In this study we present an integrative modeling framework aimed at assessing
the balance between water demand and availability and its spatial and
temporal variability over long time periods. The model was developed and
tested over the period 1971–2009 in the Hérault
(2500 km
In recent decades, climatic and anthropogenic pressures on water resources
have increased in many regions of the world. According to projections of
climate and socioeconomic changes, midlatitude areas could experience
increased water stress in the course of the 21st century
Many authors have underlined the need for integrative assessments of the
impacts of climate change on the balance between water demand and
availability (e.g.,
Studies that compare water demand and availability at the basin scale rarely
fully incorporate anthropogenic and climatic drivers. The studies generally
focus on simulating water resources (i.e., surface streamflow and/or
groundwater levels) under varying climatic conditions. In some cases, water
uses are dealt with by looking at population changes, without quantifying the
corresponding water demand (i.e., the volumes associated;
In some studies, water demand is simulated based on anthropogenic and
climatic drivers and is compared to water availability based on climatic
variability and water management rules
In this mindset, interactions between water demands and water resources
should also be accounted for. Notably, few studies have actually simulated
natural streamflow separately from influenced streamflow. In some cases,
hydrological models are calibrated on observed (i.e., influenced) streamflow
data, and the simulated streamflow is compared to water demand to assess
water stress
The balance between water demand and availability can be represented by using
indicators.
To be able to adapt effectively to future climatic and anthropogenic changes,
we need to understand the interactions between the different drivers leading
to water shortage or excessive pressure on water resources. Our review of the
literature underlined a lack of integration of anthropogenic and climatic
drivers in most modeling studies dealing with the balance between water
demand and availability in river basins, i.e., at the scale of water
management plans. River basins need to be considered as hydrosystems
(i.e., systems made of water and the associated aquatic environments within a
delimited geographical entity) that fully incorporate the different water
uses and the influence that these uses may have on water resources, including
storage and supply facilities. The need to better understand and represent
interactions between resources and demand and to account for the variability
of anthropogenic and climatic drivers is particularly pronounced in the
Mediterranean region, which faces significant climatic variability, rapid
population growth and economic development, and increasing competition
between different water uses
Before simulating changes in water demand satisfaction under prospective
water use and climate scenarios, we need to show that the modeling approach
used is able to represent past variations in demand and availability, in
space and over time. Also, giving historical context and explaining the past
variations in water demand satisfaction can help us understand the vulnerability
of hydrosystems to climatic and anthropogenic changes and design appropriate
adaptation strategies. The aim of this study was thus to (i) combine
socioeconomic and hydro-climatic data in an integrative modeling framework to
represent water stress and its spatial and temporal variations over a past
multi-decadal period, and (ii) use this framework and appropriate indicators
to assess the sustainability of current water uses. The integrative modeling
framework was developed and applied in two contrasting Mediterranean
catchments facing increasing climatic and anthropogenic pressures: the
Hérault Basin (2500 km
The Hérault and the Ebro catchments differ in their geographical
characteristics. The Hérault Basin is located in the South of France
(Fig. 1a). It is bordered in the north by the Cevennes Mountains. The Hérault
River flows 150
The Ebro Basin is located in the north of Spain (Fig. 1b). The Ebro River
flows 910
Daily climate forcings for the Hérault Basin for the period 1969–2009 were
extracted from the SAFRAN meteorological analysis system, an
8
Daily streamflow data were extracted from the French Ministry of Ecology and
Sustainable Development's database Banque Hydro
Population data were provided by national statistics institutes (INSEE for
France and INE for Spain), and information concerning irrigated areas and
crop dynamics was extracted from agricultural censuses. The efficiency of
water supply networks and of irrigation systems and data relating to unit
water consumption and industrial activities were provided by the local water
management agencies (adapted from
The Hérault catchment is characterized by a Mediterranean climate influenced
by the Cevennes Mountain range, with mild wet winters and hot dry summers.
