Groundwater availability and role of climate change

Main characteristics of selected aquifers

The effects of recent climate variations on ground-water availability are evaluated considering five wide hydrogeological structures (HSs). In each HS the shallow or outcropping aquifer is considered; three are porous, two are constituted by carbonate rocks, all are coastal aquifers.

The Apulian Tableland HS, Tavoliere HS, consists of a shallow and large porous aquifer within a conglomerate sandy-silty succession, less than 60 m deep, with a clayey impermeable bottom

Table 5. Net rainfall trend and MAP classes considering data from 1924 to 2001

MAP Class

ANR (mm)

NRT

NRV/ANR

(mm/a)

(%)

< 600 mm

85.5

-0.39

-33.1

600-900 mm

227.9

- 0.89

- 27.1

900-1300 mm

464.8

- 1.99

- 32.2

>1300 mm

967.9

- 4.30

- 32.3

ANR, average net rainfall; NRT, net rainfall trend; NRV, net rainfall variation from 1924 to 2001 and ANR ratio.

ANR, average net rainfall; NRT, net rainfall trend; NRV, net rainfall variation from 1924 to 2001 and ANR ratio.

(Polemio et al. 1999). It is deep enough to allow seawater intrusion only in the vicinity of the coast. Groundwater is phreatic inland or far from the coast, in the recharge area, whereas it is confined in the remaining part of the aquifer; maximum piezometric levels reach 300 m a.s.l.

Except for the Tavoliere, the Apulian region is characterized by the absence of rivers and the nonavailability of surface water resources due to its karstic nature. Considerable groundwater resources are located in large and deep carbonate coastal aquifers, as in the case of the Gargano (not considered in this study due to the low data availability), the Murgia and the Salentine Peninsula (Salento) HSs. The Murgia and Salento areas show some common features (Cotecchia et al. 2005). They consist of large and deep carbonate aquifers, constituted mainly of limestone and dolomite rocks. Carbonate rocks are affected by karstic and fracturing phenomena, which occur well below sea level, whereas intruded seawater underlies fresh ground-water owing to a difference in density. Confined groundwater is more widespread inland; ground-water is phreatic everywhere along a narrow coastline strip. The maximum piezometric head is about 200 m a.s.l in the Murgia area and 5 m a.s.l in the Salento area (Spizzico & Tadolini 1997).

Five rivers cross the Metaponto plain, located along 40 km of Ionian coast. Marine terraced deposits, mainly sands, conglomerates and silts, crop out in the upland sectors of the Metaponto plain, while alluvial, transitional, marine and coastal deposits crop out in the coastal plain and along the rivers (Polemio et al. 2003). Two main types of porous aquifers can be distinguished in the Metaponto plain. The first one encloses the aquifers of the marine terraces and the alluvial river valley deposits. The marine terrace aquifers display medium to high hydraulic conductivity; the river valleys regularly break their spatial extent. The aquifers of the river valleys display low to medium hydraulic conductivity and they do not generally permit an accumulation of significant groundwater resources. The second type of aquifer includes one of the coastal plain deposits and has a medium hydraulic conductivity. This aquifer is the most exploited one for practical purposes due to its extension (about 40 km wide), thickness, continuity across the plain and also because its outcropping surface is more affected by economic growth and increasing water demand.

The groundwater of the coastal plain aquifer flows in a multilayered aquifer; it is mainly phrea-tic, otherwise it is confined due to an upper, almost impervious and outcropping stratum.

The Sibari plain is located in NE Calabria and covers the final sector of the Crati river. The Sibari plain is bordered by the carbonate relief of the Pollino Massif to the north and by the intrusive and metamorphic rocks of the Sila Massif to the south; it is composed of sedimentary lithotypes, varying from sand to marl and clay and including gravel locally.

The Sibari plain houses multilayered aquifers, the recharge of which is partially ensured by groundwater flowing from massifs and by leakage of rivers. For this study the shallow sandy aquifer alone has been considered (Polemio & Petrucci 2003).

Hydrological data

Piezometric data (Table 6) and river discharge measurements (five time series ranging from 1930 to 1992 for the Tavoliere and three series ranging from 1929 to 1971 for the Metaponto plain; SIMN 1916-2000) are considered together with already analysed rainfall and temperature data. The continuous or regular monthly piezometric data are derived from gauges managed by SIMN (19162000) or by the Irrigation Development Agency (Regione Puglia 1983). Occasional and recent data were collected on-site by the IRPI hydrogeological staff (Polemio & Dragone 2003, 2004; Polemio et al. 2003, 2004b, 2004c; Polemio & Petrucci 2003) and by other sources (Regione Puglia 2002) for the Tavoliere aquifer; Lopez et al. (2003) and CASMEZ (1987) for the Sibari aquifer.

Data from 58 wells or piezometric gauges are available for the three Apulian HSs - the Tavoliere, the Murgia and the Salento (Polemio & Dragone

2004). The piezometric data sets regarding the Tavoliere are available for a minimum of 17 years and for a maximum of 55 years, covering a continuous period between 1929 and 1994 (Polemio et al. 1999). Continuous data are available from 1973 to 1978 for the Murgia and the Salento. Furthermore, sporadic recent data were collected in Apulia for the periods 1995-1997 and 2001-2003.

