Run off: controlling factors. 

Runoff is precipitation that did not get (infiltrated) absorbed into the soil or did not evaporate, and therefore, made its way from the ground surface into places that water collect. Runoff causes erosion, and also carries chemicals and substances on the ground surface along to the rivers where the water ends up. Only about 35% of precipitation ends up in the sea or ocean. The other 65% is absorbed into the soil. Some of it too will evaporate. 

1. Types of Runoff: 

a. Surface Runoff: That portion of rainfall which enters the stream immediately after the rainfall. It occurs when all loses is satisfied and rainfall is still continued and rate of rainfall [intensity] in greater than infiltration rate. 

b. Sub-Surface Runoff: That part of rainfall which first leaches into the soil and moves laterally without joining the water table, to the stream, rivers or ocean is known as sub-surface runoff. It is usually referred is inter-flow. 

c. Base flow: It is delayed flow defined as that part of rainfall, which after falling on the ground the surface, infiltrated into the soil and meets to the water table and flow the streams, ocean etc. The movement of water in this is very slow. Therefore it is also referred a delayed runoff. 

2. The surface runoff process: As the rain continues, water reaching the ground surface infiltrates into the soil until it reaches a stage where the rate of rainfall (intensity) exceeds the infiltration capacity of the soil. Thereafter, surface puddles, ditches, and other depressions are filled with water (depression storage), and after that overland flow as runoff is generated. The process of runoff generation continues as long as the rainfall intensity exceeds the actual infiltration capacity of the soil but it stops as soon as the rate of rainfall drops below the actual rate of infiltration.

3. Factors Controlling Runoff

 A. Climate factors: 

I. Types of Precipitation: It has great effect on the runoff. E.g. A precipitation which occurs in the form of rainfall starts immediately as surface runoff depending upon rainfall intensity while precipitation in the form of snow does not result in surface runoff. 

II. Rainfall Intensity: If the rainfall intensity is greater than infiltration rate of soil then runoff starts immediately after rainfall. While in case of low rainfall intensity runoff starts later. Thus high intensities of rainfall yield higher runoff.

III. Duration of Rainfall: It is directly related to the volume of runoff because infiltration rate of soil decreases with duration of rainfall. Therefore medium intensity rainfall even results in considerable amount of runoff if duration is longer. 

IV. Rainfall Distribution: Runoff from a watershed depends very much on the distribution of rainfall. It is also expressed as “distribution coefficient”. Near the outlet of watershed, runoff will be more. 

V. Direction of Prevailing Wind: If the direction of prevailing wind is same as drainage system, it results in peak low. A storm moving in the direction of stream slope produce a higher peak in shorter period of time than a storm moving in opposite direction. 

VI. Other Climate Factors: Other factors such as temperature wind velocity, relative humidity, annual rainfall etc. affect the water losses from watershed area. 

B. Physiographic Factors: 

I. Size of Watershed: A large watershed takes longer time for draining the runoff to outlet than smaller watershed. 

II. Shape of Watershed: Runoff is greatly affected by shape of watershed. Shape of watershed is generally expressed by the term “form factor” and “compactness coefficient”. Form Factor = Ratio of average width to axial length of watershed. Compactness Coefficient: Ratio off perimeter of watershed to circumference of circle whose area is equal to area of watershed. Two types of shape: Fan shape [tends to produce higher runoff very early]. Fern shape [tend to produced less runoff].

 III. Slope of Watershed: It has complex effect. It controls the time of overland flow and time of concentration of rainfall. E.g. sloppy watershed results in greater runoff due to greater runoff velocity and vice-versa. 

IV. Orientation of Watershed: This affects the evaporation and transpiration losses from the area. The north or south orientation, affects the time of melting of collected snow. 

V. Land Use: Land use and land management practices have great effect on the runoff yield. E.g. an area with forest cover or thick layer of mulch of leaves and grasses contribute less runoff because water is absorbed more into soil.

 VI. Soil moisture: Magnitude of runoff yield depends upon the initial moisture present in soil at the time of rainfall. If the rain occurs after along dry spell then infiltration rate is more, hence it contributes less runoff. 

VII. Soil type: In filtration rate vary with type of soil. So runoff is great affected by soil type. 

 VIII. Topographic characteristics: It includes those topographic features which affects the runoff. Undulating land has greater runoff than flat land. 

IX. Drainage Density: It is defined as the ratio of the total channel length [L] in the watershed to total watershed area [A]. Greater drainage density gives more runoff. Drainage density = L/A.