Modeling Rainfall Runoff and Snowmelt Modeling Rainfall Runoff and Snowmelt in the Pine Flat Watershedin the Pine Flat Watershed

ByBy

Rachael Hersh-BurdickRachael Hersh-Burdick

USACE Water Management Sacramento DistrictUSACE Water Management Sacramento District

UC Davis Civil and Environmental Engineering DepartmentUC Davis Civil and Environmental Engineering Department

Rachael.Hersh-Burdick@us.army.milRachael.Hersh-Burdick@us.army.mil

Goals of Pine Flat Modeling ProjectGoals of Pine Flat Modeling Project

Can HEC-HMS accurately model Can HEC-HMS accurately model snowmelt runoff volume?snowmelt runoff volume?

Inflow model for reservoir operators Inflow model for reservoir operators

Create comfort and familiarity with Create comfort and familiarity with HEC-HMS in Corps Sac District HEC-HMS in Corps Sac District

OverviewOverview

Characteristics of Pine FlatCharacteristics of Pine Flat

Model StructureModel Structure

Snow-Melt Modeling with HEC-HMS Snow-Melt Modeling with HEC-HMS

Why model Pine Flat?Why model Pine Flat?

Pine Flat ReservoirPine Flat ReservoirElev. 970 ft

Central Valley Project WatershedsCentral Valley Project Watersheds

Pine Flat BasinPine Flat Basin• Mean elevation: Mean elevation:

7,635 ft7,635 ft

• Elevation Range: Elevation Range: 700 – 14,000 ft700 – 14,000 ft

• Area = 1,541 miArea = 1,541 mi22

Why model snowmelt?Why model snowmelt? Average April 1 volume of water Average April 1 volume of water

stored in the snow pack = 1.8 MAFstored in the snow pack = 1.8 MAF Reservoir Storage = 1.0 MAFReservoir Storage = 1.0 MAF

2006 1997

Pine Flat DamPine Flat Dam

Pine Flat Precipitation (& Discharge) Gages ’97 Event

Kings R Above

Trimmer

Kings R Below

Trimmer

Pine Flat Flow in

(computed)

Temp

Groundmelt

Rain on snowvs

Sun on snow

Snow cold content

f(snow density, temp)

Wind

Basic Snowmelt Basic Snowmelt Modeling ConceptsModeling Concepts

Numerical Snow ModelsNumerical Snow Models

Energy BudgetEnergy Budget ComponentsComponents

• WindWind• TemperatureTemperature• Water VaporWater Vapor• Radiation (net)Radiation (net)• PrecipitationPrecipitation• Advection (rain)Advection (rain)• GroundmeltGroundmelt

Detailed (layered) Detailed (layered) snow packsnow pack

Temperature IndexTemperature Index ComponentsComponents

• TemperatureTemperature• PrecipitationPrecipitation

Single layer snowSingle layer snow Calibration of meltrate Calibration of meltrate

coefficient implicitly coefficient implicitly accounts for other accounts for other factors factors

(Dr. Steve Daly)

Temperature IndexTemperature Index Degree-day approach Degree-day approach

• Fixed amount of snowmelt for each degree Fixed amount of snowmelt for each degree above freezingabove freezing

Primary Equation:Primary Equation:• Snowmelt= (Air –Freezing Temp)*CSnowmelt= (Air –Freezing Temp)*C

C = time variant factor that includes:C = time variant factor that includes:• total heat transfer at snow surface (LW & SW total heat transfer at snow surface (LW & SW

radiation)radiation)• latent heatlatent heat• sensible heat sensible heat • wind speed, aspect, slope, vegetationwind speed, aspect, slope, vegetation• MeltrateMeltrate

Kings River Below North Fork Kings River Below North Fork ’97 Event’97 Event

Blue = Daily observed Flow; Red = Daily modeled flow

Getting a Better Calibration…. Getting a Better Calibration….

Different temperature gage Different temperature gage Adjust number of elevation bandsAdjust number of elevation bands Increase baseflowIncrease baseflow

Pine Flat SWE & Temperature Gages from ’97 Event

Snow Water Equivalent Snow Water Equivalent

Blue = Observed; Red= Modeled (in Reynold’s Creek Example)

Getting a Better Calibration…. Getting a Better Calibration….

Different temperature gage Different temperature gage Adjust number of elevation bandsAdjust number of elevation bands Increase baseflowIncrease baseflow Create more subbasinsCreate more subbasins

The EndThe End