ASRAdvances in Science and ResearchASRAdv. Sci. Res.1992-0636Copernicus GmbHGöttingen, Germany10.5194/asr-12-121-2015Ten years water and energy surface balance from the CNR-ISAC micrometeorological station in Salento peninsula (southern Italy)MartanoP.p.martano@isac.cnr.itElefanteC.GrassoF.CNR-Istituto di Scienze dell'Atmosfera e del Clima – UOS Lecce, Via Monteroni, 73100 Lecce, ItalyRipartizione Informatica, Università del Salento, Viale Gallipoli 49, 73100 Lecce, ItalyP. Martano (p.martano@isac.cnr.it)16June201512112112514January201527May20151June2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://asr.copernicus.org/articles/12/121/2015/asr-12-121-2015.htmlThe full text article is available as a PDF file from https://asr.copernicus.org/articles/12/121/2015/asr-12-121-2015.pdf
Data of surface-atmosphere energy and water transfer from a ten years
(2003–2013) period of activity of the ISAC-Lecce micrometeorological station
(http://www.basesperimentale.le.isac.cnr.it) have been analyzed: to
the authors' knowledge this is the first decadal data set of
surface-atmosphere transfer in Salento peninsula. The surface energy budget
shows a tendency to a positive bias possibly due to several reasons that
require more investigations. Some suitable indices related to the surface
water balance, such as the precipitation intensity, the aridity index and
the ground water infiltration fraction have been calculated. Possible trends
of these annual averages in the decadal period are considered, also taking
into account the statistical uncertainty associated to measurement errors
and missing data. The results indicate a significant increasing in the
precipitation intensity together with an experimental evidence of increasing
of the ground water infiltration in the measurement area, that is in
agreement with recent estimations for the whole Salento peninsula. On the
other hand, recent studies show that seawater intrusion and salinization of
the deep underground aquifer keep increasing in the same period.
Introduction
Semi-arid coastal regions are common in Mediterranean areas. The correct
assessment of the surface water and energy balance can be difficult for such
regions, due the extreme changes in the surface moisture conditions that
require direct measurements. Indeed reliable estimates of evapotranspiration
generally based on the potential evaporation concept (Allen et al., 1998)
are not easy to obtain over surfaces that are often in very dry conditions.
In this context evapotranspiration constitutes one of the less known
components of the surface water budget especially for climatic studies, and
long term time series of measurements are often not easy to be found. Long
term measurements of the surface water balance can be also helpful for the
management of the often scarce fresh water resources. Indeed coastal
groundwater aquifers are often the only fresh water supply for the local
populations, and imbalance between drainage and recharge by surface
infiltration can lead to loss of pressure and seawater intrusion in the
aquifer, significantly degrading water quality (Custodio, 2010; Delle Rose
et al., 2000). Salento peninsula, in the south-east of the Italian peninsula
(Fig. 1) is a semiarid region characterized by an almost plane surface with a
small orographic relief in the south-west part (200 m height). The
non-urbanized lands are generally covered by olive groves, vineyards or
arable lands, mostly non-irrigated, while few remaining natural extensions
are characterized by pine forests or mixed Mediterranean shrubs. The
precipitation climatology is typically Mediterranean with warm dry summers
and quite mild wet winters. The average annual precipitation is about 650 mm
but with a certain variation within tens of kilometres due to the particular
geographical shape of the southern end. This, together with the presence of
the low orographical relief, triggers air lifting and a convergence zone,
thus enhancing local precipitation in this area (Fig. 1). The average annual
temperature is about 16–17 ∘C, with a spatial variation of about
1 ∘C from the east to the west coast that is not directly exposed
to the mistral winds. Indeed the wind regime has two main components:
south-west, prevailing in cyclonic conditions, and north-west, that are
quite common in this area due to the channelling effect of the Otranto
channel, and are prevailing in anti-cyclonic conditions. Hydrology is
characterized by ephemeral stream systems and karstic geological
characteristics that allow the presence of a deep good quality ground water
aquifer, the major source of freshwater for anthropic use (more than 80%
of the total fresh water necessity in the area). Monitoring and modelling
studies of the aquifer balance in the last decades (Margiotta and Negri,
2005; Portoghese et al., 2005; Romanazzi and Polemio, 2013) showed a
decreasing aquifer pressure with increasing salty seawater intrusion. Thus,
efforts have been recently made to specifically modelling the groundwater
flow in the fractured karst aquifer (Giudici et al., 2012; De Filippis et
al., 2013). The aim of this work is to present a data set of long term
experimental measurements of the surface energy and water balance as
measured by the CNR-ISAC micrometeorological station in Salento peninsula
(the only available long term data set in Salento, to the authors'
knowledge), and to give an experimental support to the hydrological
modelling for the local water resources management.