The average seasonal temperature in the period 1971–2009 ranged from
6
Mean annual streamflow was 36
The period 1971–2009 comprises significant climate variability: statistical
breaks in temperature and discharge series were detected in both basins
around the year 1980. Temperature increased by 1
Figure 1 shows the main anthropogenic pressures on water resources in the
Hérault and the Ebro Basins. In the Hérault Basin, the north differs
significantly from the south, with low population density and sparse
agricultural areas in the north and a high concentration of urban and
agricultural areas in the south (Fig. 1a). Water demands for agricultural and
urban use inside and outside the basin amount to comparable volumes, i.e.,
40
The Ebro is a complex and highly regulated hydrosystem with a total of 234
dams, amounting to a storage capacity of 60 % of total runoff
In both basins, hot dry conditions in summer lead to a peak in irrigation
water demand associated with low flows. Since the 1970s, the population has
doubled in the Hérault Basin and irrigated areas have increased by 30 % in the
Ebro Basin. Increasing demand and drier conditions have led to water shortage
events in both basins. The EU Water Framework Directive
General framework for the integrative modeling chain developed and applied to the Hérault and the Ebro basins at a 10-day time step over the 1971–2009 period. Water demand and natural streamflow were simulated based on climatic and anthropogenic drivers. Anthropogenic influence on streamflow was assessed through the simulation of demand-driven dam management and consumptive use. Water stress was assessed by comparing demand to availability and characterized through the use of indicators.
A modeling framework including hydro-climatic and anthropogenic dynamics and accounting for interactions between water resources and water demand at a 10-day time step was designed (Fig. 2). Climatic variability affects natural streamflow and water demand (mainly through crop irrigation requirements), while human activities affect water demand through population growth, industrial activity, irrigated areas and the types of irrigated crops, and the efficiency of the water supply networks. Water demand is defined as the amount of water that users would withdraw without restrictions, i.e., the withdrawals that would enable users to have access to optimal amounts of water considering the efficiency of supply networks and irrigation techniques. Anthropogenic drivers of water balance include local water management rules through the operation of dams and canals, which affect water availability. These climatic and anthropogenic drivers were distributed and combined dynamically in space and over time to evaluate changes in water stress.
At each demand node, water demand is compared to water availability (based on streamflow and reservoir levels). If water availability is equal to or higher than water demand, then water withdrawals are equal to water demand for all types of demand. If water availability is lower than water demand, then restrictions are applied to limit withdrawals. According to the order of priority defined locally, restrictions are first applied to agricultural water demand (AWD), then to other water demands (OWD) and lastly to urban water demand (UWD). Water shortage is calculated through the difference between water demand and effective water withdrawal. Only a part of the water withdrawn is actually used, the rest is considered to return to the sub-basin outlet as return flow. The quantification of consumptive use and return flows is explained in Sect. 3.3.2. Natural streamflow is thus modified by dam management, water withdrawals and return flows.
This integrative modeling framework was applied to a long period of time to capture past variability in climatic and anthropogenic drivers. The 1971–2009 period was chosen based on data availability in both basins. This nearly 40-year period includes a wetter and colder decade (1971–1980) and a warmer and drier period (1981–2009). Calibration and validation of the hydrological model was performed over natural streamflow data (see Sect. 3.2.2.), while the simulation of water demand could not be thoroughly validated for lack of data. The simulation of influenced streamflow was validated against observed streamflow data at each resource node (see Sect. 3.3.2.).
The spatial distribution of water demand and availability was mapped to
correctly reproduce the spatial heterogeneity of water shortage in each study
area: water stress assessments can vary depending on the spatial or temporal
scale
Maps of the main physical and human spatial characteristics of the
two basins:
The map of the Hérault hydrosystem (Fig. 3a) was adapted from the work of
Mapping the Ebro hydrosystem was mainly guided by the existence of extensive irrigation systems managed in association with large storage dams (see black triangles in Fig. 3b). The Pyrenean catchments influenced by a snowmelt hydrological regime were selected to accurately represent the corresponding freshwater availability. The two main right bank systems were also selected, as they are representative of the heterogeneous climatic and hydrological conditions in the Ebro Basin.
Three types of water demand were considered: (i) UWD,
including domestic water consumption, irrigation of parks and gardens and
commercial water use, (ii) AWD for crop
irrigation, and (iii) OWD linked to industrial processes
and to power plant cooling systems. Details concerning the reconstitution of
past water demand at the sub-basin scale in the Hérault and the Ebro basins
between 1971 and 2009 can be found in
The UWD of each municipality was assessed by multiplying a unit water allocation per capita by the population at each 10-day time step. Unit water allocations were calculated from available urban water withdrawal and population data. These allocations depend on the water consumption rates of urban activities and on the efficiency of water supply networks, and were assumed to remain stable throughout the study period. Annual variations in population over time were taken into account, as were the seasonal dynamics due to summer tourism in the Hérault Basin.
