Basin characteristics From hydrological processes at the point scale to hydrological processes throughout the space continuum: point scale à river basin The watershed characteristics (shape, length, topography, ) control the response of the river basin shape of the hydrograph soil water dynamics NB River basin, drainage basin, watershed, catchment are used as synonimous can be used to identify similarity of response among different basins extrapolation and regionalisation of models 1
Basin characteristics Lecture content Skript: Ch. IV.1, IV.1.1, IV.1.3, IV.1.4 Quantitative characterization of topographic features river network characteristics soil land cover interrelationships between watershed features 2
Watershed definition watershed: area of land draining into a river network converging to an outlet the area of the watershed is delimited by the topographic (=drainage) divide outlined on a topographic map 3
Topographic vs phreatic divide TOPOGRAPHIC DIVIDE PHREATIC DIVIDE GROUNDWATER FLOW PHREATIC DIVIDE IMPERVIOUS LAYER TOPOGRAPHIC DIVIDE IMPERVIOUS LAYER the area contributing to water balance corresponds to that delimited by the topographic (=drainage) divide the area contributing to water balance DOES NOT correspond to that delimited by the topographic (=drainage) divide ê Closing the water balance requires the knowledge of the phreatic divide 4
Hypsometric curve H [m asl] 2200 1900 1600 1300 1000 700 400 elevation range [m] ΔA [km 2 ] A tot [km 2 ] 1685 1500 3.4 3.4 1500 1250 4.9 8.3 1250 1000 7.6 15.9 1000 750 4.1 20.0 750 500 2.8 22.8 500 443 1.3 24.1 H = h i A i i A tot = 1108 m 0 2 4 6 8 10 12 14 16 18 20 22 24 A [km 2 ] 1685 m asl 1500 m asl 1250 m asl 1000 m asl 750 m asl 500 m asl cumulative frequency distribution of areas with elevation watershed steepness use watershed similarity hydropower potential (identifies large contributing areas with high elevation difference) 5
Watershed average slope (Alvard-Horton s) l i = length of the average elevation line between two subsequent contour lines d i = distance between two subsequent contour lines local slope s i = Δh i /d i vs basin slope, local slope for approximately rectangular areas between two subsequent contour lines of difference h: s NB s i = h = hl i = hl i where a i = area between two contour lines d i d i l i a i basin slope for regularly spaced elevation ranges (contour lines difference = h): the degree of accuracy depends on the contour line spacing and on the accuracy of the rectangular areas approximation s i = hl 1 a 1 a 1 A + hl 2 a 2 a 2 A +...+ hl n a n where L = total length of average elevation lines A = total area a n A = h ( A l 1 + l 2 +...+ l n ) = hl A 6
Watershed average slope (grid method) local slope s i = Δh i /d i with Δh = contour lines interval d i = minimum distance between two contour lines in different gridboxes basin slope s i = i N s i NB the accuracy of the method depends on the number N of local slope estimates where N = number of grid nodes where local slopes s i are computed 7
Watershed characteristics (automatic methods) modern surveying techniques (airborne laser altimetry, satellite radar altimetry, ) make available digital terrain models (DTMs) most commonly DTMs are available in raster format à grid of squares with elevations associate to each node DTMs can be manipulated by means of Geographical Information Systems (GIS) to extract basin characteristics: watershed delineation local and basin slope digital river network flow direction flow accumulation (drainage area) Shaded relief representation of the DTM of the Maggia River basin à (grid size 25 25m) river profile aspect! 8
Digital River Network (DRN) Example of DRN extracted by GIS from a DTM actual drainage network ( blue lines ) } Digital River Network 9!
Other watershed characteristics Soil Example of Digital Soil Map (Maggia river basin, Tessin, CH) soil type soil depth use delineation of infiltration and soil storage characteristics parameterisation of infiltration models problems accuracy of information different classification / resolution used by different countries!! 10
Other watershed characteristics Land use Example of Digital Land Use Map (Maggia river basin, Tessin, CH) use parameterisation of models infiltration (e.g. SCS-CN) evapotranspiration problems representativeness of map depends on raster size different classification /resolution used by different countries 11
Other watershed characteristics Drainage Density total length of streams D= = area L i LOW à D< 3 km/km2 AVERAGE à D 7 10 km/km2 HIGH à D 18 28 km/km2 VERY HIGH à D>100 km/km2 i A it measures the efficiency of the basin drainage (i.e. of how well or how poorly a watershed is drained by rivers) it depends upon both climate (e.g. rainfall regime) and physical characteristics (e.g. geology, slope, soil, land cover) of the drainage basin for equal climatic characteristics can be used as proxy information for permeability high D à low permeability low D à high permeability 12
Interrelationships of watershed characteristics 13