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Modeling water and nutrients fluxes in the Buyuk
Menderes drainage basin, Turkey
O. F. Durdu and V. Cvetkovic
ABSTRACT
O. F. Durdu
Water Resources Research Center,
Adnan Menderes University,
Aydin 09100,
Turkey
E-mail: odurdu@adu.edu.tr
V. Cvetkovic
Department of Land and Water Resources
Engineering,
Royal Institute of Technology,
Brinellvagen 32,
Stockholm SE-100 44,
Sweden
E-mail: vdc@kth.se
Buyuk Menderes catchment, located in the southwestern part of Turkey, is one of the most
populated river basins in Turkey with 2.5 millions inhabitants. Due to increasing activities
in agriculture and industrial sectors, water resources management in the basin is one of the
biggest matters for the future. During the past decade, it has been observed a basinwide
shift to larger monocultural, intensively operated farm units. Therefore, there is land use
conversion from native lands to agriculture. The threat of nutrients pollution, nitrogen and
phosphorus, has become a preoccupation since many lands and rivers undergo a eutrophication
process. The discharge of nutrients from Buyuk Menderes basin to the Aegean Sea through
Buyuk Menderes river also needs to be reduced in order to bring the eutrophication problems
under lasting control. In this paper, the PolFlow model embedded in PCraster is applied to
the catchment for quantifying water and substances fluxes for the five-year period, 1999–2004.
The implementation of the model in the catchment allows describing the water balance and
thus nutrient transport on the landscape surface but also through the soil and aquifer’s layers.
Modeling process is complicated by the transfer of nutrients from diffuse and point-source
emissions, managed by retention and periodic release from storages within the catchment.
Modeling diffuse and point-source nutrient emissions contribution to river loads can be
improved by better knowledge about spatial and temporal distribution of this retention and
release in the basin.
Key words | basin scale modeling, GIS, nutrient loads, water discharge
INTRODUCTION
Excess nutrient loads through drainage basins lead to
eutropication of both coastal and marine waters of the
Aegean Sea region. The EU Water Framework Directive
(WFD), the legislative framework for water management
in Europe, sets clear objectives that a good water quality
status must be achieved by 2015 and that sustainable water
use is ensured throughout Europe. Convention for the
Protection of the Mediterranean Sea against Pollution
(1976) and protocols (1980, 1982) recommends that all
joint parties should take appropriate measures to prevent,
abate and combat pollution of the Mediterranean Sea area
caused by discharges from rivers, coastal establishments or
outfalls, or emanating from any other land-based sources
within their territories. Over the past three decades, the
Aegean Sea has experienced significant water quality
problems due to eutrophication caused by extensive flows
of nutrients from point and non-point sources. Due to high
level of farm practices in Western Turkey, nutrient loads
into Aegean Sea from agricultural lands seem a major
challenge for the future. The Nutrient Reduction Action
Plan for Turkey prepared with Global Environment Facility
(GEF) suggests environmentally friendly farming practices,
such as crop rotation, integrated pest management, early
warning system and improved livestock management,
doi: 10.2166/wst.2009.013
531 Q IWA Publishing 2009 Water Science & Technology—WST | 59.3 | 2009
a good management of manure storage areas, optimum
application of organic and inorganic fertilizers, and moni-
toring and evaluation of soil and water quality.
Buyuk Menderes catchment (Figure 1), a watershed
area of 24,976km2 and 3.2% of the total area of the country,
is located in the southwestern part of the Turkey. The length
of the Buyuk Menderes river is 584km. Mean annual
precipitation in the river basin is 635mm and total mean
annual evaporation is 2,122mm (Class A pan). Precipi-
tation occurs mainly in the winters while during the
summer irrigation period there is very little rain. Buyuk
Menderes basin is a graben area containing Paleozoic
metamorphic formations consisting of gneiss, schist, cryis-
talline limestone. Most of the agricultural land in the Buyuk
Menderes basin is arable land that is drained by artificial
ditches, and irrigated during summer. The land use in the
Buyuk Menderes river basin is as follows: 40% agriculture,
45% forest and scrubland, 10% meadow and pasture, 3%
empty, 1% settlement, 1% surface water. The agricultural
economy of the basin depends on the irrigated cotton
cultivation, corn, fig and olives. Total population of the
basin is 2.5million. 37% of this population is involved in
agricultural activities. Large part of the basin depends on
the Buyuk Menderes river, draining into the Aegean Sea, for
its irrigation water supply. The coastal waters of the Aegean
Sea have been plagued by algal blooms, leading to fish kills
and unpleasant conditions for tourism. These problems
indicate that the eutrophication in the Aegean Sea have
increased over the last century as a result of the increased
inputs from the catchment. Therefore, the improvement of
the water quality in the Aegean Sea requires a reduction in
nutrient pollution in the Buyuk Menderes basin.