Salento peninsula with averaged 1921–1996 precipitation
distribution (mm) (adapted from http://www.supermeteo.it). The dot
indicates the measurements site.
Data processingData collection
The CNR-ISAC micrometeorological base is placed in a suburban area 5 km
south-west from Lecce (Fig. 1), within the Salento University campus (Lat:
40∘20′12′′ Lon: 18∘7′17′′)
characterized by mixed typical local vegetation (pines, olive groves and
Mediterranean shrubs) with an increasing content of buildings and
non-natural surfaces. Buildings and trees have an average height of about
10 m and with estimated roughness length z0 and displacement height d of
about 0.5 and 7 m respectively (Martano, 2000). A 16 m high mast is
equipped with a fast response eddy covariance system and standard
meteorological sensors for routinely collecting half-hour averaged data of
air temperature and humidity. The mast height allows a flux footprint fetch
of the order of several hundreds of meters (Hsieh et al., 2000), thus taking
contributions from the campus and some immediately surrounding vegetated
areas. An ancillary Campbell meteorological station collects measurements of
air temperature, air humidity and net radiation and soil surface
measurements of temperature, soil moisture and soil heat flux. Data are
averaged and stored either in a data logger (meteorological station) or in a
dedicated netbook for fast response data processing (eddy covariance
system), that outputs half-hour averaged turbulent flux data in the
streamline coordinate system (McMillen, 1988). Before the final storage in
the web database as 30 min averages, data undergo a quality control that
eliminates out of range measurements generated by possible malfunctioning of
the sensors. A site map and a more detailed description of the station are
available elsewhere (http://www.basesperimentale.le.isac.cnr.it, Martano et al. 2013).
Time series post processing
The 30 min averaged data time series have been post processed trying to
minimize the possible uncertainties and errors coming from the measurement
procedure, such as the eddy covariance method for the turbulent fluxes of
heat and water vapor (Foken and Wichura, 1996). Massman (2000) spectral
corrections and mass flux Webb corrections (Webb et al., 1980) have been
applied. In addition an estimation of the uncertainty to be associated to
the annual cumulative and/or average value has been attempted. It has been
defined as the maximum between the uncertainty associated to the measurement
procedure (instrumental uncertainty for slow response measurements and
statistical uncertainty for eddy covariance measurements), and the
uncertainty associated to gaps of data lacking in the time series (however,
annual time series with more than 1 month total data gap have not been
used). The statistical uncertainty Eec associated to the eddy
covariance fluxes <w′q′> was estimated as the standard deviation of the
instantaneous measurements w′q′ in the averaging time (30 min).
An evaluation of the uncertainties associated to the presence of gaps of
dN data in the (annual) time series of N (30 min averaged) measurements is
proposed as (if dN≪N):
Total annual surface energy balance (Rn-G- LE -H)
compared with the measured annual net radiation (Rn, circles).
Eave=σN(dN/N)1/2 for average values and Ecum=QdN/N for
cumulant values (total sum)
where σN is the standard deviation of the (annual) time
series and Q its sum.
Besides directly measured quantities some other derived indices are used in
the following analysis:
Evaporative Fraction EVF = LE/(LE +H) where LE and H are respectively the latent and
sensible heat fluxes.
Aridity Index AI =P/ET0 where P and ET0 are the precipitation and
the potential evapotranspiration calculated by the Penman–Monteith formula
(Allen et al., 1998).
Water Infiltration Fraction WIF = (P- Ev)/P where Ev is the actual (measured)
evapotranspiration and runoff is neglected, that is a very reasonable
assumption in the measurement area, and quite generally also in Salento
peninsula (Portoghese et al., 2005).
Precipitation Intensity PI =<P>wd where <…>wd indicates the time
series average for wet days only (days in which P> 1 mm).
In addition the North Atlantic Oscillation Index NAO (difference between the measured sea level atmospheric
pressure in Lisbon and Reykjavik) has been computed for the same periods by
web data (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml).
