CRWR-PrePro Calculation of Hydrologic Parameters Francisco Olivera, Ph.D. Center for Research in Water Resources University of Texas at Austin Texas Department of Transportation (TxDOT) Water Resources Modeling Using GIS Austin, Texas

Hydrologic Parameters zWatershed yAbstractions and routing method yParameters zStream yRouting method yParameters

Watershed Abstractions and Routing Method zAbstractions: rainfall/runoff transformation ySCS curve number method yInitial loss + constant rate loss zRouting: runoff/flow transformation ySoil Conservation Service (SCS) synthetic unit hydrograph

Watershed Abstractions Soil properties Precipitation (mm/hr) Runoff (mm/hr) Runoff = f(precipitation, soil properties, moisture conditions)

Watershed Routing Runoff (mm/hr) Flow (m 3 /s) Watershed

Watershed Routing Runoff (mm/hr) Flow (m 3 /s) Time Runoff and Flow Flow = f(Runoff, Watershed hydrologic properties) Time

Hydrologic Parameters zWatershed yAbstractions and routing method yParameters zStream yRouting method yParameters

Watershed zArea zAverage curve number zInitial + constant rate loss zLength of longest flow-path zSlope of longest flow path zLag time

Watershed Abstractions Average Curve Number SCS Method Initial + Constant Method Initial Loss (mm) Constant Loss (mm/hr)

Elevation Grid

Flow Length Downstream

Flow Length Upstream

Longest Flow-Path

Flow Length Downstream to the Watershed Outlet

Hydrologic Parameters zFlow length downstream to the sub-basin outlet.

Flow Length Upstream to the Watershed Divide

Hydrologic Parameters zFlow length upstream to the sub-basin divide.  A NODATA cell is defined at the sub-basin outlets before running the flow length function.

Watershed Longest Flow-Path

Hydrologic Parameters zSub-basin longest flow- path. zThe longest flow-path is the geometric locus of the points for which the sum of both flow lengths is a maximum.

Slope of Watershed Longest Flow Path

Hydrologic Parameters t p :sub-basin lag-time (min) L W :length of sub-basin longest flow-path (ft) CN :average Curve Number in sub-basin S :slope of the sub-basin longest flow-path (%)  t: analysis time step (min) zSub-basin lag-time according to the SCS formula:

Watershed Lag-Time (SCS) Flow length upstream and downstream Average Curve Number Length and slope of the longest flow-path Identification of the longest flow-path Lag-time

Hydrologic Parameters t p :sub-basin lag-time (min) L w :length of sub-basin longest flow-path (ft) V w :average velocity sub-basin longest flow-path (m/s)  t: analysis time step (min) zSub-basin lag-time according to 0.6 L/v formula:

Watershed Lag-Time (L/V) Flow length upstream and downstream Longest flow-path average velocity Length of the longest flow-path Lag-time

Hydrologic Parameters zSub-basin parameters: Grid code, area (Km 2 ), unit hydrograph model (SCS), length of longest flow path (m), slope of longest flow path (fraction), average curve number, lag-time (min), baseflow (none).

Hydrologic Parameters zWatershed yAbstractions and routing method yParameters zStream yRouting method yParameters

Routing Method

Pure-Lag Method zFlow is delayed a fixed amount of time t lag. t lag : flow time in the reach I: inflow to the reach Q: outflow from the reach

Muskingum Method zFlow is delayed a fixed amount of time K, and redistributed around its centroid. S: storage in the reach K: flow time in the reach (  t < K <  t/2X ) X: storage parameter I: inflow to the reach Q: outflow from the reach  t: analysis time step

Routing Method Flow time (L s /V s ) L s /V s <  t Muskingum Routing L s /V s >  t Pure Lag Routing L s :length of the stream V s :flow velocity in the stream  t: analysis time step

Hydrologic Parameters zWatershed yAbstractions and routing method yParameters zStream yRouting method yParameters

Hydrologic Parameters t lag :reach lag-time (min) L s :length of reach (m) V s :reach average velocity (m/s) zReach lag-time for Pure Lag routing:

Hydrologic Parameters K :Muskingum parameter K (hr) X :Muskingum parameter X L s :reach length (m) V s :reach average velocity (m/s) n :number of sub-reaches zReach lag-time and number of sub-reaches for Muskingum routing:

Hydrologic Parameters  Pure lag routing for L s /60v s <  t, otherwise Muskingum routing. zReach parameters: Grid code, sub-basin code, length(m), velocity (m/s), routing method (Lag or Muskingum), lag time (min), Muskingum X, Muskingum K (hr), number of sub-reaches.