Stream Hydrology. Watershed 8/29/13. Area that contributes water to a point on a stream Scale is user-defined Other names: Catchment Drainage basin

Transcription

1 Stream Hydrology Watershed Area that contributes water to a point on a stream Scale is user-defined Other names: Catchment Drainage basin 1

2 Basics of watershed hydrology Inputs Precipitation (Rain, Snow) Outputs Stream How does water get to stream? Hydrograph response to precipitation input (Fig. 2.9) Precipitation (ppt) occurs Runoff to stream (pathway) Rising limb Peak flow, with some lag Recession limb Baseflow (groundwater contribution) If you divide discharge (m 3 s -1 ) by watershed area (km 2 ), you get runoff (cm hr 1- ) 2

3 Hydrograph -- time series record of streamflow at a stream cross-section: discharge plotted against time Discharge = = low rate Units: Q = L 3 /T (e.g., m 3 /sec, gallons/day, acre-feet/yr) In most simple view, Runoff = Precipitation Evapotranspiration Effect of snow storage on seasonal runoff, and high summer ET High summer precip but low runoff in this forested watershed High summer precip but much evaporation in this desert watershed Note that these hydrographs have different shapes, i.e., seasonal patterns of runoff. Why is this important? 3

4 Factors influencing shape of a hydrograph: (1) -- infiltration to groundwater vs. overland flow (2) -- how fast overland flow occurs (3) (type & extent) and ET (4) type of precipitation ( ) (5) stream size Small stream -- generally a relatively fast response to precipitation compared to Large river -- lots of small tributaries -- integrates small tributary responses These factors vary at a variety of spatial scales (e.g., elevation and type of precipitation, topography, geology and soils) (e.g., amount and seasonal timing of precipitation, vegetation cover, topography, geology, glaciation history) The shape of the hydrograph tells us a lot about watershed characteristics and biological potential of the river. (Fig. 2) stream -- one whose hydrograph responds quickly to changes in precipitation (little groundwater) Many extremes, both high and low stream -- one with more sluggish response (more groundwater flow) Less variable environment Q What is the ecological significance of HIGH discharge? LOW discharge? Temperature stability? yr day 4

5 Humans modify watershed water balance and shapes of hydrographs Example of Urbanization on hydrograph How will this change the hydrograph? (refer back to previous page) Discharge vs. Velocity 2 specific terms and one general term Discharge = streamflow = how much water moving down the channel = flow rate Units: Q = L 3 /T (e.g., m 3 /sec, gallons/day, acre-feet/yr) Stream surface Water velocity (U) at a point in the water column Streambed Current velocity = how fast water molecule is moving at a point = flow rate Units: U (or V) = L/T (e.g., m/s) = Q or U!!!! Is this U or Q? 5

6 How do we measure discharge (Q)? Example: Garden hose filling a bucket Q = A * U m 3 /s m 2 m/s How do we get the area of a river cross section? (A = W x D) Q = W * D * U So, we need the average width, the average depth and the average U. How do we do this? Divide river into a large number of cells and measure the width, depth, and velocity (at 60% of depth in each), multiply and add them up. (We ll come back to the 60% of depth part) What is the relationship between average U and Q? In the stream below, how do velocity (U) and discharge (Q) change from the pool to the riffle? Pool Riffle Is the discharge different? Why? Is the velocity different? Why? 6

7 Consider 3 different channel geometries below. How does U change between them for a given Q? Let Q = 10 m 3 /s. We know continuity of flow: Q 1 = Q 2 = Q 3 Q = A * U (= W * D * U) Riffle (wide, shallow, fast) Pool (wide, deep, slow) Chute (narrow, deep, fast) X-sec A = 10 m 2 U = X-sec A = 50 m 2 U = X-sec A = 5 m 2 U 3 = Why is current velocity (U) important ecologically? 1) this is the force organisms feel Faster veloicities = more erosional force 2) erosional force on sediment (i.e., habitat) (faster flows --> remove smaller particles) 3) delivery of nutrients, gases, food; removal of wastes Is current velocity same everywhere in stream? Is wind velocity the same over the surface of the Earth? 7

8 Is current velocity same everywhere in stream? Why or why not? Friction! As water encounters solid surface Why is velocity zero at solid surface interface? No-slip condition (velocity = 0 at boundary) Where is there friction in a stream? What would a Vertical Velocity Profile look like for Fig. B? Height from Bottom (cm) 0 1 U (m/sec) Water surface Shape of the velocity distribution (from bed to surface) is (ideally): Average U in a stream cross-section = 0.6 depth of stream Can calculate U at all depths from surface to the bottom This vertical change in velocity creates on the bottom the force exerted as one layer of water moves past another. The loss of velocity near the bed translates this force onto the bed ( wall ). It is proportional to the velocity gradient (how fast velocity changes vertically from the bed). 8

9 How does velocity affect an object projecting off the bed into the flow? Shear stress ( ) = the force per unit area on the streambed or object on the streambed Shear stress (τ) is proportional to velocity gradient, which is measured as the change in velocity ( ) divided by the change in depth( ) Δy ΔU So, for an animal on the streambed experiences a force that is proportional to the change in velocity from its ventral surface to its dorsal surface. Shear Stress ~ ΔU/Δy [Measure how fast velocity changes with depth] Which of the following have the greater ΔU/Δy? For same Δy, which has greater ΔU? max Distance from bottom (y) Δy 0 0 ΔU ΔU Velocity (U) max Take home 9

10 Physical forces acting on benthic organisms 1) Shear stress (τ) τ ~ ΔU/Δy Lift (ΔU vertical) 2) Lift Upward force caused by pressure difference created in vertical dimension (ΔU vertical, i.e. Umax - Umin) 3) Form Drag Force caused by pressure difference in longitudinal direction (ΔU longi), influenced by body s shape examples?) 4) Skin Friction Drag Drag due to friction between moving water and surface of object (mucus reduces) acceleration U max Object on bed flow separation low velocity Skin Friction U min (= 0) Form Drag (ΔU longi) For this benthic insect, where is the greatest lift? How does it prevent being lifted from bed? Lift (ΔU vertical) U max acceleration Object on bed flow separation low velocity Skin Friction U min (= 0) Form Drag (ΔU longi) 10