Introduction

Riverine ecosystems encompass ecological, social, and economic processes (ecosystem functions) that interconnect biotic components and provide goods and services for society. Degradation of these vital ecosystems has been a primary cause of increasing water insecurity, raising the need for integrated solutions to freshwater management. Sustainable management of freshwater flows is fundamental to the four dimensions of development, including social needs, economic development, ecological integrity and environmental limits. However, unplanned developmental activities during the past four decades have been altering the land cover affecting the physical integrity, bio-geo-chemical cycling, hydrological regimes, biodiversity, etc. This makes it necessary to understand i) the landscape dynamics, and its relation with the hydrological, biological entities for determining the level of services provided by the ecosystem, and, ii) linkages of ecosystem structure with its functional capabilities, which are essential to frame appropriate management strategies towards mitigation of impacts.

Aquatic ecosystems constitute the destination of water in the hydrological cycle and are broadly categorized as lentic and lotic ecosystems. Lentic ecosystems refer to stationary or relatively still water (such as lakes, ponds, etc.), while lotic ecosystems refer to flowing water (such as streams and rivers). Water sustenance in the aquatic ecosystems depends on the integrity of the catchment as vegetation helps in retarding the velocity of water by allowing impoundment and recharging of groundwater through infiltration. As water moves in the terrestrial ecosystem, part of it gets percolated, while another fraction gets back to the atmosphere through evaporation and transpiration. Forests with native vegetation acts as a sponge by retaining and regulating the transfer of water between land and atmosphere^1,2. The mechanism by which vegetation controls flow regime is dependent on various bio-physiographic characteristics namely, type of vegetation, species composition, maturity, density, structure, aerodynamic and surface resistance, root density and depth, hydro-climatic condition, etc. Roots of vegetation helps (i) in binding soil, ii) improve soil structure by enhancing the stability of aggregates, which provide habitat for diverse micro fauna and flora leading to the higher porosity of soil thereby creating the conduit for infiltration through the soil³. Native species of vegetation with the assemblage of diverse species help in recharging the groundwater, mitigate floods, and other hydro ecological processes⁴. These functions augment with the age/maturity of the forests, their diversity, density of plant species, etc. In mature forests, streams are perennial with sustained yield (during all seasons), due to infiltration and storing water in subsurfaces (which gets released to the streams during lean seasons). Also, the annual surface transpiration reduces with an increase in understory transpiration⁵. Revival of natural forest capabilities through reforestation or afforestation would take about 20-25 years in the tropical ecosystems and achievement of full potential about 40-50 years^6,7. This necessitates safeguarding and maintaining the existing native forest patches to sustain hydrological regime, which caters to biotic (ecological and societal) demands. An undisturbed native forest has consistent hydrologic regime with the sustained flows during lean seasons⁸.

Aquatic ecosystems are the most threatened ecosystems in India due to alterations in the landscape structure (changes in the land cover), anthropogenic inputs (disposal of untreated or partially treated wastewater), construction of reservoir (altering the flow regime), water abstraction, river channelization (narrowing drains and concretization), etc. which in turn affect the physical and chemical integrity of the system. The spatial and temporal variability in fresh water stock with the burgeoning societal demands has resulted in anthropocentric regulation of river flow through construction of reservoirs, diversion works, etc. causing significant alterations in the hydrological regime and river morphology⁹. In general, dams are constructed for irrigation, hydroelectric power generation, domestic and industrial water supply, recreation, etc. and for controlling floods. Size and functionality of dams affect land use, livelihoods, local climate, hydrology, and economy. Reservoirs and other storage and diversion works have impacted the hydrological regime of rivers, which include loss of interconnectivity along rivers, fragmentation of catchment, changes in hydrological processes, downstream erosion^10,11 and alterations in the flow regime of the freshwater impacting downstream biota^12,13.

