STA412.5__Lecture_3.pd Water and Soil Manage

A lecture note from the university lecturer. This module lecture is covered by the students streamed into Agricultural production strand, as well as the Agricultural business student. It pays a vital role in boosting the technological capacity of many students in the university.

1. Runoff
Runoff is the water flow that happens when rainfall exceeds the rate
at which it infiltrates into the soil, or when soil infiltration reaches its
maximum capacity. The 'extra' water that is left over after rain and
other sources flow over the land.

2. Objective
Why do I have to know runoff?
❖To comprehend the hydrologic cycle and how the surface of the Earth is affected by it
❖To determine the necessary skills and knowledge to manage factors that increase or
minimize the adverse effects of the rainfall-runoff
❖To be familiar with the topography, rainfall patterns, vegetation cover, altitude, slope,
and air concentration of a certain place.
❖To determine the flood risk when farming, constructing roads, building bridges, and
constructing buildings.
❖To understand how runoff from rainfall affects the growth of plants, soil, water quality, and
human lifestyles.
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3. Rainfall-runoff
Without considering the causes or relationship between rainfall runoff and water challenges, a
solution cannot be provided. The problems include inadequate water distribution, excessive
watering, or neither, which might restrict plant growth. An understanding of rainfall-runoffcan help
with challenges related to managing irrigation, building drainage, encouraging infiltration,
minimizing soil erosion, and conserving water.
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4. Hydrologic cycle
❖The flow of water over, through, and beneath the surface of the planet Earth makes up the
water cycle.
❖Rain, snow, and sleet are all forms of precipitation. Rainfall can either: (1) be absorbed by
plants, (2) infiltrate into the ground, (3) runoff (flowing over the surface) or (4) evaporate.
❖The hydrologic cycle is maintained through transpiration, evaporation occurs from the surface
of plants, water bodies, and soil.
❖When a portion of the rainfall reaches the soil, restores water aquafers, and slowly seeps into
rivers, streams, and oceans, the rest of the rainfall flows over the land as runoffs to streams and
rivers.
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5. Diagrams of the hydrologic cycle
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6. Rainfall is measured in millimeters (mm)
❖Estimating or measuring the rate, depth, and frequency of rainfall must be done when
designing soil and water management systems.
❖Millimeters (mm) of rainfall are measured using manual and standalone automatic rain gauges.
❖For modeling purposes, the rainfall data is precisely recorded, examined, and kept.
❖The representative quality data is helpful to scientists, engineers, project planners, and other
professionals.
❖Rainfall data can be statistically analyzed to predict storms with a given probability
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7. Example 1; Types of rain gauges
Manual rain gauge
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TB3 tipping bucket rain
gauge

8. Example 2; Table showing Quality
representative data
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10. Intensity, Duration and frequency of
rainfall
❖The standard unit of measurement for rainfall intensity and frequency is millimeters per hour
(mm/h).
❖ High-intensity storms are rapid and only cover limited areas, or they can last for several
days and cover massive areas. Such an event rarely occurs and causes floods in PNG.
❖Rarely can high intensities and long durations occur, although they do occasionally when
there is a lot of rainfall, which can cause disastrous floods and soil erosion.
❖While depth increases with duration, rainfall intensity decreases with time. The "RETURN
PERIODS" relationship indicates the likelihood of events occurring once every ten or one
hundred years.
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11. Comparison of ground water vs rainfall
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12. Storm duration, rainfall intensity, and
depth
oDuration (hours): Length of time over
which rainfall (storm event) occurs.
oDepth (inches/mm): Total amount of
rainfall occurring during the storm
duration.
oIntensity (inches/mm per hour):
Intensity is calculated by dividing the
depth by the duration.
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13. Calculating 10-year return period
storm in 100 years
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NOTE: This is an approximate method to be used when local data is not available.

