Water resources serve irreplaceable functions in human society and ecosystems. With the rapid social and economic development, ever-growing anthropogenic interventions to the hydrologic cycle have significantly strengthened the interaction between social and hydrologic systems [1
]. Over the last decades, it is well recognized that both domestic and productive water demand in most watersheds have been dramatically increased due to rapid population growth, along with accelerating agricultural and industrial expansions, all of which have led to intensifying competition and conflicts among different water use sectors [3
In addition to consuming more freshwater, human activities have also exerted large-scale impacts on the hydrologic systems either directly or indirectly. For instance, direct withdrawals of water from natural aquatic systems to satisfy domestic and productive demands have led to lack of water for environment and thus jeopardize ecosystem health [2
]. In addition, human-induced land use and land cover changes as well as direct flow regulation, e.g., dam construction and inter-basin water diversion, have further distorted the natural flow regimes. Besides, human activities can impact water resources in an indirect and long-term way, most of which are related to climate change through greenhouse gas emissions [1
]. The recently released Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) declared that climate change will significantly impact the availability, seasonality, and extreme properties of natural water resources [5
]. Global warming is supposed to cause more intense and frequent extreme events, e.g., floods and droughts, due to the increase in hydrologic variability [6
]. Among various extreme events, droughts are likely to substantially reduce water supplies and deteriorate water qualities [8
]. In turn, the potentially altered hydrological regimes will present added challenges to water managers who have already suffered from strong inter-annual variability of observed water resources [9
To confront the growing threat of failures for water resources systems to meet water requirements, some countries have managed to either propose executable administrative measures or improve existing water distribution systems. However, political and engineering measures including limiting the water use quotas allocated to different water users [11
], as well as improving the efficiency of water usage by minimizing the leakage loss of water distribution networks [13
] can reduce water consumption to some extent, but may be insufficient to alleviate the water stress. Reservoirs are among the most efficient man-made infrastructures for managing water supply and reducing the devastating impacts of droughts [15
]. The mass constructions and improving operation skills of basin-scale reservoir and water distribution network systems have provided resilience for water supply against extremes, which in turn promoted the development and application of water resources allocation models.
Some simulation-based platforms for water resources management have been developed to simulate water supply with routine reservoir operation rules and pre-set water user priorities. For example, the Aquatool model [16
], developed by the Universidad Politecnica de Valencia, has been used by two river basin agencies in Spain as a standard decision-support tool to develop their Basin Hydrological Plans. Another water supply simulation package, the Resource Allocation Model (REALM) [17
], has been used to model two case studies in Australia covering both urban and rural water supply systems with diverse forms of operating rules. The Water Evaluation and Planning model (WEAP) [18
] model, developed by the Stockholm Environment Institute, incorporates watershed-scale hydrologic processes with water management model by introducing the concept of demand priorities and supply preferences. The Mike Basin model [19
] developed by the Danish Hydraulic Institute, has been widely used by water agencies to simulate basin-scale water resources management for multi-purpose, multiple-reservoir systems by specifying associated reservoir operation rule curves and guiding water extraction from several reservoirs in order of priority. Despite their rich modules and user-friendly interfaces, the simplified reservoir operation rules and water allocation strategies of these models operated within a “what-if-then” scenario-based framework generally offer poor flexibility if changing hydro-climatic and anthropogenic factors are considered.
Motivated by abovementioned shortcomings, this paper aims to develop an integrated optimal water resources allocation model for regional water supply security analysis under changing environment in strongly human-impacted area. Optimal water allocation models have been earlier applied in agricultural area to optimize water irrigated to different crops, especially under deficit irrigation [20
]. With the improvement of living standard and the expansion of industry, water allocation has to gradually take into account multiple objectives involving social, economic, environmental and political tasks [22
]. As a result, a variety of multi-objectives algorithms such as the macro-evolutionary genetic algorithm [24
], the non-dominated sorting genetic algorithm [25
], the macro-evolutionary multi-objective immune algorithm [26
], etc. have been proposed and applied to achieve water allocation policies that are economically efficient, technically feasible as well as socially fairly in recent years. Zhou and Guo [27
] and Yang et al. [28
] derived adaptive multi-objective operating rules for the Danjiangkou reservoir in China to increase the ecological flow and water supply yield, respectively. Zhou et al. [29
] proposed a theoretical framework for optimal multi-objective allocation for a complex adaptive water resources system and applied it to the water resources planning of Dongjiang River basin.