Agricultural water demand (AWD) simulation model.
Figure 4 shows how AWD was calculated based on irrigated crops, climate
conditions and irrigated areas. In the case of irrigated vineyards in the
Hérault Basin, the maximum irrigation requirement (MIR) was considered to be
the water required to meet 80 % of crop evapotranspiration under standard
conditions ET
OWD was not taken into account in the Hérault Basin because of the very
limited industrial activity. In the Ebro Basin, industrial demand was
estimated by the
Streamflow was modeled in the 6 sub-basins in the Hérault and the 20
sub-basins in the Ebro using GR4j
To assess natural runoff in each sub-basin, the model was calibrated only
against runoff data that were considered to be natural, i.e., not influenced
by withdrawals or dam management. Withdrawals upstream from the Pyrenean dams
were considered to be negligible, so all streamflow data were used for
calibration. Data on discharge entering the dams were not directly available,
so streamflow upstream from the dams was computed based on a balance between
inflow, variations in the reservoir level, outflow, and evaporation. Runoff
produced in each downstream sub-basin was computed at a 10-day time step by
subtracting the ingoing streamflow from the streamflow at the outlet of the
section. However, these downstream sections are also strongly influenced by
water withdrawals, and natural streamflow could not be calculated based on
observed streamflow and water withdrawal data because withdrawal data were
not available with the adequate time step and time depth. Thus, assumptions
had to be made in order to evaluate when streamflow was natural, i.e., not
modified by consumptive use. Since AWD was considered to have the highest
consumptive use, we assumed that when negligible AWD was simulated, then
consumptive use was negligible and flows were considered natural. Thus, the
time steps for which simulated agricultural demand was greater than
0.1
Accurate simulation of dam management is essential for simulations of water
availability in highly equipped hydrosystems. A demand-driven dam management
model adapted from
Demand-driven dam management model. The reservoir level (
As shown in Fig. 5, the water balance of the reservoir was computed at each
10-day time step, accounting for water demand, entering streamflow,
evaporation, and the initial reservoir level. Infiltration was not included
in the water balance. Precipitation and evaporation on the reservoirs were
computed by averaging the
Target levels were defined for each 10-day time step according to maximum
observed levels over a given time period in which the dam operation rules
were similar. Changes in dam operation rules were accounted for throughout
the study period. Data on the reservoir level of the Ebro dam, for example,
revealed a change in reservoir level variability after 1980; so, target levels
were defined separately for the periods 1971–1980 and 1981–2009. Minimum
levels were provided by the stakeholders in charge of dam operations or, if
the information was not available, were defined as the minimum level observed
over the period. The minimum levels of the reservoirs are kept as a safety
reserve for a particular water use (e.g., 100
Because dam management rules are driven by the demand associated with each individual dam, simulation of dam operations is highly dependent on accurate mapping of resources and demand. In the Hérault Basin, the Salagou dam supplies water for irrigation in a clearly delimited area (Fig. 1a). In the Ebro Basin, dam operation rules are much more complex. Some dams, including the Ebro dam, release water into the river for downstream users. The volume of water released depends on the associated demand and on the runoff produced between the dam and the users (i.e., between the outlet of the Ebro dam and the beginning of the Lodosa canal in the Ebro system). In other cases, a canal is directly associated with the dam (e.g., the Bardenas canal and Yesa dam) and transports water from the reservoir to irrigated areas that may be located in a different sub-basin. The volume of water released from the dam depends on the demand associated with the canal and on the capacity of the canal. This type of dam also releases water directly in the river for downstream users or to respect minimum flows. Lastly, the management of two or more dams may be coordinated to supply a particular irrigation system. The dams may be located on the same river and regulate flows from upstream to downstream (e.g., the Escales, Canelles and Santa Ana dams on the Noguera Ribagorzana River or the Mediano and Grado dams on the Cinca River; see Fig. 3b), in which case they were simulated as if they were one large dam located downstream. Two dams located in different sub-basins can also be operated jointly: if the total volume of water demand cannot be met by the first dam, it can be supplemented by the second. This is the case of two systems in the Ebro Basin modeled in this study: the Alto Aragon and the Aragon and Catalunya areas (Fig. 3b).
