4.1 What is the runoff?
Runoff is the
water flow through the streams, rivers, and rills.
4.2 Types of runoff
(1)Surface runoff
After all losses such as
interception, infiltration, and transpiration overland flow have occurred. That
enters streams immediately as soon as.
(2)Subsurface runoff
The proportion of the rainfall
is entered into the soil vertically. Then the water flows laterally towards stream
without join to the water table. It takes little time to reach the stream.
(3)Baseflow
Delayed flow is defined
as base flow. First, enter the soil and vertically flows towards the groundwater
table.
Take relatively long
period such as month or year to join streams.
4.3 Factors
affecting runoff
(1)Climatic
-Precipitation
a) -Type of precipitation (Rainfall or snowfall)
b) -Rainfall intensity (When rainfall intensity is
relatively high, infiltration capacity is low therefore rainfall excess create
runoff.)
c) -Duration (When the duration of precipitation is
relatively high, infiltration capacity is less so runoff has occurred.)
d) -Rainfall distribution (Runoff vary according to the
proportions of the catchments. Outlets obtained relatively more precipitation
therefore peak flow has occurred more quickly.)
-Temperature
(Interception is relatively high at high temperature so most of the water evaporates
back to the atmosphere so runoff can be limited)
-Humidity
(Evaporation is limited under the high relative humid air so high runoff is
occurred.)
-Wind
velocity (Interception and transpiration is relatively high under high wind velocity
so runoff is limited.)
-Annual
rainfall (High annual rainfall forms larger rainfalls so runoff occurs
more quickly.)
(2) The direction of the prevailing wind
(When the direction of both prevailing wind and water flow is same, peak flow is
occurred very quickly.)
(3) Physiographic factors
-Factors
of watershed
(a)Size
of a watershed (Larger watershed need relatively more time to deliver water to
outlet, therefore peak flow has occurred later. Smaller watershed need
relatively less time to deliver water to the outlet, therefore peak flow is
occurred fast.)
(b)The shape of a watershed- Fan-shaped, fan-shaped (elongated) and broad shaped
(In fan shaped watershed, nearby tributaries deliver runoff toward the outer
more quickly than furthest tributaries, therefore, peak runoff is less. In a broadly
shaped watershed, all of the tributaries are delivered runoff the same time towards the outlet. Therefore peak runoff is occurred more quickly and has a large amount)
(c)Orientation of watershed
(Win wand side has occurred relatively more runoff than a leeward side because the windward side of mountains gets more rainfall than the leeward side.)
-Land
use
·
Forest (Soil of
forests have relatively more organic matter so infiltration capacity is high
therefore runoff is relatively lees.)
·
Barren land
(Soils of barren land have relatively lowest organic matter so infiltration
capacity is less therefore runoff is relatively high.)
-Factors of soil
(a)Type of
soil(Light textured soils such as sandy soils have relatively lower runoff than
heavy soils such as clay because light-textured soil has relatively larger
pores that are well connected together so infiltration capacity is high
therefore runoff is less.)
(b)Soil
moisture (High moist soils have less infiltration capacity so runoff is high.
Less moist soils have high infiltration capacity therefore runoff is low.)
-Topographic factors
(a)Slope(Steep the slope has relatively high runoff
because the water flows more rapidly along the slope so soils can’t absorb
water as much as soils in flat land so infiltration capacity is lees therefore
runoff is high.)
(b)Channel
characters-Density, slope, length, storage, and shape
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