Chapter 6: Catchment characteristics
Catchment characteristics interact with variable patterns of rainfall and determine the character and size of runoff volumes and peak flows. This is true for both natural catchments where human activity is absent or unimportant and runoff plots upon which tillage or other agricultural treatments are being tested. Generally, there is a hierarchy of influence imposed by different characteristics, but this hierarchy is often difficult to sort out and understand. For example, where a catchment has high slopes and lime vegetation, slope will play a major role in determining the runoff regime. This regime would tend to exhibit high runoff proportions; rapidly increasing flows to high peaks and equally rapid falls. Were this catchment to be of a more linear form and were its vegetation cover to increase, then peak flows would be smaller but more prolonged and total runoff volumes would probably be less. Were the slope lower, runoff would probably be less.
In the case of catchments with low slopes, the effects of vegetation cover and microtopographic features often exert a stronger influence over runoff than the overall land slope. Local slopes are often relatively high and they may direct runoff either into basins where it can infiltrate or to channels by which it can easily leave the catchment. Heavy textured soils tend to give a higher proportion of runoff. Soil textures are related to slope as well as to parent material, and the climatic regime under which the soil formed will often have been a determining factor of the soil textural type. Where human interventions have been imposed the natural conditions of a catchment may have been altered radically; grazing, tree-felling and clearance are obvious examples. Agricultural techniques; ploughing, bunds and microcatchments are introduced to reduce runoff and usually they do, but the removal of natural vegetation and badly managed systems can have the opposite effect. Below, the main catchment characteristics and their influence on runoff are discussed.