At each time step, water withdrawals were evaluated according to water availability, simulated water demand, and the following order of priority: (i) UWD (ii) OWD and (iii) AWD. Return flows were also taken into account. For each type of water demand, a part of the water withdrawn was considered to return to the environment and, in fine, to the streamflow at the outlet of the sub-basin in which the water was pumped. For UWD, two types of return flows were considered. First, 80 % of the volume actually supplied to domestic and more generally urban users was considered to return to the sub-basin outlet as treated effluent. The volume actually supplied was calculated by subtracting losses from the supply network from withdrawals. Second, part of the losses from supply networks was considered to return to the sub-basin outlet. For OWD, 80 % of withdrawn water was considered to return to the sub-basin outlet. For AWD, only part of the losses from supply networks was considered to return to the sub-basin outlet. All the water actually available for crop irrigation was considered to be used by the crops (i.e., consumptive use).
The return flow rate from losses from urban and irrigation supply networks
was estimated for the Hérault Basin as a whole, and for each sub-basin of the
Ebro Basin, to account for soil and geological heterogeneities. Return flow
rates were tested from 0 to 1 with a step of 0.1 and were calibrated by
optimizing goodness-of-fit criteria including NSE on low flows (July–September included), noted NSE
First, anthropogenic and climatic pressures on water resources were assessed. Anthropogenic impacts on streamflow, i.e., consumptive use, were estimated as the difference between natural and modified streamflow. The part of natural streamflow and anthropogenic consumptive use variations in the cause of the decrease in streamflow observed was evaluated by comparing variations in simulated natural and influenced streamflow.
Calibration and validation results of the simulation of natural streamflow in the Hérault and the Ebro basins. Optimal values of NSE, VE and VEM are 1, 0 and 0, respectively.
Second, water shortage for each type of demand was characterized by the
magnitude, frequency, and average length of withdrawal restrictions (see
Fig. 2). These indicators were calculated for each demand node considering
hydro-climatic and anthropic variability between 1971 and 2009. The
percentage of water demand that could not be satisfied due to lack of
available water was computed for each type of demand at a 10-day time step.
According to the local stakeholders, withdrawal restrictions were acceptable
as long as they did not exceed 50 % of AWD and 5 % of UWD. Water shortage
analysis concentrated mostly on agricultural demand since it was last in
order of priority (Fig. 2). The frequency of withdrawal restrictions (
Last, the current sensitivity of the two basins to water stress was evaluated
using the calibrated framework: water demand corresponding to the size of the
population, the irrigated areas, and industrial activity in the year 2009 was
compared to water availability under the prevailing hydro-climatic conditions
in the period 1971–2009, using the dam management rules applied in 2009.
Five indicators (see Fig. 2) were computed at each demand node to
characterize the balance between water uses and water availability in the
Hérault and Ebro basins with their current human activities and water
management practices, and faced with the climate variability of the recent
past. Maximum shortage (MS) was defined as the maximum simulated annual
irrigation water shortage rate and reliability (Rel) was considered to be the
rate of occurrence of satisfactory years (adapted from
Simulated and observed reservoir levels (
Simulation of influenced streamflow in the Hérault Basin from 1971
to 2009: results for the periods over which simulations of natural streamflow
were calibrated (Cal: 1981–2009) and validated (Val: 1971–1980). Optimal
values of NSE, NSE
Calibration and validation results for the simulation of natural streamflow are presented in Table 1. Overall, the results were satisfactory except for some downstream sections in which discharge was significantly modified and few calibration data were consequently available on natural streamflow. However, the sub-basins where the model performed poorly generally produced only a small proportion of total discharge.
In the Hérault Basin NSE values were over 0.80 and volume error (VE) was negligible over the calibration period in all sub-basins except in the Agde section. In the upstream Saint-Laurent and Laroque sub-basins, NSE values over the validation period were slightly lower (0.79 and 0.72, respectively), and volume errors tended to increase slightly. The lack of observed streamflow in the 1970s in the Gignac, Salagou and Agde sub-basins prevented validation of natural streamflow simulations in these areas.
