Assessing watershed sustainability using the watershed sustainability index (Case study: Bujin watershed in Hamadan province)

Introduction
In many regions, ecosystem sustainability and environmental security have become more fragile. Because watersheds are dynamic systems, their hydrological function and health are constantly changing under the influence of land use, climate change, and human interventions. Since the destruction of the ecosystems of a watershed has harmful economic and social consequences, in recent decades there has been a general tendency to evaluate the relative conditions or health of watersheds on a national and local scale. Ecologists have paid special attention to the study of how natural resource ecosystems respond to different types of stress caused by human activities. The watershed sustainability index (WSI) can be considered as an effective tool in watershed management including priorities monitoring changes and influencing factors on ecosystem management. In recent years, various studies and plans have been conducted to preserve natural resources and achieve sustainable development. The sustainability of watersheds includes four important goals of regulating the water flow regime, maintaining and improving water quality, maintaining the ecological quality of plants and animals, and energy resources. In this context, the pressure-state-response (PSR) model has been introduced and used for a comprehensive assessment of the health of an ecosystem. The conceptual model of PSR was developed using a set of criteria expressing environmental performance. This study aimed to evaluate the sustainability level of the Bujin watershed.
 
Materials and methods
One of the methods for evaluating watershed sustainability is the use of the conceptual pressure-state-response model (PSR). Applying the causal-effect PSR model using theWSI criteria in the form of four sub-criteria of hydrology (qualitative and quantitative), environment, life, and policy-making, one can evaluate the sustainability of the watershed numerically. In this method, considering the available data and information to investigate each sub-criteria, the parameter values are determined in three modes of pressure, state, and response, and in the scoring range from zero to one, five categories are converted to quantitative mode. Therefore, the PSR framework has three types of criteria: pressure criteria that evaluate environmental pressure resulting from human activities (waste, sewage), and state criteria that express environmental conditions (water quality). and the response criteria that evaluate the society's reactions (water quality) and the response criteria that evaluate the society's responses (policies, laws, management). The sub-criteria and parameters used in this research were determined based on the index selection criteria published by the HCTF Habitat Protection Fund in 2003. Sub-criteria were investigated based on three conceptual model parameters in 10 years for the Bujin watershed. The WSI criteria were calculated at three low, medium, and high levels to assess the watershed sustainability.
 
Results and Discussion
According to the results, the value of the pressure parameter and the quantitative status of the basin's hydrology in terms of available water variable is in class (C), i.e. in the range of 3400 > AW > 1700, which is a poor condition. The average scores were obtained for the water quality part (0.583), which shows the average to low status. The average score for the hydrology sub-criterion was 0.375, which indicates a poor situation in this region. The values of pressure, state, and response parameters for the sub-criterion of life in the Bujin watershed, during the 10 years studied, indicate a change in the state from weak to moderate. The results also showed that the pressure parameter with a score of 0.75 and the response parameter with a score of 0.625 had the highest and lowest scores for evaluating the sustainability of the Bujin watershed, respectively, indicating an appropriate response to reduce the pressure applied to the ecosystems. Sub-hydrology index with a score of 0.16 and environment with a score of 1 had the lowest and highest priority for the management of the basin ecosystem. According to the distribution maps of the criterion for evaluating watershed sustainability in conventional watershed systems during the period (2006-2016), the standard level of watershed sustainability at the beginning of the period was lower than the middle class (score 0.59) and in the middle of the period was in the middle class (score 0.62) and for the end of the period, it was upgraded to the upper than the middle class (score 0.7).
 
Conclusion
The priorities of achieving sustainable development (the priority in improving the conditions to promote the level of sustainability and achieve sustainable development) are different, and it is important to know which sub-criterion should be improved first and which parameter the decision-makers should pay attention to avoid wasting time, money and energy, and to take faster development steps in an area. Evaluation of relative conditions of watershed sustainability using the PSR model is very useful for providing appropriate management strategies because according to the nature of the conceptual model, a specific dimension of watershed health is explained. Bujin watershed has an unstable condition in the sub-index of hydrology and a good condition in the sub-index of life and human development, although, for this watershed, obtaining a score of 0.7 for WSI criteria in the whole watershed showed that the level of watershed sustainability in the 10 years is in the middle class and it is necessary to pay more attention to improve the level of sustainability and health of the watershed.