عنوان مقاله [English]
Introduction: Groundwater is predominantly a renewable resource, and when managed properly can ensure a long-term water supply for increasing water demand and for climate change impacted region. Surface water renews as part of the hydrologic cycle in an average time period ranging from approximately 16 days (rivers) to 17 years (lakes and reservoirs); however, the average renewal time for groundwater is approximately 1400 years for aquifers to millions years for some deep fossil groundwater. Groundwater depletion, which is the reduction in the volume of groundwater storage, can lead to land subsidence, negative impacts on water supply, reduction in surface water flow and spring discharges, and loss of wetlands. Water balancing strategy has been considered as one of the most effective options to mitigate the groundwater depletion, and thus the balancing scenarios are applied as main approach to manage ground water sustainably. The purpose of the water balancing strategy in aquifers management is that groundwater level to be returned to the primary water level and to compensate the water resources shortage of aquifers’ storage.
Materials and Methods:
1. Case study: Birjand aquifer with an area of 1100 square kilometers is situated in eastern part of Iran. The location of the aquifer is between 59o 45 and 58o 43 east longitude, and 33o 08 and 32o 34 north latitude.
2. Modeling: Laplace Equation is the basic equation for groundwater flow study in steady or unsteady states. In simulation by using numerical models, the boundary of the model, recharge and discharge resources, evaporation and recharge zones are important elements. After finding the key components of the conceptual model, the MODFLOW software was applied for simulation of groundwater. MODFLOW, which is a computer code that solves the groundwater flow equation and uses finite-difference method, is provided by the U.S. Geological Survey.
3. Sustainability Analysis: In order to achieve the objective of this study, water balancing scenarios should be evaluated for sustainability of the groundwater system using appropriate indices. Here, three indicators of reliability, vulnerability and desirability are proposed and were employed to assess the stability of groundwater system in different balancing scenarios in lumped and distributed forms. The aquifer sustainability index is expressed in Equation 4. In this equation, three indicators of aquifer reliability (Equation 1), aquifer vulnerability (Equation 2) and Desirability (Equation 3) have been used to assess the stability of groundwater system. The aquifer reliability index means in what extent the withdrawal scenario has been able to return the aquifer to its original state using the Equation 1 as follows:
In which the number of periods where the groundwater level is above the desired level (equilibrium balance) and the total number of time steps in simulation. The vulnerability index indicates the amount of shortage in the groundwater storage and expresses the severity of the system failures using the Equation 2 as follows:
In this equation, the desired groundwater level at time step t, the groundwater level simulated in t time period for each scenario, the groundwater level without scenarios and n the number of periods where the groundwater level is lower than the desired level. The index of the likelihood of returning the system to a favorable state is presented as an indicator of the desirability of the system using the Equation 3 as follows:.
In this equation, indicates the f ground water level after the depletion, is the desired level of groundwater and is the groundwater level (without the scenario). After estimating three indicators of reliability, vulnerability and desirability, the sustainability index for each scenario can be appraised using Equation 4.
In this equation, groundwater sustainability index, reliability index, desirability index and vulnerability index.
Results and Discussion: In this study, six water balancing strategies were employed to reduce 1, 1.5, 2, 2.5, 3 and 3.5 percent water withdrawing for agricultural water use. Results of the simulation of different water balancing strategies demonstrated that with reducing in water use, the stability index has been improved significantly. The improvement changes from 32% increase in the index for 1% water withdrawing reduction scenario to 88% increase in the index for the 3.5% water withdrawing reduction scenario. Moreover, the reviewing of the stability indices of the system in various scenarios reveals that a 2.5% reduction in water use will assistance the aquifer status achieve to a stable state.
Conclusion: In order to manage groundwater withdrawal, it is easier to assess the impact of the water balancing scenarios using the groundwater sustainability index. The review of sustainability indices in the studied aquifer shows that by reducing 1% of the water harvest, 32% of the system's stability increases, and if water harvest reduction reaches 3.5%, the index increases 88%. Considering the distributed potential and possibility of the investigation of different scenarios by proposed indices in this study, they can be applied to assess and manage other similar aquifers.