions. Approximately 15.7% of the world’s coastlines are composed of carbonate rocks. Regarding climatic zones, the most significant proportion of karst is found in temperate regions, where 19.1% of the land surface consists of carbonate rocks, followed by cold climates (16.8%) and arid zones (14.8%). In contrast, tropical and polar regions have significantly lower proportions of carbonate rocks, with 8.8% and 7.7%, respectively.

The main objective of this work is to provide an approximate assessment of the current natural recharge in karst aquifers worldwide, using the recharge-to-precipitation rate of 34.3% obtained from Sanz and Menéndez Pidal (2013) [13], and to quantify the importance of this type of aquifers’ contribution to the global water balance.

The results provide the world’s first global picture of natural recharge in karst aquifers. They will be of particular value for assessing potential global water resources and modeling the influence of climate–surface–subsurface interactions on global climate change. Some assessments are made based on assumptions of future climate change, but an attempt is also made to roughly infer what karst recharge may have been like in the past during the last ice age.

2. Materials and Methods

2.1. Methodology

This study applies a new method for estimating the average natural recharge of large regions based on establishing recharge rates as a function of rainfall for six lithological groups of different permeability (we could also call them hydro-lithological units).

The method was initially tested using a very large sample of springs, allowing researchers to establish specific distribution functions for water supply (or contribution) based on flow rates across different lithological groups in the Iberian Peninsula [22]. This test confirms the validity of the general lognormal function established by Sanz Pérez (1996) [23].

Subsequently, in a large region such as Spain (506,000 km²), statistical inferences were applied to a sample of 875 springs for which the mean flow and the lithology of their feeding area were known. The analysis grouped springs according to regions with different rainfall and showed that mean annual recharge was a fixed fraction of mean rainfall for each lithology and independent of temperature. Recharge rates, as a function of precipitation, were obtained by adjusting the provisional recharge rates obtained to the estimated total recharge in the natural regime in Spain as assessed by the Libro Blanco del Agua (MIMAM, 2000) [24] so that diffuse discharges were also considered.

Thus, the authors determined the recharge rates with respect to average precipitation for six lithological groups with different permeability (hydro-lithological units) [13]: sands, gravels, and alluvial formations in general, 8.3%; conglomerates, 5.6%; sandstones, 7.3%; limestones and dolomites, 34.3%; marls, marlstones, silts, and clays, 3.3%; hard rocks, 1.3%. Among these, the karstic aquifers stand out, with a high average recharge rate of 34.3%.

Spain has a remarkable lithological variability, an abundance of calcareous soils (15%), varied pluviometry, topography, etc. These features then entail a certain representativeness as a sample. Thus, recharge rates relative to precipitation, and especially those of karst, are quasi-universal values that can be used to estimate average recharge or average groundwater resources for large regions anywhere in the world except in singular areas, such as those with permafrost. In this sense, precipitation and lithology data are widely available, so the method is applicable elsewhere to complete water balances. The method applied here in Spain has not yet been extended to the rest of the world because the cartography of the hydro-lithological units available worldwide (e.g., Gleeson et al., 2011) is not exactly the same as the one we used [3,25,26,27]. It would be necessary to translate the geological maps into hydro-lithological maps where the lithostratigraphic formations, and not the chronostratigraphic ones, would be clearly distinguished.

The authors have previously applied this procedure to a river basin in Spain (Cuenca del Duero, 90,000 km²) and four European countries (Spain, Portugal, Ireland, and Italy) to check the goodness of the method. For this purpose, the results have been compared with official statistics (Eurostat, 1998) [28,29,30] obtained by different methods. These countries have a sufficiently large surface area, a varied lithology, high participation of karst aquifers, and very different rainfall (Spain is the driest country in Europe, with an average annual rainfall of 687 mm/yr, and Ireland is the rainiest, with 1150 mm/yr). Therefore, the proposed method has already been adjusted and calibrated and does not require a historical series. The calculation of average recharge builds on knowing the surface distribution of a river basin, region, or country according to these six lithological groups. Once this distribution is known, the analysis can be applied to the average or annual recharge. This method only requires knowing the yearly rainfall in each lithological group. The recharge rates were obtained for Spain but can be considered globally representative, given that Spain has a very diverse geology and lithology. However, these rates could be refined in regions where they could be better adapted. In this way, the method can be applied to any region or country, except in very cold regions or other extreme cases (such as permafrost), since precipitation and lithology data are readily available. Our method allows an approximate assessment of potential groundwater resources worldwide by using readily available data: lithology and mean precipitation.