Subsequently, water allocation models have been incorporated in the water supply risks analysis using the reliability and resilience criteria proposed by Hashimoto et al. [31
] to evaluate regional socio-economic drought conditions [32
] or the drought mitigation abilities of local water supply systems [33
]. Rajagopalan et al. [35
] assessed the annual risk of the Colorado River water supply and suggested flexible management practices to mitigate the increased risk due to future reduction in flows. Milano et al. [36
] develop a synthetic modeling framework driven by a conceptual GR2M hydrological model and a storage dam model to evaluate current and future capacity of water resources to meet different water demands. However, in regions where water resources have been excessively overexploited and overused, the building-up pressure of water demand in the future will impose an added vulnerability to local water supply system, causing societal, economic and environmental damages [37
]. Moreover, while most current studies emphasize great importance on the impact of climate change on local water resources planning, few of them argue the effects of large-scale inter-basin water diversion projects on the water resources area, especially on the lower reach of the water intake after water diversion in the upstream.
The rest of this paper is organized as follows: in Section 2
, a brief introduction of study area and dataset are presented, followed by the methods used in this study in Section 3
. Then, results are shown in Section 4
. Conclusions and discussion is drawn in Section 5
4.1. Trends of Future Water Demand
On the basis of the Integrated Water Resources Planning of Hanjiang River Basin, off-stream water demand in various water sectors (i.e., urban domestic water demand WD1
, rural domestic water demand WD2
, industrial water demand WD3
, agricultural water demand WD4
) for each unit at present and in the two planning horizons (short-term and long-term periods) under different frequency conditions along with their monthly distributions are estimated. In-stream ecological water demand WD5
of each zone is calculated by the Tennant method [44
] based on the local water resources.
The annual water demand of four off-stream water users (WD1
, and WD4
) for the whole study area summed up over all units are listed in Table 3
. It can be seen from Table 3
that the annual total water requirement in the MLHRB will be gradually increasing with the extension of planning period, from 10,744–12,841 million m3
/year at the base year to 12,532–14,185 m3
/year at the long-term planning horizon.
According to Table 3
, both the urban domestic water demand (WD1
) and industrial water demand (WD3
) are projected to significantly increase in the planning horizons resulted from living condition improvement, population growth and dramatically socio-economic development. On the contrary, the agricultural water demand (WD4
) is reduced slowly, partially because of promotion of water saving consciousness and improvement of water usage efficiency in most units. Conversely, the rural domestic water demand (WD2
) shows no significant inclination as a combined result of rising living standard but decreasing rural population due to expanding urbanization. With the increase of domestic and productive water requirement, the water supply security and sustainable development of the MLHRB will be faced with tremendous threat.
4.2. Optimum Allocation of Water Resources
Long time series dataset (1956–2010) of historical hydro-climatic information, reservoir inflow, water demand and diversion scenarios are all treated as inputs to the water management module operated under the reservoir operation rules and water resources allocation rules. It should be noted that for the planning horizon, the real inflow to the Danjiangkou reservoir is obtained by subtracting the water diversion volume through the HWWDP from the natural inflow since the water source of the project (Huangjinxia and Sanhekou Reservoir) are both located in the upstream of the Danjiangkou reservoir. By optimizing the water allocated among various water sectors in different operation units based on the integrated water resources allocation model, the results of water balance between water demand and supply for the MLHRB under different frequencies in the three water demand scenarios are calculated and listed in Table 4
It is observed from Table 4
that under the water demand scenario of the base period, the water shortage problem over the MLHRB is not serious, with an average total water deficit volume of 1029 million m3
(6.75%) and off-stream water deficit of 605 million m3
(5.33%), respectively. The total water supply shortage ratio for the MLHRB in the normal year (50%), moderate dry year (90%), severe dry year (90%) and extreme dry year (95%) is 6.11%, 8.90%, 11.84% and 14.95%, respectively. For the short-term planning horizon, with the synchronous increase of water demand and inter-basin water diversion capacities, the water supply satisfaction over the MLHRB will be deteriorated with total water deficit ratio of 7.67% and off-stream water deficit ratio of 6.25% for the mean annual case. It is even obvious when it comes to the extreme dry year, a 20.41% of insufficient water supply for all water sectors and 17.68% of unsatisfied off-stream water demand may present a challenge on regional water security. For the long-term planning period, the water supply scarcity in the MLHRB will be even aggravated since both water requirement and water transferred volume are further raised. The shortage ratio of mean annual total water supply and off-stream water supply would reach up to 10.01% and 8.84%. The gaps between water supply and water demand under all frequencies are considerably large, with the water supply deficit ratio of 8.49%, 12.73%, 18.76% and 26.20% for the normal year, moderate dry year, severe dry year and extreme dry year, respectively.
