The Storm Hydrograph refers to graphs that show how a drainage basin responds to a period of rainfall. Storm hydrographs are very important in predicting the flood risk and in making the necessary precautions to avoid damage to property and loss of life. Physical factors are the biotic factors which affect organisms and their surroundings, their surroundings in this case being the drainage basin and therefore the storm hydrograph is affected. These physical factors would be factors such as Basin, Size Shape and Relief, types of precipitation, land use; they contribute to regulating the ways in which a river responds to precipitation.
The size, shape and relief of the basin are very important controls. For instance if the basin is small it is most likely expected that the rainfall will reach the main channel more rapidly than in a larger basin where the rainfall has more area to cover and further to travel therefore reaching the main channel at a slower rate. Therefore lag time would be shorter in a small basin than that of a large basin. Shape also contributes to the varying lag times of basins. Drainage basins with a circular shape or profile are more likely to have a shorter lag time and a higher peak flow. This is because all the points on the watershed are equally distant from the gauging station. Whereas drainage basin with a more elongated shape or profile tends to have a longer lag time than the circular shape basin because of the extremities of the basin to reach the gauging station. Relief on the other hand refers to the general unevenness of the basin or the elevation or difference in elevation of the landscape. Where gradients are steep with a lot of sloping uneven land such as a valley, water runs-off faster and reaches the river channel more quickly than that of a gentler more even landscape where water will slowly crawl off in to the river channel such as plains.
Percitipitation refers to rain, sleet, snow, hail and other forms of water falling from the sky. Different types and amount of precipitation can have a varying effect on the storm hydrograph. Heavy rainfall such as conventional rainfall results in a lot of excess rain water which was not able to be infiltrated into the soil, there is also a lot of run-off, a shorter lag time and a rapid rise in river levels. Whereas a slow light rainfall can be quickly absorbed by infiltration into the soil causing less excess water therefore a reduction in surface run-off and a longer lag time. When there is heavy snowfall large amounts of water are kept in temporary storage on the surface therefore river levels drop. However when temperatures rise rapidly the melted water soon reaches the river channel and increases the river levels with a short lag time.
Land use also influences and changes the response of the hydrograph to rainfall. Land with a lot of vegetation, help to prevent flooding by intercepting rainfall, this is the temporary storage of moisture on the leaves of vegetation before it evaporates back into the atmosphere. Also plant roots especially those of trees reduce through flow by taking up water from the soil. Therefore there is less surface run-off and river levels drop and there is a longer lag time. Whereas regions where there is a lot of deforestation flooding is more likely to occur. When land is being cleared for the use of agriculture and large settlement maybe resulting from urbanization, more and more land is needed to facilitate the increasing number of persons. This therefore increase the flood risk as water cannot infiltrate through tarmac and concrete therefore causing an increase of surface run-off with a very short lag time.
All these various factors contribute to how different drainage basins regulate and the way in which a river responds to precipitation.