Morphometric Analysis of Mun River Basin, Thailand: A Geographical Information System Approach

The aim of present study is to investigate morphometric analysis of Mun river basin in Thailand. The drainage parameters performed such as linear, aerial and relief aspects of 23 sub watersheds in the concentrated area. The analysis shows that watershed conatins 6611 drainage segments and stream order from I to VII. From that 5275 segments are comes under I order stream, 1025 are II order, 235 are in III order, 57 are IV order, 16 are in V order, 2 and 1 segments are comes under VI and VII order, respectively. The total stream length of Mun river basin is 40353.8 km. The majority of basin contains the bifurcation ratio value is >5. This indicates that geologically hard rock terrain, less infiltration and high flash flood. This analysis helps to better understanding the management and planning activities in study area .


Introduction
It is a tool for measuring Morphometric, mathematical analysis of the configuration of the earth's surface, form, and dimensions of its landforms (Clarke,1966). This examination provides a quantitative explanation of the basin geometry to grab initial slope or inequalities within the rock hardness, structural controls, recent geologic process and geomorphic history of the basin (Strahler, 1964). The current investigation deals with the fluvialmorphometry, which includes the consideration of linear, aerial and relief aspects of the Sub-watersheds. The study area has been divided into 23 sub-watersheds. GIS techniques are used for assessing various terrain and morphometric parameters of the watershed, as they provide a flexible environment and a powerful tool for the manipulation and analysis of abstraction data significantly for the feature identification and also the extraction of knowledge for higher understanding.

Materials and Methods
The general purpose of this analysis of watershed, from a Survey of India, toposheet on 1:2,50,000 scale have been used for preparation maps like base map and drainage network map of the watershed and demarcate the 23 sub-watershed in the concentrated area based on the elevation, slope and outlet points. The numbers were given in the Figure 2. The watersheds have been digitized through the ArcGIS Software 10.1 and calculated the stream orders, which was proposed by Strahler (1952). The various Quantitative morphometric analysis was carried out in 23 micro-watersheds separately for assessing their linear, areal and relief aspects.  Hierarchical rank of streams Strahler (1952) Bifurcation Ratio (Rb) Rb=Nu/Nu+1 Nu=Total no. of stream segments of order "u' Nu+1= Number of segments of the next higher order Schumm (1956) Mean Bifurcation Ratio (Rbm) Rbm= Average of Bifurcation ratios of all orders Strahler (1957) Stream Length (Lu) Length of the stream (Km) Horton (1945) Mean Stream Length (Lsm) Lsm=Lu/Nu Lu=Total stream length of order 'u' Lu-1=Total stream length of its next lower order Horton (1945) Stream Length Ratio (Rl) Rl= Lu/Lu-1 Lu=Total stream length of the order 'u' Lu-1=Total stream length of its next lower order Horton (1945) (1956) The morphometric analysis of a drainage basin and its drainage network can be better achieved through the latest technologies like GIS, since conventional measurement of these parameters is laborious and cumbersome. The methodology adopted for the computation of morphometric parameters given in Table 1.

Results and Discussion
In the present study, the morphometric parameters such as linear, areal and relief aspects for the delineated sub-watersheds are calculated based on formulas suggested by various workers and the results are discussed below.

Linear Aspects
Linear aspects of the basin are related to the channel patterns of the drainage network wherein the topological characteristics of the stream segment in terms of open links of the network, which consists of all of the segment of stream of a particular river, is reduced to the level of graphs, where stream junctions act as points (nodes) and streams, which connect the points (junctions) become links or lines wherein the numbers in all segments are counted, their hierarchical orders are determined, the length of all stream segments are measured and their different interrelationship are studied. The nature of flow paths in in terms of sinuosity is equally important in the study of linear aspects of the drainage basins. Thus, the linear aspect includes the discussion and analysis of stream order (µ), stream number (nµ), bifurcation ratio (Rb), stream lengths (Lµ), length ratio (Rᶫ), length of overland flow (Lᵍ), sinuosity indices etc.