The simulated natural streamflow in the Ebro Basin was in good agreement with observed streamflow in the sub-basins that contribute most to discharge at the outlet, i.e., the Pyrenean sub-basins and the Ebro upstream from the Aragon River. In these sub-basins, NSE values were above 0.60 and VEM and absolute values of VE were below 18 % over the calibration and validation periods, with the exception of the Noguera Ribagorzana sub-basin upstream from the Santa Ana dam. In this sub-basin the three dams (Escales, Canelles and Santa Ana) were simulated as one large dam in the location of Santa Ana (see Sect. 3.3.1), thus, multiplying possible errors in the calculation of the natural discharge at Santa Ana. In sub-basins 13, 14 and 20 (Ebro at Zaragoza, Mequinenza and Tortosa; see Table 1) results indicated poor calibration and validation scores. This can be explained by the high level of influence of withdrawals on streamflow, leaving very few data for calibration. The simulation of natural streamflow in these three sub-basins of the semi-arid Ebro Valley was thus considered to be unreliable. However, the climatic and topographic conditions of the middle and lower Ebro Valley suggest that the contribution of these areas to total discharge is minor. For the rest of the study, the contributions of sub-basins 13, 14 and 20 to the natural discharge of the Ebro River were set to zero.
Figure 6 shows the simulated reservoir levels in comparison with observed reservoir levels. Seasonal dam operations were well represented for the majority of dams. Moreover, simulated interannual variations of the reservoir levels were in good agreement with observations (e.g., the Ebro, Sotonera or Tranquera dams). Despite the complexity of their management, the reservoir levels of the Yesa and Grado dams were satisfactorily simulated, with NSE values of 0.68 and 0.56, respectively, and mean volume errors of 9 and 8 %.
Simulations of the Salagou reservoir level led to scores of 0.53 for NSE and 1 % for VEM for the period 1990–2009 (reservoir level data were only available after 1990). As can be seen in Fig. 6a, the reservoir level started showing significant seasonal variations in 1986, which is consistent with the start-up of a hydropower turbine and with increasing water demand for irrigation since the 1990s.
Influenced streamflow was accurately simulated in the Hérault Basin (see Fig. 7) with NSE values above 0.80 in all sub-basins for the calibration and validation periods, except at the outlet of the Salagou dam where the outflow was only moderately well simulated by the dam management model. However, the contribution of the Salagou dam to streamflow at the outlet of the basin is very low.
Results at the outlet of the Ebro Basin (see Fig. 8) showed that the
influenced discharge in this complex hydrosystem was well simulated:
aggregated simulations at the basin outlet led to NSE and volume error scores
of 0.68 and
Simulation of influenced streamflow in the Ebro Basin from 1971 to
2009: results for the periods over which simulations of natural streamflow
were calibrated (Cal: 1981–2009) and validated (Val: 1971–1980). Optimal
values of NSE, NSE
The ability to represent observed low flows, which are highly influenced by
storage and water withdrawals, is an indicator of the modeling chain's
efficiency. Low flows were well simulated at the outlet of the Hérault Basin
(Hérault at Agde) with NSE
Comparison of natural vs. influenced simulated streamflow:
anthropogenic impacts (consumptive use and water storage) on
Frequency and intensity of agricultural and urban water shortage in
the period 1971–2009 considering the spatial and temporal dynamics of water
uses and hydro-climatic conditions
Total water demand in the Hérault Basin doubled between the 1970s
(24
Total demand in the Ebro Basin increased from 4330
Simulated changes in natural streamflow at the outlet of the Hérault and the Ebro basins are illustrated in Fig. 9. The 1980s and 2000s appear to have been particularly dry compared to the 1970s and the 1990s, mainly in the Hérault Basin. In both basins, simulated natural streamflow decreased by approximately 20 % between 1971–1980 and 1981–2009.
At Agde, the natural streamflow of the Hérault River decreased in the winter,
spring and summer (
The changes in natural streamflow simulated in both basins are in agreement
with the climatic trends described in Sect. 2.3., i.e., a decrease in winter
precipitation and an increase in fall precipitation between 1971–1980 and
1981–2009, associated with a 1
Figure 9 also shows that the consumptive use increased between 1971 and 2009
in both basins, in absolute terms and relative to natural streamflow: average
annual consumptive use increased from 11
In the Hérault Basin the impact of water use was highest in the summer: it reached 30 % of natural flow between mid-July and mid-August in the 2000s (Fig. 9a). In the Ebro Basin the storage role of reservoirs is clearly visible, with anthropogenic impacts decreasing in July and August, when withdrawals were made from reservoirs (Fig. 9b).
