Models - Repositories of Knowledge(Proceedings ModelCARE2011 held at Leipzig, Germany, in September 2011) (IAHS Publ. 3XX, 201X).

1

CLIMATE AND LAND USE CHANGE IMPACTS ON
GROUNDWATER RECHARGEBERTRAND LETERME1 & DIRK MALLANTS1
1 Performance Assessment Unit, SCK•CEN, Boeretang 200, 2400 Mol, Belgium
bleterme@sckcen.be

Abstract We show the effects of both climate and land use changes on long term groundwater recharge. The
study was conducted in the context of a safety assessment of a near-surface disposal facility for low andintermediate level waste; this includes estimating groundwater recharge for the next millenia. Climatechange impact on groundwater recharge was simulated using HYDRUS-1D and weather time series fromso-called analogue stations. Results showed that transition to a warmer climate is expected to yield adecrease in groundwater recharge. For land use change impact on groundwater recharge in the Netecatchment, conversion to crop (maize) and coniferous forest resulted in respectively the highest positive(recharge increase by 31%) and negative (recharge decrease by 42%) sensitivities. Further improvements ofthe method may consider correlation and feedback between combined land use change and climate change.
Key words groundwater recharge; climate change; land use change, Nete catchment

INTRODUCTION
The Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS)
aims to develop a surface disposal facility for low and intermediate level waste (L/ILW) in Dessel(north-eastern Belgium). In the context of long-term safety assessment ot the facility, groundwater
recharge is a key factor determining the dispersion and dilution of radionuclides in theenvironment. Climate change may lead to modifications of groundwater recharge or groundwaterlevel thus affecting the dispersion and dilution of radionuclides outside the L/ILW facility. Landuse conversions may affect the atmosphere-vegetation-soil water balance in the vicinity of the sitethus changing the amount of groundwater recharge and possibly the magnitude ofdispersion/dilution.
The objective of this study is to provide estimates for the next few millennia of groundwater
recharge in the vicinity of the Dessel site, characterized by a current maritime temperate climatethat will gradually evolve to a warmer subtropical climate, possibly combined with climate-driven
land use changes. In this respect, the relative importance of climate and land use changes on theregional groundwater recharge needs to be assessed.
MATERIAL AND METHODS
Study area
The study area is the Nete catchment (1673 km²), in the North-East of Belgium. The topography isrelatively flat (from 2 m.a.s.l. in the west to 67 m.a.s.l. in the east). Dominant land uses, as visible
in Fig. 1, are cropland (37%), grassland (22%), coniferous (12%) and deciduous (9%) forests andbuilt areas (16%). The present study considered the first four of these, thus representing 80% of
the study area. Maize is the dominant crop in the area (Mestbank, 2011) and was taken as therepresentative cover for cropland (crop rotation schemes were not considered).
Within the Nete catchment boundaries, the groundwater flow originates mostly from theatmospheric infiltration and to a lesser extent from external sources (navigation canals across thecatchment boundaries). Infiltrated water travels towards groundwater sinks i.e. mainly drains andrivers. The influence of two rivers, the (northern) Kleine Nete and the (southern) Grote Nete,reaches down to the Boom Clay aquitard. The latter is considered the bottom boundary of thecatchment groundwater system (outcropping in the south-west to ~270 m below surface in the
Copyright  201X IAHS Press

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Leterme & Mallants

north-east, and even deeper east of the boundary of the Rur graben; Beerten et al., 2010).

Fig. 1 Land use in the Nete catchment (source: Landsat 2001 supervised classification; AGIV, 2011).