Annual averages of precipitation (m, circles), water infiltration
fraction (squares) and aridity index (triangles) with estimated maximum
uncertainties. The continuous (P) dashed (WIF) and dash-dotted (AI) lines are least
squares linear regressions.
Results and discussion
The annual averages of the energy balance (Rn-G- LE -H, where Rn is the net radiation and
G the soil heat flux at 2 cm depth) are shown in Fig. 2. Although the
budget appears to be generally closed within the quite large uncertainties,
a tendency to a positive imbalance is also apparent. Several reasons can
contribute: from the divergence of the soil heat flux between the surface
and 2 cm depth, to the tendency of the eddy correlation system to
underestimate the turbulent fluxes due to the finite averaging time (Cava et
al., 2008). Even possible uncertainties in the calibration of the net
radiometer after suffering strong rain events with dome breaking cannot be
excluded, and all will require a separate analysis. Figure 3 shows some
trends for the water balance: total precipitation, water infiltration
fraction and aridity index. Here and in the next figure two tests have been
performed to check the decadal trends. In the first the sign of the trend
has been checked after deleting the first and the last annual average in the
series, while in the second the statistical uncertainty of the regression
slope was calculated to verify whether a change of sign of the slope were
possible within the slope uncertainty. The trend is considered significant
if both tests give negative result (no change in the slope sign), marginally
significant if only one of them gives negative result, and not significant
if both give positive result. In Fig. 3 the trends are marginally
significant and positive for both total precipitation and water infiltration
fraction, while the AI is decreasing, indicating a probable decrease in the
year-averaged surface soil moisture, as AI is quite well correlated with local
soil moisture measurements when available (correlation coefficient 0.85 for
annual averages). The apparent disagreement between these last results
(increasing precipitation and infiltration, and decreasing surface soil
moisture) is perhaps less surprising when observing Fig. 4, that shows a
significant positive trend for the total precipitation intensity. It is
possible that the increasing concentration of precipitation events in an
otherwise semi-arid region with karstic geologic features and generally
negligible slopes and runoff is likely to increase infiltration more than
evaporation. This is because the soil surface becomes wet within short time
intervals during the year, while it is going to dry up quickly in a few days
after rain, preventing strong long term evaporation. The NAO negative trend
also shown in Fig. 4 suggests that the locally measured increasing
precipitation intensity could be linked to a regional trend of increasing
precipitation associated to the decreasing NAO phase during the decade of
observations (Willems, 2013). A recent statistical downscaling analysis also
found a scenario of increasing precipitation intensity, over this region for
the next decades, especially in the dry season (Palatella et al., 2010).
These results suggest a potentially increasing availability of water
recharge for the groundwater aquifers during the past decade in the
measurement area. Although the extension of this experimental result to the
whole Salento peninsula would require more measurement sites, this
possibility is suggested by the following remarks. Indeed the measurement
site is far from the maximum of the precipitation distribution (Fig. 1) and
characterized by Mediterranean arboreus vegetation that tends to enhance and
stabilize evapotranspiration with respect to bare soil or dry shrub
Mediterranean areas (Scanlon et al., 2006). Thus the annual difference
between measured precipitation and evapotranspiration is not expected to
overestimate the average over the Salento peninsula and the measured
positive trend of the surface infiltration also appears as an experimental
confirmation of recent estimations for the whole peninsula (Portoghese et
al., 2013). This trend can be constrasted with a recent analysis of the
available data from monitoring wells by Fidelibus and Tulipano (2014),
showing that the freshwater column thickness of the local deep aquifer keeps
decreasing during the last decade, together with an increasing height of the
freshwater-seawater interface under Salento peninsula. In synthesis, the
water surface budget measurements presented here give an experimental
evidence of an increasing availability of potential groundwater recharge by
infiltration, still not affecting the increasing salinization and marine
intrusion in the Salento deep aquifer in the last decade.
Annual averages of precipitation intensity (m, circles) and
NAO (squares). The straight lines are least squares regressions.
Acknowledgements
The authors wish to acknowledge the PON 2007-13 I-AMICA (OR1) project
(http://www.i-amica.it), and the CNR GIIDA project
(http://www.dta.cnr.it/content/view/2735/244/lang,en/), for partially supporting
the micrometeorological base and the database, and the HyMeX project
(http://www.hymex.org) for the scientific support.
Edited by: F. C. Bosveld
Reviewed by: S. Margiotta and another anonymous referee
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