2.2. Limitations of the Method

This work attempts to assess natural recharge in karst aquifers at a global level. The authors are aware of the difficulties and uncertainties involved in a study that focuses on the Spanish case as a prototype, although it serves to compare different geographical areas and changing situations (climate change and past epochs).

This method is only suitable for large regions where the weights of the various factors involved in aquifer recharge cancel each other out, leaving precipitation and lithology as the determining factors for mean annual recharge. Karst hydrogeology is highly heterogeneous, and recharge is often influenced by diverse factors such as conduit development, land cover, vegetation, and seasonal dynamics, none of which are considered here. Therefore, this method is unsuitable for small geographical areas where these particular local characteristics may be more critical than lithology. The minimum scale at which it is valid is unknown, but it has proven to work for Spanish regions up to 90,000 km². This method should be used cautiously in smaller areas since local characteristics can influence recharge significantly. The mean annual recharge rates have been derived from statistical analysis of large areas so that they can be applied accurately on a global scale.

The assessments here refer to the karst’s spatial distribution as aquifers exposed to precipitation or outcropping, where predominantly diffuse autogenous recharge is verified. Allogenic recharge from losing rivers that infiltrate the karst and deep confined or semi-confined aquifers below other impermeable geological formations, even if they receive delayed recharge and constitute groundwater resources and reserves, is not included in the calculation. Transfers between aquifers are also neglected. Although this does not substantially change the large numbers obtained, it is indeed a systemic underestimate.

The current approach disregards karst areas covered by glacier ice, as the former mainly affects continental regions. This assumption is a simplification that neglects some exceptional cases, such as the recognition of glaciokarst depressions where recharge occurs at the bottom of the glacier. Areas with permanent or temporarily frozen ground will also not be considered. However, karst development and the behavior of karst aquifers are very different and limited under permafrost conditions [15,31], and recharge is also known to occur in some instances.

Another secondary limitation of the method is the resolution of the data used in the GIS application, which may be limited. For instance, the raster files employed (for rainfall maps, digital elevation models, and similar datasets) have relatively coarse grid sizes. In addition, certain attributes in vector files remain unknown due to the unavailability of data in some regions of the world. The assumed projection system is global, and some inaccuracies may arise in calculating areas

2.3. Data Sources and the Application of Geographic Information Systems (GISs)

Regarding data source and lithology distribution in particular, this study built on the 1:400,000 Map from Riba Arderiu (1969) for Spain [32] and the CEC-Eurostat GISCO/1995 Map Series for Portugal, Ireland, and Italy, which group the multiple lithologies differentiated in the six categories considered above [33]. Figure 1 illustrates the lithological distribution involved in the study. Current global precipitation data, along with another map for the period 1941–2060, were taken from WorldClim [34] to analyze the future scenario of the variation in global mean precipitation due to climate change. The results yield a 1–3% decrease from the original values. Likewise, the map information concerning karst aquifers was taken from WHYMAP (BGR, IAH, KIT & UNESCO, 2017) [34], that of the distribution of glaciated areas from Natural Earth [35], and that of current permafrost from the FAO-UNESCO Soil Map of the World [36].

The procedure used to gather all the information related to the karst terrain surface and the associated precipitation in a single layer requires the use of the Intersection tool, which superimposes the vector layer of the karst from the WHYMAP on the raster maps of current and future precipitation from the Natural Earth website [35]. The high latitude and high mountain glacial and permafrost layers were subtracted later with the aid of a GIS tool.

3. Examples of Application of the Proposed Method for the Assessment of Total Annual Mean Natural Recharge

3.1. Application to the Assessment of Natural Recharge in the Duero River Basin (Spain)

The Duero River basin is located in the northwest of Spain and covers an area of almost 90,000 km². It receives an average rainfall of 621.3 mm/yr. It is a large sedimentary basin containing detrital and marly deposits, surrounded by predominantly carbonate mountain ranges (Figure 1).

