Impact of landuse changes on the flow regime and sustenance of environmental flows of Tawi river at Jammu

Abstract

Flow regime of a river is an important component of project hydrology, on the basis of which development of water resources of a river for various beneficial uses is thought of. During recent decades, concerns about the impacts of changing patterns of landuse associated with deforestation and agricultural transformation on water resources have created social and political tensions from local to national levels. Major concerns focus on consequences of landuse change for water supply and demand, for local and downstream hydrological hazards, and for biodiversity conservation. Also, the increased competition for water and alterations in land use in the upstream of many rivers, are argued to have contributed to change in hydrological regimes and consequently, the river ecology of many rivers. While that has been acknowledged, few studies have been conducted in developing countries. The river Tawi, the left bank tributary of Chenab river is endowed with vast water resources with irrigation, domestic water and hydropower potential which are yet to be assessed in details. The increasing demand of the development of Tawi river for beneficial uses of the population of Jammu, Udhampur and Doda districts calls for the systematic hydrological studies for the river. Since last four decades, few minor schemes for irrigation, hydropower and domestic water supply have also come up. Tawi river is the major source of water supply to the Jammu city. Recently, one project for recreational activity is also coming up on Tawi river at Jammu. Based on the discussions with J&K state forest deptt. officials, it is concluded that Tawi river has seen a lot of changes in land use in the upper catchment, which may pose a threat to the river flows and the river ecosystem. In this limelight, the present study was taken up to study and understand how land use/cover changes affect the flow regime and the sustenance of environmental flows of a river. The present study has been planned to cover the four aspects: (i) To evaluate the land use/cover change in the Tawi basin; (ii) To model the flows of Tawi river at Jammu using SWAT model; (iii) To assess the environmental flows of Tawi river at Jammu; (iv) To assess the impact of land use/cover change on flow regime of Tawi river. The LULC maps have been prepared for the years 1985, 1995 and 2005 by using the LANDSAT imageries through the unsupervised classification in ERDAS software. The thematic maps have been prepared considering the six LULC classes i.e. Built-up (urban & rural), Crop land, Evergreen Broad Leaf Forest, Grass Land, Shrubs Land and Water Bodies. There has been decrease in the area under Evergreen Broad Leaf Forest and Crop Land. This decrease has been due to increase in the area under Built-up, Grass Land and Shrubs Land. As the major objective of the study was to assess the impact of LULC change on the flow regime of Tawi river and land use component is strongly built in the SWAT model, ArcSWAT software has been calibrated and validated for the runoff simulation in this study. The data required for the model set up has been procured, processed and digitized. The model has been calibrated for the period from 1983 to 1992 and validated for the years from 1993 to 1997. The LULC map pertaining to the year 1995 has been used for the calibration and validation. The model simulated the discharge of Tawi catchment upto Jammu satisfactorily with Coefficient of Correlation (CC) and Nash–Sutcliffe efficiency (NSE) as 0.715 and 0.453 during calibration and 0.855 and 0.835 respectively during validation. Further, the discharge simulations have been made by using the LULC maps for the years 1985 and 2005 for the calibration and validation periods.While observing the simulated discharges for the LULC pertaining to years 1985 and 2005, it is found out that the discharges of the Tawi river in general are decreasing due to the changing landuse except for the months of July and August. It may be concluded that due to conversion of forest area into grassland or agricultural land into built-up area, the monsoon discharges are increasing while non-monsoon discharges are decreasing. For the assessment of environmental flows, the hydrological desktop approach has been selected considering the limited data on baseline biodiversity of the Tawi River. Under the hydrological desktop approaches for assessing E-Flows, the Global Environmental Flow Calculator (GEFC) developed by International Water Management Institute (IWMI) has been applied in this study. The results of ArcSWAT and GEFC have been compared to assess the scenario of environmental flows in the Tawi catchment. It has been concluded that the simulated discharges for the LULC of 1985 are mostly representing the natural or pristine condition while the simulated discharges for the LULC of 2005 are representing EMC B (slightly modified) for Jan-Feb; EMC C (moderately modified) for Oct; EMC D-F (largely to critically modified) for Mar-May & Nov. The flow regime is shifting in such a way due to change in LULC that although the discharges are decreasing in general but the monsoon discharges are increasing and the non-monsoon discharges are decreasing due to less baseflow contributions. It is also observed that the lean season discharges are the most affected due to landuse changes. At present, there is no major abstraction of water from the Tawi river, it is highly desirable that the flow regime should represent the EMC A or B (Almost Natural or Slightly Modified). Then only, in future, as the various water demands may increase and there will be more abstraction for Tawi river, the flow regime could be maintained in EMC B or C condition. For achieving this, the efforts are required to rejuvenate the degraded forest cover to the natural conditions as existed prior to 1985.