Agricultural Water Runoff: Causes and Management Tips

Runoff, defined as the overflow of water when the land's capacity for absorption is exceeded, can result from a combination of natural phenomena and human interventions. This excess liquid traverses the terrain, eventually finding its way into nearby creeks, streams, or ponds. In essence, runoff comprises the portion of rainfall making its journey to streams and rivers, amidst the initial losses.

Three primary types of water runoff include surface runoff, subsurface runoff, and base flow.

Surface Runoff:

This is the portion of rainfall that enters the stream immediately after it has fallen. When all losses are satisfied, and rain continues at a rate greater than its filtration rate, the excess water forms a head over the ground surface (surface detention) and tends to move from one location to another; this phenomenon is known as overland flow.

Surface runoff occurs when overland flow meets streams, channels, or oceans.

Sub-surface Runoff:

It is the portion of rainfall that first leaches into the soil and moves laterally without connecting to streams, rivers, or oceans. Subsurface runoff is sometimes referred to as service runoff because it takes so little time to reach the river or channel in comparison to groundwater.

Interflow is the term used to describe subsurface runoff.

Base flow:

It is defined as that part of rainfall that, after falling on the ground surface, fills the soil, meets the water table, and flows to the streams or oceans. The movement of water in this type of runoff is very slow, which is why it is also referred to as delayed runoff. It takes a long period of time to join rivers or oceans. Sometimes, base flow is also known as groundwater flow.

Thus, Total Runoff = surface runoff + base flow (Including sub-surface runoff).

Factors Affecting Water Runoff

Water runoff is significantly influenced by two key factors: climatic factors and physiographic factors. Among the climatic factors, the watershed's runoff is intricately tied to precipitation characteristics, encompassing the types of precipitation, rainfall intensity, forms of precipitation, duration of rainfall, and rainfall distribution.

The direction of the prevailing wind and other climatic elements play a pivotal role in shaping the dynamics of runoff within a given area.

i). Climatic Factors Affecting Water Runoff

a). Type of precipitation:

Different types of precipitation have a significant impact on runoff dynamics. Rainfall, for instance, initiates surface flow immediately as it falls, its intensity and magnitude determining the runoff's extent.

In contrast, precipitation in the form of snow or hail doesn't contribute to immediate runoff; it takes place only after the gradual melting of the frozen forms. During this melting process, the water infiltrates the soil, leading to minimal surface runoff generation.

b). Rainfall intensity:

The intensity of rainfall plays a pivotal role in determining runoff yield, with rainfall intensity surpassing the soil surface infiltration rate leading to rapid runoff, while lower rainfall intensities exhibit the opposite trend. Consequently, higher intensities of rainfall result in increased runoff, and conversely, lower intensities yield diminished runoff.

c). Duration of rainfall:

The duration of rainfall is intricately linked to the volume of runoff, primarily because the soil's infiltration rate gradually diminishes as rainfall persists, eventually stabilizing. Consequently, even a mild-intensity rainfall that lingers for an extended period can produce a substantial and lasting runoff.

d). Rainfall distribution:

The runoff in a watershed is significantly influenced by the pattern of rainfall distribution. This distribution can be quantified using the "distribution coefficient," which is the ratio of the maximum rainfall at a specific point to the mean rainfall across the entire watershed.

When all other factors remain constant and the total rainfall is consistent, a higher distribution coefficient leads to increased runoff, while a lower coefficient results in reduced runoff.

It's important to note that even with the same distribution coefficient, the amount of runoff can vary depending on where the storm predominantly occurs within the basin, particularly closer to the outlet or the lower part of the watershed.

e). The direction of the prevailing wind:

The direction of the prevailing wind plays a pivotal role in shaping runoff dynamics, with profound implications for drainage systems.

When the prevailing wind direction aligns with the natural flow patterns, the drainage system exerts a more pronounced influence on both the magnitude of peak flow and the duration it takes for surface runoff to reach the outlet.