Climate change
The following climate states (Trewartha, 1968) are considered in the evaluation of the evolution ofclimate for the Dessel site: DO (maritime temperate, present-day climate in Dessel), Cs/Cr(subtropical with dry summers/no rainfall seasonality), EO (cold without permafrost) and FT (coldwith permafrost). The reference evolution of climate adopted here (assuming a scenario of highanthropogenic CO2 increase; BIOCLIM, 2003) foresees a Cs/Cr climate for the next ~150 000
years. In alternative sequences of climate states, a colder climate state EO or FT will not occur
before ~50 000 AP (Leterme et al., 2011).
Different approaches exist to generate weather time series of future climate conditions. A widelyused approach consists of applying dynamical or statistical downscaling to the output of global
climate model simulations. Similarly, time series can be created using weather generators
conditioned on site-specific weather statistics. An alternative approach, already used in several
long-term performance assessment studies (Palutikof & Goodess, 1991; Bechtel, 2004), implies
the use of so-called analogue stations. In this approach, time series are obtained frommeteorological data of analogue stations selected to minimize differences due to latitude/longitude
effects on insolation and oceanic/continental influences compared to the reference site.
Criteria for the selection of analogue stations included elevation (<150 m.a.s.l.) and the distance tothe nearest source of moisture or shoreline (<120 km taking dominant wind direction into
account). Analogue stations located in the Atlantic circulation system (Northern hemisphere) werepreferred. Potential analogue stations were ranked based on their deviation from the annual
average temperature and precipitation. The two stations with the smallest deviation from theannual averages were selected as reference stations. The two stations with the highest and lowest
precipitation regimes were also selected to represent bounding cases. This allowed to investigate
as much as possible the existing variability in precipitation, temperature and other climateparameters within a climate class and its influence on soil infiltration and groundwater recharge.
In this paper, only the results of Dessel (DO), Gijon (Cr) and Sisimiut (FT) analogue stations will
be presented. For the Cs/Cr climate state, Gijon was chosen as the most appropriate to estimate

Climate and land use impacts on groundwater recharge

3

future groundwater recharge rate, because its precipitation record (about +5% compared to Dessel)
is more in accordance with the predicted increase for the near future (median +9% on annual total;
IPCC, 2007). Sisimiut was chosen to be the representative analogue of both EO and FT colder
climate states, for being drier (geological records of the last glacial period suggest thatprecipitation was much lower than today) but not the most extreme case in the FT climate class;and because the EO class shows a too large variability in groundwater recharge rates to allow adefensible designation of any representative EO station (Leterme et al., 2011). Climatographs forthe three analogue stations selected are presented in Fig. 2.
CURRENT
Dessel, Belgium (DO)

(mm)
150

(°C)
30

Precip
Temp

(mm)
150

Precip
Temp

100

15

50

0

0

-15

WARMER Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Gijon, Spain (Cr)(°C)
COLDER
Sisimiut, Greenland (FT)
(mm)
30
150
Precip

100

15

50

0

0

-15
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(°C)
30

Temp

100

15

50

0

0

-15
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Fig. 2 Climatographs of Dessel (DO), Gijon (Cr) and Sisimiut (FT) analogue stations.

Land use change
The sensitivity of groundwater recharge to land use change was estimated by arbitrarily converting
the whole study area to a given land use. No detailed (more realistic) scenarios of land use change
were assessed. Such scenarios are discussed for example in Dams et al. (2008) in the Kleine Netecatchment (approximately the northern half of the Nete catchment) but were limited to the next 20
years. The four scenarios of land use change in the latter study all predict a decrease in agriculturalland use (from –8 to –18% between 2000 and 2020) and a significant increase in urban land use(+14 to +58%). The present study aimed to assess the impact of more drastic changes on the longterm and therefore only the sensitivity to a complete conversion of land use was assessed as asensitivity measure. This indicator was also calculated by Dams et al. (2008).
Potential evapotranspiration and throughfall
Time series (between 20 and 30 years depending on data availability) of daily meteorologicalobservations were used to derive potential evapotranspiration (ET) for the climate analogue
stations. Potential ET was calculated using Penman-Monteith equation (as outlined in theguidelines of Allen et al., 1998) implemented in the REF-ET software.
Then, the daily interception, throughfall, evaporation of intercepted water, potential evaporationand potential transpiration are calculated in an Excel worksheet (see description in Jacques et al.,2011). The daily variability of vegetation parameters is shown in Fig. 3

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Leterme & Mallants
8

1.2

Grass

1.0

5

Coniferous

0.8

4

Deciduous

LAI (-)

6

Kc (-)

Maize

7

3
2
1

Maize

0.6

Grass

0.4

Coniferous

0.2

Deciduous

0.0

0
1

74

147
220
Day of year

293

1

366

74

0.6
4

0.5

2

0.3 Maize

0.2
1

Lm (cm)

