1. INTRODUCTION TO HYDROLOGY
January 9, 2018

2. Hydrology
The study of the occurrence, circulation, storage, and distribution of surface water and ground water on the earth
CEVE 412
Hydrologic Cycle
Meteorology
Statistical Hydrology & Extreme Events
Rainfall/Runoff & Land Use Effects
Hydraulics & Open Channel Flow
Flood Routing in River Systems
Flood Policy & Climate Change

3. U.S. Weather Disasters (2017)
16 disasters >$1 billion
2017 Est. > $300 billion
average is 5.8 events/yr
average is 11.6 events/yr

4. Oroville Dam, February 2017
Rains brought on by an atmospheric river caused Oroville Dam to overflow
Spillway Failure

5. Hurricanes Harvey, Irma, Maria

6. West Coast Wildfires $65 billion, 1.4 million acres
5/20 most destructive fires in state history

7. Nepal September, 2017 Monsoon Flooding 1200 fatalities
40 million affected

8. The Hydrologic Cycle
Continuous process in which water is evaporated from water surfaces and oceans, moves inland and precipitation generates runoff

9. Water Balance I – Q = ΔS = (dS/dt) I = Inflow (L3/t)
Q = outflow (L3/t)
dS/dt = change in storage (L3/t)
Find a better water balance/conservation of mass picture
1.0 ac inch = cfs - hr

10. Water Balance P – F –R –E – T = ΔS
Precipitation (P) – Rainfall, snow, etc.
Infiltration (F) – water entering the soil system, function of soil moisture, soil type
Runoff (R) – Overland flow, portion of precipitation that does not infiltrate
Evapotranspiration (ET)
Evaporation (E) – conversion of water to water vapor from a water surface
Transpiration (T) – loss of water vapor through plant tissue and leaves

11. Example Lake Problem

12. Solution

13. Watershed Contiguous topographic area that drains to a single outlet
Basic hydrologic unit within which all measurements, calculations and predictions are made

14. Mississippi River Watershed

15. Rainfall Measurement Why measure rainfall?
Water resources planning (annual & daily)
Large scale water supply studies (annual)
Long term climate planning
Urban drainage studies (5 min to 60 min)
Reduce localized flooding
Need intensity and duration of rainfall
Spatial variation inside watershed

16. Point Measurement Rainfall gage networks Methods of representation
Maintained by NWS, USGS or local organizations
Methods of representation
Accumulated total rainfall
“Cumulative mass curve”
Rainfall Intensity vs. time
“Hyetograph”

17. Areal Precipitation
The average depth of precipitation over a specific area (watershed)
Use point measurements to determine average rainfall
Three Methods
Arithmetic Mean
Thiessen Polygon Method
Isohyetal Method

18. Arithmetic Mean
Takes arithmetic mean of rainfalls from available gages
Not accurate for large areas with variable distribution
Only works if gages are uniformly distributed

19. Thiessen Polygon Method
Areal weighting of rainfall for each gage
Series of polygons created by connecting each gauge and drawing perpendicular bisectors
ratio of polygon area to total area of interest
Most widely used method

20. Isohyetal Method
Draw contours of equal precipitation based on gauge data
Needs an extensive gauge network
Most accurate method

21. Next-Generation Radar (NEXRAD)
Allows for measurement of rainfall rates and cumulative totals
Gridded rainfall data used
Rainfall data not lumped and applied over entire watershed
Benefits of radar
Provides a more complete coverage than gages alone
Can be very accurate when compared to gauge network

22. Distributed Radar Rainfall

23. Simple Rainfall Runoff
Rational Method
QP = CiA
QP = peak flow (cfs)
C = runoff coefficient
i = rainfall intensity (in/hr) for a duration equal to time of concentration (tc)
tc = time for a wave of water to propagate from the most distant point of a watershed to the outlet
A = area of watershed (acres)

24. Parameters that affect Watershed Response
Rainfall
intensity and duration
Size, Slope, Shape, Soil
Channel morphology
Channel type
Land use/land cover
Imperviousness

25. Hydrographs Rising Limb Crest Segment Recession Curve Inflection Point
Increase in flow
Crest Segment
Peak flow rate
Recession Curve
Decrease in flow
Inflection Point
Point where direct runoff ends

26. Response to Development
Natural:
Developed:

27. The Woodlands, TX
Master planned community idealized by George P. Mitchell in 1974
97,000 residents
6,000 acres of green space
160 miles hike &
bike trails

28. Impacts of Development Types