It is usual that streams of different size, shape and order flow in an area. It is seen that streams merged together to form bigger streams. So, it is important to know the order of a stream and how do it position in the stream’s hierarchy, and stream order is the way to know that (Horton, 1945; Strahler, 1957). Actually, morphometric analysis starts with the finding out of stream orders. It provides very important information regarding drainage network of an area. Strahler’s (1964) method for stream ranking was used here in GIS environment. It was found that there are 3420 streams of different ranks flowing as drainage network in the investigated area. Highest rank of the stream order is 6. Lower ranked streams mostly found in the relatively elevated regions compared to higher rank. Figure 4 showed the stream ordering of the area of study.

In this investigation, the maximum number of streams is observed in the first-order and counted to 1745, whereas the minimum number of 81 streams is observed for 6^th order. Second order streams are 820. The third and fourth order was relatively close with numbers 374 and 318. Fifth order is counted to 82 and very close to the 6^th order. It is closely observed that stream numbers significantly decreasing with increasing stream orders. Table 1 and Figure 5 showed stream ordering and stream numbers. The first three as well as fifth stream orders show normal tendency of stream branching, but it was not found for the fourth and sixth stream orders. This type of stream branching results a poor drainage network, and often cause flooding or slow stream flow which makes the study area a good potential for groundwater recharge.

GIS environment profoundly used to calculate and measure stream length in the area of study and tabulated in Table 2. It showed different stream lengths for different stream orders. Surface runoff characteristics significantly influenced by stream length which actually consider as an important hydrological parameter. Smaller streams are found in areas with steep slope. Flatter plain provides opportunity to have longer stream lengths (Strahler, 1964). Stream length of 1031.90 km measured in the first order found to be maximum among the total extracted lengths of 1774.72 km. Sixth order streams have the minimum length of 23.95 km. Among others second, third, fourth and fifth order streams occupied 405.61 km, 162.86 km, 117.97 km and 32.42 km respectively. It is closely observed that stream lengths shrank with increasing stream order (Figure 6).

The characteristics of an area are revealed from mean stream length and have significant influence on the surface. It can be found by different component analysis of drainage network (Strahler, 1964). It was calculated using Equation (1). It revealed that L_m values confined in the 0.59 to 0.30 range. The L_m value of 0.59 was found for first order and 0.30 for sixth order. Other values of 0.49, 0.44, 0.37 and 0.40 associated with second, third, fourth and fifth order respectively (Table 2).

With the help of Equation (2) this parameter has been calculated and tabulated in Table 2. Highest and lowest value of this parameter were 0.74 and 0.27 observed for sixth order and fifth order stream respectively, the average being 0.42. Higher value of R_l indicates the abnormality in drainage network. Abnormalities are found in between stream 4 and stream 3 lengths as well as in streams 6 and stream 5. These types of drainage network often cause very low drainage water movement, which facilitates a good chance for water to percolate.

It is very important morphometric parameter which can predict flood prone zones (Prabhakaran & Raj, 2018). Using Equation (3), this parameter has been estimated and shown in Table 1. Highest and lowest value of the parameter was 3.88 and 1.01 found for fifth order and sixth order stream respectively, whereas the average value was 2.08. Lower mean bifurcation ratio means the area is flat or rolling surface more likely to be affected by the flood and highly favorable for groundwater recharge. Higher bifurcation ratio is seen near areas near close to rivers.

Proposed method of Schumm (1956) was used here to calculate this parameter. The R_bwm found in this study is 2.12 (Table 1). It is close to average value of bifurcation ratio. Low value of this parameter indicates areas with very low slope and often affected by flood.

It is considered to be a vital parameter in morphometric analysis of an area. Here, it was calculated using Equation (4) and tabulated in Table 1. Higher the value of q, the more is the storage in the drainage network which in turn can cause flooding. Parameter value confined in between 0.06 and 0.73. The zones having lower slope and faulty drainage pattern with higher stream order facilitate low water current. Hence, it makes a greater scope of flooding and groundwater recharge.

