SPATIAL AND TEMPORAL VARIATION OF SEDIMENT YIELD AND RUNOFF IN BIDA WATERSHED

CHAPTER ONE

1. INTRODUCTION

1.1     Background of the study

Sediment yield refers to the amount of sediment exported by a basin over a specific period of time, with amount that will enter a reservoir located at the downstream limit of the basin also Runoff is described as the part of water cycle that flow over land as surface water instead of been absorbed into ground water or evaporation.

With current pace of development and increase in population  water resource shortage  and soil  loss as become  a major  threat in Bida watershed basin which as cause major disaster around the Vicinity.

Sediment transport is an important part in the quantitave and qualitative management of river in engineering.

The rate of sediment transport in water bodies depends on availability of transport materials and transport capability of the water bodies, The subject of sediment yield modeling has attracted the attention of many scientists but lack of data, resources and widely accepted methods to predict/estimate sediment yields are some of the barriers against this direction of research, Summer et al., 1992.

Cases that distributed and process based mathematical models such as SWAT could be a potential tool in predicting and estimating sediment yield especially at a catchment scale. Application of the distributed and process based models could minimize the uncertainty resulting from assuming lumped, stationary and linear systems. Besides, the SWAT model has particular advantages for the study of basin change impacts and applications to basins with limited records (Bathurst, 2002; Ndomba, 2007).

The runoff component of the SWAT model supplies estimates of runoff volume and peak runoff rate using the curve number method (SCS, 1972) and modified rational method, respectively, which, along with the sub-basin area, are used to calculate the runoff erosive energy variable. The crop management factor or cover management factor is recalculated every day that runoff occurs. It is a function of above-ground biomass, residue on the soil surface and the minimum cover factor for the plant. The KUSLE factor is estimated using an equation proposed by Mulengera and Payton (1999) for tropics. Other factors of the erosion equation are estimated as described by Neitsch et al. (2005).

Watershed is an area of land that drains all the streams and rainfall to a common outlet. Essentially a watershed is all the land and water area which contributes runoff to an outlet in the main flow channel, Watersheds are significant because the stream flow and the water quality of a river are affected, human-induced or natural, happening in the land area “above” the river-outflow point. The environment worsening of a watershed is a common occurrence in most parts of the world.

Among several causes the major one are improper and unwise utilization of watershed resources observed in developing countries, Watershed management implies rational utilization of land, soil, and water resources for optimum and sustained production with minimum hazards to natural resources and environment, Soil erosion is a serious global issue because of its rigorous adverse economic and environmental impacts.

Economic impacts on productivity may be due to direct effects on crops/plants both on-site and off-site, and environmental consequences are primarily off-site related to the damage to civil structure, siltation of water ways and reservoirs, and additional costs involved in water treatment . This study employs the SWAT (Soil and Water Assessment Tool) model to take advantage of its integration with GIS (Geographic Information System) and locally available data and data from similar areas that can be used to calibrate and validate the model, so that runoff and sediment yield from the watershed can be predicted and most problematic sub-basins can be identified.

1.2 STATEMENT OF PROBLEM

Temporal variation of sediment yield and runoff are important parameters for sustainable management of watershed around the world. As commercial development continues at record levels, both the quantity of runoff generation  and sediment yield are issues that need to be looked at, carefully. The challenge of how to know the runoff coefficient  due to heavy rain fall, and sediment yield in the catchment to be able define for the subsequent year.