Stream Order
The designation of stream orders in the first step in drainage basin analysis based on a hierarchic ranking of streams. In the present study, ranking of streams has been carried out basin on the method proposed by Strahler (1964). According to him "each finger-tip channel is designated as segment of 1 st order. At the junction of any two 1 st order segments, a channel of 2 nd order is produced and extends down to the point, where it joins another 2 nd order segments whereupon a segment of 3 rd results and so forth". These streams may have additional stream segments of lower orders than their own order IJARSG-An Open Access Journal (ISSN 2320 -0243)

2943
and thus these do not affect the classification (Figure 1). It may be mentioned that the hierarchical order increases only when two stream segments of equal meet and form a junction. The order does not increase if a lower order stream segment meets a stream segment of high order.
The entire basin consists 6611 segments of river in the range of stream order I to VII. Out of which 5275 segments are comes under I order stream, 1025 are II order, 235 are in III order, 57 are IV order, 16 are in V order, 2 and 1 segments are comes under VI and VII order respectively.
Taking into sub watershed individually seven and eight sub-watershed consist above 500 stream segments, 2,12,16,17 and 23 sub watersheds are consist above 400 stream segments, 6 and 13 subwatersheds are consist above 300 stream segments, 1,3,4,5,9,15 and 18 sub watersheds are consist above 200 stream segments, 10,11,14,19,20, and sub-watersheds are consist of above 100 stream segments and 22 sub watershed is consist below 100 stream segments. The sub-watersheds with higher no of stream segments are characterized by bigger watershed area.

Stream Length
The numbers of streams of various orders in watershed are counted and their lengths from mouth to drainage divided are measured with the help of GIS software's, (Table 2) the stream length (Lu) has been computed based on the law proposed by Horton (1945) for all the 23 sub watersheds (Table 1). Generally, the total length stream segments are maximum in first order streams and decreases as the stream order increases.
The total stream length of Mun river basin is about 40353.8 km of which first order stream length is about 21334.6 km. The second order stream length is 7346.8 km. The third order stream length is 3983.8 km. The fourth order stream length is 3514.1 km. The fifth order stream length is 2691.8 km.
The sixth order stream length is 1364.3 km. and the seventh order stream length is 118.1 km.
The large sub-watershed such as, sub watershed 8, 16, 23, 12 and 17 are compressing the total stream lengths of 3324.4, 3098.5, 2920, 2782.1 and 2767 km are respectively. Obviously, the lesser total stream length is observed in the watershed of lesser area. Higher of I order stream segments are notably observed in sub watershed 8, 16, 17, 2, 12, 7,13, 23 and 18. Whereas in II order, the stream lengths are higher in sub watershed such as, Sub watershed 8, 17, 12, 16 and 13. Generally, the total length of stream segments is maximum in first order streams and decreases as the stream order increases.

Mean Stream Length
According to Strahler (1964), the mean stream length is a characteristics property related to the drainage network and is associated surface. The mean stream length (Lsm) has been calculated by dividing the total stream length of order 'u' and number of stream length of order 'u' (Table 1) it is noted from ( Table 3) that Lsm varies from 4.08 to 12.61 and Lsm of any given order is greater than that of the higher order, this might be due variations in slope and topography.

Stream Length Ratio
Stream length ratio (RL) may be defined as the ratio of the mean stream length of the one order to the next lower order of stream segment (Table 1) Horton's law (1945) of stream length states that stream length segments of each of the successive orders of a basin tends to approximate a direct geometric series with streams length increasing towards higher of streams.

Bifurcation and Mean Bifurcation Ratio (Rb and Rbm)
The term bifurcation ratio (Rb) may be defined as the ratio of the number of stream segments of given order to the number of segments of the next higher order (Schumm, 1956). Strahler (1957) demonstrated that bifurcation ratio shows a small range of variation for different environmental except where the powerful geological control dominates. The mean bifurcation ratio (Rbm) may be defined as the average of bifurcation ratio of all orders (Strahler, 1957). The Mun river basin the higher mean bifurcation ratio has been observed for the sub watershed 11, 15 and 21.
Mean bifurcation shows stable trends in a region of uniform geological structure and lithologies but they show variable trends over varying geological structures, (Sing et al., 1984) have remarked that "geological structure and associated lithologies do not cause significant variations in bifurcation ratios and this observation holds parity with the conclusions of (Miller, 1953)". Horton (1945) classified the mean bifurcation ratio into two classes: 0 to 2 flat or rolling basin; 3 to 4 mountainous or hilly regions.  Table 4 reveals  sub watershed 3 , 9, 10, 14, 15, 19, 20 and 22 are identified with the mean bifurcation ratio values of less than 2.5 and these sub watersheds are observed with the soil types of Palaviduthi or Aeolian or Vyologam, geological these sub watersheds are identified with either silliminate or recent alluvium.