Ecological integrity of riverine ecosystem depends on the river morphology, river connectivity, water quality^14–17, quantum, duration and velocity of water flow which influences the aquatic biodiversity. Ecosystem fragility refers to the extent a system experience a damage caused by the sustained exposure of different stress agents that can cause environmental changes or changes in ecosystem functions^{16, 17}. Sustenance of water in rivers, streams and wetlands during all seasons is crucial to maintain the aquatic health and sustain biodiversity. The fresh water flows in terms of quantity and timing are essential to maintain the process and functioning of freshwater resource^18,19. The health of the river (water bodies) deteriorate when the flow is either reduced or inhibited below a threshold required to sustain aquatic life²⁰ or environmental flow²⁰ (also known as ecological flow, in-stream flow, minimal flow). Maintaining environmental flow in streams and rivers is necessary to meet the needs of aquatic biota along with the societal demand²¹, sustain the health of an aquatic system²², manage flow and protect the water bodies and river networks²³, maintain and enhance the ecological character and functions of floodplains, wetlands and riverine ecosystems which may be subject to stress from drought, climate change or water resource development^24,25.

Four river basins in the central Western Ghats with the varied levels of anthropogenic stress have been chosen to understand the implications of large scale changes in the respective landscape structure on the hydrologic regime, social needs, economic development, ecological integrity and environmental limits.

The Western Ghats (WG) are a range of ancient hills that runs parallel to the west coast of India covering an approximate area of 160,000 sq. km that extend between 8° N and 21° N latitude, and 73° E and 77° E longitude. The region is endowed with diverse ecological regions depending upon the altitude, latitude, rainfall and soil characteristics²⁸. WG are among 8 hottest hotspots of biodiversity²⁹ and 36 global biodiversity hotspots³⁰ with the exceptional endemic flora and fauna. Natural forests of Western Ghats have been providing various goods and services³¹ and are endowed with species of 4,600+ flowering plants (38% endemics), 330 butterflies (11% endemics), 156 reptiles (62% endemics), 508 birds (4% endemics), 120 mammals (12% endemics), 289 fishes (41% endemics) and 135 amphibians (75% endemics)³². Numerous streams originate in the WG, which drain millions of hectares, ensuring water and food security for 245 million people and hence are aptly known as ‘water tower’ of peninsular India. The region has tropical evergreen forests, moist deciduous forests, scrub jungles, sholas, savanna including the high rainfall savannas of which 10% of the forest area is under legal protection. Areca nut, coconut, coffee, rubber, sugarcane, tea, are the horticultural crops and spices, paddy, cereals, cotton are major agricultural crops grown across the regions.

The Western Ghats (WG) landscape consist of heterogeneous interacting dynamic elements with complex ecological, economic, and cultural attributes. The interactions among the landscape elements result in the flow of nutrients, minerals and energy, which contribute to the functioning of the landscape. This complex interaction helps in the sustenance of natural resources through bio-geochemical and hydrological cycles. The changes in the landscape structure have been altering the ecosystem functions.

The landscape with relic forests and perennial rivers have been catering to the societal water demand, while ensuring food security in the peninsular India. The region is rich in biodiversity with numerous species of flora and fauna. Fragmentation of large contiguous forests to small and isolated forest patches either by natural phenomenon or anthropogenic activities has led to drastic changes in forest patch sizes, shape, connectivity and internal heterogeneity, which restricts the movement of species leading to inbreeding among Meta population with extirpation of species.

The impacts of unplanned developmental activities²⁶ are evident with i) the existence of barren hill tops, ii) conversion of perennial streams to intermittent or seasonal streams, iii) flash floods during monsoon and droughts during summer, iv) pollution of ecosystems, v) change in water quality, vi) soil erosion and sedimentation²⁷, vii) extinction of endemic flora and fauna, viii) loss of habitats, breeding grounds, etc. The region is ecologically fragile and vulnerable with high susceptibility to anthropogenic stress. This necessitates assessment of eco-hydrological footprint which will aid in the prudent management of fragile ecosystems to sustain i) natural flow regime, ii) ecosystem goods and services and iii) livelihood of the people. The current study involves investigation of land use dynamics with hydrologic regime to assess eco-hydrological footprints (water availability with demand to meet the societal and ecological needs), across the riverscapes in the Central Western Ghats with varied levels of anthropogenic stress. The study outcome would help toward evolving appropriate integrated management strategies to ensure sustenance of water, supporting biodiversity and people’s livelihood.