Canopy and Surface Methods within HECHMS Hydrologic Engineering

Canopy and Surface Methods within HEC-HMS Hydrologic Engineering Center 1

Losses within HEC-HMS Interception / Canopy Storage Losses Surface / Depression Storage Infiltration / Soil Storage Hydrologic Engineering Center Evapotranspiration Excess Precipitation 2

Objectives • Provide an overview of the available canopy and surface methods within HEC-HMS • Supply commonly used parameter values • Discuss factors that affect canopy and surface losses Hydrologic Engineering Center 3

Canopy Interception • Vegetation can intercept precipitation and reduce the amount of that arrives at the ground surface • Vegetation can also extract water from the soil in a process called transpiration • Evaporation and transpiration are often combined and referred to as evapotranspiration Hydrologic Engineering Center 4

Available Canopy Methods • Dynamic Canopy • Gridded Simple Canopy • Simple Canopy Hydrologic Engineering Center 5

Common Canopy Storage Values • Zinke (1967) gives the following canopy interception depths: • These values must be calibrated and validated! Hydrologic Engineering Center Canopy Description Storage (in) General Vegetation 0. 05 Grasses and Deciduous Trees 0. 08 Coniferous Trees 0. 1 6

Surface / Depression Storage • Surficial depressions can temporarily store water – Puddles – Rough areas • Captured precipitation empties over time due to infiltration and/or evapotranspiration Hydrologic Engineering Center 7

Available Surface Methods • Gridded Simple Surface • Simple Surface Hydrologic Engineering Center 8

Common Depression Storage Values • Bennet (1998) gives the following surface depression depths: • These values must be calibrated and validated! Surface Description Storage (in) Steep, smooth slopes 0. 04 Most areas with impervious surface Most areas with pervious surface Flat agricultural land with conservation tillage Hydrologic Engineering Center 0. 125 – 0. 250 0. 25 – 0. 50 2. 0 9

Hydrologic Engineering Center 10

Factors that Affect Canopy Interception and Surface / Depression Storage • Storm character – Duration – Intensity – Precipitation form • Time of year • Vegetation – Type – Density • Catchment slope • Drought • Wildfire Hydrologic Engineering Center Blue River Lake, Sep 2020 11

Review • Three canopy and two surface methods are available within HEC-HMS • In general, canopy and surface losses are small • Multiple phenomena can affect the amount of canopy and surface losses • Next lecture will focus on infiltration methods within HEC-HMS Hydrologic Engineering Center 12