Carbonate rocks belong mainly to the Paleozoic limestones of the Cantabrian Mountains to the north (e.g., “Calizas de Montaña”—“Mountain Carbonates”—of the Carboniferous of Picos de Europa). To the east, the Jurassic and Upper Cretaceous limestones and dolomites of the Iberian Cordillera predominate. To the south is the Upper Cretaceous limestone rim bordering the Sistema Central. There are also horizontal Miocene limestones forming plateaus (‘Calizas del Páramo’) in the central areas of the basin.

The Duero basin (object of this study) was divided into 11 areas defined by isohyets at intervals of 100 L/m², ranging from 350 L/m² to 1450 L/m². The class marks of each area or interval were rounded with precipitations of 400 L/m² and 500 L/m², i.e., in multiples of 100. The lithological groups’ occupied areas were calculated for each of these 11 areas.

These data were derived from the 1:400,000-scale lithological map [32], in which each 5 × 5 km² grid cell indicates both the area occupied by each lithological group and the corresponding average rainfall.

Table 1. Calculation of hydraulic contributions to the Duero catchment (Spain).

Precipitation (L/m2)		Lithological Areas (km2)
Intervals	Class Marks	Alluvials	Conglomerates	Sandstones	Limestones	Marls	Others	Total
350–449	400	6311	1126	80	1122	7327	852	16,818
450–549	500	5296	1803	193	3580	7184	3500	21,556
550–649	600	2403	2209	237	1387	2992	3169	12,397
650–749	700	1020	1431	223	893	1072	3336	7975
750–849	800	793	523	190	164	269	6352	8291
850–949	900	1035	1047	165	329	1094	1857	5527
950–1049	1000	141	262	152	180	513	2858	4106
1050–1149	1100	-	-	19	106	-	980	1105
1150–1249	1200	-	-	-	-	-	-	-
1250–1349	1300	-	-	16	56		481	553
1350–1449	1400	10	-	-	88	-	534	632
All		17,009	8401	1275	7905	2,0451	23,919	78,960
%		21.5	10.6	1.6	10	25.9	30.4	100
Spain %		16.1	6.3	3.9	15	24.8	33.9	100
Precipitation		532.02	631.08	722.35	583.93	527.17	768.81	621.3
Hydraulic contribution (hm3)		751	296.9	67.2	1583.2	355.8	257.4	3311.5

shows the distribution results for the entire Duero basin.

For the sake of brevity, in Table 1 and the following ones, which correspond to the assessment in several European countries, we have selected a few lithologies out of the nine shown in Figure 1. Thus, the rest have been gathered in a group called “others”, which collects the results of the remaining rock types: quartzite, slate, plutonic, gypsum, and volcanic rocks. However, the recharge rates corresponding to each of these lithologies have been included in the recharge estimation calculations.

Thus, a total contribution of 3311.5 hm³/yr has been obtained, very similar to that obtained by Estrela et al. (1999) [38], from a distributed mathematical model applied to the whole of Spain, which gave an average annual natural recharge value of 3000 hm³/yr for the Duero basin. We used the hydrological model SIMPA (Simulation of Precipitation Input, developed by CEDEX, Centro de Estudios y Experimentación de Obras Públicas, Spain) for these calculations. This distributed conceptual model simulates average monthly flows in a natural regime at any point in a basin’s hydrographic network. It reproduces the essential processes of water transport that occur during the diverse phases of the hydrological cycle. The approach applies the principle of continuity, establishing distribution and transfer laws between storages in each cell of the discretized territory. The temporal resolution was set to one month, and the spatial resolution to a cell size of 1 km². The model assumes that recharge to the aquifer coincides with infiltration within each cell and that there are no transfers between aquifers or losses from allogenic rivers. Although the latter is a limitation of the method, it does not detract from its value in national-scale assessments.

3.2. Application to the Calculation of Natural Recharge in Several European Countries

To demonstrate the versatility of this method, the approach has been applied to four Western European countries: Spain, Portugal, Ireland, and Italy, which have sufficiently large surface areas and assorted lithologies and rainfall. In addition, Spain is the most arid country in Europe, and Ireland is the wettest (Figure 2). It is also assumed that natural recharge has been assessed with sufficient accuracy by other very different methods.

Table 2. Calculation of average annual recharge for four European countries (Spain, Portugal, Ireland, and Italy) according to their lithology. Data source: MIMAM (2004) [40]. $P_m$ accounts for the average yearly precipitation.