Simulating natural and influenced streamflow made it possible to distinguish between the impacts of climate and consumptive use variations. In the upstream sections of the Hérault Basin (Saint-Laurent, Laroque and Lodève, data not shown) the decrease in streamflow between 1971–1980 and 1981–2009 was linked to a natural decrease in streamflow only, whereas in Gignac and Agde, respectively, 1 and 3 % of the decrease in annual influenced streamflow was linked to an increase in consumptive use. Simulations linked 80 % of the decrease in influenced discharge at the outlet of the Ebro Basin between 1971–1980 and 1981–2009 to a decrease in natural discharge, and 20 % to the increase in consumptive use. These proportions varied throughout the basin: 75–25 % in the Aragon sub-basin, 50–50 % in the Cinca sub-basin and a decrease in consumptive use in the Segre sub-basin.
Figure 10 shows the water shortages simulated between 1971 and 2009 in the Hérault and the Ebro basins. Results are only presented for the Gignac and Agde sections of the Hérault Basin, as they concentrate most of the water uses and our simulations did not identify any water shortage in the other sub-basins. Figure 9a shows the frequent occurrence of agricultural water shortages in the Gignac area from the 1980s on. Although AWD decreased slightly in the 2000s due to recent efforts to improve the efficiency of the Gignac canal, water shortage events continued to occur 3 years out of 5. In Agde, increasing UWD and AWD continued to be satisfied until the 2000s. Restrictions on agricultural and urban water withdrawals appeared in 2005, a dry year. These results are consistent with the information provided by local stakeholders on the occurrence of water supply problems in the Hérault Basin in the past 40 years: the main stakes concerning the supply of agricultural water are concentrated around the Gignac canal, while user conflicts may appear in the downstream area of Agde. The year 2005 was indeed notable for tensions around water resources, with strict regulations concerning the use of water and special negotiations that led to the release of additional water from the Salagou dam to compensate for low flows in the Hérault River.
Figure 10b shows the results of the simulation of water shortage for the
eight main management systems in the Ebro Basin. The systems on the right
bank are very exposed to water shortage, particularly the Guadalope
sub-basin. Although some agricultural water withdrawal restrictions may have
been applied in all areas except the lower Ebro, between 1971 and 2009, only
the Ebro Valley, Bardenas and Alto Aragon areas faced shortages exceeding
50 % of demand. In the case of the Bardenas irrigation system, the increase
in water shortage mirrored an increase in water demand. The storage capacity
of the Yesa dam is currently being increased by about
600
Frequency and intensity of agricultural and urban water shortage
under current water uses (2009) and hydro-climatic conditions in the period
1971–2009
In the Gignac area of the Hérault Basin, the impact of the improved
efficiency of the canal in the 2000s (see Sect. 2.3) is clear: while AWD
reached 13
According to the results shown in Fig. 12, the Aragon and Catalunya, Ebro
Valley, Segre–Urgel and lower Ebro sections in the Ebro Basin appear to have
found a sustainable balance between water use and availability if climatic
conditions remain in the range of variability as that observed in the recent
past. Comparison of Figs. 10b and 11b shows an improvement in the water
balance in the Segre–Urgel irrigation system due to the construction of the
Rialb dam in the early 2000s, which was added to the storage capacity of the
Oliana dam. The combined operation of the two dams reduces annual
agricultural water shortage rates (Fig. 11b) compared to the Oliana dam
without the additional storage capacity of Rialb (Fig. 10b). On the other
hand, the expansion of irrigated areas in the Bardenas and Alto Aragon
systems appears to have contributed to the increase in water stress revealed
in Fig. 10b. Figure 11b shows that the current uses of water in the
Bardenas system would not match water availability in the hydro-climatic
conditions of the recent past. With its current water use and water
management and under unchanged climate variability, in the future, this area
could face many long severe shortage events (MS
Sustainability of current water uses (2009) under the hydro-climatic
conditions of the recent past (1971–2009) for
The purpose of this study was to (i) combine socioeconomic and
hydro-climatic data in an integrative modeling framework to represent water
stress and its spatial and temporal variations over a past multi-decadal
period, and (ii) use this framework and appropriate indicators to assess the
sustainability of current water uses. The approach presented in this paper
enabled better identification of the drivers of water stress in basins facing
rapid climatic and anthropogenic changes, which is at the heart of the
challenges put forward in the Panta Rhei hydrological scientific decade of
the IAHS (International Association of Hydrological Sciences)
The modeling framework was validated by comparing simulated reservoir levels and influenced streamflow with observed data over a nearly 40-year period. Influenced streamflow was represented by accounting for the significant variability of water use and climate during the period, and by properly simulating anthropogenic pressures on water resources (i.e., water withdrawals, consumptive use and dam management). A combination of socioeconomic and hydro-climatic data based primarily on the mapping of resource and demand nodes enabled us to account for heterogeneities in water use and management practices in the two basins. The main heterogeneities in resource availability were identified by isolating the catchments that contributed most to runoff in the Hérault Basin and the catchments with a snowmelt hydrological regime in the Ebro Basin, and by defining the dams most important for the satisfaction of water demands. Also, local water management was taken into account and the demand nodes in our study represent actual management units, a scale at which water allocations are discussed.