Piezometric time series of monthly data are available in the Metaponto plain for 60 wells in two periods, 1927-1940 and 1951-1984 (Polemio & Dragone 2003). Occasional but high density data are available in the whole plain for 1953 and 1990 and in a selected study area for each season of 2002 (Polemio et al. 2004a).

Data from 121 wells in the Sibari plain were considered, for which discontinuous piezometric data are available from 1932 to 2002. Data are concentrated in the 1930s, the 1950s and the 1970s. The surveying was managed by different institutions in these different periods: the location and the identification of wells are not detailed enough to permit linking of the series. The oldest data were regularly collected from 1932 to 1940 in 27 wells; this data set has been used as a reference for spatial analyses. In June 2002 a high density piezometric survey was carried out. Due to the shortness of regular piezo-metric time series, the analysis for the Sibari plain is limited to the spatial analysis of piezometric surface modifications.

Data analysis

Piezometric data are explored by typical approaches of time series analysis such as autocorrelation, cross-correlation and trend analysis tools, and of spatial analysis, using kriging to obtain grid data of piezometric surfaces to compare using simple arithmetic operations and volume determinations.

The piezometric value recorded in any month is strongly dependent upon the values of the previous months, the link being significant, diminishing as the time lag increases. The duration and the

Table 6. Piezometric data availability for each hydrogeological structure (HS) and straight line trend (AC)

HS Well number Data range Minimum AC (m/a) Trend more probable at 2002 or 2003

Tavoliere

Murgia

Salento

Metaponto

Sibari

1929-2002 1965-2003 1965-2003 1927-2002 1932-2002

High decrease High decrease Decrease Decrease Decrease

*The number of wells available for occasional years is higher and variable.

fIn the periods 1927-1940 and 1951-1984.

^Determination not available due the characteristics of data set.

intensity of this dependence, called the memory effect, is a function of specific yield, saturated thickness, hydraulic conductivity and extent of the aquifer. High values of these parameters are typical of aquifers of high quality to tap ground-water; in these cases the autocorrelation decreases slowly as the lag increases.

The cross-correlation for each piezometric series is determined by comparing it pairwise with data from a hydrogeologically significant series of rainfall, temperature and, where available, river discharge, the data of which are utilized step by step with increasing lag from 1 to 12 months. The cross-correlation coefficient expresses a measure of effect of the latter variable, rainfall, temperature or river discharge, on the variability of the former variable, the piezometric height or level.

The spatial analysis is utilized to complete the trend analysis of piezometric data when sporadic but high density data are available.

The three considered porous aquifers are subject to similar hydrological conditions: the range of mean annual rainfall and temperature is, respectively, 440-600 mm and 15.9-16.9°C. The Tavoliere area is a little cooler and drier than the other two areas. With regard to the monitored river discharges, the whole range of mean annual values is 1.0-20.0 m3/s.

In the Tavoliere aquifer the autocorrelation coefficient always decreases slightly and quite linearly. There is a very high autocorrelation in 56% and 22% of wells, decreasing the coefficient respectively from 1 to about 0.8 and to 0.5, increasing the lag up to 12 months. The autocorrelation of remaining wells is insignificant after six lags.

The piezometric height is cross-correlated with river discharge, temperature (in this case this is an anticorrelation) and rainfall in decreasing order of maximum absolute value of coefficient. The river bottom is generally higher than the piezometric height in the monitored locations.

The maximum of the cross-correlation coefficient (MCC, as absolute value if the coefficient is negative, as in the case of temperature) and of maximum significant lag (MSL, the maximum lag for which there is statistical significance of cross-correlation) are, respectively, 0.5 and 2 months for discharge, 0.4 and 3 months for temperature, and 0.3 and 5 months for rainfall. MSL seems well correlated to depth to water, and thus appears to be a useful parameter to evaluate the time necessary to transfer a surface water impulse to groundwater.

The fact that even temperature variations are significant, more widespread than rainfall, has already been observed in similar hydrogeological conditions (Polemio et al. 1999). This can be explained by considering the nature of the climate, which is semi-arid everywhere for the selected aquifers. In this type of climate, the temperature is significant because of two separate and cyclic phenomena. The first is a natural phenomenon, real evapotranspiration, which 'regulates' the availability of net rainfall for infiltration from autumn to spring. The second is anthropogenic and is mainly linked to groundwater discharge from spring to autumn, due to high temperatures and potential evapotranspiration: the farms use more groundwater to offset the water deficit. In this way, temperature variation more than rainfall explains piezometric variations over the whole hydrologic year.

The trend of river discharge has been characterized by five gauges: the trend is clearly decreasing, especially since 1980, as in the case of rainfall in the whole Apulian region. It should be recognized that this variable is also influenced by human activity due to depletion by dams or by diffuse withdrawals from rivers for farm use, which strongly increased by 1980.