Country	Lithology	$\mathbf{Area}\left({\mathbf{k}\mathbf{m}}^2\right)$	${\mathit{P}}_{\mathit{m}}(\mathbf{L}/{\mathbf{m}}^2)$	${\mathit{P}}_{\mathit{i}}$	Recharge ${\mathbf{h}\mathbf{m}}^3/\mathbf{y}\mathbf{r}$
Peninsular	Alluvials	80,104	687	0.083	4567.6
Spain	Conglomerates	31,141	687	0.056	1198.1
	Sandstones	19,213	687	0.073	963.6
	Limestones	74,582	687	0.343	17,574.6
	Marts	123,464	687	0.033	2799.1
	Others	168,973	687	0.014	1625.2
	Total	497,477	687	-	28,728.2
Portugal	Alluvials	12,485	882	0.083	913.9
	Conglomerates	0	882	0.056	0
	Sandstones	13,516	882	0.073	870.24
	Limestones	5500	882	0.343	1663.8
	Marls	1678	882	0.033	48.83
	Others	56,720	882	0.014	700.37
	Total	89,898	882	-	4197.14
Ireland	Alluvials	3384	1150	0.083	323
	Conglomerates	0	1150	0.056	0
	Sandstones	11,758	1150	0.073	987.1
	Limestones	24,446	1150	0.343	9642.7
	Marls	0	1150	0.033	0
	Others	29,989	1150	0.014	482.8
	Total	69,577	1150	-	11,435.6
Italy	Alluvials	80,722	982	0.083	6579.3
	Conglomerates	0	982	0.056	0
	Sandstones	31,812	982	0.073	2280.4
	Limestones	71,911	982	0.343	24,421.5
	Marls	59,212	982	0.033	1918.8
	Others	41,204	982	0.014	566.48
	Volcanics	17,694	982	0.2 *	3539
	Total	302,557	982	-	39,305.5

Note: * Estimate.

summarizes the area, the average rainfall period 1901/1902–1995/1996 [39], the average recharge calculated in previous work, and the recharge calculated in the current study.

As for Portugal, recharge calculations were based on separating the base flow from selected rivers. Using the average recharge values obtained for various basins, regression lines were fitted to mean precipitation data and subsequently extrapolated to the remaining basins [41,42,43].

In Ireland, water balance calculations were based on actual evapotranspiration using the Penman formula. Where possible, the resulting recharge estimates were compared with values derived from base flow separations in river hydrographs.

In Italy, water balance studies were conducted in well-documented basins, assessing basin outflows (river base flow, direct discharge to the sea, and extractions) using both mathematical and analog models. Recharge values obtained from these studies were subsequently extrapolated to other aquifers.

The 1:400,000 Map from Riba Arderiu (1969) [32] was used for the lithological data for Spain. The CEC-Eurostat GISCO/1995 Maps were used for Portugal, Ireland, and Italy, where the numerous lithologies differentiated are grouped into the six groups considered above. Table 2 shows the lithological distribution. To calculate the recharge, the $P_i$ coefficients of

Table 3. Natural recharge rate to precipitation $\left(P_i\right)$ according to lithological groups in Spain (source: Sanz and Menéndez-Pidal, 2013 [13]).

			Recharge of Peninsular Spain
Lithologies	${\mathit{P}}_{\mathit{i}}(\%)$	km2	%
Alluvials, sands, gravels	8.29	4562	15.9
Conglomerates	5.63	1204	4.2
Sandstones	7.29	962	3.4
Limestones, dolomites	34.31	17,580	61.4
Marls, silts, clays	3.32	2816	9.8
Others	1.32	1532	5.3
Total		28,656	100

and the average precipitation of each country were used.

The lack of precision associated with using mean precipitation introduces some errors, but the large spatial scale of the analysis partly compensates for this. Except for Italy, the results obtained (Table 3 and

Table 4. Comparison of estimated mean recharge in earlier published works, and in the present study for Spain, Portugal, Ireland, and Italy during the period 1940–1941 to 1995–1996.

			Mean Recharge (hm3/yr)
	Area (km2)	Average Precipitation (mm/yr)	Previous Works *	Estimates of This Work
Spain (Peninsular)	497,477	687	28,908 1	28,728.2
Portugal	89,898	882	4000 2	4197.14
Ireland	69,577	1150	10,800 3	11,435.6
Italy	302,557	982	43,000 4	39,305.5

Notes: * Sources: ¹: Estrela et al. (1999) [38]; ²: MIMAM (2000) [24]; Carmona (1999) [43]; ³: EEA (2001) [44]; WRI (2001); MIMAM (2004) [40]; ⁴: IRSA (1997) [45].