0.4

k (-)

wc (mm)3

Grass

Coniferous

0.1 Deciduous
0.0

0
1

74

147
220
Day of year

293

366

1

74

147
220
Day of year

293

366

147
220
Day of year

200
180
160
140
120
100
80
60
40
20
0

293

366

Maize

Maize

Grass

Grass

Coniferous

Coniferous

Deciduous

Deciduous

1

74

147
220
Day of year

293

366

Fig. 3 Daily variability of vegetation parameters used for the modelling of potential evapotranspirationand groundwater recharge. LAI: leaf area index (grass: Jacques et al. (2011), coniferous: Vincke &Thiry (2008), deciduous: Lawrence (pers.comm.) based on Lawrence & Slingo (2004)); Kc: crop
coefficient (maize: Allen et al. (1998), coniferous: Meiresonne et al. (2003) based on GellensMeulenberghs & Gellens (1992), deciduous: City of Riverside Planning Department (1994) cited in
Amador et al. (2007)); wc: interception capacity (maize: wc (mm) = LAI×0.5 so that mean interception
fraction (fint) is equal to 0.10, grass: wc = 0.55 mm so that fint = 0.15 (Meyus et al., 2004), coniferous:
Breuer et al. (2003), deciduous: wc (mm) = LAI+0.2 so that mean fint = 0.15 (Breuer et al., 2003)); k:light extinction coefficient (maize: Flénet et al. (1996), grass: Jacques et al. (2011), coniferous anddeciduous: Granier et al. (1999); Lm: maximum root depth (grass: Jacques et al. (2011), coniferous:
Vincke & Thiry (2008), deciduous: Breuer et al. (2003)).

Groundwater recharge modelling
Regional groundwater recharge is calculated by coupling the Hydrus-1D model (Šimůnek et al.,2005) with a Geographical Information System (GIS). Climate-dependent time series ofthroughfall and potential evapotranspiration are used as top boundary condition. Soil hydraulicproperties are combined with root depth parameters for a given land use (cropland, meadow,
coniferous and deciduous forests) and the net downwards flow is taken as model output forgroundwater recharge. Maximum root depth over time is shown in Fig. 3. Root distributions forthe different vegetation types were: linear for maize and grass, derived from observations (Vincke& Thiry (2008) for coniferous forest, and derived from the function Y = 1–βd (d is the depth in cm)
with β = 0.966 (Jackson et al., 1996) for deciduous forest.
Simulations were performed on sandy soils (podzol) characteristic of the study area. Three-meterdeep soil profiles of the Zcg (6 horizons) and Zeg soil series (5 horizons) were used. A constant 1m deep groundwater table was taken as bottom BC for the calculations under current climate
conditions. Observations under a pine forest stand in the Dessel area (Vincke & Thiry, 2008) andsimulations indicate that the aquifer can supply considerable water for ET. However, the validityof a constant 1-m deep water table as bottom BC is highly questionable under different climate
conditions. Simulations for the warmer Cs/Cr climate showed that a groundwater table at 1 m led
to negative annual drainage (i.e. upward flow from groundwater) under a precipitation deficit(compared to potential ET) because groundwater then acted as a supplier for ET. A consequenceof the high ET demand would be a drop in groundwater level. However, with the constant BC this

Climate and land use impacts on groundwater recharge

5will never happen.
An alternative is to use the deep drainage BC implemented in HYDRUS-1D, for which discharge
rate q(n) at the bottom of the soil profile at node n is defined as:(1)q(n)  q(h)  Aqh exp Bqh  h  GWL0L





-1

where q(h) is the discharge rate (L.T ), h (L) is the pressure head at the bottom of the soil profile,Aqh (L.T-1) and Bqh (L-1) are empirical parameters and GWL0L (L) is the reference (initial)
groundwater depth (L). In this case vertical drainage across the lower boundary of the soil profile
is approximated by a flux which depends on the position of the groundwater level (Hopmans &Stricker, 1989). Parameters for Eq. (1) were obtained from a series of measured groundwaterlevels and corresponding calculated fluxes (Leterme et al., 2011).
RESULTS AND DISCUSSION
Sensitivity to climate change
Table 1 shows the simulation results for Dessel (DO), Gijon (Cs/Cr) and Sisimiut (EO/FT)
analogue stations. Current groundwater recharge in the Nete catchment calculated for Dessel
conditions gives a mean annual value of 391 mm. A decrease of groundwater recharge is simulated
under (warmer) Gijon climate, due to higher potential ET combined with the presence ofgroundwater at a depth of 3 m (on average for Gijon) allowing limited water supply to planttranspiration.
Table 1 Mean annual groundwater recharge (mm) per land use and percentage of annual precipitation.