It is one of the most significant morphometric parameters of an area. Groundwater recharge potentiality exclusively depends on this factor as well. It indicates how closely drainage channels are distributed in an area. Its value depends on many surface and sub-surface parameters. Low permeability of subsoil materials, flat or very small steepness of lands and high vegetation results very small drainage density (Nag, 1998). Low drainage density values provide higher chance of flooding and groundwater recharge hence good groundwater potentiality. High relief, small vegetation and higher permeability provide good reasons for higher drainage density. The D_d value of the area ranging from 0.01 to 23.06 km/km² (Figure 7) was calculated using Equation (5). It is seen that most of area has lower drainage in the range of 0.01 to 4.90 km/km². North-east part of the area near to the river Gorai has very high drainage density value ranging from 8.80 to 23.06 km/km². Gentle drainage density from 3 to 5 km/km² is seen in one-third areas distributed evenly.

Here Equation (6) was used to calculate this parameter. F_s values of this study are in the range of 1.00 - 5.64 no. of streams per km². The stream frequency values as shown in Figure 8 were positively correlated with D_d values of the area. It was revealed that the regions of lower D_d had lower stream frequency and higher D_d were followed by higher F_s. But higher stream frequency having range of 3.36 to 5.64 is distributed in the area sporadically. It is due to streams of very small lengths.

It denotes relative positioning of drainage network. Smith (1950) classified drainage texture to 5 categories based on drainage density. In this study, D_t values were calculated using Equation (7) and depicted in Figure 9. The parameter varied in between 0.25 to 7.70, and it lies in the range of very coarse to moderate coarse based on drainage density. It is revealed from the results that it followed a very closer drainage texture pattern.

This morphometric parameter was calculated using Equation (8) and represented in Figure 10. The C values of this area are in the range of 0.04 to 90.09 km²/km. In most of the areas C value is below 0.24 m and next channel length is as much as 28.25 m covers almost entire area. Areas with lower C values mean quick water discharge hence less favorable for groundwater recharge and vice versa.

The calculated values of L_g using Equation (9) are shown in Figure 11. It is seen that L_g values for the area are ranging from 0.02 km to 45.05 km. But comparatively low relief having values of 0.02 km to 0.24 km is dominating most of the area. It means the area is densely populated with streams of all orders. Overland flow length above 14.12 m is hardly seen.

The estimated circulatory ratio of the area using Equation (10) is shown in Figure 12. Here, R_c values varied from 0.39 to 0.84. Low R_c values indicate mainly elongated areas whereas a value close to unity means more circular area. Most of the areas have higher R_c values ranging from 0.77 to 0.84, and hence relatively quick flow through the unit area results. Somehow it is less favorable for percolation.

This morphometric parameter measures different types of erosional process, movements of sediments, flood formation and corridor of flood. For a perfect circle F_f value is 1, when it is nearing zero it means elongated area. Here it was calculated using Equation (11) and found its value as 0.31 which indicates the area slightly elongated in shape. Hence, flood probability is high as transport rate is low. This area is favorable for groundwater recharge potential.

Generally, R_e values lie between 0.6 to 1 depending on different climatic and geologic types (Rudraiah et al., 2008). Equation (12) was used to calculate this parameter, and its value is 0.62 for the area studied. So, this area is moderately elongated. It provides relatively larger time to travel from one side to another of the study area, which in turn preferable for groundwater recharge.

Absolute relief and relative relief are 3D morphometric parameters of an area. Absolute relief was readily available from SRTM (DEM) data. It was presented in Figure 13. The relative relief was estimated using Equation (13) with the help of GIS software and depicted in Figure 14. Areas with low relative relief increase the probability of surface water body, flooding and groundwater recharge.

Overall steepness of an area is described by the relief ratio, a key indicator of entire erosional process of an area. Relief ratio calculated using Equation (14) for this study is found to be 0 ­ 1.81 (Figure 15). It is highly connected with the hydrologic character of the study area and erosional process. This value is high near rivers whereas the remaining areas are mostly flat indicating the favorability for groundwater recharge.

This morphometric parameter mostly used to measure undulation of surface topography (Selvan et al., 2011). This parameter was calculated using Equation (15). Ruggedness number of the area studied ranges from 0.04 to 214.72 as shown in Figure 16. When drainage density is high with high relief of a basin, the ruggedness number of that river basin will also be high (Chow, 1964). High R_n value is observed in the riverine area where dominant part is below the value 25.24 spreading over the area.