37 research outputs found
Optimal adaptation strategies to face shocks on groundwater resources
We consider an exogenous and reversible shock to a groundwater resource, namely a decrease in the recharge rate of the aquifer. We compare optimal extraction paths and the social costs of optimal adaptation in two cases: under certainty, i.e. when the date of occurrence of the shock is known, and under uncertainty, when the date of occurrence of the shock is a random variable. We show that an increase in uncertainty leads to a decrease in precautionary behavior in the short run and to an increase in precautionary behavior in the long run. We apply our model to the particular case of the Western la Mancha aquifer in Spain. We show that, in this context, it is advantageous for the water agency to acquire information on the date of the shock, especially for high-intensity and intermediate-risk events
Measurements and estimation of the columnar optical depth of tropospheric aerosols in the UV spectral region
International audienceWe report values of the columnar tropospheric aerosol optical depth at UV wavelengths based on experimental measurements of the direct spectral irradiances carried out by a commercial spectroradiometer (Li1800 of Licor company) covering the range from 300?1100 nm at two stations with different climate characteristics in Spain. The first station is located in a rural site in north central Spain with continental climate. The data extend from March to the end of October of 1995. The other station is a coastal site in the Gulf of CĂĄdiz (southwest Spain) of maritime climate type. This study is mainly focused on the capability of estimating aerosol optical depth values in the UV region based on the extracted information in the visible and near infrared ranges. A first method has been used based on the Ă
ngström turbidity parameters. However, since this method requires detailed spectral information, a second method has also been used, based on the correlation between wavelengths. A correlation has been established between the experimental aerosol optical depth values at 350 nm and 500 nm wavelengths. Although the type of aerosol seems to be the key factor that determines the quality of these estimations, the evaluation of the associated error is necessary to know the behaviour of these estimations in each area of study
Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) â Part 2: Analysis of site aerosol influence
Several validation studies have shown a notable overestimation of the clear
sky ultraviolet (UV) irradiance at the Earth's surface derived from
satellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) and
the Ozone Monitoring Instrument (OMI) with respect to ground-based UV data
at many locations. Most of this positive bias is attributed to boundary
layer aerosol absorption that is not accounted for in the TOMS/OMI
operational UV algorithm. Therefore, the main objective of this study is to
analyse the aerosol effect on the bias between OMI erythemal UV irradiance
(UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances and
ground-based Brewer spectroradiometer measurements from October 2004 to
December 2008 at El Arenosillo station (37.1° N, 6.7° W,
20 m a.s.l.), with meteorological conditions representative of the South-West of
Spain.
<br><br>
The effects of other factors as clouds, ozone and the solar elevation over
this intercomparison were analysed in detail in a companion paper (AntĂłn
et al., 2010). In that paper the aerosol effects were studied making only a
rough evaluation based on aerosol optical depth (AOD) information at 440 nm
wavelength (visible range) without applying any correction. We have used the
precise information given by single scattering albedo (SSA) from AERONET for
the determination of absorbing aerosols which has allowed the correction of
the OMI UV data.
<br><br>
An aerosol correction expression was applied to the OMI operational UV data
using two approaches to estimate the UV absorption aerosol optical depth,
AAOD. The first approach was based on an assumption of constant SSA value of
0.91. This approach reduces the OMI UVER bias against the reference Brewer
data from 13.4% to 8.4%. Second approach uses daily AERONET SSA values
reducing the bias only to 11.6%. Therefore we have obtained a 37% and
12% of improvement respectively. For the spectral irradiance at 324 nm,
the OMI bias is reduced from 10.5% to 6.98% for constant SSA and to
9.03% for variable SSA. Similar results were obtained for spectral
irradiances at 305 nm, and 310 nm.
<br><br>
Contrary to what was expected, the constant SSA approach has a greater bias
reduction than variable SSA, but this is a reasonable result according to
the discussion about the reliability of SSA values. Our results reflect the
level of accuracy that may be reached at the present time in this type of
comparison, which may be considered as satisfactory taking into account the
remaining dependence on other factors. Nevertheless, improvements must be
accomplished to determine reliable absorbing aerosol properties, which
appear as a limiting factor for improving OMI retrievals
Impact of long-range transport over the Atlantic Ocean on Saharan dust optical and microphysical properties based on AERONET data
Arid regions are a major source of mineral dust aerosol.
