Spatial characterization of long-term hydrological change in the Arkavathy watershed adjacent to Bangalore, India

Sally Thompson,Veena Srinivasan,Sharachchandra Lele,Iryna Dronova,Gopal Penny

doi:10.5194/hess-22-595-2018

Abstract

Abstract. The complexity and heterogeneity of human water use over large spatial areas and decadal timescales can impede the understanding of hydrological change, particularly in regions with sparse monitoring of the water cycle. In the Arkavathy watershed in southern India, surface water inflows to major reservoirs decreased over a 40-year period during which urbanization, groundwater depletion, modification of the river network, and changes in agricultural practices also occurred. These multiple, interacting drivers combined with limited hydrological monitoring make attribution of the causes of diminishing water resources in the watershed challenging and impede effective policy responses. To mitigate these challenges, we developed a novel, spatially distributed dataset to understand hydrological change by characterizing the residual trends in surface water extent that remain after controlling for precipitation variations and comparing the trends with historical land use maps to assess human drivers of change. Using an automated classification approach with subpixel unmixing, we classified water extent in nearly 1700 man-made lakes, or tanks, in Landsat images from 1973 to 2010. The classification results compared well with a reference dataset of water extent of tanks (R2 = 0.95). We modeled the water extent of 42 clusters of tanks in a multiple regression on simple hydrological covariates (including precipitation) and time. Inter-annual variability in precipitation accounted for 63 % of the predicted variability in water extent. However, precipitation did not exhibit statistically significant trends in any part of the watershed. After controlling for precipitation variability, we found statistically significant temporal trends in water extent, both positive and negative, in 13 of the clusters. Based on a water balance argument, we inferred that these trends likely reflect a non-stationary relationship between precipitation and watershed runoff. Independently of precipitation, water extent increased in a region downstream of Bangalore, likely due to increased urban effluents, and declined in the northern portion of the Arkavathy. Comparison of the drying trends with land use indicated that they were most strongly associated with irrigated agriculture, sourced almost exclusively by groundwater. This suggests that groundwater abstraction was a major driver of hydrological change in this watershed. Disaggregating the watershed-scale hydrological response via remote sensing of surface water bodies over multiple decades yielded a spatially resolved characterization of hydrological change in an otherwise poorly monitored watershed. This approach presents an opportunity to understand hydrological change in heavily managed watersheds where surface water bodies integrate upstream runoff and can be delineated using satellite imagery.

Highlights

Human water consumption is straining water resources worldwide (Vogel et al, 2015; Gleick, 2014; Wada et al, 2012; Lall et al, 2008), with developing nations vulnerable to water scarcity (Vörösmarty et al, 2010)
Statistical analysis of the tank water extents suggests that while inter-annual variability in tank water extent is largely explained by precipitation, this variability is superimposed on a longer-term trend in tank water extent that is independent of precipitation, representing a non-stationarity in inflows
The Arkavathy watershed embodies many of the water security challenges confronting southern India

Summary

Introduction

Human water consumption is straining water resources worldwide (Vogel et al, 2015; Gleick, 2014; Wada et al, 2012; Lall et al, 2008), with developing nations vulnerable to water scarcity (Vörösmarty et al, 2010). Human interventions in the water cycle often occur due to decisions made at local scales, and exhibit considerable spatial heterogeneity when considered at larger scales. This is problematic in this region because most research linking human drivers to hydrological responses focuses on either the local scale (Perrin et al, 2012; Van Meter et al, 2016) or regional to national scales (Gosain et al, 2011; Devineni et al, 2013; Tiwari et al, 2009). The gap in scientific understanding at managementrelevant scales is strongly associated with a lack of data resolution at these scales, and forces water managers to make decisions without sufficient information about cause and effect within watersheds (Batchelor et al, 2003; Glendenning et al, 2012; Lele et al, 2013; Srinivasan et al, 2015)

Methods

Results

Discussion

Conclusion