Exercises in Basin Morphometry
Rivers carve basins and valleys in which they flow. Every river consists of a major trunk segment, fed by a number of mutually adjusted tributaries that diminish in size away from the trunk stream. The tributaries define a network of channels that drain water from a specific, finite area which constitutes the drainage basin or watershed of the river system.
Morphometry involves the measurement and mathematical analysis of the shape and dimensions of landforms. Morphometric properties of a drainage basin are quantitative attributes of the landscape that are derived from the terrain and drainage network within a drainage basin. With digital elevation data becoming available since the early 2001, GIS techniques are increasingly being used for morphometric analysis of drainage basins throughout the world. Terrain characterization became an important part in modelling surface processes. Morphometric characteristics are helpful in deciphering surface-water hydrology and are used for mapping areas vulnerable to floods and high rates of erosion. Basin morphometry has also been used to assess infiltration capacity, groundwater potential and identification of suitable sites for construction of check dams for artificial recharge structures. Several investigators have also used morphometric properties of drainage basins and drainage networks as a reconnaissance tool for drawing inferences regarding tectonic activity to which a basin has been subjected. Geomorphic indices like hypsometric integral, drainage basin asymmetry, stream length gradient index, mountain front sinuosity etc. are used as a measure of active tectonics of the basin. Combined with data on subsurface geology and structure, the results of morphometric analysis can yield accurate results having far reaching applications in watershed and resource management and vulnerability mapping.
The first step in basin morphometry involves delineating streams and watersheds, and getting some basic watershed properties such as area, slope, flow length, and stream network density. Traditionally this was done manually by using topographic/contour maps. The results obtained were neither accurate nor replicable. With the availability of digital elevation models (DEMs) and GIS tools, watershed properties can be extracted by using automated procedures. The processing of DEM to delineate watersheds is referred to as terrain pre-processing. In these exercises, we will use the results of such pre-processing in the form of delineated watershed, sub-watersheds, stream network and some other watershed characteristics that collectively describe the drainage patterns of a basin. The results from this exercise can be used for hydrologic modeling and deciphering active tectonics in the region.
The drainage basin is the fundamental landscape unit concerned with the collection and distribution of water and sediment. Each basin is separated from its neighbor by a divide, or interfluve. Thus, the basin can be viewed as a geomorphic system or unit which is inseparably linked with hillslope processes that contribute water and sediment to the river network depending upon the regional climate, underlying bedrock, tectonic regime, and land use by human activity. Any feature or portion of the basin can be considered a subsystem having its own unique set of processes, geology, and energy gains and losses. It is possible to measure the geometric properties of the basin which are influenced, apart from other factors, by the tectonic activity in the region. Thus on the basis of basin morphometry, it is possible to assess the active tectonic status of the basin, as indeed of the region surrounding the basin.
Furthermore, because it is possible to measure the amount of water entering the basin as precipitation and the volume leaving the basin as stream discharge, hydrologic events can be readily analyzed on a basinal scale. Likewise, much of the sediment produced within the basin is ultimately exported from the basin through the trunk river. Considered on a long temporal scale, the rate of lowering of the basin surface can be estimated.
Every basin possesses a quantifiable set of geometric properties that define the linear, areal, and relief characteristics of the watershed. These variables correlate with stream order, and various combinations of parameters obey statistical relationships that hold for a large number of basins. Two general types of numbers have been used to describe basin morphometry or network characteristics – Linear scale measurements which allow size comparisons of topographic units, and measurements consisting of dimensionless numbers, often derived as ratios of length parameters. Linear parameters include the length of streams, relief, basin perimeter or basin length. The ratios may include length to width ratios, bifurcation ratios, relief ratios and valley width to height ratios. These measures permit comparisons of basins or networks.
Basin morphometric parameters play an important role in hydrological processes, as they largely control a catchment’s hydrologic response. Their analysis becomes significant when studying runoff reaction to intense rainfall, especially in the case of ungauged, flash flood prone basins. Morphometric parameters are also influenced by the presence of and activity along structural and tectonic features, for instance long straight segments of stream reaches are located along faults, or a highly elongated basin may be developing along an active fault system. For these exercises we have selected the Goriganga River Basin in Eastern Kumaon Himalaya, tectonically one of the most active regions in the world, and also one that receives high rainfall. The exercises dispensed through this site take advantage of GIS assisted computation of morphometric characteristics like Channel Sinuosity, Form Factor, Basin Asymmetry, Drainage Density, Elongation Ratio, Hypsometric Integral, Valley Floor Width to Height Ratio, Relief Ratio, etc. to subsequently use these indices to predict hydrological and tectonic characteristics of the Goriganga Basin.
Methodology Adopted for Computation of Morphometric Parameters
For these exercises in basin morphometry, digital elevation models (DEMs) derived from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) have been used. The boundary of the Goriganga River, as also 32 sub-basins within this, and the drainage network on a fairly detailed scale have already been delineated using standard procedures.
All the data that is necessary for these exercises can be downloaded from here:
Exercise 1: Relief Ratio (Rr)
Exercise 2: Channel sinuosity (Cs)
Exercise 3: Bifurcation Ratio (Rb)
Exercise 4: Asymmetry factor (Af)
Exercise 5: Drainage density (Dd)
Exercise 6: Basin elongation ratio (Re)
Exercise 7: Basin Shape (Bs)
Exercise 8: Hypsometric integral (HI)
Exercise 9: Valley floor width-to-height ratio (Vf)
Exercise 10: Stream-length gradient index (SL)
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)