Several studies have addressed different aspects of
nutrient pollution in the Buyuk Menderes River basin.
Altınbas et al. (1999) analyzed nutrient inputs from
fertilizers and agricultural chemicals to the surface waters
of Buyuk Menderes basin and investigated water quality of
the Buyuk Menderes river during the irrigation period.
Considering in particular nutrient fluxes, small amount of
work has been done in the basin. For the implementation
of the Water Framework Directive, Netherlands and
Turkey made an agreement (MATRA) to investigate the
status of water quality in the basin.
The study presented in this paper differs from the above
mentioned studies, because it takes point and diffuse sources
into account, and the nutrient transport from pollution
sources to the river outlet. It presents a large scale analysis
and a first attempt to spatially model nutrient transport
within the Buyuk Menders drainage basin. The theory of
the water fluxes and nutrients transport modeling applied
in the study is essentially based on the approach of
De Wit (1999, 2001), that proposes two models to describe
both water flow and nutrients transport through the soil and
surface water (PolFlow model) as well as comprehensive
method to evaluate diffuse and punctual pollution emis-
sions at the river basin scale. This modeling approach was
successfully applied for the Rhine, Elbe, Po and Lake Peipsi-
Chudskoe basins (DeWit 2001;DeWit&Bendoricchio 2001;
Mourad & van der Perk 2004). The objectives of this study is
to: (i) construct and implement a first spatial information
database for the modeling of water fluxes and Nitrogen (N)
and Phosphorus (P) surface and subsurface transport in
the Buyuk Menderes catchment; (ii) give an overview of
the average N and P fluxes in the Buyuk Menderes basin;
and (iii) identify key parameters/processes that are most
influential for determining accurate nutrient loading.
METHODS
Modeling water flow
The approach used for modeling is based on the GIS-based
PolFlow model (De Wit 1999). The PolFlow model isFigure 1 | Buyuk Menderes Drainage Basin.
532 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
embedded in PCRaster, a raster GIS suitable for both static
and dynamic modeling. All spatial inputs have been
converted to a generic map format using a 1km2 grid cell
size (Mourad & van der Perk 2004). The water flow module
of PolFlow simulates long-term water fluxes within the
drainage basin. The module is based on three determinant
factors: (i) the long-term average total runoff or precipi-
tation surplus Q (mm/yr): Q ¼ (P 2 Ea) where P is the long-
term average annual runoff (mm/yr) and Ea is the long-term
average actual evapotranspiration (mm/yr). The denomina-
tion “long term” always refers to an annual average made
over several years (here ten years); (ii) the groundwater
recharge index Qgw/Q, where Qgw is the long-term average
total groundwater recharge (mm/yr). In this study, deep
groundwater recharge is considered, i.e. the groundwater
recharge that flow in the deeper part of the aquifer and has
a longer average residence time that the shallow ground-
water; (iii) the average groundwater residence time RTgw
(year) (Darracq et al. (2005)). The groundwater residence
times are based on Wendland (1992) and Meinardi et al.
(1994), and is described in detail in De Wit et al. (2000).
These hydrological characteristics are estimated for each
km2 in the Buyuk Menderes basin as a function of average
annual precipitation, average annual temperature, eleva-
tion, land cover, soil data, and lithology.
Nutrients transport through the soil/groundwater
system
Nutrients are assumed to follow water flow paths and the
results from the water model, therefore, build the basis for
the nutrient transport module of PolFlow (Darracq et al.
2005). The fraction of the five-year average surplus at the
soil surface (SSS) that is leached, immobilized, volatilized,
or discharged to the surface water is calculated as a func-
tion of average total runoff, groundwater recharge indices,
groundwater residence time, soil, lithology, slope, land
cover, and the type of pollutant (De Wit & Bendoricchio
2001). PolFlow model works on a five years time step.