Crop (maize)
Meadow (grass)
Coniferous forest
Deciduous forest
Nete catchment

Dessel (current DO)
P = 899 mm
495 (55%)
307 (34%)239 (27%)375 (42%)391

Gijon (warmer Cs/Cr)
P = 947 mm
473 (50%)276 (29%)
211 (22%)315 (33%)
361

Sisimiut (colder EO/FT)P = 306 mm
128 (42%)96 (31%)73 (24%)104 (34%)
108

Sensitivity to land use change
Using the results from Table 1, the sensitivity of groundwater recharge to the conversion of thewhole Nete catchment into a single land use can easily be estimated. Table 2 shows thatconversion of the entire catchment to crop (maize) land use would increase groundwater recharge,
while conversion to other land uses would decrease it. Although crop (maize) is the dominant land
use in the study area, groundwater recharge sensitivity is high for this land use. The main reason isthe absence of a vegetation cover in winter (i.e. during relatively high precipitation and low ET0
period), which results in a higher recharge.
Table 2 Sensitivity of groundwater recharge to land use change in the Nete catchment. The sensitivity,
given in parenthesis as a percentage, is calculated by assuming a complete conversion of the Netecatchment to one land use for a given climate state.

Climate state
Dessel (DO)Gijon (Cs/Cr)
Sisimiut (EO/FT)

Mean annual groundwater recharge in Nete catchment (mm)Current land Crop (maize)
Meadow
Coniferous
Deciduoususe(grass)forestforest391
495 (+26%)
307 (–21%)239 (–39%)
375 (–4%)361
473 (+31%)
276 (–24%)
211 (–42%) 315 (–13%)
108
128 (+18%)
96 (–11%)73 (–33%)
104 (–4%)

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Leterme & Mallants

Conversion to coniferous forest would result in the strongest decrease of groundwater recharge
under all climate states.
Using the same indicator, Dams et al. (2008) found the following sensitivity values: +12% fordeciduous forest, +9% for wet meadow +5% for meadow and agriculture 5%, and +1% for
coniferous forest. The sensitivity was negative for industry (–19%) and urban (–24%) land uses.
These values are given only as an indication and not for comparison because Dams et al. (2008)
considered different land use categories and different modelling assumptions than in the present
study.
In the present study, not taking built areas into account has the consequence that groundwater
recharge is probably overestimated at the catchment scale. Therefore the absolute sensitivity of theconversion to a given land use is maybe biased, but qualitatively the relative sensitivity of the land
uses analysed here would not change.
CONCLUSION
Climatic analogues provided robust estimations of future (long term) climatological parameters
needed for groundwater recharge estimation, even though considerable uncertainty is unavoidable
given the time scale considered. Although no optimal choice of analogue stations may exist due todifferent weather circulation systems (e.g. Gijon vs. Dessel), this approach allows to include
variations of all meteorological parameters in a transparent and simple way. This study suggested
that transition to a warmer climate by AD 2100 (IPCC projection) is expected to yield a decreasein groundwater recharge (–9%) in the Nete catchment.
Land use changes can also significantly affect the regional balance. A diminution of agriculturalareas (as in the scenarios analysed by Dams et al., 2008) could further reduce recharge. Among thefour land use classes included in the present analysis, crop (maize) and coniferous forest provided
respectively the highest positive (recharge increase) and negative (recharge decrease) sensitivities.
On the time scale considered, it is important to take into account the depenency between climate
and land use. Furthermore, it is expected that climate change will have a significant impact on landuse. Also, some combinations of land use and climate are unrealistic or will not occupy large areasof land and some vegetation parameters of a given land use may depend on climate variables.
Therefore, effective approaches to estimate long-term regional groundwater recharge need toconsider both climate and land use changes while accounting for such dependencies.
Finally, coupling HYDRUS-1D model with the available groundwater model (Gedeon & Mallants,2010) instead of using the deep drainage boundary condition could better describe the interactions
between the unsaturated zone and groundwater.
ACKNOWLEDGEMENTS
This work has been performed as part of the project on disposal of category A waste – short-lived
low and intermediate level waste (LILW-SL) – that is carried out by ONDRAF/NIRAS, theBelgian Agency for Radioactive Waste and enriched Fissile Materials.
The authors wish also to acknowledge the Royal Meteorological Institute (Belgium), the Spanish
Meteorology Agency (Agencia Estatal de Meteorologia, Ministerio de Medio Ambiente y Medio
Rural y Marino), the Danish Meteorological Institute (Danmarks Meteorologiske Institut) and theNorwegian Meteorological Institute (Meteorologisk Institutt) for providing weather observationdata.

Climate and land use impacts on groundwater recharge

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