Study of watershed characteristics using Google Elevation Service

Study of watershed characteristics using Google Elevation Service

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Rambabu Palaka
Department of Civil Engineering
Bharat Institute of Engineering and Technology
Hyderabad -India
Prof. G. Jai Sankar
Department of Geo-Engineering, Andhra University
Visakhapatnam -India

Protection, improvement and rehabilitation of watersheds are of critical importance in the achievement of overall development goals. GIS-based approach facilitates analysis of different morphometric parameters of watershed delineated based on DEM data. Here is a look at a study of morphometric parameters in linear, aerial, relief aspects of three micro-watersheds using Google Maps & Elevation API

Rambabu Palaka
Department of Civil Engineering, Bharat Institute of Engineering and Technology, Hyderabad

Prof. G. Jai Sankar
Department of Geo-Engineering, Andhra University, Visakhapatnam

Abstract
Watershed characteristics such as size, slope, shape, drainage density, land use/land cover, geology and soils, and vegetation are important factors affecting various aspects of runoff. This article presents general watershed/morphometric characteristics associated with the watershed which is located near Kosigi village in Kurnool district, Andhra Pradesh, India. The input data used for this analysis is downloaded from Open Data Archive (CartoDEM) of ISRO’s Bhuvan website. Thematic maps such as watershed boundary, drainage network, and contour maps are prepared from the DEM data using ArcGIS 10 Hydrology tools. To study watershed characteristics, an online tool is developed using Google Maps API and Google Elevation Service. The Google Elevation Service provides elevation data for locations on the surface of the earth which is useful to find the gross slope of the terrain which is one of important factor affecting runoff. Other characteristics such as size, shape, drainage density of watershed can be analysed using Google Maps API.

Introduction

A watershed is a basin-like landform defined by highpoints and ridgelines that descend into lower elevations and stream valleys. In other words, a watershed describes an area of land that contains a common set of streams and rivers that all drain into a single larger body of water, such as a larger river, a lake or an ocean.

Objective of protecting or conserving watersheds

Protection, improvement and rehabilitation of watersheds are of critical importance in the achievement of overall development goals. Recognising this, many developing countries are turning increasing attention and resources to the field of watershed management.

Watershed degradation: Watershed degradation is the loss of value over time, including the productive potential of land and water, accompanied by marked changes in the hydrological behaviour of a river system resulting in inferior quality, quantity and timing of water flow. Watershed degradation results from the interaction of physiographic features, climate and poor land use.

Watershed management: Watershed management is the process of formulating and carrying out a course of action involving the manipulation of resources in a watershed to provide goods and services without adversely affecting the soil and water base. Usually, watershed management must consider the social, economic and institutional factors operating within and outside the watershed area.

Watershed survey and planning: Watershed survey and planning is the preparatory work which, if properly conceptualised and carried out, permits the successful implementation of actual watershed management.

Objectives of proper planning and prioritization of watershed development

Setting main objectives: After collecting existing data, identifying major watershed problems and considering management possibilities, the main objectives of the proposed project should be defined. The following are some of the most common ones:
– to rehabilitate the watershed through proper land use and protection/conservation measures in order to minimise erosion and increase the productivity of the land and the income of the farmers;
– to protect, improve or manage the watershed for the benefit of water resources development (domestic water supply, irrigation, etc.);
– to develop rural areas in the watershed for the benefit of the people and the economies of the region;

Different objectives call for different techniques, manpower, inputs and approaches in planning. The monitoring and evaluation criteria will also be different. Therefore, main objectives should be identified and defined as early as possible.

Establishing priorities: Priority watersheds or sub-watersheds should be identified during the preparatory stage. As work cannot be carried out at the same time in all the sub-watersheds due to manpower and resource constraints, a priority list must be set. Priorities are usually given to those sub-watersheds which are in critical condition many times; priority areas are also selected because of people: their enthusiasm, strategic locations, poverty or others.

Significance of study of watershed geology and geomorphology characteristics

Geology characteristics: In many countries, geological maps and information may be already available. However, the map scale may often be small and the information is not specific enough to cover the watershed in question. Some rechecking and refinement are usually needed. If there is no existing information, a brief survey is required. The basic geologic information needed is related to erosion and sedimentation. Rock types, depth of weathering, structures, among others, are the main concerns.

Geomorphology characteristics: Geomorphology deals with land forms in a watershed. A survey of land forms will result in a better understanding of the erosion process, hazards. For instance, a valley at youth stage will have more active erosion than one at old stage. High stream density usually means quick surface runoff and flash floods, etc. This kind of information, together with rock types and structures, permits proper selection of sites for dams and roads as well as estimation of peak flows and timing, etc. In addition to collection of descriptive land form information, there are some quantitative analysis methods which can be used for comparison or interpretation.