14. Rainfall factor
The rainfall erosivity factor (R-factor) is based on kinetic energy considerations of falling rain
(Whelan 1980) and represents a measure of the erosive force and intensity of rain in a normal
year (Goldman et al. 1986)
Question: Determine rainfall intensity and total rainfall 24-hour duration storm that will occur
once in 10 years at PNG UNRE, Rabaul PNG.
❖Read the intensity of 0.2 mm/h for a 24-hour storm in the previous slide. By interpolation,
the geographic rainfall factor is 1.1. The average rainfall intensity is ;
❖i, = 0.2 mm/h x 1.1 = 0.22 in/h (5.6 mm/h)
❖The rainfall amount in 24 h is = i , 0.22 in/h x 24 h = 5.3 in (135 mm)
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15. Runoff
❖When rainfall contacts the land surface, infiltrate and some will be stored in surface
depression and the excess flows as runoffs.
❖Peak runoff rate and total runoff volume must be estimated for planning water drainage
systems, water control structures, and reservoirs.
❖Storm properties are; depth, duration, distribution, and return periods of a given event.
Intense storms = greater peak runoff rates, longer duration storms produce watershed
properties or total runoff , soil hydrologic features, land use, farming, etc.
❖The topographical features: length, slope, shape, and relationship of the watershed to the
direction of storm travel.
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16. Rainfall hydrographs
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17. Peak runoff rate
❖There are many methods for determining
peak runoff rates from simple to complex
computer models.
❖It depends on length, slope, area of the
watershed, curve number, and time of
concentration (the time it takes to travel
from the watershed to the outlet). Refer
to the hydrograph shown above.
❖To calculate peak runoff, the time of
concentration and the features above
should be estimated using the equation
below.
❖Tc = L^0.8(1000-9)/1140s^0.7
❖Tc = L^0.8(1000-9)/1140s^0.7
❖Tc = time of concentration in hours, L is the
watershed in length,
❖s is the average % slope of the watershed.
❖Determine the time of concentration peak
runoff rate for a 50-year return period storm
if the watershed of the Vudal area is 50,000
m² and the max flow length is 609.6 m.
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18. Peak runoff rate (Rational method)
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Rational method
Q = CiA
Whereas Q = discharge, C = co coefficient, A = area of
the runoff catchment
The rainfall total for the month of July at Lihir was 305
mm. The entire open pit catchment area of the mine is
0.5 km² with 50% of rehabilitated land with trees.
Use a co-efficient of 0.7 to
calculate the total runoff
that occurred in July.

19. Calculating peak runoff using the
rational method
Q = CiA
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20. Example #1: Runoff co-efficient
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21. Time of concentration (tc)
❖Note that the goal of calculating the duration of concentration is to identify the place
from which the drainage area outlet will require the longest travel time.
❖The distance between this point and the drainage exit does not always have to be the
longest. Why?
❖There are various techniques used to calculate the time of concentration. The most
popular one calculates the total time of travel or concentration by adding the times for
sheet flow, shallow concentrated flow, and open channel flow which are computed
independently.
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22. Time of concentration
Where,
❖tc = time of concentration (minutes)
❖i = rainfall intensity (in/hr)
❖L = overland flow length (ft)
❖n = Manning’s roughness coefficient
❖S = slope of the surface (ft/ft or m/m
Applying this equation involves a repeated procedure in which the time of concentration must
be calculated after choosing the rainfall intensity for a specific duration during the return period.
This process must be repeated until the selected duration roughly corresponds to the calculated
concentration time.
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23. Tc equation
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24. Curve Numbers
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What percentage of an event's rainfall turns into the runoff is represented by the curve
number. Based on various soil groups, curve numbers are assigned. With the aid of the
registered topographic maps, the soil texture map of the research region was traced,
scanned, and corrected in ArcGIS.

