INTRODUCTION TO HYDROLOGY January 9, 2018

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

U.S. Weather Disasters (2017) 16 disasters >$1 billion 2017 Est. > $300 billion 1980-2017 average is 5.8 events/yr 2013-2017 average is 11.6 events/yr

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

Hurricanes Harvey, Irma, Maria

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

Nepal September, 2017 Monsoon Flooding 1200 fatalities 40 million affected

The Hydrologic Cycle Continuous process in which water is evaporated from water surfaces and oceans, moves inland and precipitation generates runoff http://www.buffer.forestry.iastate.edu/Photogallery/illustrations/illustrations-1.htm

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 = 1.008 cfs - hr

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

Example Lake Problem

Solution

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

Mississippi River Watershed

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

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”

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

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

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

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

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

Distributed Radar Rainfall

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)

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

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

Response to Development Natural: http://www.pennsylvaniawatersheds.org/?page_id=3 Developed: http://www.flickr.com/photos/eddeedaniel/3519995531/

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

Impacts of Development Types