Understanding Watershed Drainage Systems: Characteristics and Analysis
Drainage System
The drainage system of a watershed consists of the river and its tributaries. Studying its ramifications and development is crucial because it indicates the rate at which water leaves the watershed.
Types of Currents
Rivers are commonly classified based on their flow continuity:
- Perennial: These rivers contain water year-round, fed by a water table that never falls below the channel’s water level, even during droughts.
- Intermittent: These rivers flow during rainy periods and dry up during dry periods. They carry runoff and groundwater during wet seasons because the water table remains above the riverbed. This changes during dry seasons when the water table falls below the riverbed.
- Ephemeral: These rivers exist only during or immediately after precipitation, carrying only surface runoff. The water table always stays below the riverbed, preventing groundwater flow into the river.
Current Order
River order reflects the degree of branching within a watershed. Following Horton’s approach, rivers are classified as shown in Figure 3.1:
- First-order streams are the smallest channels with no tributaries.
- Two first-order streams combine to form a second-order stream.
- Two second-order streams form a third-order stream, and so on.
- Two nth-order streams create an (n+1)th-order stream.
The main river’s order indicates the extent of branching in the basin.
Drainage Density
Drainage density (Dd) indicates the development of a drainage system. It’s the ratio of the total length (L) of all watercourses (ephemeral, intermittent, or perennial) to the total watershed area (A).
Drainage density is inversely proportional to stream length, reflecting drainage efficiency. It typically ranges from 0.5 km/km2 in poorly drained basins to 3.5 km/km2 in well-drained basins.
Terrain Features
A catchment area’s relief significantly influences meteorological and hydrological factors. Surface runoff speed depends on the basin’s slope, while temperature, precipitation, and evaporation are functions of altitude.
Slope
Basin slope controls surface runoff rate, affecting the time rainwater takes to reach riverbeds, flood peak magnitude, infiltration chances, and soil erosion susceptibility.
Mean Elevation
A basin’s altitude variation and mean elevation influence precipitation, evapotranspiration, and average flow. Significant altitude differences lead to variations in rainfall and temperature, affecting evapotranspiration.
Mean elevation can be determined using the hypsometric curve (Figure 3.3) or the following equation:
(3.4)
where E is the mean elevation, e is the average elevation between consecutive contour lines, a is the area between contour lines, and A is the total basin area.
Hypsometric Curve
The hypsometric curve graphically represents the relief of a watershed, showing elevation changes relative to sea level. It indicates the percentage of drainage area above or below different elevations.
The hypsometric curve can be calculated using the method described above or by planimetric grids of areas between contours. Table 3.2 details the steps for calculating a watershed’s hypsometric curve.
Main Channel Slope
Rainwater concentrates in riverbeds after surface and underground drainage. Watercourse slope significantly influences river discharge because it affects the speed at which headwater contributions reach the outlet. Steeper slopes result in higher flow rates and more pronounced, narrower flood hydrographs.
Equivalent Rectangle
Introduced by French hydrologists, the equivalent rectangle helps compare watershed characteristics’ influence on surface runoff. It’s a rectangle with the same area as the basin, with sides L and l, and contours parallel to the short side, respecting the basin’s hypsometry.
The rectangle’s sides are calculated using equations (3.6) and (3.7), derived from the rectangle’s area and perimeter and the compactness coefficient (equation 3.1):
A = L * l
P = 2 (L + l)