Hydrologic Cycle and Its Components

After reading this article you will learn about the hydrologic cycle and its components.

Runoff, a potential carrier of nutrients and sediment to streams and lakes is best described in the science of hydrology. Two basic physical principles governing the amount and distribution of water on the earth are those of mass conservation and energy conservation.

These principles, along with several empirical relationships, form the basis for most mathematical descriptions of the hydrologic phenomena. The principle of mass conservation is frequently illustrated by hydrologic cycle or by water budget for an arbitrary volume of soil.

In the most-simple form the hydrologic cycle can be represented as below:

From soil erosion and conservation point of view, the balance equation for surface runoff can be written as follows:

Components of the Hydrologic Cycle:

(i) Interception:

When rain begins, drops strike plant leaves and stems and are retained on those surfaces by the forces of adhesion and cohesion until a sufficiently thick film of water accumulates. If rain continues, the storage on an individual leaf will be­come nearly constant, with as much water falling from the leaf as falls upon it. Some water will also be lost from the film on vegetation by evaporation.

Rain intercepted by the canopy may subsequently reach the ground by dripping from the leaves or flowing down the stem. If stem flow is significant, it can produce substantial differences in soil-water content over rather small distances. The interception storage amounts for rain may range from 0.25 to 9.14 mm. For most of the agricultural crops the average value could be taken as 2.5 mm and 1.3 mm for most grasses.

Interception as such does not have an important in­fluence on runoff or deep percolation from fields. However, an increase in interception is partly responsible for the reduction in runoff caused by conver­sion from clean tilled crops to pasture.

Interception by dense cover of forest or shrubs may be as much as 25% of the annual precipitation. Trees may intercept nearly 1.25 cm from a long gentle storm. This interception also has a detention storage effect, delaying the progress of precipitation that reaches the ground only after running the plant or dropped from the leaves.

(ii) Depression Storage:

After interception storage has been filled and the infiltration capacity of the soil is exceeded so that all or part of the soil surface is saturated, water will accumulate in surface depressions. Water stored in depressions either evaporates or infiltrates into the soil, none of it runs off the surface.

Depression storage can be increased by cultural (agronomic or engineering) practices and, therefore, can be important in reducing direct runoff from fields. For example, under ideal conditions, as much as 6.25 cm of water may be stored in contour furrows and about 5 cm in level bench terraces.

The potential surface water storage decreases as land slope increases and is approximately half as great for a 7% slope as for a 1% slope. With annual cropping systems, storage capacity usually is maximum in the planting to first cultivation period.

The storage capacity then decreases and reaches a minimum during the harvest to plowing period. Although manipulation of depression storage is an obvious method, affecting surface runoff, the amount of change can be deduced only indirectly by analysis of rainfall and runoff.

(iii) Infiltration:

As rain falls on the soil surface or drips from the vegetation, the phenomenon of infiltration governs the amount of water that will enter the soil and thereby greatly affects the amount of surface runoff. Characteristics of both porous media and fluid affect the infiltration. The porosity, pore-size distribution and tortuosity of soil pores substantially affect the infiltration rate.

Sands have higher infiltration rates than silts or clays which have a higher porosity but much smaller size pores (Table 3.1). Soil compaction by use of heavy machines or trampling of livestock reduces infiltration capacity and increases surface runoff. Thus, changing the infiltration capacity by manipulating soil physical status can profoundly affect the amount of surface runoff.

Raindrop impact on bare soil breaks up soil aggregates into their com­ponent particles or much smaller aggregates which can be carried into larger pores by water and form a thin surface layer of low hydraulic conductivity. This surface layer then controls the infiltration rate.

Dense vegetation with massive root system and farming system that leave substantial amount of plant residues near the surface maintain high soil organic matter content and promote ag­gregate stability thus maintaining high infiltration rates. Since tillage increases the volume of large pores near the soil surface, it increases the infiltration rate.

(iv) Soil Water and Groundwater:

Water, stored in the soil and rock, is frequently separated into two components, i.e., the saturated or groundwater zone, and the unsaturated zone between the groundwater and the surface. Within the unsaturated zone water moves in response to gravitational and capillary potential gradients.

It may move generally downward during rainfall and generally upward after a long dry period, or it may move upward near the surface and downward in the lower part of the profile simultaneously. In general, water movement in the un­saturated zone will be predominantly vertical.

In some soils, a rather permeable top soil is underlain by a slowly permeable clay layer. If infiltration is rapid enough, the surface soil may become saturated, resulting into flow which is predominantly in a lateral downslope direction and is known as interflow. This water may reappear on the surface some distance downslope or at the foot of the slope.

(v) Evapotranspiration:

The sum of evaporation from the soil surface and transpiration from plants is called evapotranspiration which represents the transport of water from the earth to the atmosphere. It is important in agriculture, because it is required for crop growth and it affects the volume of direct runoff and deep percolation to the saturated zone.

Evapotranspiration transports more than 60% of the precipitation and, therefore, forms a major component in the hydrologic cycle. Its contribution could vary from 100% in arid regions to about 50% or less in hill regions.

Three physical requirements must be met for evapotranspiration from a sur­face to continue:

(1) There must be a supply of heat to convert liquid water to vapour;

(2) The vapour pressure of the air must be less than that of evaporating surface; and

(3) Water must be continually available at the evaporation surface.

When water is not limiting, the evapotranspiration rate is limited by the radiant energy and the adverted energy available. Wherever, the ground surface is not fully covered by the transpiring crop the evapotranspiration is less than from grasses, and the total runoff is greater. This is one reason why conversion from row crops to meadow or pasture usually reduces runoff.

(vi) Runoff:

Runoff is that portion of precipitation which makes its way toward stream, channels, lakes or oceans as surface or sub-surface flow. Commonly speaking, the term runoff implies to surface runoff only. Before runoff can occur precipitation must satisfy the demands of evaporation, interception, infiltra­tion, surface storage, surface detention and channel detention.

In general, runoff will occur only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. After the infiltration rate is satisfied, water begins to fill the depressions, small and large on the soil surface. As the depressions are filled overflow begins. Sometimes interception may be great enough to prevent light rains from wetting the soil.

As a transport medium for sediments with adsorbed nutrients, surface runoff is an important link between fields and streams or lakes. Surface runoff is classified somewhat arbitrarily as either overland flow or channel flow. Overland flow is sometimes considered to be thin sheet flow over a relatively smooth surface.

However, a more general and realistic definition would be the flow that is outside of the well-defined channel system. The mean velocity of overland flow (laminar flow) is directly related to the slope and is inversely related to the hydraulic resistance of the surface.

The hydraulic resistance varies widely, depending on the surface characteristics, from a Manning’s resis­tance coefficient of 0.02 for bare soil to 0.4 for a dense turf.

Such differences in hydraulic resistance would result in water being about six times deeper on the turf than on the bare soil for the same discharge. The velocity, of course, would be only one-sixth of that on the bare soil.

Greater depth on the dense sod would allow much more time for infiltration after the rainfall is stopped, resulting in less runoff even if the infiltration characteristics of the soils were the same. The decreased shear stress on the soil with a sod cover would also result in a much lower erosion potential. Since surface runoff is a major soil transporting agent.