Different parameters or factors are used to describe and
evaluate different paths of circulation for nutrients at the
surface of the soil and through the soil and groundwater
(Darracq et al. (2005)). The data used to model water fluxes
in the Buyuk Menderes basin is shown at Table 1.
Nutrients transport through the river network
The local drainage direction map (ldd map) is used to route
the nutrients through the river system. A ldd map is a
network of connected cells and developed using a digital
elevation map (DEM by USGS) by connecting each cell
(1 km2) to the lowest neighboring cell all the way down to
the outlet of the basin. Nutrients transport is governed by
a transport fraction and in each cell, there is a certain
fraction of the nutrient loss. The fraction that is transported
from a cell to its lowest neighboring cell is described as
a function of the average annual discharge (accumulated
average annual total runoff), slope, and the presence of
lakes (De Wit & Bendoricchio 2001). In case of increasing
discharge and slope, the relative loss in a cell decreases.
As shown in Figure 2, Lx21 and Lx represent the nutrient
load respectively in the cells x 2 1 and x.
Table 1 | Data used to model water fluxes in the Buyuk Menderes River Basin
Data Resolution Sources
Average annual precipitation – GLOBALSOD NOAA http://www1.ncdc.noaa.gov/pub/data/globalsod
Average annual temperature – http://www1.ncdc.noaa.gov/pub/data/globalsod
Hydrogeological map 1/250,000 DSI and MTA resources
Digital Elevation Model (DEM) 1 degree, DEM 30 arcsecond or 1/250,000
GTOPO30, US Geological Survey
Slope 1 km2 Derived from digital elevation model
Land cover 1/250,000 http://glcf.umiacs.umd.edu/data/landcover/
Soil Map 1/5,000,000 http://www.lib.berkeley.edu/EART/fao.html
Discharge data for rivers www.dsi.gov.tr
533 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
The factor tf represents the fraction of nutrients that is
transported from one cell to the downstream one. (1 2 tf)
represents the retention, loss and decay in the river. tf is a
function of the average discharge in the cell and the terrain
slope. A tf value of 1 means that there is absolutely no
retention, loss or decay in the cell (Darracq et al. (2005)).
The factor tf can be expressed as:
tf ¼ 121
1þ ðrn1:ð1000:slopeÞ þ 1ÞÞ:qrn2
d
h i
in which rn1 is the loss in the river network (s/m3); rn2 is
the loss in the river network; qd is average discharge in
the cell (m3/s); slope is derived from digital elevation model.
The data used for the analysis of nutrients fluxes in the
Buyuk Menderes basin is shown at Table 2.
RESULTS
Water flux model
The water flux model was only validated by measured
discharges on rivers in the Buyuk Menderes catchment.
For water flux, no calibration was done in the model since
none of parameter had to be calibrated. Concerning the
discharge measurements, the monthly data were available
from the EIE and DSI resources which measures dis-
charges at 15 stations (Figure 3). Those data was used to
have a value of measured annual average discharge (m3/s)
so that it could be compared with modeled values of
discharges. Total groundwater recharge index (Qgw/Q)
should have been compared to a ratio between average
annual discharge and average weekly lowest discharge.
Since weekly measurements were not available, this
comparison was not implemented. As demonstrated in
Figure 4a, the comparison between modeled and measured
discharge values indicates that the results are globally in
good agreement.
Figure 2 | Nutrient transportation through the river network.
Table 2 | Data used for the analysis of nutrient fluxes in the Buyuk Menderes River Basin
Data Resolution Reference
Population numbers Basin sub-catchments Turkish Statistical Institute
Connection rate to sewage system and WWTP Basin sub-catchments Basin municipalities
Livestock numbers Basin sub-catchments Department of Agriculture
Agricultural land use Basin cities Department of Agriculture
Crop yields Basin cities Department of Agriculture
Fertilizer use Basin sub-catchments Department of Agriculture
Industrial emissions Basin cities Local Chamber of Industry
Long term average total runoff (Q) 1 km2 Modelling the water flow
Shallow groundwater recharge index (Qgsw) 1 km2 Modelling the water flow
Deep groundwater recharge index (Qgwd) 1 km2 Modelling the water flow
Average annual discharge (qd) 1 km2 Modelling the water flow
Average residence time shallow groundwater (RTsgw) 1 km2 Modelling the water flow
Average residence time deep groundwater (RTdgw) 1 km2 Modelling the water flow
Local drainage direction map 1km2 Derived from elevation map,GTOPO30/HYDRO1K, USGS
534 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
Water flux model uses two different approaches (Wend-
land 1992; Meinardi et al. 1994) for calculating average
annual discharges (De Wit 2001). It appears that both
methods give results in the same range of values. However,
the results from Meinardi et al. (1994) approach are closer
to measured values. The results from Wendland (1992) are
slightly over estimating discharge values. Therefore, annual
average discharge values calculated byMeinardi et al. (1994)
approach were chosen to use in the following nutrients
fluxes modeling. Analysis on modeled and measured
discharge values indicates that absolute error or difference
increases with the discharge value (Figure 4). This is
reasonable because it is acceptable to make an error of
5m3/s on a 50m3/s value rather than on a 0.1m3/s value.
