Watershed Modeling using HEC-HMS and EPA-SWMM ©T. G. Cleveland, Ph.D., P.E. 10 July 2012 Lesson 2

Outline Hydrologic Principles Interpolation of irregular spaced data – Exercise: Interpolate an empirical hyetograph Digitizing Charts and Maps – Exercise: Delineate and measure a watershed using a digital planimeter.

Hydrologic Principles The Watershed The Hydrologic cycle – Precipitation – Losses – Runoff

Watershed The fundamental unit in surface water hydrology is the watershed. – It can be as small as a parking lot draining to an inlet, or continental sized (Bedient et al., 2008). A watershed is defined as the area on the surface of the earth that drains to a specific location. – The watershed is therefore defined both by the drainage location as well as topographic features that govern flow to that point.

Watershed Watershed physical properties are characteristics such as: – Area – Main channel length (if a main channel exists) – Slope (requires the specification of path), – Soil moisture/permeability, and similar measurable characteristics.

Watershed Watersheds also have descriptive properties such as: – %-developed – %- polluted, and so forth. These properties are certainly physical, but are called descriptive because there will be analyst interpretation in the specification of the values.

Watershed Physical properties are those things than can be measured from a topographic map. – Area, slope, length Descriptive properties are everything else. – Soil texture (and infiltration rate) – Fraction developed

Watershed An analysis or even design will likely start with watershed delineation. – Aerial imagery – Topographic map – Sewer drawings

Watershed What is the process of delineating watersheds? – Manual delineation involves drawing lines on a topographic map, and connecting the slope or ridge tops. Assuming the water will drain away from those points, the watershed is delineated by enclosing a polygon.

Watershed What is the process of delineating watersheds? – Automated delineation involves some technical skills with GIS-like databases. Digital Elevation Model (DEM) Stream network, and stream outlets. – The DEM and stream outlets can either be downloaded or constructed. If a new DEM must be constructed, consider cost-sharing with the U.S. Geological Survey to do so.

Manual delineation example in course reference CD – Topowatershed.pdf

Watershed Watershed physical and descriptive characteristics determined after delineation. – Areas and lengths Use Acrobat Pro tools to find areas and lengths on map image, convert to actual lengths using map scale. – Slopes Change in elevation along a path – Special concerns Sewers can cross topographic watershed boundaries Flat terrain – channels hard to define

Watershed Watershed physical and descriptive characteristics determined after delineation. – Descriptive characteristics Google Earth/aerial imagery can be used to define cover types and fractions. Soil maps for selected properties.

Watershed Minimal descriptions – Watershed boundary on a map Outlet Subsurface storm sewer network – Area that drains to the outlet – Main channel length Outlet to highest point in watershed

Watershed Minimal descriptions – Slope(s) – Descriptive (any or all) Soil type Fraction developed/impermeable etc.

Hydrologic Cycle Precipitation (Input) Runoff (Output) Loss

Precipitation – Rainfall (by far most important in Texas) – Snow, Sleet, Hail Meteorology – Synoptic storms – Cyclonic storms Data – NWS, local networks, SAO, NCDC (historical)

Losses – Infiltration Hortonian Loss Model Green-Ampt Loss Model NRCS Runoff Generation Model Initial Abstraction, Constant Rate Model – Evapotranspiration Thornwaithe Energy Balance Models

Transformation Transform the spatially and temporally distributed precipitation input to the outlet – Unit Hydrograph is an example of a lumped transform model

Storage and Routing Storage – Reservoirs, ponds, depressions store water and release later in time (as compared to the input) Routing – Moving water from one location to another on the watershed occurs over a path (route). – Routing develops the temporal relationship of input to the outlet from this process

Rainfall-Runoff Process Watershed –Losses –Transformation –Storage –Routing Precipitation –Meterology, Climate Runoff – Fraction of precipitation signal remaining after losses

Hydrologic Principles Watershed is fundamental unit – Area, Length, Slope, etc. Rainfall is the input function Losses and Storage are Watershed functions – Excess rainfall is fraction of rainfall signal remaining after storage and losses are satisfied – Runoff is the excess rain redistributed in time

Irregular Spaced Data Use linear interpolation to map irregular spaced data to fixed time or distance steps. – Seemingly simple task, but of immense importance. – Illustrate by example – won’t actually need to use this method until we get to historical data representation. Important point is that data need preparation outside of the modeling software!

Digitizing Maps and Charts Delineation first. Use a tool to find X-Y pairs of area of interest AND A REFERENCE area of known dimension. Describe enclosing polygon, measure its area, then scale to the reference area. Charts are similar except reference area has no meaning.

Exercise Exercise 2 – Delineate and measure a small watershed in Virginia (later we will pretend it is in Texas)

Documenting Your Work Like the prior exercise you are completing a table. As before complete the exercise and fill in the table, then print the report and hand-in.