Modeling Rainfall Runoff and Snowmelt in the Pine Flat Watershed By Rachael Hersh-Burdick USACE Water Management Sacramento District UC Davis Civil and Environmental Engineering Department

Goals of Pine Flat Modeling Project Can HEC-HMS accurately model snowmelt runoff volume? Can HEC-HMS accurately model snowmelt runoff volume? Inflow model for reservoir operators Inflow model for reservoir operators Create comfort and familiarity with HEC-HMS in Corps Sac District Create comfort and familiarity with HEC-HMS in Corps Sac District

Overview Characteristics of Pine Flat Characteristics of Pine Flat Model Structure Model Structure Snow-Melt Modeling with HEC-HMS Snow-Melt Modeling with HEC-HMS

Why model Pine Flat? Pine Flat Reservoir Elev. 970 ft

Central Valley Project Watersheds Pine Flat Basin Pine Flat Basin Mean elevation: 7,635 ftMean elevation: 7,635 ft Elevation Range: 700 – 14,000 ftElevation Range: 700 – 14,000 ft Area = 1,541 mi 2Area = 1,541 mi 2

Why model snowmelt? Average April 1 volume of water stored in the snow pack = 1.8 MAF Average April 1 volume of water stored in the snow pack = 1.8 MAF Reservoir Storage = 1.0 MAF Reservoir Storage = 1.0 MAF

Pine 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 snow vs Sun on snow Snow cold content f(snow density, temp) Wind Basic Snowmelt Modeling Concepts

Numerical Snow Models Energy Budget Components Components WindWind TemperatureTemperature Water VaporWater Vapor Radiation (net)Radiation (net) PrecipitationPrecipitation Advection (rain)Advection (rain) GroundmeltGroundmelt Detailed (layered) snow pack Detailed (layered) snow pack Temperature Index Components Components TemperatureTemperature PrecipitationPrecipitation Single layer snow Single layer snow Calibration of meltrate coefficient implicitly accounts for other factors Calibration of meltrate coefficient implicitly accounts for other factors (Dr. Steve Daly)

Temperature Index Degree-day approach Degree-day approach Fixed amount of snowmelt for each degree above freezingFixed amount of snowmelt for each degree above freezing Primary Equation: Primary Equation: Snowmelt= (Air –Freezing Temp)*C C = time variant factor that includes:Snowmelt= (Air –Freezing Temp)*C C = time variant factor that includes: total heat transfer at snow surface (LW & SW radiation)total heat transfer at snow surface (LW & SW radiation) latent heatlatent heat sensible heatsensible heat wind speed, aspect, slope, vegetationwind speed, aspect, slope, vegetation MeltrateMeltrate

Kings River Below North Fork ’97 Event Blue = Daily observed Flow; Red = Daily modeled flow

Getting a Better Calibration…. Different temperature gage Different temperature gage Adjust number of elevation bands Adjust number of elevation bands Increase baseflow Increase baseflow

Pine Flat SWE & Temperature Gages from ’97 Event

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

Getting a Better Calibration…. Different temperature gage Different temperature gage Adjust number of elevation bands Adjust number of elevation bands Increase baseflow Increase baseflow Create more subbasins Create more subbasins

The End