Lecture 13: Runoff Key Questions 1. What is Hortonian overland flow? 2. What is saturation from below and interflow? 3. What is subsurface flow? 4. Ho...
Lecture 13: Runoff Key Questions 1. What is Hortonian overland flow? 2. What is saturation from below and interflow? 3. What is subsurface flow? 4. How do the different runoff mechanisms affect the shape of the hydrograph? 5. What is the effective rainfall depth and why does if vary with season? 6. What is the variable‐source area concept?
Looking southeast down Lake Whatcom
Photo by Margaret Landis
Lake Whatcom Water Budget inputs – outputs = change in storage
Lake Whatocm Water Budget for the WY2010 Inputs Direct Precipitation Diversion Runoff
Volume (MG) % of total 7350 23.7 860 2.8 22762 73.5
Outputs Whatcom Creek 22311 Hatchery 875 Puget Sound Co‐Genera 51 City of Bellingham 3522 LW Water & Sewer District 239 Evaporation 2592 Change in Storage
1384
75.4 3.0 0.2 11.9 0.8 8.8
Runoff (millions of gallons)
Modeled Runoff into Lake Whatcom: WY 2010
Oct
Apr
Sep
The hydrograph response shape depends on the rainfall and basin characteristics Hyetograph
Rain
Hydrograph
Q
Time Time
basin characteristics
Basin Characteristics
1. Basin size (area) 2. Topography 3. Drainage dentistry (length of streams per area) 4. Vegetation type and distributions 5. Geology 6. Soil type and thickness 7. Runoff processes
Q = stream discharge
Lake Whatcom Watershed Geology
Lake Whatcom Watershed Soils
Lake Whatcom Watershed Soil Depth
Runoff Processes: Hortonian Overland Flow When the rainfall rate exceeds the infiltration rate the soil will ‘saturate from above’ and produce surface runoff.
infiltration
percolation
Runoff Process: Hortonian Overland Flow
rainfall rate exceeds the infiltration rate
silt and clay soils have slow infiltration rates, hence more surface runoff
Runoff Process: Hortonian Overland Flow
rainfall rate exceeds the infiltration rate
Impervious surfaces in urban settings don’t allow any infiltration, hence very rapid surface runoff
Runoff Process: Hortonian Overland Flow
rainfall rate exceeds the infiltration rate
Frozen soils don’t allow any infiltration, hence very rapid surface runoff
Runoff Process: Hortonian Overland Flow
rainfall rate exceeds the infiltration rate
fractured bedrock (Chuckanut Formation)
Exposed bedrock in the Canyonlands, Utah: Hortonian overland flow
Runoff Process: Hortonian Overland Flow exposed bedrock Lake Whatcom watershed
the first water to arrive at the stream is surface overland flow
Time
fractured bedrock (Chuckanut Formation)
Runoff Process: Subsurface Flow (saturation from below) soil on bedrock
fractured bedrock (Chuckanut Formation)
Runoff Process: Subsurface Flow (saturation from below) In forested watersheds (like Lake Whatcom) there is a very permeable (high hydraulic conductivity) humus layer above the soil. The layer is soil, broken up by roots, critter burrows, and contains aggregated soil and organic matter.
fractured bedrock (Chuckanut Formation)
Runoff Process: Subsurface Flow (saturation from below) In forested watersheds (like Lake Whatcom) there is a very permeable (high hydraulic conductivity) humus layer above the soil. The layer is soil, broken up by roots, critter burrows, and contains aggregated soil and organic matter.
rain infiltrates and percolates quickly into the humus layer and ponds on top of the silt
silt loam
Runoff Process: Subsurface Flow (saturation from below) Water flows laterally through the humus layer along the surface of the soil layer contact—its also called interflow.
Hydrograph
Q
the water from interflow arrives at the stream at a later time because it moves slower through the humus Time
fractured bedrock (Chuckanut Formation)
Runoff Process: Subsurface Flow (movement within the soil) Water that percolates through the humus layer and ponds on top of the soil, can infiltrate into the soil and move laterally as subsurface flow
water infiltrating into the soil (silt loam)
silt loam
Runoff Process: Subsurface Flow Water flows laterally through the soil layer (silt loam)—also called interflow
Hydrograph
Q
Time
fractured bedrock (Chuckanut Formation)
the water moving through the soil arrives at the stream at a much later time because it moves even slower through the soil (naturally it depends on the type of soil)
Analyze the response of the Smith Creek basin to rainfall events Hyetoograph
Rain
Hydrograph
Q
Time Time
basin characteristics
soil type
soil thickness
North Shore MET Station
Smith Creek Stream Gauge
North Shore MET Station
inches
January Hyetograph
January Hyetograph
inches
1.32 inches total rain
Q (cfs)
Storm hydrograph as a result of a 1.32 inch rain event in January
Time (hrs)
Q (cfs)
storm event
baseflow Time (hrs)
Q (cfs)
response hydrograph‐ discharge in the stream due to runoff from the rain event with the baseflow removed
Time (hrs)
Q (cfs)
Response Hydrograph
Time (hrs)
the event flow volume (EFV) is the area under the curve
Q (cfs)
Response Hydrograph
Time (hrs)
EFV = 5781252 cubic feet
If the EVF is spread evenly over the basin area, it corresponds to the effective rainfall depth (Weff). basin area = 5 sq‐miles (139392000 sq feet)
Weff =
5781252 139392000
Weff =
0.5 inches
Weff is 38% of 1.32 inches
1.32 inches of rain fell on the basin during a January rain event, but only 38% of it discharged out of Smith Creek. Where did the rest of the rain go? Hyetoograph
Rain
Hydrograph
Q
Time Time
basin characteristics
July Hyetograph July rainfall event (2.03 inches)
0.4 0.3
Q (cfs)
Precipitation (inch)
Response hydrograph due to the 2.03 in rain event.
0.2 0.1 0 1
12
23
34 45 56 Time (hour)
67
Time (hrs)
Q (cfs)
The event flow volume for the July rainfall event is 749493 cubic feet
Time (hrs)
If the EVF is spread evenly over the basin area, it corresponds to the effective rainfall depth (Weff). basin area = 5 sq‐miles (139392000 sq feet)
Weff =
749493 139392000
Weff =
0.065 inches
Weff is 3.2% of 2.03 inches
1.32 inches of rain fell on the basin during a January rain event, but only 38% of it discharged out of Smith Creek. 2.03 inches of rain fell on the basin during a July rain event, but only 3.2% of it discharged out of Smith Creek. Where did it go?
Variable Source Area (VSA): Only certain regions of the watershed have moist enough soils to allow surface runoff and interflow—the SVA size varies with season. The SVA is small in July
The SVA is large in January
As the SVA increases, so does the travel path of runoff, and hence the time it takes to drain the basin back to baseflow conditions.
