Watershed: Definition, Delineation and Characteristics

1 year ago

Atiqur Rahman

10 minutes

Watershed Definition

The Watershed is a natural geomorphic unit. It may be defined as the area which contributes water to a particular stream or sets of streams. Defined by topographic divides, it is an area of land which drains the water, sediment and dissolved materials to a common outlet. Thus, the watershed is an area that drains surface water to a common outlet.

A Watershed provides the opportunity to estimate the amount of erosion because measurement of river flow, and by knowing the area of the watershed and by assuming a diversity of material, the rate of land erosion over the whole catchment may be deducted.

Watershed is a limited, convenient and usually clearly defined and unambiguous topographic unit available in a nested hierarchy of sizes based on stream order. It provides the best way to measure precipitation and solar radiation inputs and outputs of discharge.

Watershed is ecologically and geomorphologically a relevant management unit. Its analysis provides a practical analytical framework for spatially explicit, process-oriented scientific assessment that provides information useful for guiding management decisions.

Watershed Nomenclature

The nomenclature related to the watershed is diagrammatically illustrated in Figure 1. The boundary between two watersheds is referred to as a watershed boundary or water divide. The watershed area is normally defined as the total area flowing to a given outlet or pour point, or mouth of the watershed (Fig.).

The pour point is where water flows out of a watershed. This is the lowest point along the boundary of a watershed. The cell in the source raster is used on the pour point about which the contributing area is determined. The source cell may be a feature such as a stream gauging station, dam site or watershed mouth for which characteristics of the contributing area are determined.

A Watershed has a stream or network of streams of different orders having specific flow directions. The confluence of two streams is known as a stream junction. Other dimensions of a watershed are length, width, perimeter and area (Fig.).

Watershed Delineation

Delineation of watersheds is the first step to proceed further on integrated watershed modelling and management. There are two ways of watershed delineation. These are the traditional way through topographic sheets and automated watershed delineation using GIS technology.

Traditional Way of Watershed Delineation

The traditional way of watershed delineation is delineating watershed boundaries through topographic sheets. This method draws the watershed boundary manually on a topographic map using a pattern of contours (Fig.).

Delineation of the watershed or the total runoff contributory area to a part depends on the watershed drainage pattern. The person who draws the boundary uses topographic features on the map to determine where a divide is located.

For watershed delineation at a 1:25000 scale, maps are suitable, and the boundary for a particular watershed is drawn as a line which surrounds all the drainage lines and depressions in the watershed and passes through the highest points between the stream and adjacent one.

Watershed delineation: by traditional method through contours. — Watershed delineation: by the traditional method through contours.

Automated Watershed Delineation

Today GIS software is used to delineate watershed boundaries automatically through computers. One can generate watershed boundaries in a fraction of the time through this technique. Watershed boundary through a computer is determined by using the Digital elevation model (DEM) as data input. A stream network can be derived from the DEM. From DEM, there are two ways for watershed delineation. These are point-based and watershed area-wide.

Point-Based Watershed- From the DEM-based stream network, individual watersheds can be delineated based on points of interest such as gauging site, dam site or watershed mouth. Using these points as pour points, the area of the individual watershed may be delineated (Fig).

Area-wide Watersheds- The area-wide watershed for each steam section is delineated by selecting threshold values in terms of area or cells. A large threshold value will have fewer but larger watersheds. The figure illustrates the determination of area-wide watershed delineation at different threshold values. The derivation is based on a threshold accumulation value in terms of area, i.e. 500 hectares (Fig. above) and 50 hectares (Fig. below).

Watershed delineation: by automated by threshold values,i.e.,500 hec (above) and 50 hec (below). — Watershed delineation: automated by threshold values,i.e.,500 sec (above) and 50 hec (below).

Watershed: An Open System

The famous geomorphologist W.M. Davis treated the river like the vein of a leaf; broadly viewed, it is like an entire leaf. Like a leaf, the watershed is an open system (Fig.2). Close systems are those which possess clearly defined boundaries across which no import or export of materials or energy takes place. The open system requires a continuing energy supply and is, the effect, maintained by constant supply and removal of energy. The watershed is an excellent example of a geomorphological system.