) are reasonably reliable. In the former case, two factors can explain the estimated mean recharge value: First, limestone outcrops are mainly located in areas with the highest precipitation, which means that above-average precipitation should have been used in the calculations. Second, Italy has a significant amount of volcanic rock, for which the recharge rate relative to precipitation remains uncertain, but is assumed to be relatively high.

3.3. Assessment of World Average Natural Recharge in Karst Aquifers

Figure 3 shows the map of karst outcrops totaling 20,259,600 km², representing 15.17% of the Earth’s surface. The mean annual precipitation isohyets and areas covered by glaciers and permafrost are superimposed on this layer. The spatial information has been harmonized by back-projection to WGS 84. Likewise, raster files have been resampled to a standard resolution of ~1 km² (30 arc-seconds), which ensures geometric consistency between the layers used (WorldClim v2, WHYMAP, Natural Earth, FAO-UNESCO). Then, 2350 km² of karst surface (11.6% out of the total) is located in regions topped by permafrost and glaciers, having either low or null recharge rates. Consequently, these are disregarded in the assessment, leaving a net karst area of 18,509,000 km² (13.8% of the Earth’s surface). According to data provided by WorldClim on monthly precipitation averages recorded during the period 1980–2010, the estimated recharge worldwide is 4,381,063.7 hm³/yr.

Table 5. Breakdown of variables for calculating total natural recharge by continent.

Continent	Total Karst (km2)	Glaciers and Permafrost in Karst (km2)	Net Karst (km2)	Net Karst Percentage (%)	Average Yearly Precipitation (mm/yr)	Recharge Rate (%)	Natural Recharge (hm3)	Natural Recharge (mm)
Africa	4,053,400	0	4,053,400	13.5	496.71	34.31	690,785.3	170,421
Asia	8,340,000	818,000	7,522,000	18.6	694.17	34.31	1,791,512.7	238,170
Oceania	501,300	600	500,700	6.2	1266.78	34.31	217,620.4	434,632
Europe	2,166,000	67,000	2,099,000	21.8	671.87	34.31	483,858.5	230,519
North America	4,431,000	742,000	3,689,000	19.6	726.14	34.31	919,072.4	249,139
South America	767,900	123,000	644,900	4.3	1257.38	34.31	278,214.4	431,407
Global	20,259,600	1,750,600	18,509,000				4,381,063.7

shows its distribution per continent, with Asia contributing the most and having the highest recharge rate per unit area of karst surface (434 mm/m²).

3.4. Recharge at the 2050 Horizon Taking into Account Climate Change

This analysis considers a future scenario in 2050, where average annual precipitation over karstic surfaces drops by 2% from the current 732.8 mm/yr to 718.1 mm/yr (source: WorldClim) [34]. However, this scenario’s spatial distribution of precipitation does not mirror the current pattern proportionally. The hypothesis assumes that rising temperatures eliminate all seasonal permafrost, allowing frozen karstic terrains to function as new aquifers. As a result, total recharge declines to 3,918,530.9 hm³/yr (

Table 6. Estimation of the decrease in total and continent-specific natural recharge according to climate change predictions in 2050.

Continent	Estimated Precipitation in 2050 (mm/Yr)	Recharge Rate (%)	Estimated Natural Recharge in 2050 (hm3)
Africa	407.54	34.31	566,779.72
Asia	595.83	34.31	1,537,717.14
Oceania	820.38	34.31	140,932.48
Europe	830.77	34.31	598,292.92
North America	559.50	34.31	708,161.32
South America	1657.05	34.31	366,647.32
Global			3,918,530.9

). This assumption implies a decrease in recharge of approximately 10.55%. It should be noted that the reduction in precipitation to 2% is an average and that, according to predictions, this reduction is more significant in areas with high rainfall.

Finally, although it is a mere testing exercise of very relative value, an estimate has been made of the recharge in the karst during the Last Maximum Glacial (LMG), assuming that the distribution of precipitation and recharge rate is the same as at present, which may be highly debatable. Thus, according to the distribution of glaciated and permafrost areas in the LMG [47], the total recharge was 3,800,000 hm³/year, which is a 12% reduction, mainly in Europe, Asia, and North America.

