RUNOFF CYCLE Runoff defined Runoff is defined as

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RUNOFF CYCLE Runoff defined: Runoff is defined as the surface flow of water following

RUNOFF CYCLE Runoff defined: Runoff is defined as the surface flow of water following the slope of the ground. Horton considered it as overland flow in excess of infiltration. There may be a sub surface flow at the same time following the down slope direction of the terrain. However, Hortonian overland flow is common in arid and semi arid regions, while saturation overland flow is common in humid climates (Kirkby, 1969). W G Hoyt elaborated the existence of runoff cycle in five phases.

First phase It is related to a rainless period when surface storage and most

First phase It is related to a rainless period when surface storage and most channel storage have been depleted and the stream flow is maintained by outflow from natural subterranean storage. This subterranean flow is known as base flow, or effluent seepage or sustained flow. The draining out of base flow with little or no replenishment results in gradual lowering of ground water table. When water table drops below the channel, stream flow ceases.

 In semi arid, arid and sub humid regions such a condition is common

In semi arid, arid and sub humid regions such a condition is common but in humid regions after prolonged drought such a condition may appear at the heads of the stream branches. Evaporation takes place from the surface of the stream, zone of aeration and saturation where the ground water is near the surface, and transpiration occurs from plants. If snow, ice, or frost is present, water will be released after air temperature goes above freezing.

Diagram of First phase

Diagram of First phase

Second phase The second phase of the runoff cycle is marked by the end

Second phase The second phase of the runoff cycle is marked by the end of the rainless period an occurrence of rain. The rain water is distributed in a variety of storage. Part of it directly falls on the stream flow, another part is retained by the surface depressions, part of it is intercepted by vegetation, and a part infiltrates into soil. The last one causes an increase of water in the zone of aeration. Overland runoff is little at this phase except from roads, buildings, and impervious areas. Evaporation and transpiration occur at meagre amount. If snow is present, it will absorb part of the rainwater. If frost is present, it will reduce infiltration and will cause an increase in surface runoff.

Diagram of Second phase

Diagram of Second phase

Third phase The third phase is envisaged to be associated with continuation of rain

Third phase The third phase is envisaged to be associated with continuation of rain at varying rates of intensity. Additional rain water to the depression storage will cause an outflow as overland runoff, or infiltration may absorb part of it. Overland flow dominate the situation. After the replenishment of the zone of saturation ground water table will start rising. The zone of aeration will also reach its capacity. Subsurface storm flow may reach the stream channels. Hence, the water level of streams will rise. Evaporation and transpiration occur at a very low rate. If snow is present and its capacity to hold rain water is reached, surface runoff will increase.

Diagram of Third and Fourth phase

Diagram of Third and Fourth phase

Fourth phase In the fourth phase of the cycle rainfall is continued and all

Fourth phase In the fourth phase of the cycle rainfall is continued and all the available natural storages have been utilized. Infiltration may take place at a rate equal to the rate of transmission of water through the zone of aeration either to the water table or stream channel as subsurface storm flow. In areas of low permeability downward percolation is prevented and infiltrated water comes out on the surface to reach stream channels as overland flow. As rain continued water table rises persistently until it reaches the land surface almost. An extreme peak of ground water table is only reached in flat swampy areas. The effects of snow and frost remain the same as it is in the third phase.

Fifth phase The fifth phase is marked by cessation of rainfall although sufficient time

Fifth phase The fifth phase is marked by cessation of rainfall although sufficient time has not elapsed. Surface retention will be depleted through evaporation and infiltration. Transpiration will be done by plants. Water in the zone of aeration will reach the water table or stream channel. Stream flow is sustained by draining out of storage in stream channels, subsurface storm flow, and base flow. Evaporation from ground and water level of streams will continue. If the temperature is above freezing the net effect of snow and ice is to prolong this phase by the time necessary for the depletion of snow and ice storage. If it is below freezing the conditions are not materially modified.

Diagram of Fifth phase

Diagram of Fifth phase

Criticism Snyder has pointed out that over a basin of considerable size with variable

Criticism Snyder has pointed out that over a basin of considerable size with variable conditions of slope, vegetative cover, soil and geology the processes of runoff cycle do not synchronize and each process differs in magnitude in different parts of the drainage basin.