Impact of land use land cover change on runoff and water quality of an increasingly urbanized tropical watershed in java, indonesia
Astuti, Ike Sari
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Increasing water resource problems such as accelerated sedimentation, drying-up springs, and downstream eutrophication were reported in Indonesian water bodies, especially in Java. The growing pressures on watershed due to land use land cover change (LULC) was perceived as the cause of watershed degradation. Information about the nature of LULC change and drivers were poorly understood, how LULC plays a role in generating runoff was unknown, and what factors governing the variability of water quality (TSS and Chl-a) in water ecosystems like reservoirs is unavailable. This study aims at improving the understanding on relationship between LULC change (1995-2015), drivers, impacts on runoff and water quality. Upper Brantas was chosen as the study state, representing a typical urbanized tropical watershed in developing countries. Varying methods were used covering remote sensing, GIS and SWAT modelling. Findings revealed the direct and indirect causes of LULC change with agricultural expansion, population growth, infrastructure, accessibility, economic, social and cultural factors jointly affected the LULC change. Major LULC change trajectories were forest to dryland agriculture, dryland-agriculture to rice-field, and rice-field/dryland agriculture to settlement. Urban areas became the land use type exhibits the fastest growth, being doubled in a 21-year period. Forest conversion, farm loss, and rapid uncontrollable settlement development appear to be the challenges for land use planning. SWAT-based assessment results showed that the LULC change over the past 21 years led to subtle increase in surface runoff, water yield, and decrease in ground water. Among these, surface runoff was the most affected variable with relative increase of 8%. Forest conversion both to settlement or dryland agriculture generated greater impact in generating runoff, followed by dryland agriculture conversion to urban. Long-term TSS and Chl-a assessment showed that TSS in all reservoirs was closely associated with rainfall and discharge. This amplifies the needs for runoff management from existing land use practices. On the other hand, the Chl-a variability appeared to be influenced both by internal and external conditions of each reservoir as well as the corresponding draining watershed. The findings suggest the watershed management cannot be generalized due to large variability in land use and watershed physical conditions.