Changes to anthropogenic pressures on reach-scale rivers in South and Southeast Asia from 1990 to 2014

Abstract

Rivers are essential to human livelihoods and agricultural production, yet human usage and irrigation are jeopardizing river sustainability. It is thus crucial to investigate the fine-scaled spatiotemporal dynamics of anthropogenic pressures on rivers. Most research, however, is conducted at the grid-scale, which impedes detailed investigations. In this study, by tracking anthropogenic pressures at the scale of river reaches (the length of river between river confluences) in South and Southeast Asia from 1990 to 2014, we provide new insights into anthropogenic pressures on river reaches using a simple and straightforward approach. We selected human usage (represented by built-up area) and irrigation (represented by irrigated area) as two fundamental indicators of anthropogenic pressure. We divided the study area into 5 × 5 km grids and calculated anthropogenic pressures on each grid to its nearest river reach. Pressures were calculated as the ratio of built-up and irrigated area to the distance between grids and reaches. Groundwater was also included to adjust for additional irrigation-induced pressures on reaches. Anthropogenic pressures on each reach were then calculated by summing pressures from the two indicators of all grids attached to it. Results indicate that >50% of reaches are affected by anthropogenic activities and that average pressures increase by ∼15% from 1990 to 2014, with hotspots concentrated in eastern Pakistan and northern India. Irrigation is the dominant pressure on ∼33% of reaches, while human usage is dominant for ∼24% of reaches. Anthropogenic pressures within transboundary river basins vary longitudinally, increasing as distance from the ocean declines. Pressures also vary significantly with reach size. Although large rivers suffer from greater anthropogenic pressures, they are rising more rapidly for small rivers. Empirically, this study reveals the increasing and heterogeneous nature of anthropogenic pressures on river reaches in South and Southeast Asia. Methodologically, it suggests that reach-scale river sustainability assessment can serve as a promising approach for researching and managing regional and transboundary rivers.