Energy Inputs and Outputs of Watershed

The watershed has energy inputs to regulate its system. Therefore, the watershed can be envisaged as receiving energy or input from the climate over the watershed surface, and it loses energy (or output) through the water discharge, sediment and mineral flow through the watershed mouth and evaporation and transpiration from its surface.

The endogenic forces below the watershed surface are the second source of energy input of the watershed. The endogenic forces are responsible for developing an initial form of a watershed.

On the initial landform, various kinds of denudational processes originated from climatic energy, i.e., rainfall and temperature; and the landforms are developed in watersheds of different forms and characteristics.

The energy is transformed from one watershed system to another through hydrological and denudation processes from its common outlet, i.e., mouth or through its surface (Fig.). The environment, ecology and the form of the watershed are a function of all these energy inputs and watershed responses (outputs).

Watershed as an open system (after, Rawat,1985)

Watershed Characteristics

The energy provided by the climate is regularized within the watershed system, which is controlled by watershed characteristics, i.e., geology, soils, relief morphometry, drainage morphometry, morphology, vegetation etc. Geological variability in the watershed causes different rates of discharge and silt delivery.

Under identical rain inputs, the different rock units of the watershed may produce water discharge and silt delivery at different rates. Soil depth and texture directly influence rainwater infiltration, soil moisture storage and groundwater recharge. The relief morphometric parameters such as altitudinal zones (Fig.), slope and aspect play an important role in controlling the hydrological and denudation processes. The aspect plays a very important role in the distribution of temperature.

The drainage morphometric parameters drainage density, stream frequency, bifurcation ratio, drainage texture, watershed shape, stream order and watershed size play a significant role in controlling various watershed processes.

Watersheds with high drainage density, stream frequency, bifurcation ratio and drainage texture are subject to high overland flow, low infiltration, low water balance and high denudation rates. The shape of the watershed influences the time taken for water from the remote part of the watershed to arrive at the outlet.

Thus, the occurrence of the peak and the shape of the hydrograph are affected by the watershed shape. Stream ordering (Fig.), watershed size and hierarchy (Fig.) also play a significant role in the regularization of climatic energy inputs within the watershed.

Morphologically a watershed is constituted of three major zones, i.e., the crest zone, the mid-crest zone and the valley zone. These are also known as upland areas, midland areas and lowland areas. These morphological zones are characterized by different natural landforms of different genetics, i.e., pluvial, fluvial etc., depending upon the location of the watershed.

Vegetation directly controls hydrological processes such as interception loss, infiltration, overland flow, rain splash erosion, sheet wash erosion and mass wasting processes. Given the significant controlling roles, it is necessary to study in detail the watershed characteristics for wise management of watersheds.

Watershed Input and Output Balance

The environment and form of a watershed are a function of interactions of watershed input parameters (i.e., climate and tectonic processes) with watershed characteristics (i.e., geology, soils, morphometry, morphology and vegetation etc.). Balance or equilibrium between the input and output parameters of a watershed means no environmental deterioration, pollution or a healthy environment.

An equilibrium state is one in which the many parameters of a watershed system are dynamically balanced. When the entire watershed remains in physical balance, nutrients like water and soils are detained longer within the watershed. The soil builds and retains more moisture, further retaining water and simultaneously encouraging life. The built-up soils have increased nutrient storage sites, and the biogeochemical cycle looks more, keeping the chemicals a richer lingering life (Warshall, 1976).

In a balance watershed system, a hill slope profile is a product of a balance between runoff, infiltration and erosion. All of these are modified by the degree of soil developed since that, in turn, controls what grows on the hillside and thus controls runoff, infiltration and erosion (Curry,1976).

In the disequilibrium state of the watershed, runoff intensity usually increases, thus, causing erosion on hill slopes. Simultaneously, flood waters reach watercourses faster due to the denser and more effective integration of ephemeral channels on the hill slopes, and flood heights increase coupled with increased sedimentation.

Increased sedimentation and increased flooding lead to increased lateral erosion of stream channels that must change their channel geometry to accommodate a greater percentage of sediment load to water discharge. Under such a disequilibrium state, a headwater anthropogenic change can affect a change in the shape of the whole watershed below it (Curry,1976).

Watershed: Definition, Delineation and Characteristics

Watershed Definition

Watershed Nomenclature