Analysis on relative error, ratio between discharge absolute
error and measured discharge, is demonstrated in Figure 5a.
It appears that the error on discharge lies under 25% for
most of them. The results obtained for the deep ground-
water recharge index were compared with the reference
values (Figure 5b). The comparison results indicated that
some of the values were overestimated, some of them were
underestimated, and only the half of the results were close
to the supposed real indexes values. These variations can be
explained in several ways: 1) the reference recharge indexes,
the model results are compared with, are calculated since
no direct measurements can be done to evaluate them. An
empirical ratio was used to compute them, which is rather
rough and could probably be improved; 2) the results
concerning deep groundwater recharge indexes are very
sensitive and differ a lot with a short distance, since they
depend on many parameters.
The eastern part of the catchment and the corridor
along the river bed have higher recharges in comparison to
the other part of the drainage basin. Model results point out
that the total groundwater recharge index is ranged from 0.5
and 1, except for the lakes where it is considered as zero
(Figure 6a). The deep groundwater recharge is much more
dependent on the aquifer quality. Thus, the soil and rock
characteristics are assign the distribution of the deep
groundwater recharge in the region. Figure 6b demonstrates
the deep groundwater recharge index and it is mostly
between the values of 0.15 and 0.2. The results for residence
times in shallow groundwater mainly between 0 and 5 years
Figure 4 | (a) Modelled vs. measured annual average discharge in a logarithmic scale, (b) Discharge absolute error vs. measured discharge (logarithmic scale) for each of the 15
stations.
Figure 3 | Location of monitoring stations used for water flow validation: 1-Soke,
2-Kocarlı, 3-Cine, 4-Aydın, 5-Yenipazar, 6-Kemer sonrası, 7-Nazilli, 8-Feslek,
9-Cubukdag, 10-Kızıldere, 11-Saraykoy, 12-Yenice, 13-Curuksu, 14-Adıguzel,
15-Bekilli.
535 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
indicate that the eastern part of the catchment has higher
values in comparison to the western part (Figure 6c).
The residence time in deep ground water, mainly situated
between 0 and 200 years, is also higher on the flat
landscapes in the eastern part of the basin (Figure 6d).
From above explanations, the modeled discharge values
appeared to be correct and the groundwater recharge
indexes were reasonable. Therefore, those results were
validated and used in the nutrients fluxes modeling.
Nutrients fluxes model
The nutrient flux model works on a five years time step.
Different parameters or factors are used to describe and
evaluate different paths of circulation for nutrients at the
surface of the soil and through the soil and groundwater.
Calibration for nutrient flux model was carried out to
estimate different parameters: the weighted effect of surface
runoff (sr), the maximum storage capacity of the soil (pms),
the weighted effect of groundwater recharge (gr), the loss
in the river network (rn1), the loss in the river network (rn2).
Calibration was made using measurements of total N and P
concentrations in the main rivers of the Buyuk Menderes
basin. For the sr parameter it was assumed that emissions to
the surface water and nutrient contents of the soils in the
Buyuk Menderes basin were close to those used in the
original model by De Wit (1999). Therefore the sr parameter
for N and P model was accepted as 0.00025. pms parameter
was not a local parameter and thus the value used by De
Wit (1999) could apply to the Buyuk Menderes basin since it
was pollutant specific. pms values for N and P model
chosen as 1.75 and 100, respectively. gr value, Qgw effect on
the leaching of nutrients, was increased slightly to calibrate
the model. The assigned gr value for N and P model was
230. rn1 and rn2 parameters, which govern loss, retention
and decay through the river network, appeared to be the
key for calibration. The Buyuk Menderes basin is largely
spread with alluvial farm lands, therefore, these parameters
were raised compared to values used by De Wit (1999).
