2.1 Delineation and physical characterization Watershed Analysis 2.1 Delineation and physical characterization

Short description This part of the course will cover the methods, data and tools used in watershed delineation and characterization.

Learning outcomes identify data/tools sources, type of data used in watershed delineation and characterization, and limitations in these data/tools able to delineate and characterize watersheds able to assess the accuracy of the delineation results

Definitions Outlet: the location at which the design is being made Watershed: the land area that sheds water to the outlet during a rainstorm Watershed boundary:

Watershed Delineation Watershed delineation is the process of identifying the drainage area of a point or set of points. For many years, Water Engineers have been using EMA paper maps for watershed delineation. The maps are in UTM coordinates and in 1:50,000 scale.

Simple Analogy Consider a gabled roof house which has the ridge in the middle. Every drop of water falling in the left side of the ridge center line flows to the left and vice versa. The ridge is the water divide for the gutters in the opposite side. The roof area to the left and right of the roof ridge are the areas draining to their respective gutters and hence are drainage areas.

W A=LxW L Q=CIA

Short description This part of the course will cover the methods, data and tools used in watershed delineation and characterization.

Watershed Delineation Water flows in the direction of the terrain steepest downhill slope. Streamlines are orthogonal to the elevation contour lines.

Watershed Delineation Streamlines do not flow towards drainage divides, and do not intersect them. Drainage divides are found along the highest points of the terrain.

DEMs Digital Elevation Models (DEMs) are grids of elevation. based on DEMs rather than contour lines. DEMs store the same type of information contour lines do, but with a different data structure. Watershed delineation can be based on DEMs rather than contour lines.

90-Meter DEMs For Ethiopia, 90-meter DEMs have been developed by USGS (SRTM mission). 90-meter DEMs are more or less consistent with 1:50,000 paper maps.

Hydrologic Functions 1 2 4 8 16 32 64 128 The eight-direction pour point algorithm (D-8) assigns a flow direction code to each cell, based on the steepest downhill slope as defined by the DEM. Flow direction codes The flow direction indicates the cell – out of the eight neighbor cells – towards which the water flows. Flow direction cannot be defined for cells within a terrain depression. DEMs have to be filled before determining flow directions.

Hydrologic Functions 1 1 67 56 49 53 44 37 58 55 22 67 56 49 53 44 37 58 55 22 Slope:

DEM filling The Fill sinks will 'remove' from a Digital Elevation Model (DEM): depressions that consist of a single pixel, i.e. any pixel with a smaller height value than all of its 8 neighboring pixels, depressions that consist of multiple pixels, i.e. any group of adjacent pixels where the pixels that have smaller height values than all pixels that surround such a depression.

Hydrologic Functions DEM Flow direction grid Flow network (not a grid) 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM 2 4 1 8 128 Flow direction grid Flow network (not a grid) Function: Flow direction Argument: DEM 32

Flow Direction Water flows to one of its neighbor cells according to the direction of the steepest descent. Flow direction takes one value out of eight possible values.

Hydrologic Functions Function: Flow accumulation Argument: Flow direction 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 1 2 3 8 5 20 24 Flow accumulation grid DEM Flow network (not a grid)

Flow Accumulation Flow accumulation is a measure of the drainage area in units of grid cells. The cell itself is not included.

Hydrologic Functions Process: Stream delineation 1 2 3 8 5 20 24 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM Flow accumulation (> 2 cells) Process: Stream delineation Argument: Flow accumulation

Stream Network All grid cells draining more than a user-defined threshold value (blue streams) are part of the stream network. All grid cells located downstream of user-defined cells (red streams) are also part of the stream network.

Hydrologic Functions Function: Stream links Argument: Delineated streams 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM Delineated streams Streams links Stream links are assigned an identification number at random.

Hydrologic Functions Process: Outlets Argument: Stream links DEM 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM Stream links Outlets Outlets are assigned the identification number of their link.

Watershed Outlets The most downstream cells of the stream segments are watershed outlets. User-defined points indicated as white dots are watershed outlets. Outlets

Hydrologic Functions Function: Watershed Argument: Flow direction and Outlets 71 56 44 53 69 74 78 72 47 68 58 55 21 31 67 49 46 37 38 64 22 61 16 DEM Outlets Watersheds Watersheds are assigned the identification number of their outlet.

Watershed Delineation The drainage area of each watershed outlet is delineated.

Raster to Vector Conversion stream links obtained by vectorizing the stream grids Both datasets include a field Grid-Code in their attribute tables, which stores the stream link and watershed identification number. Since the identification number of the streams and their corresponding watershed is the same, the attribute tables can be related.