The piezometric trend everywhere is decreasing (Table 6). The continuous piezometric lowering has transformed many confined wells into phreatic wells; after that, the shallow groundwater of the Tavoliere is completely depleted in places. In terms of straight line trend, the trend everywhere is strongly negative, constituting a severe problem for groundwater discharge by wells (Table 6). The trend is confirmed by the spatial analysis of sporadic 2002 data: the spatial mean of piezometric decrease is 7.93 m over about 15 years.

In the Metaponto plain, the autocorrelation coefficient is quite linear from the maximum, a bit lower than 1, to the minimum, equal to not less than 0.3, increasing the lag up to MSL, equal to 6 months (after that the piezometric values are independent). The memory effect is high everywhere, but generally higher where groundwater is confined.

The MCC is generally less than 0.5 while MSL is 3 months, with some exceptions up to 4 months in the case of rainfall. As in the case of the Tavoliere, the MCC is low and lower than the absolute value of temperature MCC which is, in this case, also greater than the river discharge MCC. The temperature MCC is generally less than 0.7 while MSL is 2 months, with some exceptions of up to 4 months; everywhere, the coefficient is negative. The river impulse in terms of piezometric variations is extremely quick and important: MSL is generally 1 month and MCC less than about 0.6.

The trend of the piezometric series has been defined both on a synoptic scale, for the whole plain, and on a detailed scale, in the selected study area. With regard to the synoptic scale, the trend is described by AC while for the detailed scale it is the result of a comparison of the piezo-metric surfaces of different years.

The piezometric minimum occurred generally between 1952 and 1954 (up to 1984), when the exploitation of the aquifers was very high, after the end of land reclamation works (Polemio & Dragone 2003).

The time series analysis shows a widespread negative trend for the period 1927-1940 with a piezometric drop, on average, equal to 0.05 m/a. In the same period of time, the rainfall trend is slightly positive locally. This figure can be explained, as the groundwater represented the only irrigation resource during these years. Conversely, a positive trend is observed from 1951 to 1984 (75% of the total available time series), even if rainfall and river discharge trends are not positive. This figure can be explained considering the fact that many dams were built in this period; the dams started to supply more irrigation water than ever before. This allowed a reduction of groundwater tapping and also created a sort of artificial recharge, due to over-irrigation.

A new trend variation, a widespread downward trend, started during the 1980s; this trend remains unchanged. During 1988-1991 a heavy drought hit the area. The effect was the depletion of artificial lakes and the massive re-utilization of groundwater. The piezometric effects of drought in 1990 were relevant, particularly as compared to the situation in 1953, defined almost as a minimum until 1984. Negative piezometric variations are generally also prevalent in the case of the selected study area, where a more detailed analysis was carried out during 2002 (Polemio et al. 2003). For this area, the 2002 spatial mean of piezometric height is 1.12 m less than that of 1953, while that of 1990 is 0.34 m higher.

The maps of the piezometric variations in the Sibari plain highlight a decreasing trend which started around the 1950s, assuming as reference piezometric surface that of the 1930s. The plain as a whole is characterized by piezometric heights in the 1950s which were higher than in the 1930s (positive piezometric variation). Major decreases of piezometric height (negative variations) appear inland during the 1970s while two separate positive variation areas remain, involving main rivers and portions of the coast. Both a lowering of the piezo-metric negative variations, in term of absolute values, and a narrowing of the positive variation area are observed in 2002, the positive variation area becoming quite similar to a strip along the coast. The spatial mean of piezometric variation of 2002 is the lowest or the worst observed; it is equal to a lowering of 4.42 m with respect to the 1930s.

In the case of the considered Apulian carbonate hydrogeological structures, the autocorrelation piezometric coefficients consistently show a progressively declining trend, starting from one to the statistically significant minimum, everywhere not less than 0.5, increasing the lag to 4 months for the Murgia and 5-6 months for the Salento area. The consistent memory effect of Apulian groundwater is a characteristic feature which is of great importance during droughts or dry spells. The Salento has shown very strong and long-lasting memory effects, which is only further proof of the good hydrogeological characteristics of these aquifers.

There is a cross-correlation between the piezo-metric and climatic variables of an acceptable significance level for a time lag up to 4 months. The effects of rainfall are perceptible up to a maximum period of 2-3 months, whereas the best correlation with temperature is felt with a time lag of 4 months. The temperature variations are more significant than rainfall in some portions of the Murgia and the Salento.

The calculated piezometric trend, generally speaking, is downward, since there is a widespread tendency, albeit in some cases a very slow one, towards a piezometric drop. The lowest piezometric decrements have been observed in the Salento area, which has an AC in the range of —0.060 to — 0.012 m/a; worse AC values are typical of the Murgia HS (Table 6). In the Murgia, as in each HS, the AC approaches zero the closer one gets to the coastal areas, as would be expected.

During 2002, a widespread and dramatic drought period ended. On the basis of the available data set, the most likely piezometric trend, ending in the second half of 2002, was very negative and serious in terms of the sustainability of the ground-water demand, over the entire area covered by porous aquifers, as in the case of the Tavoliere, the Metaponto plain and the Sibari plain (Table 6). This situation is confirmed by sporadic data of 2003 in the case of the Murgia and the Salento, notwithstanding the effect of more than a year of abundant rainfall.

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