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a mapping exercise delineating catchments

    Topic: a mapping exercise delineating catchments

    The aim of this tutorial exercise is to become acquainted with the process of delineating catchments manually and to consider

    drainage patterns within catchments. Topographical maps upon which students can trace catchment boundaries should be obtained

    (desirably maps of local areas of interest) either by downloading appropriate maps from a website (see below), purchasing an

    appropriate map or using maps in your place of work or in a library.
    1) If available use 1:250,000 topographic maps to choose 2 small subcatchments from within a relevant catchment of interest.

    Ideally, the subcatchments should have different features such as topography or rainfall or show different land use e.g.

    forested versus agriculture, cropland versus pasture, agriculture versus urban.
    2) Identify the name(s) of your stream(s) subcatchment(s).
    3) Use 1:250,000 maps to roughly define the subcatchments. Then use
    1:100,000 topographic maps to more accurately define your subcatchments by marking the watershed (basin or catchment

    boundary) directly on the map
    if laminated or on tracing paper. The method of delineating catchments will
    follow that stated by Gordon et al.pdf, wherein: “boundaries are drawn by following the ridge tops which appear on

    topographical maps as downhill-pointing V-shaped crenulations. The boundary should be perpendicular to the contour lines it

    intersects. The tops of mountains, often marked as dots on a map, and the location of roads which follow ridges are other

    clues.” Some “extrapolation” may be needed in subcatchments with low topographic relief by drawing lines approximately half

    way between streams flowing in opposite directions.
    4) Using the contour lines on the 1:100,000 maps, describe the topography of your subcatchments (are they flat or hilly?).

    Are there differences in topography between your subcatchments?
    5) Compare the stream drainage system (network) of your subcatchments.
    Determine the type of drainage pattern using Figure 4.9 in Gordon et al.pdf. If many intermittent stream channels are shown

    on your map the subcatchment either has low rainfall or very porous soils. The drainage network is a geohydraulic continuum

    between surface waters and groundwater. Water flowing at the surface at one place may be underground at another; streams may

    cease to flow at one point but recommence to flow downstream. Interaction zones between surface water and groundwater are

    important attributes of the landscape. You may be able to find information on depth to groundwater in your subcatchment which

    would indicate near surface groundwater and thus potential areas of recharge of
    groundwater by surface waters or of discharge of groundwater to surface waters.
    6) Determine the area of your subcatchments using either the “counting squares” or the weight method (see Gordon et al.pdf).

    Trace your subcatchments onto a piece of graph paper and count the total number of squares (1cm x 1cm). This need only be

    approximate. Using the same scale as on your map trace out a square of known area e.g. 10 km x 10 km (this may vary depending

    upon the size of your subcatchments). Count the
    number of squares in your known area and then divide the number of squares
    for each subcatchment by this number. Alternatively, trace your subcatchments onto another piece of tracing paper and cut out

    the subcatchments. Using the same scale as on your map, trace out a square of known area e.g. 10 km x 10 km (this may vary

    depending upon the size of your subcatchments). Cut out this known area and weigh it. Measure the weight of your

    subcatchments. Calculate the area of your subcatchments using the formula in Gordon et al.pdf.
    – 1:250,000 maps if available.
    – 1:100,000 maps (laminated if possible).
    – Tracing paper.
    – Pencil.
    – Eraser.
    – Felt tip pen.
    – Scissors.

    The above web link is for topographic maps and river basin outlines.
    Gordon, N.A., McMahon, T.A., and Finlayson, B.L. 1992, Stream Hydrology: An
    Introduction for Ecologists, John Wiley, Chichester, UK