Merging Watersheds Adjacent watershed polygons can be merged into a single polygon, if they share their outlet or one flows into the other.

Watershed characterization Drainage area (A): the single most important watershed characteristic as it reflects the volume of water that can be generated from rainfall Watershed Length (L): the distance measured along the main channel from the watershed outlet to the basin divide important in hydrologic computations; for example, in time of concentration calculations

Characterization cont. Watershed Slope (S): reflects the rate of change of elevation with respect to distance along the principal flow path. is computed as the difference in elevation () between the end points of the principal flow path divided by the length of the flow path (L)

Characterization cont. Hypsometric Curve: is a description of the cumulative relationship between elevation and the area within elevation intervals. plotted with the elevation as the ordinate and the area within the watershed above the elevation as the abscissa.

Characterization cont. Watershed Shape: typically used are, Length to the center of area (Lca): the distance in miles measured along the main channel from the basin outlet to the point on the main channel opposite the center of area Shape factor (Ll): where L is the length of the watershed

Characterization cont. Circularity ratio (Fc): Where P and A are the perimeter and area of the watershed, respectively Circularity ratio (Rc): where A, is the area of a circle having a perimeter equal to the perimeter of the basin Elongation ratio (Re): where L, is the maximum length of the basin parallel to the principal drainage lines

Characterization cont. Surface Roughness: Roughness implies unevenness of texture Manning's roughness coefficient (n) is the most frequently used index of surface roughness Values of n for hydrologic overland flow surfaces are often tabulated

Characterization cont. Soil characteristics: Soil Profile: are referenced as follows, O – Horizon: surface litter consisting primarily of organic matter A – Horizon: topsoil consisting of humus and inorganic minerals E – Horizon: zone of leaching where percolating water dissolves water-soluble matter

Characterization cont. B – Horizon: the subsoil below the A – or E –horizons that contains minerals and humic compounds C – Horizon: a zone consisting primarily of under composed mineral particles and rock fragments R – Horizon: bedrock, an impermeable layer

Characterization cont. SOIL CHARACTERISTICS: Soil texture: is a physical characteristic of a soil; it refers to the size of the mineral particles and the fraction of the particles in different size classes Clay: d < 0.002 mm Silt: 0.002  d  0.02 mm Sand: 0.02  d  2 mm

Characterization cont. Soil texture: example 35% Sand 25% Clay and 40% Silt is a LOAM soil

Characterization cont. Soil structure: affects the hydrologic response of a watershed refers to the tendency of soil particles to aggregate into lumps and clods a function of the soil texture, the texture and type of minerals present, and the amount of biological activity in the soil column

Characterization cont. Volumetric Characteristics of Soils Total Volume: includes volume of solid, volume of water and volume of air Porosity Where Pv , Vv and Vt are the void ration, volume of void and total volume respectively

Characterization cont. Volumetric Characteristics of Soils Void ratio Bulk density specific yield specific retention The porosity, effective porosity, and specific retention are related by specific storage (Ss ) in which C1, and C2 are constants for the aquifer

Characterization cont. Soil Moisture gravity water: is water that drains under the force of gravity Field capacity: refers to the soil moisture content at which rapid drainage has ceased Permanent wilting point: level of soil moisture where plants wilt and cannot sustain growth Unavailable water: water below permanent wilting point

Characterization cont. Channel geomorphology Channel Length (Lc): distance measured along the main channel from the watershed outlet to the end of the channel has ceased Channel Slope (Sc): The channel slope can be described with any one of a number of computational schemes Where Ec is the difference in elevation between the points defining the upper and lower ends of the channel, and Lc is the length of the channel between the same two points.

Characterization cont. Channel geomorphology Drainage Density (DD): is the ratio of the total length of streams within a watershed to the total area of the watershed. high value of drainage density would indicate a relatively high density of streams and thus a rapid storm response.

Characterization cont. Stream Density (Ds): ratio of total length of streams watershed to total area of watershed The shape of a drainage basin can generally be expressed by: form factor compactness coefficient Wb , Lb = width & length of basin 2=circumference of circular area that equals basin area

Characterization cont. Fan shaped Fern (leaf) shaped

Characterization cont. Horton's Laws: Law of stream numbers: relates the number of streams of order i (Ni) to the bifurcation ratio and the principal order (k): Law of stream lengths: relates the average length of streams of order i (Li) to the stream length ratio (ri ) and the average length of first – order streams (L1):

Characterization cont. Law of stream areas: relates the mean tributary area of streams of order i (Ai) to the mean drainage area of first – order basins (A1) and the stream area ratio (ra): Law of stream slopes: relates the average slope of streams of order i (Si) to the average slope of first – order streams (S1) and the stream slope ratio (rs):