Evaluation of morphometric characteristics of the watershed to determine the erosion of the GIS Case Study: Nazloochaei Watershed

Watershed is area of land that surface water of rain and melting snow conduct towards a single point, which is usually out of the basin. Check of watershed is one of the main strategies for integrated management of natural resources and sustainable development. Recently, the availability of remote sensing (RS) data and geographical information system (GIS) technologies has allowed for improved understanding of the morphometric properties and surface drainage characteristics of many watersheds in different parts of the world (Parveenet al., 2012; Nayar& Natarajan, 2013). For example, Shrimaliet al. (2001) presented a case study of the 42 km Sukhana lake catchment in the Shiwalik hills for the delineation and prioritization of soil erosion areas. In addition, Srinivasaet al. (2004) used GIS techniques for morphometric analysis of sub­watersheds in the Pawagada area, Tumkur district, Karnataka. Nookaratnamet al. (2005) carried out a study on dam positioning through prioritization of micro­watersheds using the sediment yield index (SYI) model and morphometric analysis. Khan et al. (2001), used RS and GIS techniques for watershed prioritization in the Guhiya basin and subwatersheds in Odisha, India respectively.
The study area is one of sub-watershed of rivers of Urmia (Nazloochaei) that located in North West of Iran with an area of 948.75 km2. The study area wasselectedfordetailedmorphometric analysis using Geography information system (GIS). The input data for morphometric analysis was DEM with resolution of 30 m from ASTER satellite. The steps of stream extraction consist of: 1.Extraction of drainage networks from the DEM using the flow direction method, which consists of the followingsteps (O’Callaghan&Mark,1984): i.FillSinks:Asinkisanuncompletedvaluelowerthanthevaluesofits neighborhood.Toensureproper drainage mapping, these sinkswere filled by increasingelevationsofsinkpointstotheirlowestoutflow point.
ii.Calculate FlowDirection: Using the filledDEMproduced inStep1,theflowdirectionswerecalculatedusing theeight-direction flow model,whichassigns flowfromeachgridcelltoone of itseightadjacentcellsinthe direction with thesteepest downward slope.
iii.CalculateFlowAccumulation: Using theoutputflowdirectionrastercreatedin Step2,the numberofupslopecells flowingto alocation wascomputed.
iv.Define Stream Network: The next step is todetermine acriticalsupportareathatdefinesthe minimumdrainage area that is requiredtoinitiate achannelusinga threshold value.
v.Stream Segmentation:After the extractionof drainagenetworks,a unique value wasgivenforeachsectionofthenetworkassociatedwitha flow direction.
Morphometricanalysisofthe study area consist of: Stream number (Nu)
Nu is number of segments in order U
Stream order (U)
Cumulative length of streams (L), L = ∑Nu, L is calculated as the number of streams in each order and total length of each order is computed at subwatershed level (Horton, 1945).
Bifurcation ratio (Rb)
Rb=Nu/N(u) N(u)=Numberofsegments of the nexthigherorder (Schumms, 1956),
Watershed relief (Bb),Bb = Hmax – Hmin ,Bb is defined as the maximum vertical distance between the lowest and the highest points of a subwatershed. Hmax and Hmin are maximum and minimum elevations respectively (Schumms, 1956)
Drainage density (Dd)
Dd=Lu/A, A=Watershed area(km2), L(u) is totalstream length (Horton, 1932)
Stream frequency (Fs),Fs = Nu/A, Fs is computed as the ratio between the total number of streams and area of the watershed (Horton, 1932)
Form factor (Rf)
Rf=A/Lb2, Rf is computed as the ratio between the watershed area and square of the watershed length. 𝐿 is the watershed length (Horton, 1932)
Circularity ratio (Rc)
Rc=4π*A/P2, P is the watershed perimeter (km)
Elongation ratio (Re)
Re=(2/Lb)*(A/π)0.5
The results show that according to high streams (489 streams), streams of 1, 2 and 3, high length of rivers, the high of the catchment area, the high watershed relief (Bb) that represent the high mountains and slope in the study area. Using morphologic characteristics can be determined sensitivity landforms to erosion in the region. So that after preparing landform map using the topography position index (TPI), and considering the areas susceptible to erosion through morphometric features, landforms susceptible to erosion in the study area were determined. With the increasing number and length of channels increases the erosion. By comparing the landform map and waterways map of the study area was found that landforms Class 4 (U-shaped valleys) and landforms in class 3 (high draining) had the highest erosion. Also with increasing relief factor, the erosion was more.
So that ridge landforms such as in high altitude (landforms in class 9 and 10), had the highest erosion and were therefore the most sensitive landforms. The drainage density features as the most important factor for determination of erosion and its relation to landforms were used. The results showed that by increasing the amount of drainage density increases the erosion that they were for landforms Class 4 and Class 6. This study has demonstrated that morphometric characteristics can be used to predict other watershed characteristics.