To our knowledge, other studies did not simulate natural and influenced
streamflow at the mesoscale and over this long period, and did not achieve a
historical reconstruction of water demand and the influence of water use on
streamflow over such a long period.
Because the indicators used to measure water stress were defined with local
stakeholders, they were appropriate for water management issues. These
indicators are sensitive to the spatial and temporal dynamics of
anthropogenic pressures, and should help anticipate undesirable situations
and/or make projections, as recommended by
In the Hérault Basin, the irrigation needs of vineyards increased and were subject to great variability (standard deviation reached 60 % of the mean water demand over 1971–2009). Urban water demand also increased significantly, and water use conflicts started to appear in the basin. AWD satisfaction in the Gignac area appears to be sensitive to hydro-climatic variability (the 1970s were wetter whereas the 1980s and the 2000s were warmer and drier). However, the improvement in irrigation efficiency in the 2000s succeeded in limiting agricultural water shortages in the Gignac area. In the Agde section, an imbalance between demand and availability emerged in the 2000s because of the combined increase in AWD and UWD and the decrease in natural streamflow. Although UWD also increased in the Ebro Basin, it remained very low compared to AWD. The main driver of the decrease in water demand satisfaction in the Ebro was the increase in demand triggered by the expansion of irrigated land, which exceeded the water supply capacity in some areas (under current management and storage capacity).
Although key areas of the Hérault Basin were highly sensitive to hydro-climatic variability, the balance between water uses and availability in the Ebro Basin appears to be more critical, owing to high agricultural pressure on water resources. All the same, water demand and availability were shown to be in balance in large systems of the Ebro Basin such as the Ebro Valley, Aragon and Catalunya, Segre, and lower Ebro, which concentrate 60 % of the water demand from the large irrigation systems in the basin. In comparison, water demand in the Gignac area represented, on average, 70 % of total AWD over the Hérault Basin over the past 40 years. What is more, the vineyards in the Hérault Basin may be more vulnerable to an unreliable water supply than the cereal and fodder crops grown in the Ebro Basin, where cropping patterns can be adjusted on a yearly basis to adapt them to the hydro-climatic conditions. The spatial distribution of our integrative approach enabled us to identify the most vulnerable areas: the main stakes for maintaining or reaching water balance are concentrated in the Gignac and Agde areas in the Hérault Basin, and on the right bank and in the Bardenas and Alto Aragon systems in the Ebro Basin.
The widespread anthropogenic influence on streamflow made calibration and
validation of natural streamflow difficult, particularly in the Ebro Basin.
The challenge was that no water withdrawal data were available at a finer
timescale than annual values; consequently reconstructing series of natural
streamflow through data analysis would have been impossible. Another way of
simulating natural flow would have been to integrate the water withdrawals
and return flows in the calibration procedure by simulating natural flows,
modifying them with simulated withdrawals (according to simulated demand and
availability) and return flows, and applying the performance criteria to the
influenced flow
The other components of the modeling chain (the dam management model, the simulation of water demand, and the simulation of influenced streamflow) were validated against observed data, if available. As shown in Sect. 4.1.2, the functioning of some dams was poorly simulated. In the case of the Caspe and Talarn dams, this may be due to the streamflow regulations by other dams located upstream and not included in this study. Furthermore, the Talarn dam is mostly operated for hydropower production, which was not included in this study. In the case of the Caspe dam, the irrigated areas actually linked to the dam may have been underestimated. The Sotonera and Grado dams are operated jointly since 1981. The reconstruction of the changes in irrigated areas and their attribution to one dam or the other was a difficult task and a misrepresentation of the links between irrigated areas and each dam might have led, for example, to the Sotonera dam to be undersolicited in our simulations. The same can be said of the jointly operated Barasona and Santa Ana dams. Moreover our simulations considered the three dams Escales, Canelles and Santa Ana as one large dam in the location of Santa Ana (see Sect. 3.3.1). This may be an oversimplification of the real-life management of these dams.