Areal Aspect
The areal aspect of the drainage basin include the study of basin perimeter, geometry of closed links i.e. basin shape, law of basin area, law of allometric growth, stream frequency, drainage density, drainage texture, form factor, elongation ratio, and circularity ratio etc.

Basin Area
Basin area is very important Morphometric attribute as it is related to the spatial distribution of a number of significant attributes such as drainage density, stream frequency, drainage texture, slopes absolute and relative reliefs, dissection index etc. The drainage area is delineated on the basis of water divides and the areas of all stream segments of each order. All of the ground surface, which directly feeds the first order basins. The area of second order stream segments include the area of first order segments plus the areas of inter-basins, which are triangular patches of ground surfaces contributing directly to the second order segments. The same principle works for all the increasing successive order segments (Sing and Srivasava, 1974). Thus, the basin area becomes automatically cumulative from the first order to the successive higher orders. For the present study area of the sub watersheds is 116226 km² notably sub watershed 23, 16, 12, 8, 9 and 7 sub watersheds are the highest basin respectively (12316,11148,8322,8172,6871, and 6246 km²) ( Table 5).

Basin Shape
The geometry of basin shape is a paramount significance as it helps in the description and comparison of different forms of the drainage basins and it is also related to the functioning of the units of the basins and its genesis. On the average 3 sub-categories of basin shapes have been recognized viz. (i) circular, (ii) elongated, and (iii) indented.

Basin Length
Length in a straight line from the mouth of a stream to the farthest point on the drainage dived of its basin. The study area has total length of 3163.99 km and the 23 and 16 sub watersheds are the high basin length.

Form Factor (Rf)
According to Horton (1932), form factor (Rf) may be defined as the ratio of basin area to square of the basin length. With the reference to the Table 6 it is observed than form factor of the entire Mun river basin is 0.29. The sub watersheds are 2, 3, 6, 7, 8, 12, 14, 20 and 21 have observed the form factor value above 0.3 implies that the sub watersheds are relatively circular in shape, whereas the remaining watersheds have observed with less than 0.3 are comes under elongated in shape.
Particularly sub watershed 23 have registered with the lowest value (0.09) and highly elongated. The elongated basin with low form factor indicates that the watershed will have flatter peak of flow for longer duration. Flood flows of such elongated basins are easier to manage of the circular basin.

Elongation Ratio (Re)
Schumm (1956) defined elongation ratio (Re) as the ratio between the diameter of the circle of the same area as the drainage basin and the maximum length of the basin. In general the values of elongation ratio vary from 0.6 to 1.0 over a wide variety of climatic and geological types. Values close to 1.0 are typical of regions of very low relief, whereas values in the range 0.6 to 0.8 are usually associated with high relief and steep ground slope (Strahler, 1964). These values can be grouped in to 3 categories namely: i) Circular (>0.9) ii) Oval (0.8 to 0.9) iii) Less elongated (<0.7) Elongation ratio of the sub watershed of the study area varies from (0.34 to 0.72) ( Table 6). The lowest elongation ratio (0.32) in case of sub watershed 8 is indicates high relief (2236 m) and steep slope and less elongated in shape. Most of the study area covers the less elongated in shape. Especially the sub watershed IX and X are no relief or flat surface and sub watershed 21 and 22 are the circular in shape with low relief (191 and 253) and shape. A circular basin is more efficient in the discharge of runoff than an elongated basin (Sing, 1997).