The transport model for both N and P is the same except
that denitrification from soil and groundwater is taken into
consideration for N but not for P since this phenomenon
does not occur for this nutrient. As demonstrated in
Figure 7, measured N and P concentrations at 15 different
stations along the Buyuk Menderes basin are very variable
(Altınbas et al. 1999; Guven 2004; DSI data). The reason for
this is that water samples do not always stand for the
Figure 5 | (a) Discharge relative error vs. measured discharge (logarithmic scale) for each of the 15 stations, (b) Modelled groundwater recharge index vs. reference groundwater
recharge index.
Figure 6 | (a) Total groundwater recharge index, (b) Deep groundwater recharge index,
(c) Residence time in shallow ground water in years, (d) Residence time in
deep ground water in years.
536 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
average concentration of the entire river discharge at a
certain point in the river.
For each monitoring station on the 1999–2004 period,
an annual average nutrient concentration was calculated
using the monthly gross values of the measurement data.
Calculated concentration value was multiplied by the
modeled annual average water discharge at the location of
the corresponding station where measured data were not
always available. This process produces a measured or
observed average amount or load of nutrients that passes by
a station in one year. Figure 8 demonstrates total N and P
loads in the river network for the period of 1999–2004.
As shown in the figures, there is an overestimation of the
nutrient loads in the Upper Buyuk Menderes basin (Yenice
Reg., Adıguzel, Bekilli, Curuksu stations) which is especially
visible concerning phosphorus. This might be because of
the transmission factor tf from one cell to the downstream
one (during the transport process) is higher in the east part
of Buyuk Menderes basin than the west part. The reason
for that is that tf depends among other factor on the slope,
which is more important in the east region. In turn a
higher transmission factor leads to less retention and more
nutrients flowing into the streams. The results were
however considered as satisfactory and validated since
they reflected the measurements at most of the monitoring
stations.
In the Middle Buyuk Menderes basin (Nazilli, Feslek,
Kızıldere, Saraykoy, Cubukdag stations), there are signifi-
cant differences between modeled and observed values of
the nutrient loads, which is more prominent in nitrogen.
This might partly be due to fact that the city of Denizli and
surrounding cities have high population and great industrial
sectors and none of those cities have an active waste water
treatment plant. The estimation of point sources has been
Figure 7 | (a) Average measured N concentrations along the Buyuk Menderes river basin, (b) Average measured P concentrations along the Buyuk Menderes river basin.
Figure 8 | (a) Modelled vs measured N load (T. year21) in a logarithmic scale, (b) Modelled vs measured P load (T. year21) in a logarithmic scale.
537 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
based on average relations between emission and popu-
lation and waste water treatment connection and efficiency
rates, which gave reasonable results in the past for densely
populated areas (De Wit 1999; De Wit 2001; Mourad & van
der Perk 2004). Moreover, Kızıldere geothermal power
plant, which produces 57–80MW/year of electricity and
discharges 6million tones of wastewater annually to the
Greater Menderes river, is one of the main direct source of
nutrient pollution of the middle part of the basin. Discharge
of the wastewater into the river causes both ecological and
environmental problems because the discharge temperature
of the water is approximately 1408C (Durdu 2005). Due to
discharges from power plant and other thermal spring into
the river, there are indicative differences between measured
and modeled N and P loads. Figure 8a indicates that the
total N load in the Lower Buyuk Menderes Basin (Nazilli,
Aydın, Soke stations) is underestimated with PolFlow.
The reason is that there are extensive agricultural practices
in this region. Especially chemical pollution by pesticides
and fertilizers, conversion of native and forest lands to
agriculture and soil loss due to agricultural practices are
main diffuse sources of pollution. Figure 8b shows that the
total P loads at each sampling locations are underestimated.
Figure 9 is the map of nutrient loads and it appears that
nutrients accumulate along rivers resulting in an increasing
load moving to the downstream direction.
