INTRODUCTION
1.1 Background to the study
Water is a vital resource for every human activity. Water makes life possible. Without it, life and civilization cannot develop or survive (Ojekunle, 2011). Water forms the largest part of most living matter and is vital to man just as air and food are (Ayoade, 2003). The management and maintenance of water is thus very important (Fiorilloa, 2007). The accelerating growth of human population, the rapid advances made in industry and agriculture have resulted in a rapidly increasing use of water by man, to the extent that the availability of water as well as the control of excessive water has become a critical factor in the development of every regions of the world (Williams, 2010).
Over the decades, water supply management has proved to be insufficient to deal with strong competition for water with growing per capita water use, increasing population, urbanization pollution and storages (Wang Xiao – Jun et al, 2009). In addition, the need for domestic, industrial and agricultural water supply is growing, but the absence of demand management strategies means that the increase in demand will likely outstrip the available supply, hence water scarcity (UNESCO, 2006). One-third of the world’s total population of 5.7 billion lives under conditions of relative water scarcity and 450 million people are under severe water stress (UN, 1997). The issue of water scarcity in the world and its implication on development of new political and economic relations among countries may result to crisis in the future. Thus, there is need for the implementation of effective water resources management which becomes particularly important towards determining how much water is available for human use and economic activities that water should be shared between users.
Population growth is a major contributor to water scarcity. The global population is expanding by 80 million people annually, increasing the demand for freshwater by about 64 billion m3a year (Population Institute, 2010). Rapid population growth and urbanization could expose more people to water shortages, with negative implications for livelihoods, health, and security. These demographic trends, coupled with increasing per-capita water consumption, will be a huge development challenge (Bates, Kundzewicz, Wu and Palutikof, 2008). Growthin population implies mounting demand and competition of water for domestic, industrial, and municipal uses (Population Action International, 2011). Population growth leads directly to increases in overall water demand, while other demographic factors such as population distribution and age structure modifies the pattern in demand and determines increases in household water demand. Overall, the amount of water each person uses is expected to increase as incomes grow and consumption increases (UN-Water and FAO. 2007).
Evidences are ample that there is an explosion in the population of cities in Nigeria (Eja, Inah, YaroandInyang, 2011; Nwosu, 2013). The effect of the rapid urban population growth is noticeable through the provision of municipal services such as pipe-borne water. Expectations of the populace on the activities of policy makers for the supply of water are quite high. (Sule, 2008). Water can be said to be adequate when an individual is availed a quantity of at least 50 litres per day (World Health Organization, 2003).
The unavailability of water in required proportion for man’s use has assume global crises dimension. According to the Population Institute (2010), only 20 percent of the global population has access to running water and over 1 billion people do not have access to clean water. The Population Institute noted further that with a projected population of the world to expand to 9 billion people by 2050, it is estimated that 90 percent of the additional 3 billion people will be living in developing countriess, many of which are already experiencing water stress or scarcity therefore, it is pertinent to manage water resources sustainably.
Water resources management has a significant impact on the socio-economic development of a catchment. The water demands and availability depends on the economic, ecological, land use, and climatic changes of a region (Droogers,2012).Water resource management is a multifaceted issue that becomes more complex when considering multiple nations’ interdependence upon a single shared trans boundary river basin (Teasley and McKenney, 2011). The management of water resources as a common resource would require trade-off among countries and water users (Yang and Zehnder, 2007). The need therefore to devise means by which available water can be consumed and allocated among the various uses is pertinent.
The planning of human activities involving rivers and their floodplains must consider hydrological facts; (…) the flows and storage volumes vary over space and time (Loucks et al, 2005). The necessity of predicting the hydrological patterns is essential to the reservoir management. The reservoirs have to insure not only the water quality, but also the human, the industrial and the agricultural consumption. Nowadays the environmental concerns such as the aquatic biodiversity and the environmental pressure have an increased influence in the decision-making.
Asa River is one of the major sources of water supply in Kwara state. This study simulates the hydrological process of Asawatershed that allows for the estimation of available water resources, so that sustainable and rational utilization, conservation and management of available water resources will be adopted using Soil and Water Assessment Tool (SWAT) model. The study also proffers alternative strategies for water conservation that will meet water demand within the basin.

1.2 Problem Statement
In the study area, there has been an increase of population over the last three decades which leads to strong competition for water with growing per capital water use. Also, there is possibility of scarcity due to the potential effects of global climate change on water resources. Asa river basin faces freshwater management challenges, some of which includes allocation of limited water resources, inadequate environmental quality monitoring, and policies for sustainable water use.

1.3. Aim and Objectives of the study
The aim of the study was to assess water availability and water demand of Asacatchment using GIS-based hydrological model.The specific objectives achieved in this research are:
1. topredict water balance components of Asa river basin during the modelling period.
2. to predict total water yield potential of the basin.
3. toevaluate water demand of Asa river basin using projected population of the inhabitant of the basin.
4. toevaluate available water resources of Asa river basin from 1986-2015.
5. tocompare the volume of available water resources with the actual water demand of the catchmentarea within the modelling period.

1.4 Justification for the study
Water is vital resource for every human activity. Water is scarce and there is need for efforts to improve its availability and explore it sustainability. To achieve reliable prediction of the various hydrologic parameters including rainfall, runoff etc. for river catchment, it is very tough and time consuming by conventional methods. So it is very important to search suitable methods and techniques for quantifying the hydrological parameters. The fundamental objective of hydrology modeling is to gain an understanding of hydrological system in order to provide reliable information for managing water resources in a sustained manner.

1.5 Scope of Work
In this study, thirty years meteorological data such as rainfall, temperature, humidity, solar radiation etc. were used. Validation and calibration of model were not included due to non-availability of observed data. Population projection using geometric method was adopted.