It can also be concluded from Table 4
that even though the urban domestic water requirement will be nearly twice in the long-term planning period as the base period, domestic water demands, both urban water demand and rural water demand, for the MLHRB are satisfied with rather high guarantee since domesticity is the most important sector and should be supplied first among all water users. Despite the gradual reduction of agricultural water demand, the water supply shortages of agricultural sectors for different typical dry year are gradually increasing, from 4.18%–11.41% in the base period to 7.06%–21.82% in the long-term planning period. The main reasons of this increase are the relatively low priority of agricultural water supply and the growth of other water users with higher priorities. Disregarding the higher priority and assurance rate of industrial water user compared to the agricultural sector, the relative industrial water supply deficit is larger than that of the agricultural counterpart, partially resulted from the operation of reservoirs located within the study area whose major function is irrigation. According to Table 4
, the satisfying degree of in-stream ecological water demand is the lowest as a result of the compromise between ecology protection and relatively low tolerances of domestic and productive water deficits.
It should be noted that since the optimal water resources allocation is operated for some kind of “ideal” scenario under some arbitrary simplification, the choice of parameters of the integrated model (both
in objective function and
in constraints functions) are supposed to impact the optimization results, especially for large water user sectors (WD3
, see Table 3
and Table 4
). For example, if the
of all sectors are all reduced to zero, available water would be allocated completely based on the
, the annual average total water supply deficit for the whole MLHRB will increase from 7.43% in the base year to 11.06% in the long-term planning horizon and water supply deficit in the industrial sector will become the severest case. Ignoring the adaptabilities of some water users (i.e., agricultural and ecological sectors) to adjust to extreme drought scenarios, the available water will be used to sustain healthy in-stream ecological supply after satisfying domestic water users, thus significantly reduce the water supply to industrial and agricultural activities, which are both large water consumers. It can also be inferred from the results that whether an ecological protection-oriented water supply policy is implemented may have a significant impact on the water allocation. If the net economic return of in-stream ecological water supply is reduced to the minimum among all users (equal to the agricultural sector), the average total water supply deficit for the whole MLHRB will increase from 4.41% in the base year to 7.54% in the long-term planning period. Due to the smallest values of
parameters, water would not remain in the river channel to meet the requirement of in-stream ecological water, but to fulfill domestic and productive water demands. Trade-off between optimizing off-stream water supply and protecting in-stream ecological health needs deeper investigation. On the other hand, though the guarantee degree of urban and rural domestic water supplies are always high in terms of both large values of
, the water demand amounts of these sectors are relatively small, thus the impacts of adjusting corresponding parameters are also small.
4.3. Evaluation of Water Supply Risk
To assess the spatial distribution of water supply risk, the
indicators of each units in the MLHRB under different water demand scenarios are calculated and mapped in Figure 4
and Figure 5
For the water supply reliability , obviously, most units along the mainstream of the Hanjiang River will almost never () or seldom () experience water supply failure under the current water demand scenario, except the two units located in northern Hubei Province, i.e., U5 and U6. The operation zones away from the mainstream may suffer from a rather unsatisfactory reliability of water supply with likely () or frequent () water supply shortage such as U8. It should be noted that U5–U8 are located within the famous humpy ground of northern Hubei in China, which is long been regarded as a drought-prone area. For the short-term planning period, water volume transferred through SNWDP will increase up to 9500 million m3 along with 1000 million m3 water upstream the Danjiangkou reservoir to be diverted outside the Hanjiang River basin through the HWWDP. The simultaneous increases of water demand and inter-basin water diversion are supposed to reduce the water supply reliability in the MLHRB, especially those units along the mainstream of Hanjiang River basin. For example, the frequency of water supply failure in the U17 will upgrade from seldom () to sometimes (). This is intuitive because the water diversion projects only directly reduce the flow in the mainstream, thus impact mainly the units along the river. For the long-term planning horizon, water demand will be further increased, and the water diversion capacities of both the SNWDP and HWWDP would be extended. As a result, supply risks of many units along the mainstream of Hanjiang River are supposed to stay at the same degree under the joint effect of increasing water demand and newly operated water diversion projects. However, since part of the newly diverted water from QWDP (about 480 million m3 of 770 million m3) is transferred within the Hanjiang River basin to the northern Hubei, i.e., U5–U8, the ever-growing water demands in these areas will not worsen the local water supply deficits.