DISCUSSION
During the past decade, it has become more and more
obvious that environmental research has to consider how to
upscale results from the field scale to a larger scale in order
to enforce local environmental planning. Recent attention
has therefore focused on how to combine simple empirical
and more process-oriented models for hydrology, diffuse
nutrient losses and nutrient turnover in integrated models
(Kronvang et al. 1999). Integrating with a GIS environment,
such models can be significant tools for environmental
management, monitoring and research since they provide
insight into the coupling between the location and devel-
opment of deriving forces and pressures that determine
nutrient loss within a catchment and the resulting environ-
mental impact. The model demonstrated in this study linked
PolFlow model with PCRaster environment for diffuse
nutrient loss to surface water and nutrient turnover in
stream channels and riparian areas with a hydrodynamic
model and a GIS platform.
This study indicated that the distribution characteristics
of N in the Buyuk Menderes river were similar. The
concentration increased in the middle reaches and kept
relatively lower in the upper reaches (Figure 7a). This
distribution pattern of N in the Buyuk Menderes river was
mainly due to the differences in human activities. There is
limited anthropogenic disturbance in the upper reaches due
to the low population density (1–20 people per square
kilometer). In the middle reaches of Buyuk Menderes basin,
population density increases to 20–100 people per square
kilometer, which results in intensive anthropogenic disturb-
ance, through industrial and agricultural production, stock-
breeding, municipal swage, etc. Also a geothermal power
plant near Saraykoy (station 12) and geothermal touristy
hotels discharge their wastewater into the river. Therefore,
N concentrations increased dramatically in the middle
reaches. In the upper and lower reaches, N concentrations
are relatively low due to less differences in anthropogenic
disturbance. This results are also consistent with the Aydın
& Denizli Environmental Case Reports (2006). The study
also showed that the N concentrations in the tributaries
such as stations 3, 7 and 13, were generally clearly lower
than those in the mainstream water.
The distribution of P was different from N in the Buyuk
Menderes basin. In the middle and lower reaches, P
concentrations remained somewhat stable due to sus-
pended matter content (Figure 7b). Dissolved P is easily
adsorbed by suspended matter. The adsorption quantity is
depended to the physical and chemical properties of
suspended matter. In natural waters, P exists almost
exclusively in the form of phosphate ion. Therefore, P is
Figure 9 | (a) Annual modeled N load in the Buyuk Menderes basin in kilos for the
years 1999–2004 and river network, (b) Annual modeled P load in the
Buyuk Menderes basin in kilos for the years 1999–2004 and river network.
538 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
almost always in the particulate phase (Shen & Liu 2008).
The suspended matter in the middle and lower reaches of
the Buyuk Menderes river mainly comes from the upper
reaches and its tributaries. P concentrations in the Buyuk
Menderes river was related to the large amounts of soil
washed into the river by heavy rain, catching high
concentration of P in the form of particulate. With the
increasing runoff in the Buyuk Menderes river from the
upper reaches to lower reaches, the concentrations of N
and P also increased. This pattern in the lower reaches,
however is more obvious than in the middle and upper
reaches (Figure 10a, b). The continuous import of N and
P from the precipitation and surface water to the river
arises mainly from agricultural non-point sources. This
fact also supported by the Aydın & Denizli Environmental
Case Reports (2006).
In the Buyuk Menderes main stream, there are correla-
tions between N and runoff (Figure 10a). However, if we
consider the correlation in the lower reaches, we could find
that are clear positive linear correlation between the
concentrations of N and main stream runoff (r 2 ¼ 0.72,
p , 0.01). The difference in the correlations between N
concentration and runoff in the upper, middle and lower
reaches is related to the N sources of the Buyuk Menderes
river. N in the upper and lower reaches of the river water
mainly come from agricultural fertilizer, and soil erosion
etc. This fact also supported by Boyacioglu & Boyacioglu
(2007). Buyuk Menderes basin is a large agricultural region
where agricultural activities comprise the primary economic
activity. Increase of N concentration in the Buyuk
Menderes river has led to the increase N concentration in
the sea areas adjacent to the Buyuk Menderes estuarine.
This has resulted in abnormal nutrient ratios and important
changes in the ecological environment. There are significant
difference in scale and nature of human activities along
the middle reaches, which lead to the dramatic difference in
non-point sources N imported into the river. In the upper
reaches, however, the scale and nature of human activities
have little difference, the N concentration in the river
seems relatively low. There are no correlations between P
concentrations and runoff in the Buyuk Menderes main
stream, which shows that P concentration is mainly con-
trolled by suspended matter rather than runoff (Figure 10b).