Consequently, storms traveling in sync with the stream's slope yield a more rapid and intensified peak flow compared to those moving against the natural course of the waterway.

f). Other climatic factors:

Various climatic factors, including temperature, wind velocity, relative humidity, and annual rainfall, exert a significant influence on the overall water loss within a watershed area, subsequently impacting the volume of runoff. In essence, when these losses are more pronounced, they lead to a reduction in runoff, and conversely, if these losses are minimized, runoff increases in response.

ii). Physiographic Factors Affecting Water Runoff

The physiographic factors of a watershed encompass both the characteristics of the watershed and its channel. These combined attributes play a pivotal role in influencing runoff.

The various elements that impact runoff include the size and shape of the watershed, its slope and orientation, the land use within it, soil moisture and type, topographic features, and drainage density.

These factors collectively shape the hydrological behavior of a watershed and its associated channels, highlighting the interconnectedness of these variables in governing water flow and runoff patterns.

a). Size of the watershed:

Regarding the size of the watershed, if all other factors, including the depth and intensity of rainfall, are the same, then two watersheds, irrespective of their size, will produce approximately the same amount of runoff. However, a large watershed takes longer to drain the runoff to the outlet; as a result, the peak flow and depth are smaller, and vice versa.

b). Shape of the watershed:

The shape of the watershed has a significant impact on runoff. The terms "form factor, compactness, and coefficient" are commonly used to describe the shape of a watershed.

c). Slope of the watershed:

The slope of the watershed plays an important role in runoff, but its effect is complex. It controls the time of overland flow and the time of concentration of rainfall in the drainage channel, which accumulates the effect on the resulting peak runoff.

For example, in the case of a steep watershed, the time to reach the flow at the outlet is less due to the greater runoff velocity, resulting in the formation of peak runoff very soon, and vice versa.

d). Orientation of the watershed:

This factor affects evaporation and transpiration losses in the area by influencing the amount of heat received from the sun. The north or south orientation of the watershed affects the melting time of collected snow. In a mountainous watershed, the windward side of the mountain receives a high intensity of rainfall, resulting in more runoff yield, while the leeward side of the watershed experiences the opposite, receiving a lower intensity of rainfall.

e). Land use:

The land use pattern and land management practices employed have a significant impact on runoff yield. For example, in a forest-covered area where a thick layer of leaves and grasses has accumulated, there is little surface runoff because more rainwater is absorbed by the soil. A reverse trend is observed in a barren field where no type of cover is available.

f). Soil moisture:

The magnitude of the runoff yield is determined by the amount of moisture in the soil at the time of rainfall. If rain falls on moist soil, the infiltration rate drops dramatically, resulting in a higher runoff yield. Similarly, if rain falls after a long period of dryness, the soil absorbs a large amount of rainwater. Runoff yield, on the other hand, has the opposite effect when rain falls in quick succession, as it does during the rainy season.

g). Soil type:

In the watershed, surface runoff is greatly influenced by the type of soil, as the loss of water from the soil is highly dependent on the infiltration rate, which varies with the type of soil.

h). Topographic characteristics:

These include more topographical features of the watershed that influence runoff. It is primarily the undulating nature of the runoff water that allows more power to flow due to the slope of the surface and the time it takes the water to infiltrate into the soil.

Note: Regarding channel characteristics to describe their effect on runoff, channel cross-section, roughness, storage, and channel density are mainly considered. These also have a significant effect on runoff.

Conclusion

In conclusion, understanding the complexities of water runoff is crucial for managing our environment and water resources effectively. It involves a delicate interplay of natural and human factors, from the type of precipitation and its intensity to the size and shape of watersheds. The climatic factors, including rainfall patterns and wind direction, influence the dynamics of runoff within a given area, while physiographic factors such as watershed size, slope, and land use further shape the hydrological behavior.

The impact of human activities on runoff cannot be overstated, as urbanization, deforestation, and land management practices can alter runoff patterns, leading to issues like increased flooding or decreased water availability. Therefore, managing and mitigating the effects of water runoff requires a comprehensive approach that takes into account both natural and anthropogenic factors.

By gaining a deeper understanding of the factors affecting water runoff, we can better conserve and manage our precious water resources, reduce the risk of flooding, and make more informed decisions about land use and environmental conservation. This knowledge is essential for sustainable water management in a changing world.