Factors Affecting Surface Runoff and Its Estimation: A Case Study of a Greenfield Land in New South Wales

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Introduction
Over the recent past, there has been an increase in the quantity of paved surface all over the
world. Paved surfaces have been majorly influenced by developments that are centered on
human inhabitation. Population increase is therefore the major factor that has led to an increase
in the quality of surface runoff (Norbiato, 2009). The increase in impermeable land and a
consequent decrease in natural ground leads to an increase in the amount of water that is
transported over land and decreases the rate of infiltration (Meyles, 2003). Therefore, surface
runoff may be described as the excess water that flows over the earth’s surface after a prolonged
rainfall (Huang & Tu, 2012). The runoff generally flows over the land surface and may create
channels that only disappear at a discharge point or at a location where the surface runoff does
not have sufficient energy to flow.
There are a number of stages that are involved in the formation of surface runoff. However, these
factors may be limited to some regions more than others and as such, it is important for the
hydrologist to establish the type of land prior to making any conclusions. To begin with, runoffs
are only created during a prolonged rainfall event (Rodriguez-blanco & M, 2012). When it
begins raining, the first drops of rain usually fall on the surface of the leaves as well as the stems
and other parts of vegetation. Therefore, these drops do not reach the ground surface but are
retained on the plants. It is usually the first stage in any rainfall event and is known as the
retention stage (Scherrer, 2007). The second stage comes after a prolonged rainfall event
whereby the amount of water that infiltrates into the soil exceeds the capacity of infiltration.
When the infiltration capacity of the soil is overcome by continuous rainfall, the excess water
tends to form puddles. Soils have different infiltration capacities but all this depends on texture.
Furthermore, this may also depend on the initial moisture content of the soil whereby soils that
have higher moisture content tend to have a lower infiltration capacity while the vice versa is
also true.Therefore,a land surface will continue to generate runoff as long as the infiltration
capacity is exceeded by the rainfall (Latron, 2008). When the infiltration capacity exceeds the
rainfall amount, the amount of water that may be transported as runoff decreases.
Surface runoff may either flow to a natural water stream or body or may move downslope to a
water treatment facility (Merz & Blöschl, 2006). However, this is more prevalent in urban
centers where there are sewer systems that are used for the transportation of wastewater. The two

most common sewer types in urban settings are the combined and separate sewer systems (Sun,
2015). The two systems may be used for the transportation of storm runoff to a treatment facility
or a water body. However, the combined system combines both the wastewater from households
with the runoff while the separate system has two channels the first one transporting wastewater
while the second transporting the runoff (FEMA, 2016).
The runoff from a particular location depends on a number of factors. Regardless, the rainfall
duration and intensity are the most fundamental factors that determine the amount and rate of
runoff (Penna, 2011). To begin with, the runoff depends on the type of soil. The arrangement of
the particles determines how the water flows into the deeper layers and the resistance to
infiltration (Scherrer, 2007). Furthermore, the arrangement of particles usually determines the
water storage and retention capacity of the soil and therefore plays a fundamental role in the
generation of surface runoff. The arrangement of the particles in sandy soils allows the
infiltration to be much more than that of other compact soils such as clay. Therefore the runoff in
clay soil is likely to be more than that of sandy soil after exposure to the same rainfall intensity
as well as duration.
There is the likelihood that rainstorms are much more likely to lead to runoffs than the normal
rains regardless of the type of soil. With an increase in the rainfall there is an increase in the
energy that the drops possess which result into finer soil particles rising to the top layers and
clogging the pores (Endale, et al., 2006). Clogging of the pores results to a decrease in the
infiltration capacity of the soils which means that there is an increase in the amount of runoff
generated after a light storm. The phenomenon can easily be observed in arid and semi-arid lands
where there are frequent rainstorms and runoffs. The runoffs are common regardless of the
rainfall duration because of the clogging of the pores.
Secondly, the runoff depends on the vegetation that is available. Interception of rainfall depends
on the type of vegetation as well as its growth stage. The interception storage depends on the
type of crop with particular concern on the size of foliage (Donker, 2001). The values used to
determine the interception storage vary between 1 and 4 (Blume, et al., 2007).Furthermore, the
amount of vegetation over a particular land surface reduces the amount of rainfall reaching the
soil surface and therefore limits the amount of water that may be available for infiltration.
Foliage also limits the amount of raindrops reaching the soil surface and as such limits the

energy that may be used to crumble the soil particles. The inability of the raindrops to crumble
the soil particles and clog the pores is a limitation to the amount of runoff that may be generated
from a particular surface.
Finally, runoff is affected by the type of slope as well as the size of the catchment. To begin
with, there is an increase in the amount of runoff with the increase in the slope. Furthermore, the
length of the slope also determines the amount of runoff that may be generated. Runoff increases
with a decrease in the length of the slope (Balram & Megh , 2016). Considering the size of the
catchment, an increase in the size means that there is lesser time required for concentration but
the runoff efficiency tends to be lower. The runoff efficiency is the unit of measurement of the
concentration of runoff per unit time and tends to be more when the size of the catchment is
smaller (García-Ruíz, 2008). The runoff may be measured using empirical or graphical method.
Methods such as unit hydrographs, the SCS-CN models require the proper estimation of the
rainfall as well as the duration in determining runoff.
The aforementioned are some of the factors that affect the runoff that is generated from a
particular area. However, there are various graphical as well as mathematical models that are
used in the estimation of the surface runoff and they play a fundamental role in project
development. Prior to implementing a new development project, it is wise to consider the amount
of runoff that was generated on the undeveloped land and the amount that will be generated after
the land has been developed. Estimating the new amount of runoff will be useful in the design
and construction of drainage channels as well as the upgrade of the current discharge channels.