Coastal eutrophication is becoming more and more
serious and tending to increase in the Aegean Sea adjacent
to the estuary of the Buyuk Menderes river, which has
become popular site for harmful algal bloom. Such trends
are presumably related to the nutrients transported by the
Buyuk Menderes river. It is important to construct a
possible dam to reduce the sediment load into the estuary
and improve the transparence of seawater, due to settle-
ment of suspended matter up to 60–70% in the reservoir.
However, it could be expected that, with a construction of a
possible dam, population increase and rapid economic
development in the Buyuk Menderes catchment, more
ecological and environmental issues might arise in the
estuary.
The model employed in this study aims to predict long-
term nutrient loads from a watershed. The mean absolute
(%) error of estimations of yearly nitrogen and phosphate
loads during the years 1999–2004 is 28% and 14% and
the corresponding r 2 values is 0.77 and 0.85, respectively.
Considering the simplicity of the model and the defective-
ness of input data this can be considered a quite good result.
Figure 10 | (a) The relationships between N concentrations and the river runoff r 2 ¼ 0.254, p , 0.01; (b) The relationships between P concentrations and the river runoff r 2 ¼ 0.146,
p , 0.01.
539 O. F. Durdu and V. Cvetkovic | Modeling water and nutrients fluxes Water Science & Technology—WST | 59.3 | 2009
During the validation steps, the model underestimates
nitrogen loads by approximately 28% even though stream-
flow is quite well predicted during the study period
(Figure 8a). This is probable since the use of fertilizers
dropped dramatically in the beginning of the 200000s due
to the economical problems in Turkey. The model, as
implemented, produces good results for prediction of
phosphorus loads. The modeled phosphorus loads are
about 15% less than observed loads (Figure 8b). A possible
cause to this underestimation could be an underestimation
of phosphorus loads from erosion. The erosion factor,
the rainfall erosivity and the sediment delivery ratio are
dependent on parameters such as agricultural management
practices (Wallin 2005). Since no such data are available,
the chosen value is a very rough approximation. A more
probable explanation for the underestimation of phos-
phorus loads is that the observed values of point source
phosphorus loads in the basin are less than the actual values
and that phosphorus from anthropogenic sources. Water
from the Buyuk Menderes river basin is the main con-
tributor to the Aegean Sea region. The lack of sewage
treatment facilities in the basin contributes to very large
input of phosphorus to the sea.
The quality of data is of major importance for the result
of any modelling. In the Buyuk Menderes basin it is difficult
to find reliable data. In order for modelling results of
nutrient flows from the entire Aegean Sea basin to be
trustworthy for management and future scenario building,
more reliable data are therefore needed. It is important that
such data are easily accessed and homogenous in order to
support sustainable water management in the Aegean Sea
region. The current model helps to identify what data may
be most needed and at what locations.
CONCLUSIONS
A quantitative description of the water balance and nutrient
loads consistent with observations within the Buyuk
Menderes basin for the years 1999–2004 was illustrated
in this study. PolFlow model embedded in PCraster is
applied to the basin for quantifying water and substances
fluxes for the five year period. PolFlow has been developed
and calibrated for the Rhine basin, and it appeared also to
be applicable for the Buyuk Menderes basin. Results from
the study indicate that PolFlow model constitutes a
relatively simple and robust approach to quantifying water
flow and nutrient transport.
The integration of both surface and groundwater
transport within the same model is another asset that
gives also value to this approach. GIS based environment
makes the model outputs easier to analyze.
The modelled nutrient fluxes for the Buyuk Menderes
basin show a reasonable agreement with measured values.
Because of huge industrial sectors, thermal water tourism
and population, nutrient loads in the middle part of the
basin are increasing. In the lower part of the basin, there is
general perception that the emissions from point sources
have been remarkably stable, agriculture has become
the main source of nutrient pollution in the basin rivers.
Several aspects might however been highlighted as points
that could be improved and investigation that could be
carried out further. The quality, accuracy, and resolution of
the input data should be improved. In order to evaluate
better the consequences of climate change, population
increase, and agricultural practices on the nutrient load in
the Buyuk Menderes basin, several alternative scenarios
can now be developed using the presented modeling
methodology.
ACKNOWLEDGEMENTS
The research described in this paper was supported by the
Swedish-Turkish scholarships for European Studies, as part
of the Swedish Institute (SI) exchange program.
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