Excess Rainfall Reading for today’s material: Sections Slides prepared by V.M. Merwade Quote for today (contributed by Tyler Jantzen) "How many times it thundered before Franklin took the hint! Nature is always hinting at us. It hints over and over again. And suddenly we take the hint.“ Robert Frost

Excess rainfall Rainfall that is neither retained on the land surface nor infiltrated into the soil Graph of excess rainfall versus time is called excess rainfall hyetograph Direct runoff = observed streamflow - baseflow Excess rainfall = observed rainfall - abstractions Abstractions/losses – difference between total rainfall hyetograph and excess rainfall hyetograph

 -index  -index: Constant rate of abstraction yielding excess rainfall hyetograph with depth equal to depth of direct runoff Used to compute excess rainfall hyetograph when observed rainfall and streamflow data are available

 -index method Goal: pick  t, and adjust value of M to satisfy the equation Steps 1.Estimate baseflow 2.DRH = streamflow hydrograph – baseflow 3.Compute r d, r d = V d /watershed area 4.Adjust M until you get a satisfactory value of  5.ERH = R m -  t

Example TimeObserved RainFlow incfs 8: : : : : : : : : : : : : : : : : No direct runoff until after 9:30 And little precip after 11:00 Have precipitation and streamflow data, need to estimate losses Basin area A = 7.03 mi2

Example (Cont.) Estimate baseflow (straight line method) –Constant = 400 cfs baseflow

Example (Cont.) Calculate Direct Runoff Hydrograph –Subtract 400 cfs Total = 43,550 cfs

Example (Cont.) Compute volume of direct runoff Compute depth of direct runoff

Example (Cont.) Neglect all precipitation intervals that occur before the onset of direct runoff (before 9:30) Select R m as the precipitation values in the 1.5 hour period from 10:00 – 11:30

Example (Cont.)  t=0.27

SCS method Soil conservation service (SCS) method is an experimentally derived method to determine rainfall excess using information about soils, vegetative cover, hydrologic condition and antecedent moisture conditions The method is based on the simple relationship that P e = P - F a – I a P e is runoff volume, P is precipitation volume, F a is continuing abstraction, and I a is the sum of initial losses (depression storage, interception, ET) Time Precipitation

Abstractions – SCS Method In general After runoff begins Potential runoff SCS Assumption Combining SCS assumption with P=P e +I a +F a Time Precipitation

SCS Method (Cont.) Experiments showed So Surface –Impervious: CN = 100 –Natural: CN < 100

SCS Method (Cont.) S and CN depend on antecedent rainfall conditions Normal conditions, AMC(II) Dry conditions, AMC(I) Wet conditions, AMC(III)

SCS Method (Cont.) SCS Curve Numbers depend on soil conditions GroupMinimum Infiltration Rate (in/hr) Soil type A0.3 – 0.45High infiltration rates. Deep, well drained sands and gravels B0.15 – 0.30Moderate infiltration rates. Moderately deep, moderately well drained soils with moderately coarse textures (silt, silt loam) C0.05 – 0.15Slow infiltration rates. Soils with layers, or soils with moderately fine textures (clay loams) D0.00 – 0.05Very slow infiltration rates. Clayey soils, high water table, or shallow impervious layer

Example - SCS Method - 1 Rainfall: 5 in. Area: 1000-ac Soils: –Class B: 50% –Class C: 50% Antecedent moisture: AMC(II) Land use –Residential 40% with 30% impervious cover 12% with 65% impervious cover –Paved roads: 18% with curbs and storm sewers –Open land: 16% 50% fair grass cover 50% good grass cover –Parking lots, etc.: 14%

Example (SCS Method – 1, Cont.) Hydrologic Soil Group BC Land use%CNProduct%CNProduct Residential (30% imp cover) Residential (65% imp cover) Roads Open land: good cover Open land: Fair cover Parking lots, etc Total CN values come from Table 5.5.2

Example (SCS Method – 1 Cont.) Average AMC Wet AMC

Example (SCS Method – 2) Given P, CN = 80, AMC(II) Find: Cumulative abstractions and excess rainfall hyetograph Time (hr) Cumulativ e Rainfall (in) Cumulative Abstractions (in) Cumulative Excess Rainfall (in) Excess Rainfall Hyetograph (in) PIaFaPe

Example (SCS Method – 2) Calculate storage Calculate initial abstraction Initial abstraction removes –0.2 in. in 1 st period (all the precip) –0.3 in. in the 2 nd period (only part of the precip) Calculate continuing abstraction Time (hr) Cumulative Rainfall (in) P

Example (SCS method – 2) Cumulative abstractions can now be calculated Time (hr) Cumulati ve Rainfall (in) Cumulative Abstractions (in) PIaFa

Example (SCS method – 2) Cumulative excess rainfall can now be calculated Excess Rainfall Hyetograph can be calculated Time (hr) Cumulative Rainfall (in) Cumulative Abstractions (in) Cumulative Excess Rainfall (in) Excess Rainfall Hyetograph (in) PIaFaPe

Example (SCS method – 2) Cumulative excess rainfall can now be calculated Excess Rainfall Hyetograph can be calculated Time (hr) Cumulative Rainfall (in) Cumulative Abstractions (in) Cumulative Excess Rainfall (in) Excess Rainfall Hyetograph (in) PIaFaPe

Time of Concentration Different areas of a watershed contribute to runoff at different times after precipitation begins Time of concentration –Time at which all parts of the watershed begin contributing to the runoff from the basin –Time of flow from the farthest point in the watershed Isochrones: boundaries of contributing areas with equal time of flow to the watershed outlet

Stream ordering Quantitative way of studying streams. Developed by Horton and then modified by Strahler. Each headwater stream is designated as first order stream When two first order stream combine, they produce second order stream Only when two streams of the same order combine, the stream order increases by one When a lower order stream combines with a higher order stream, the higher order is retained in the combined stream