Circularity Ratio (Rc)
The circularity ratio was proposed by Miller (1953) as comparison of the basin area with the area of the circular having the same perimeter. The circularity ratio is influenced by the length and frequency of streams, geological structure, landuse/landcover, climate relief and slope of the basin. In this study circularity value varies between 0.03 to 0.67 (Table 6). High circularity ratio values more than 0.4 have IJARSG-An Open Access Journal (ISSN 2320 -0243)

International Journal of Advanced Remote Sensing and GIS
2949 observed for the sub watershed 2, 3, 4, 5, 6, 8, 10, 11, 12, 14, 17, 18, 20 and 22 sub watersheds, and one can said it is more or less circular and notably characterised by the drainage density between 0.15 to 0.35 km/sq km.
The remaining watersheds are observed with the circularity value between 0.3 to 0.4 and it can be said the sub watersheds are elongated in shape, and the drainage density are in this area are observed with either lower density 3.5 to 10.5. Soil, geomorphology, land use and geological formation of this area implies no direct relationship with the variations in the circularity ratio. Notably there is a close relationship between number of stream and circularity ratio, the higher circularity ratio values are usually associated lower number of stream segments and vice versa.

Drainage Density (d)
Drainage density refers total stream lengths per unit area. Horton (1945) defines drainage density as ratio of total length of all stream segments in a given drainage basin to the total area of the basin.
The drainage density can classified into five categories (Savindra Sing, 1978). According to Nag (1998) low drainage density generally results in area of highly resistant (or) permeable sub soil material, and low relief. High drainage density is the result of weak (or) impermeable sub surface material and mountainous relief. Low drainage density leads to coarse drainage texture while high density leads to fine drainage texture.

2950
The low drainage density of the study area are varies from (0.17 to 0.25) indicating low drainage density with low relief ratio of 67m, 162m and 0m respectively coarse drainage texture. Whereas the sub watershed 4, 5, 6, 12, 16, 19 and 21 sub watersheds indicating moderate drainage density (0.25 to 0.35) with high relief 529m, 729m, 2020m, 400m, 938m, 460m, 414m, and 365m respectively and it has the coarse to related coarse drainage texture.

Stream Frequency (Fs)
Stream frequency or drainage frequency is the measure of number of stream per unit area (Horton, 1932

Drainage Texture (Rt)
According to Horton (1945) drainage texture is the total number of stream segments of all orders per perimeter of that area. It is the one of the important concept of geomorphology which means that the relative spacing of drainage lines. Drainage lines are numerous over impermeable areas than permeable areas. Smith (1950) has classified drainage density in to five deferent textures.
 The drainage density less than 2 indicates very coarse  The drainage density between 2 to 4 is coarse  The drainage density between 4 to 6 is moderate  The drainage density between 6 to 8 is fine  The drainage density is greater than 8 is very fine In the present study the drainage density is less than 2, it indicates very coarse drainage texture.

Length of Overland Flow (Lg)
The length of overland flow, considered as a dominant hydrologic and morphometric factor is the mean horizontal length of flow path from the divided to the stream in a first order basin and is a measure of stream spacing and degree of dissection and approximately one half the reciprocal of the drainage density (Brice, 1964). It is the length of water before it gets concentrated into definite stream channels (Horton, 1945).
The Figure reveals that the length of overland flow is less in, Sub watershed 1, 2, 10, 11, 15, 17 and 18 sub watersheds, as drainage density is high in these sub watersheds, when comparing remaining sub watersheds. The computed value of length of overland flow for all sub watersheds varies from 0.87 to 2.94.

Relief Aspects
The relief aspects of the drainage basins are related to the study of three dimensional features of the basin involving area, volume and altitude of vertical dimensions of land forms wherein different morphometric methods are used to analysis the terrain characteristics, which are the result of basin process. Thus, this aspect includes the analysis of the relationships between area and altitude (hypsometric analysis), altitude and slope angle (clinographic analysis), average ground slope, relative relief, relief ratio, dissection index, profiles of terrains and the rivers. The stream elevation can be estimated from the contour crossings on the topographic sheets. The total drop in elevation from the source to the mouth can be found for the elevation from the source to the mouth for the tributaries and the horizontal distances can be measured along the channel using a map measures.

Relative Relief
Relative relief also termed as "amplitude available relief" or "local relief" is defining as the difference in height between the highest and the lowest points (height) in a unit area. Relative relief is very important morphometric variable which is used for the overall assessment of morphological characteristics of terrain and degree of dissection (Glock, 1932).