Study of watershed/morphometric characteristics: Morphometric analysis is refers as the quantitative evaluation of form characteristics of the earth surface and any landform unit. It incorporates quantitative study of the various components such as, stream segments, basin length, basin parameters, basin area, altitude, slope, profiles of the land which indicates the nature of development of the basin.

In general, the watersheds are selected for the morphometric analysis in following heads:
– Linear Aspect: one dimension
– Areal Aspect: two dimensions
– Relief Aspect: three dimensions

Linear Aspect: The drainage network transport water and the sediments of a basin through a single outlet, which is marked as the maximum order of the basin and conventionally the highest order stream available in the basin considered as the order of the basin. The size of rivers and basins varies greatly with the order of the basin. Ordering of streams is the first stage of basin analysis.

Stream Order (U)
Strahler (1952) system of ordering streams has been followed in general because of its simplicity where the smallest, un-branched fingertip streams are designated as 1st order, the confluence of two 1st order channels give a channel segments of 2nd order, two 2nd order streams join to form a segment of 3rd order and so on. When two channel of different order join then the higher order is maintained.

Stream Number (Nu)
The total number of stream segments present in each order is the stream number (Nu).

Stream Length (Lu)
The total length of individual stream segments of each order is the stream length of that order.

Basin length (Lb):
Gregory and Walling (1973) defined the basin length as the longest in the basin in which are end being the mouth.

Basin Area (A)
The area of the watershed is another important parameter like the length of the stream drainage. Schumm (1956) established an interesting relation between the total watershed areas and the total stream lengths, which are supported by the contributing areas.

Basin Perimeter (P)
Basin perimeter is the outer boundary of the watershed that enclosed its area. It is measured along the divides between watersheds and may be used as an indicator of watershed size and shape.

Stream Frequency (Fs)
The drainage frequency introduced by Horton (1932, p. 357 and 1945, p. 285) means stream frequency (or channel frequency) Fs as the number of stream segments per unit area.

Length of Overland Flow (Lo)
The average length of overland flow is approximately half the average distance between stream channels and is therefore approximately equals to half of reciprocal of drainage density (Horton, 1945).

Drainage Density (Dd)
Drainage density is the stream length per unit area in region of watershed (Horton, 1945, p.243 and 1932, p. 357; Strahler, 1952, and 1958; Melton 1958) is another element of drainage analysis. Drainage density is a better quantitative expression to the dissection and analysis of landform

Texture Ratio (Rt)
According to Schumm (1965), texture ratio is an important factor in the drainage morphometric analysis which is depending on the underlying lithology, infiltration capacity and relief aspect of the terrain. The texture ratio is expressed as the ratio between the first order streams and perimeter of the basin (Rt = Nl / P)

Drainage Texture (Dt)
Drainage texture is one of the important concept of geomorphology which means that the relative spacing of drainage lines. Drainage texture is also depends on the underlying lithology, infiltration capacity and relief aspect of the terrain. Dt is total number of stream segments of all orders per perimeter of that area (Horton, 1945). (Smith, 1950) has classified drainage texture into five different textures i.e., very coarse (<2), coarse (2 to 4), moderate (4 to 6), fine (6 to 8) and very fine (>8).

Drainage Intensity (Di)
Faniran (1968) defines the drainage intensity, as the ratio of the stream frequency to the drainage density. Low value of drainage intensity implies that drainage density and stream frequency have little effect on the extent to which the surface has been lowered by agents of denudation. With these low values of drainage density, stream frequency and drainage intensity, surface runoff is not quickly removed from the watershed, making it highly susceptible to flooding, gully erosion and landslides.

Infiltration Number (If)
The infiltration number of a watershed is defined as the product of drainage density and stream frequency and given an idea about the infiltration characteristics of the watershed. The higher the infiltration number, the lower will be the infiltration and the higher ran-off.

Aerial Aspect (Shape Parameters)

The areal aspect is the two dimensional properties of a basin.

Form Factor (Ff)
According to Horton (1932), ‘form factor’ may be defined as the ratio of basin area to square of the basin length. The value of form factor would always be less than 0.754 (for a perfectly circular watershed). Smaller the value of form factor, more elongated will be the watershed. The watershed with high form factors have high peak flows of shorter duration, whereas elongated watershed with low form factor ranges from 0.42 indicating them to be elongated in shape and flow for longer duration.

Elongation Ratio (Re)
According to Schumm (1965), ‘elongation ratio’ is defined as the ratio of diameter of a circle of the same area as the basin to the maximum basin length. Strahler states that this ratio runs between 0.6 and 1.0 over a wide variety of climatic and geologic types. The varying slopes of watershed can be classified with the help of the index of elongation ratio, i.e. circular (0.9-0.10), oval (0.8-0.9), less elongated (0.7-0.8), elongated (0.5-0.7), and more elongated (< 0.5).