Transport from these sources can have a great impact on aerosol climatology
in distant regions. In order to assess the impact of dust on climate we must
understand how dust properties change after long distance transport from
sources. This study addresses the changes in columnar aerosol properties when
mineral dust outbreaks from western Africa arrive over the eastern Caribbean
after transport across the Atlantic Ocean, a transit of 5â7Â days. We use
data from the NASA Aerosol Robotic Network (AERONET) located at five
Caribbean and two western Africa sites to characterize changes in columnar
aerosol properties: aerosol optical depth (AOD), size distribution, single
scattering albedo, and refractive indexes. We first characterized the local
aerosol climatology at each site and then using air mass back trajectories we
identified those days when trajectories over Caribbean sites back-tracked to
western Africa. Over the period 1996â2014 we identify 3174Â days, an average of
167Â days per year, when the air mass over the Caribbean sites could be linked
to at least one of the two western Africa sites. For 1162 of these days, AOD
data are available for the Caribbean sites as well as for the corresponding
western Africa sites about 5â7Â days earlier, when the air mass passed over
these sites. We identified dust outbreaks as those air masses yielding
AODâââ„ââ0.2 and an Ă
ngström exponent below 0.6. On this basis of
the total 1162Â days, 484Â meet the criteria for mineral dust outbreaks. We
observe that the AOD at 440ânm decreases by about 0.16 or 30â% during
transport. The volume particle size distribution shows a similar decrease in
the volume concentration, mainly in the coarse mode. The single scattering
albedo, refractive indexes, and asymmetry factor remain unchanged. The
difference in the effective radius over western Africa sites with respect to
Caribbean sites ranges between 0 and â0.3â”m. Finally we conclude
that in about half of the cases only non-spherical dust particles are present
in the atmosphere over the western Africa and Caribbean sites, while in the other
cases dust particles were mixed with other types of aerosol particles.</p
Retrieval of aerosol properties from zenith sky radiance measurements
This study explores the potential to retrieve aerosol properties
with the GRASP algorithm (Generalized Retrieval of Atmosphere and Surface
Properties) using as input measurements of zenith sky radiance (ZSR), which
are sky radiance values measured in the zenith direction, recorded at four
wavelengths by a ZEN-R52 radiometer. To this end, the ZSR measured at 440,
500, 675 and 870ânm by a ZEN-R52 (ZSRZEN), installed in Valladolid
(Spain), is employed. This instrument is calibrated by intercomparing the
signal of each channel with coincident ZSR values simulated (ZSRSIM) at
the same wavelengths with a radiative transfer model (RTM). These
simulations are carried out using the GRASP forward module as RTM and the
aerosol information from a co-located CE318 photometer belonging to
AERONET (AErosol RObotic NETwork) as input. The dark signal and the
signal dependence on temperature are characterized and included in the
calibration process. The uncertainties for each channel are quantified by an
intercomparison with a co-located CE318 photometer, obtaining lower values
for shorter wavelengths; they are between 3â% for 440ânm and 21â% for 870ânm. The
proposed inversion strategy for the aerosol retrieval using the ZSRZEN
measurements as input, i.e. so-called GRASP-ZEN, assumes the aerosol as an
external mixture of five pre-calculated aerosol types. A sensitivity
analysis is conducted using synthetic ZSRZEN measurements, pointing out
that these measurements are sensitive to aerosol load and type. It also
assesses that the retrieved aerosol optical depth (AOD) values in general
overestimate the reference ones by 0.03, 0.02, 0.02 and 0.01 for 440, 500,
675 and 870ânm, respectively. The calibrated ZSRZEN measurements, recorded
during 2.5Â years at Valladolid, are inverted by the GRASP-ZEN strategy
to retrieve some aerosol properties like AOD. The retrieved AOD shows a high
correlation with respect to independent values obtained from a co-located
AERONET CE318 photometer, with determination coefficients (r2) of
0.86, 0.85, 0.79 and 0.72 for 440, 500, 675 and 870ânm, respectively, and
finding uncertainties between 0.02 and 0.03 with respect to the AERONET
values. Finally, the retrieval of other aerosol properties, like aerosol
volume concentration for total, fine and coarse modes (VCT, VCF and VCC, respectively), is
also explored. The comparison against independent values from AERONET
presents r2 values of 0.57, 0.56 and 0.66 and uncertainties of 0.009,
0.016 and 0.02â”m3â”mâ2 for VCT, VCF and VCC, respectively.</p
