WATERSHED SUSTAINABILITY INDEX FRAMEWORK AND ITS ESTIMATION FOR A WATERSHED

Abstract

There are several issues which affect the water sustainability in a watershed. Among these are hydrological, climatological, social, economic and technological. Most of the times these are considered separately, and not as an integrated process. Keeping this in mind, this study focuses on the water sustainability framework at regional and watershed level. The philosophy of the proposed watershed sustainability index is illustrated through a case study of Piperiya watershed located in the relatively new state of Chhattisgarh in India. The seasonal and annual analysis has been carried out to study the behavior of rainfall pattern in the past so as to identify the drought or non-drought years. Further, rainfall variability is analyzed and discussed at each of the district level and then aggregated at the state level. For the regional level analysis, a time-series of historical rainfall for all districts during the period of 1901 to 2002 is considered. Monthly, seasonal, and annual rainfall statistics as well as the dependability of rainfall is assessed. The regional level water sustainability study is carried out through the concept of Standardized Precipitation Index (SPI). A ‘Distance based approach’ has been suggested for assessment of regional level water sustainability index (WSI) which is based on performance parameters such as reliability, resilience, and vulnerability of SPI as well as the index of optimism of the decision maker. The WSI for the state of Chhattisgarh was found to be 0.47 which is ‘moderate’ in nature. In order to study the impact of climate change on regional level WSI, statistical downscaling technique using multi linear regression (MLR) has been suggested for possible rainfall (predictand) projections under two possible emission scenarios of HadCM3 A2 and B2. The model parameters are refined so as to its suitability to the state of Chhattisgarh and the model is calibrated and validated before actually being used for the purpose. Performance of the model is checked through various performance criteria. Further, bias correction is applied for improving the model performance. The predicted rainfall is represented into four different time steps, i.e., past (1961-2010), 2020s (2011-2040), 2050s (2041-2070) and 2080s (2071-2099) under both the emission scenarios. Thereafter, the WSI at regional scale is assessed using the distance based approach as discussed earlier. Under A2 scenario, the WSI is assessed as 0.54 (Moderate) and under B2, it is assessed as 0.63 (Good). This is basically due to the reason that rainfall under both the scenarios is expected to increase in future. In order to carry out the water sustainability assessment at watershed level, it is desired that first the characteristics of watershed are analyzed. Therefore, in this study, first the morphometric analysis is carried for the Piperiya watershed. For this purpose, sub-watersheds have been vi identified and prioritized for the action against soil erosion. This has been achieved through the geospatial technique. The sub-watersheds have been ranked on the basis of the morphometric parameters such as stream order, stream length, stream frequency, drainage density, texture ratio, form factor, circulatory ratio, elongation ratio, bifurcation ratio and compactness ratio. Using the morphometric analysis, it was found that the Piperiya watershed is fifth order basin. Further, it was noticed that the SWS-9, SWS-4 and SWS-5 are higher erosion and soil loss prone areas. Therefore, these sub-watersheds need suitable soil erosion control measures. Further, it is important to detect the behavior and rate of land-use change. For this purpose, the present study advocates the use of Markov chain model. Using the Markov chain model, future expected landuse and the rate of land-use change is estimated for the case study. It has been noticed that in the Piperiya watershed, settlements, agricultural, barren land and the water body areas have increased in the past and shrubs and forest area have decreased. In order to assess the impact of land-use and climate change on runoff at the watershed level, a popular Soil and Water assessment Tool (SWAT) (designed by USDA) is used. Model is calibrated and validated with past observed data during 1990-2000 and 2001-2006 respectively. The performance of the model is examined through various performance parameters. Further, different scenarios (combination of climate and land-use) are visualized for study of climate and land-use change impact on runoff in the watershed. It was noticed that the impact of climate change is more as compare to land-use change on runoff. Further, in order to assess the WSI at watershed level, a framework involving four major indicators i.e., Hydrology (H), Environment (E), Life (L) and Policy (P), is suggested. Hydrology (H) indicator is further sub-divide into quantity (HQI) and quality (HWQI) sub-indicators. E, L, and P indicators are studied through pressure, state and response sub-indicators. Since in water sustainability of the watershed, hydrology plays an important role, it has been studied and analyzed in detail. For the HQI, a threshold value of 75 % dependable flow is considered. Using the threshold flow value, the performance parameters such as reliability, resilience and vulnerability are assessed and used for the purpose. For assessing the HWQI, fuzzy-rule based approach has been recommended. Rule-bases are designed and used for various parameters and the categories of parameters. Thereafter, a new rule base is designed for aggregation of quality parameters in a single platform. In the E, L, and P indicators, again a new rule base is designed for the pressure, state and response. Further, all the four indicators, i.e., HELP are aggregated by yet another rule-base to quantify the overall assessment of WSI in quantitative and qualitative terms. For Piperiya watershed, the WSI has been assessed as 0.65 which can be termed as ‘Good’.