For , a similar distribution pattern of water deficit level as can be detected, indicating the synchronism of degree between the frequency () and the intensity () of water supply failure. It also sustains the rationality from sides about the classification of water supply deficit level for both indicators. For the base period, it can also be seen that operation zones that located away from the mainstream of Hanjiang River basin are more likely to face to water supply risk in intensity, such as U1, U8, U10, U12, U14 and U21. After evaluating the water supply vulnerability for the base scenario with historical inflows and current water diversion capacity, the proposed model was then run for each planning period but with added water transfer projections and new water demand scenario. Clearly, operation zones that collecting water from the mainstream of Hanjiang River basin are more sensitive to the direct human-induced alteration. The expanding water diversion is likely to aggravate water supply risk in intensity and threaten their water supply security. For instance, values of most units along the central middle MLHRB are supposed to upgrade gradually from not (), to slight () deficit, or from slight (), to moderate () deficit in the planning period. Generally speaking, the variation range of is smaller than that of . For operation zones that have suffered a quite frequent and severe water shortage, i.e., U10, the imbalance between water requirement and supply will be deteriorating.
According to the indicators obtained from the optimal water resources allocation model, more water supply measures are necessary to increase the water supply capacity in the planning periods. Moreover, outside-basin water is expected to be transferred into MLHRB. The proposed risk assessment framework can provide a practical approach towards more robust decision-making of long-term water resources planning in similar strongly human-impacted areas.
5. Discussion and Conclusions
It is well recognized that the rapid exploitation that took place over the last decades, has tied the hydrologic and social systems more and more closely nowadays. The building-up water demand pressure is likely to aggravate the water scarcity around the world and is projected to intensify in the future [46
To monitor, model and seek adaptive strategies against the potentially enhanced water supply risk in basins that are strongly impacted by human activities, this paper proposed an integrated water resources management model incorporating three modules to evaluate the spatial and temporal distribution of water supply risk for the mid-lower reach of Hanjiang River basin, which considers the increase of water demand and planning expanding water diversion projects in the future. The ever-growing water demand presents challenges to the water supply reliability of the operation units in the MLHRB. Despite the large available capacity of the Danjiangkou reservoir, the planned expansion of inter-basin water diversion capacity is likely to have an adverse impact on operation zones that take water directly from the mainstream of Hanjiang River. The framework proposed by the study for assessing the combined effect of increasing water demand and expanding water diversion on local water supply safety is beneficial for the water resources planning and management.
A better adaptive planning to proactively manage the water supply risk in the future usually requires long prediction of the variability of water supplies, which is always carried out by stochastic resampling or simulation of historical records. Unfortunately, historically observed data in most watersheds are no longer adequate to support robust decision-making of long-term water resources planning and management due to nonstationarity in water supply and demand [2
]. Besides, persistent climate change will further enlarge this uncertainty, thus puzzling future water resources planning. All of these challenges have necessitated the development of integrated water resources management models that fully take into consideration the hydrological modeling and climate change assessment.
However, due to the limitation of existing hydrological and atmospheric models, the accuracies of simulated hydrologic regimes and projected future climate scenarios are usually accompanied with large uncertainties, which might mislead the water management decision. The ensemble of available hydrological models, General Circulation Models (GCMs) and downscaling methods within a probabilistic (e.g., Bayesian) framework seems to be a plausible approach to provide more synthetic information and reduce uncertainty, which is going to be carried out in our further studies.