Gestion dâune ressource en eau souterraine sujette aux sĂ©cheresses : analyse des stratĂ©gies dâadaptation
The management of a groundwater resource used for irrigation is a phenomenon that depends on several factors and concerning various actors (users and manager). Moreover, the resource can be subject to droughts. In this case, the management of the resource becomes a more complex problem. Adaptation to droughts is important because they can have significant impacts on agricultural activity and on the water table of the resource. This adaptation hinges crucially on the information available to the manager and the resource users. In chapters 2 and 3, we analyze the impact of an hydrological drought on the optimal management of the resource, before and after its arrival. In particular, in chapter 2, we show how the manager can adapt as good as possible to this drought according to the nature of information he has. In chapter 3, we show that taking into account strategic and dynamic interactions between the users of the ressource leads to less efficient resource use. We apply models of chapters 2 and 3 to the aquifer Western La Mancha, in Spain. In chapter 4, we study the impact of an agronomic drought on the optimal management of a farm in the area of Central Beauce, in France. We take into account hydrological, agronomic and economic informations. In particular, we analyze the impact of a dry period on the annual benefits of the farm and on the groundwater resource level. Furthermore, we study optimal strategic behavior of farmers in a dry year, whether they are subject to water restrictions or not.We show that a regulation policy is necessary to avoid the overexploitation of the ressource in a dry year.La gestion dâune ressource en eau souterraine utilisĂ©e pour lâirrigation est un phĂ©nomĂšne dĂ©pendant de plusieurs facteurs et concernant diffĂ©rents acteurs (utilisateurs et gestionnaire). En cas dâalĂ©a climatique comme la sĂ©cheresse, gĂ©rer une ressource devient un problĂšme plus complexe. Il est justement important de mieux comprendre et dâanticiper les sĂ©cheresses car ils peuvent avoir des impacts significatifs sur lâactivitĂ© Ă©conomique agricole et sur les niveaux de la ressource. Pour cela, le type dâinformation dont disposent les utilisateurs et/ou les gestionnaire est essentiel. Dans les chapitres 2 et 3, nous analysons lâimpact dâune sĂ©cheresse « hydrologique » sur la gestion optimale de la ressource, avant et aprĂšs son arrivĂ©e. Dans le chapitre 2, nous montrons comment le gestionnaire de la ressource peut sâadapter le mieux possible Ă cette sĂ©cheresse selon lâinformation dont il dispose. Dans le chapitre 3, nous montrons que la prise en compte des interactions stratĂ©giques et dynamiques entre les utilisateurs de la ressource entraĂźne une exploitation moins efficace de la ressource. Nous appliquons les modĂšles des chapitres 2 et 3 Ă lâaquifĂšre La Mancha Occidentale au Sud de lâEspagne. Dans le chapitre 4, nous analysons lâimpact dâune sĂ©cheresse « agronomique » sur la gestion optimale dâune exploitation agricole situĂ©e dans la zone de la Beauce centrale, en France. Nous prenons en compte des informations de caractĂšre hydrologique, agronomique et Ă©conomique. En particulier, nous Ă©tudions lâimpact dâune annĂ©e sĂšche sur la valeur ajoutĂ©e de lâexploitation et sur la ressource en eau utilisĂ©e. De plus, nous nous intĂ©ressons au comportement stratĂ©gique que les agriculteurs peuvent avoir en annĂ©e sĂšche, que ce soit sans ou avec restrictions des usages de lâeau. Nous montrons quâune politique de rĂ©gulation est nĂ©cessaire pour Ă©viter la surexploitation de la nappe en annĂ©e sĂšche
Partage d'une ressource en eau souterraine dans le contexte de régime shifts
National audienceWe study the occurrence of shocks in a common groundwater resource problem using a differential game. In particular, we use Rubio and Casino's adaptation of the Gisser and SĂĄnchez model where we introduce a sudden change in the dynamics of the resource, namely a decrease in the recharge rate of the aquifer. We compare the pareto optimal solution with open-loop and feedback equilibria. First, we show analytically how diffrent solutions, at the steady state, depend on the intensity of the shock. Moreover, we show that the cost and the strategic effects are decreasing functions of the intensity of the shock, i.e. that all the solutions get closer at the steady state for more intense shocks. We finally apply the game to the particular case of the Western La Mancha aquifer. The aim of this application is to estimate how shocks influence the ineffciency of open loop and feedback strategies in terms of welfare. We show that this inefficiency decreases the earlier the shock occurs or the higher the intensity of the shock