The ability to represent observed low flows, which are highly influenced by storage and water withdrawals, is an indicator of the modeling chain's efficiency. Low flows were well simulated at the outlet of the Hérault Basin. As indicated in Sect. 4.1.3, the poor simulations of low flows in the upstream sub-basins of the Vis at Saint-Laurent and the Hérault at Laroque are most likely due to difficulties of the hydrological model to reproduce the functioning of the karstic system. Indeed, withdrawals are very scarce in these areas; therefore, their influence on streamflow is limited and many natural streamflow data were available for the calibration of the model. The simulated low flows in the Hérault at Gignac could be biased because of the upstream natural streamflow simulations and because of a biased simulation of withdrawals from the Gignac canal and their influence on streamflow. Low flows in the Ebro Basin were more problematic, particularly in the Cinca and Segre sub-basins, where low flows depend almost exclusively on the outflow from the various storage dams. Streamflow is influenced by the management of the Grado dam (operated jointly with the Sotonera dam) and of the Barasona dam (operated jointly with the Santa Ana dam) at the outlet of the Cinca sub-basin, and by the management of the Santa Ana dam and of the jointly operated Talarn and Oliana dams at the outlet of the Segre Basin.
Although the long-term reconstruction of past water demands produced valuable
information that was not available using regular data sources, accurate
validation of water demand simulations in space and over time was not
possible due to the lack of appropriate data on withdrawals. Nonetheless, the
orders of magnitude, seasonal distributions, and past dynamics appear to be
in agreement with the data that were available as well as with the knowledge
of local managers (see
Another point to consider is the assumption made on groundwater simulation
and the groundwater–surface flow links. The hydrological model used in this
study does not fully account for groundwater and groundwater–surface flow
links. It simulates streamflow coming from both surface and groundwater
sources. Other models that properly account for surface–groundwater
interactions exist
Simulations of water shortages at a 10-day time step should be interpreted
with caution, as no real-time water management adjustments were considered in
this modeling framework. Indeed, the order of priority for the supply of
water to different uses was fixed in our model, and no restrictions were
imposed on industrial and urban demand before agricultural withdrawal
restrictions reached 100 % of demand. In real life, water use restrictions
can be decided on in advance to limit non-essential urban uses before
supplies to irrigators are entirely cut off. Likewise, in the Ebro Basin each
Junta de Explotación can decide on the water volumes allocated to
different users at the beginning of the irrigation season, based on the
filling level of the associated reservoirs. Thus, if the irrigation season
begins with low reservoir levels, withdrawals can be partially restricted
throughout the season instead of waiting until the reservoirs are at their
minimum level before limiting withdrawals. Sometimes farmers decide on their
cropping patterns according to the reservoir level and/or snow cover
Finally, the simulation of UWD, OWD and AWD does not provide a comprehensive view of water uses. Notably, the demand for hydropower production was only partly accounted for through reserved flows at the dams concerned for the satisfaction of AWD. This could be a source of bias in the modeling of influenced streamflow, as the operation of dams for the production of hydropower can have a major impact on the downstream streamflow and hence significantly affect downstream users. Also, despite the increasing attention paid to environmental flows, these flows were not included in this study, except for reserved flows at dam outlets in the Ebro Basin. However, considering the time depth of the study, one could argue that water withdrawals would not have been limited by environmental concerns during the majority of the study period. In future studies, environmental flows should be taken into account when assessing the sustainability of a hydrosystem's water balance.
Despite the limitations mentioned above, the approach described in this paper
will enable the identification of the drivers of water stress in basins that
are likely to face rapid climatic and anthropogenic changes in the coming
decades. Such information will be extremely useful to policy makers when
designing water plans for coming decades. The next step of this work, which
is currently being conducted in the framework of the GICC REMedHE project
(
This work was carried out as part of the GICC REMedHE project funded by the French Ministry of Ecology, Sustainable Development and Energy for the period 2012–2015. The authors thank the Syndicat Mixte du Bassin du Fleuve Hérault and the Confederación Hidrográfica del Ebro for providing the necessary data and documents for this study as well as for sharing their knowledge of water resources management in the studied basins. Climatic data for the Ebro Basin was provided by the Agencia Estatal de Meteorología (AEMET). The authors wish to thank L. Menzel, C. Steele, P. van der Zaag and two anonymous referees for their valuable, detailed comments and suggestions on the manuscript. Edited by: P. van der Zaag