INTRODUCTION

Water is the essential limited natural resource [1] for all life forms on the planet for their growth and sustenance; production of food, economic development, social wellbeing [2]. Water is one of the unique natural resource capable of being transported, diverted, stored and recycled [2], these properties of water imparts itself as a great utility for agro-socio-economic development to the mankind. The water resource is being over utilized [3, 4] to cater the increasing agriculture and human demands in order to maintain the food security of a region. Water allocation priorities according to National Water Policy of India, 2002 [5] are: (i) drinking water, (ii) irrigation, (iii) hydropower, (iv) ecology, (v) agro-industries and non-agricultural industries, and (vi) navigation. Traditionally the water resource allocation was focused mainly upon human and agricultural needs ignoring the environmental sustainability [6, 7] leading to diversions and variations in the natural flow regime. These variations in the natural flow of water has led to massive loss of wetlands, decline of riparian’s, unauthorized cultivation practices in the river beds and banks [8], loss of aquatic species [9], etc. Recent studies highlight trend of increasing global awareness in maintaining the hydrology (flow regime) that acts as a key driver in maintaining the ecology of aquatic ecosystem including wetlands [10, 11, 12, 13], land cover in a watershed [4] the flow cycles as flooding and drying that are critical for sustaining variety of plants and animals [14, 15],  recognizing the need for making provision for the environmental requirements. Rivers are also the water users, and has to have an untouchable reserve [16] which has led to the concept of the ecological flow that emphasized on maintaining a flow that resembles the natural flow regime is critically important in sustaining the native biodiversity and ecosystem integrity in aquatic ecosystems [17], trying to cater the environmental water requirements while ensuring the availability of water to the basic domestic needs.

Environmental water describes the quality, quantity and timing of water flow required in the stream to sustain freshwater ecosystems, human livelihood and wellbeing that depend on the surrounding environment [18]. Environmental water availability as hydrologic variation in any river basin is dependent upon the land use and the climatological factors [19]. The change in land use adversely affects the catchment dynamics in maintaining the hydrologic conditions. Large scale land use [20, 21, 22] alterations has transformed water resource carrying capacity [23, 24, 25] required to cater the environmental water needs in the basin. In order to analyse and manage the capacity of the basins to supply resources without losing its current potential through water balance studies [26, 27] in combination with GIS and remote sensing [28, 29, 30]. Studies have used land use information derived from the remote sensing data is integrated with the long term climatological information such as rainfall [31, 32, 33], temperature and solar radiation [31, 34] etc. to evaluate the water balance in the watershed by quantifying hydrological parameters such as surface runoff, evapotranspiration, Ground water recharge, agro-domestic-livestock demands etc. Several watershed management studies in India [35, 36, 37, 38, 39, 40, 41] have been carried out in order to assess the potential of the rivers or streams in catering the demands in the basin. In this respect the objective of the study is to quantify the environmental water availability through water balance studies by understanding the dynamics of land use and its implication on the river capabilities in catering the demands.