Circularity Ratio (Rc)
For the out-line form of watershed (Strahler, 1964, and Miller, 1953) used a dimensionless circularity ratio as a quantitative method. Circularity ratio is defined as the ratio of watershed area to the area of a circle having the same perimeter as the watershed and it is pretentious by the lithological character of the watershed. Miller (1953) has described the basin of the circularity ratios range 0.4 to 0.5, which indicates strongly elongated and highly permeable homogenous geologic materials.

Compactness Coefficient (Cc)
Compactness Coefficient is used to express the relationship of a hydrologic basin to that of a circular basin having the same area as the hydrologic basin. A circular basin is the most susceptible from a drainage point of view because it will yield shortest time of concentration before peak flow occurs in the basin (Nooka Ratnam et al. 2005).

Relief Aspect

Linear and areal features have been considered as the two dimensional aspect lie on a plan. The third dimension introduces the concept of relief. By measuring the vertical fall from the head of each stream segment to the point where it joins the higher order stream and dividing the total by the number of streams of that order, it is possible to obtain the average vertical fall.

Basin Relief (H)
Basin relief is the elevation difference of the highest and lowest point of the valley floor.

Relief Ratio (Rh)
Relief ratio is defined as the ratio between the total relief of a basin i.e. elevation difference of lowest and highest points of a basin, and the longest dimension of the basin parallel to the principal drainage line (Schumn 1956). The high values of Rh indicate steep slope and high relief and vice-versa. Run-off is generally faster in steeper basins, producing more peaked basin discharges and greater erosive power

Study Area

Kosigi watershed is located near Kosigi village (longitude 77°10’ to 77°17’ E and latitude 15°48’ to 15°56’ N) in Kurnool district, Andhra Pradesh, India, which about 212 km from, Hyderabad. It has an average elevation of 398 meters (1309 feet). It covers around 75 km2 area. Normal annual rainfall is 670mm. Soils are red earth and black cotton.

Data Used and Methodology
Data collection
– Source of data is ISRO’s Geoportal (bhuvan.nrsc.gov.in)
– Type of data downloaded from Open Data Archive is CartoDEM version 1.1 R1
– Data vertical accuracy is of 8m at 90% confidence
– Data having horizontal resolution of one arc-second (approximately 30 meters)

Data Preparation
The following thematic maps were prepared after delineation of watershed based on DEM data
• Watershed Map (Attributes: WatershedNo, PoutPointLatitude, PourPointLongitude)
• Drainage Network Map (Attributes: WatershedNo, StreamOrder)
• Contour Map (Attributes: ContourInterval)

Delineation of watershed using ArcGIS 10 Spatial Analyst (Hydrology Tool)

Delineation is part of the process known as watershed segmentation, i.e., dividing the watershed into discrete land and channel segments to analyze watershed behavior.

Once layers were generated and the same can be viewed on Online Watershed Analysis Tool which is developed based on Google Maps API at

Results and Discussion

Comparison of drainage basin characteristics

The details of the morphometric analysis and comparison of drainage basin characteristics of Kosigi watershed are presented in Table 1.

Table 1: Morphometric Analysis of Kosigi Watershed – Comparative Characteristics

* It is observed that the basin relief which is automatically calculated is showing wrong values. Even though the procedure and data is correct but this is happening due to presence of small hills on the perimeter of watershed. This problem can be resolved by visual interpretation and manual calculation using Google Maps & Elevation API.

Table 2. Modified Relief Parameters (Three Dimensional)

Conclusion

GIS based approach facilitates analysis of different morphometric parameters of watershed delineated based on DEM data using ArcGIS software. In this study, an attempt is made to study morphometric parameters in linear, aerial, relief aspects of three micro-watersheds using Google Maps & Elevation API. It is observed that the shape of these watersheds are nearly circular, less elongated with gentle slope.

References

NOOKA, RATNAM, K., SRIVASTAVA, Y.K., VENKATESHWARA RAO, V., AMMINEDU, E. and MURTHY, K.S.R. (2005) Check dam positioning by prioritization of micro-watersheds using SYI model and morphometric analysis – Remote Sensing and GIS perspective. Jour. Indian Soc. Remote Sensing, v.33 (1), pp.25-38.
KULDEEP PARETA, UPASANA PARETA (2011) Quantitative Morphometric Analysis of a Watershed of Yamuna Basin, India using ASTER (DEM) Data and GIS. International Journal of Geomatics and Geosciences Volume 2, No 1, 2011

CENTRAL GROUND WATER BOARD REPORT on Ground Water Information, Kurnool District, Andhra Pradesh (July 2007)
AKRAM JAVED, Md YOUSUF KHANDAY, SUBAH RAIS (July 2011) watershed Prioritization Using Morphometric and Land Use/Land Cover Parameters: A Remote Sensing and GIS Based Approach
SENADEERA K.P.G.W, PIYASIRI S, NANDALAL K.D.W The evaluation of Morphmetric Characteristics of Kotmale Reservoir catchment using GIS as a tool, Sri Lanka