4. Discussion

The intrinsic characteristics of karst aquifers—such as their heterogeneity, degree of karstification, and variable recharge rates—and the limited data available for many poorly studied systems make it extremely difficult to carry out global recharge and water resources assessments. This results in a high level of uncertainty. Assessing both the degree of uncertainty and the statistical error is challenging because, although the natural recharge assessment method has been tested in other Spanish and European basins, insufficient data is available to calibrate it in areas outside Europe. Furthermore, there are no recharge assessment data on karst at a global level, nor on specific aquifers or general recharge at the country level. Using a universal recharge rate relative to precipitation might seem like an oversimplification that overlooks the complexity of karst systems. We suggest that future work should incorporate more detailed karst typologies where such data are available. However, nothing can truly be understood without measurement, which requires representing reality through a limited set of measurable attributes and quantifiable and operational variables. In this context, the recharge rate relative to precipitation used in this study (34.31%) appears reasonable and falls within the range reported in numerous karst hydrogeology studies. For instance, it closely aligns with the average values identified by Hartmann et al. (2015) for European karst regions (35–40%) [19].

This way, total recharge across exposed karst areas worldwide reaches 4,381,063.7 hm³/year, corresponding to 236 mm/yr per square meter of karst terrain. This estimate exceeds the initial assessment of Stevanović (2019) [18], who reported 3165 km³.

Nine evaluations compiled by Margat and Van der Gun (2013) [48] have been considered to estimate global groundwater resources. These range between 12,000 and 15,000 km³/yr, although the most frequent figures fall between 12,000 and 13,000 km³/yr—such as Yoshikawa et al. (2011), reporting 11,968 km³/yr [49], and Döll and Fiedler (2008), who reported 12,700 km³/yr [50].

Using the latter as a reference, the recharge estimated in this study represents 34.5% of global groundwater resources, highlighting the significance of karst-related water reserves and their potential availability for supply purposes. Analysis shows that 50% of total natural recharge in the world’s karst regions occurs within the precipitation range of 600 to 1600 mm/yr (Figure 4). In contrast, areas with extremely low precipitation (0–400 mm/yr) contribute only 5.57% to global recharge. Likewise, regions receiving over 3500 mm/yr account for just 5% of the total.

These figures indicate that extreme climate zones—where deviations from typical recharge behaviors and rates could occur—have a limited influence on the global balance. Notably, the 34.3% recharge rate used in this study was derived from a region that falls within the 600–1600 mm/yr range, which lends further credibility to its applicability.

According to climate change projections for 2050, Table 6 shows that global natural recharge decreases to 3,918,530.9 hm³/yr, representing a 10.55% reduction compared to current levels. Some of the losses caused by increased temperature and evaporation have been partially offset by the exposure of ice from newly thawed karst terrains.

However, the behavior of these areas remains uncertain. It is unclear whether they possess inherited karstification or will function effectively as aquifers. As Ford and Williams (2007) note, karstic lithologies may require several tens of thousands of years to evolve into fully functional aquifers [51].

5. Conclusions

Despite the uncertainty in estimating the global average natural recharge of aquifers, this study attempts to provide a reasonable approximation in the case of karst systems. The approach is based on a method that uses the recharge–precipitation ratio derived for lithological groups with similar hydrogeological behavior, initially established in Spain.

This method is useful for large-scale assessments, where the numerous factors influencing recharge ultimately reduce to two key variables: lithology and precipitation. The researchers validated the method at smaller scales in a Spanish river basin and at broader scales across several European countries by comparing the results with those obtained through other methodologies.

Researchers have calculated a natural recharge-to-precipitation ratio of 34.31% for karst aquifers, proposed here as a globally representative average value, though it can be later regionally adjusted. Upon using this ratio, the method was applied to a global assessment of natural recharge, excluding areas currently covered by glaciers and permafrost.

As a result, total recharge in exposed karst outcrops worldwide reaches 4,381,063.7 hm³/year. Although karst terrains cover only 15.17% of the Earth’s surface, they account for 34.5% of global groundwater resources—a figure that underscores the significance of karst systems in global water availability. Climate change projections for 2050 suggest a 10.55% reduction in recharge compared to present-day values.