Water resources are divisible into two distinct categories : the surface-water resources and the ground-water resources. Each of these categories is a part of the earth's water circulatory system, called the hydrologic cycle, and is ultimately derived from precipitation, which is rainfall plus snow. They are interdependent, and frequently the loss of one is the gain of the other. The brief description of the run-off cycle, which is a part of the hydrologic cycle, will help us to understand the origin and the interdependence of these two categories of water resources.

The precipitation that falls upon the land and is the ultimate source for both the categories of water resources is dispersed in several ways. A sizeable portion is intercepted by the vegetal cover or temporarily detained in surface depressions. Most of it is later lost through evaporation. When the available interception or the depression storage are completely exhausted and when the rainfall intensity at the soil surface exceeds the infiltration capacity of the soils, the overland flow begins. Once the overland flow reaches a stream channel, it is called surface run-off, which together with other components of flow, forms the total run-off.

Factors affecting water resources.

1) Climate factors

a) Rainfall: its intensity, duration and distribution

b) Snow

c) Evapo-transpiration.

2) Physiographic factors

A) Basic characteristics

1. Geometric factors: Drainage area, shape, slope and stream density.

2. Physical factors: Land use, surface infiltration conditions, soil types etc.

B) Channel characteristics: carrying capacity and storage capacity

3) Geological Factors

A) Lithologic including composition, texture, sequence of rock types and the thickness of rock formations.

B) Structural, including chief faults and folds that interrupt the uniformity of occurrence of rock types or sequence of rock types also beds, joints, fissures, cracks, etc

C) Hydrologic characteristics of the aquifers permeability, porosity, transmissivity, storability, etc