25. Definition of SCS Hydrologic Soil
Groups
Group A
Even when fully wet, group A soils have strong infiltration rates and little potential for
runoff. They are mostly made up of deep, well-to-overly-drained sands or gravels, and
they transmit water at a high pace (more than 0.30 inches per hour).
Group B
Group B soils are mostly medium-deep to deep, moderately well to well-drained, and
have textures that range from moderately fine to moderately coarse. These soils have
moderate infiltration rates when fully wet. The rate at which water moves through these
soils is moderate (0.15 to 0.30 in./hr).
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26. Curve Numbers (CN) Continue….
Group C
Group C soils are characterized by fairly coarse textures and a layer that prevents water
from moving downward. These soils have poor infiltration rates when completely wetted.
The rate at which water moves through these soils is moderate (0.05-0.15 in./hr).
Group D
The possibility for runoff is significant in Group D soils. They are mostly clay soils with a high
swelling potential, soils with a persistent high-water table, soils with a claypan or clay layer
at or near the surface, and shallow soils over practically impermeable material. They have
very poor infiltration rates when thoroughly wetted. The rate of water conveyance in these
soils is relatively low, 0.00 to 0.05 in./hr.
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27. Calculating 10 years period in 100
years of rainfall
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27Year JanuaryFebruaryMarch April May June July AugustSeptember OctoberNovemberDecemberMonthly Max Annual Total
2003 436 188.8 434.8 204 237.2 45.7 169.6 73.1 131.4 248.9 159.4 155.2 436.0 2,484
2004 189.3 271.5 271.5 461
2005 134.6 39.2 54.8 269.2 259.8 129.6 196.4 285.5 285.8 248.2 100.5 255.4 285.8 2,259
2006 188.8 188.7 313.8 216.5 176 216.5 236.6 361.5 318.6 109.5 310.8 213 361.5 2,850
2007 232.3 128.2 165 294 101 126.6 139.7 96.2 234.8 194.9 272.2 38.6 294.0 2,024
2008 145.4 318.7 145.4 97 185 143.6 135 318.7 1,170
2009 341.9 130.9 305.1 173.9 520.8 192.4 277.6 239.3 520.8 2,182
2010 229.4 260.8 384.3 202 344.8 39.2 52.4 95.5 108.5 206.6 384.3 1,924
2011 20.2 325.3 146.8 212.4 169.3 325.3 874
2012 159 454.9 316 479.6 151.3 104.2 104.4 88 222.7 479.6 2,080
2013 150.2 310.2 141.4 141 57.9 64 159.6 310.2 1,024
Average 202.5 240.1 253.4 227.8 185.6 134.4 149.2 167.7 260.4 198.8 224.1 184.7 202.4 1,821
Max 436.0 454.9 434.8 479.6 344.8 216.5 236.6 361.5 318.6 248.2 310.8 255.4 341.5 2,850
Min 341.9 39.2 54.8 97.0 101.0 126.6 135.0 96.2 234.8 109.5 100.5 38.6 122.9 460.8

28. Steps to calculate Probability & return
periods
1)Sort out the data from the largest to
the smallest
2)Rank the data 1 – 10
3)Use the probability formula to
calculate Frequency Analysis ( %)
4)Use the Reoccurrence interval formula
to calculate return periods
❖Probability = rank /n + 1 x 100
❖Reoccurrence interval = n + 1/rank
❖n = number of years
❖Rank = Rearranging data from the
smaller to the larger.
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29. Data sorting
(Refer to data calc in xl)
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Note: Data re-arranged from the largest to the
smallest

30. Questions
❖What are the factors that affect rainfall runoff?
❖How would you measure the storm duration?
❖In a watershed, how do you compute probability and recurrence intervals for rainfall and
water level/stream discharges?
❖What's the importance of calculating the probability and return periods?
❖Why is it necessary to know the soil groups and types when dealing with infiltration and
permeability?
❖Why is it important to know about infiltration?
❖What is the rational method to calculate peak runoff?
❖ How does groundwater correlate with rainfall events as shown in the previous slide?
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31. Summary
❖The types of soil, vegetation, landscape/topography, and precipitation significantly affect runoffs
❖ The calculation of flood events based on probabilities and reoccurrences gives much confidence
for managing soil, drainage, available water, and cost
❖ The curve numbers give you preliminary details of soil types and this is derived from the use of
satellite imaginary systems or GIS technology.
❖The numbers can be used to determine the duration of water movement from the initial saturation
point to the exit to rivers or sea
❖Quality data is accurate data collected by trained personnel consistently following standard
operating procedures
❖ Seepage tests and other methods can be used to determine the rate at which water percolates
through the soil, which is dependent upon the groups and types of soil.
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32. Reference
❖Mutha R, Porchelvan P (2009) Estimation of surface runoff in Malattar sub-
watershed using SCS-CN method. J Soc Remote Sens 37(2):291–304
❖Bansode A, Patil KA (2014) Estimation of runoff by using SCS curve number
method and arc GIS. Int J Sci Eng Res 5(7):1283–1287
❖Bhura CS et al (2015), Estimation of surface runoff for Ahmedabad urban
area using SCS–CN method and GIS, IJSTE. Int J Sci Technol Eng 1(11):2349–
2784
❖Chow VT, Maidment DK, Mays LW (2002) Applied Hydrology, McGraw -Hill
Book Company, New York, USA
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