In this chapter the procedure of creating hydrological model are shown and elaborated with full details. It explains procedures and data required to conduct a successful hydrological model.
3.2 METHODOLOGY FLOW CHART STUDY
FIGURE 3.0 Procedure of conducting the project.
3.2.1 DATA COLLECTION
In this project enormous amount of data collection is needed to execute a perfect hydrological model. First data required in hydrological modelling is Digital Elevation Model (DEM) also known as Digital Terrain Model (DTM). This data is digital elevation of earths surface which is used in ArcGIS-ArcMap and HEC-Geo-HMS software for terrain pre-processing in order to generate vector layers of catchments and streams. It is also used to develop drainage lines, and stream segmentation. DEM datas main function is to create sub-basins and rivers to form a catchment before the data is exported to HEC-HMS software.
Digital Elevation Model (DEM) of Malaysia will be obtained as Shuttle Radar Topography Mission (SRTM) with 30 meters grid resolution and freely downloaded from DIVA-GIS online website and the data obtained shows elevation of peninsular Malaysia as well as Sabah and Sarawak as shown in FIGURE 3.1 while in this study only needs digital elevation model of Semenyih basin, thus various steps and techniques will be undertaken to create the required digital elevation model of the project.
FIGURE 3.1 DEM data [adapted DIVA-GIS]
After creating the basin from ArcGIS-ArcMap and HEC-Geo-HMS software using digital elevation model data, the catchment or the basin is exported into HEC-HMS which also require large amount of data input. The Department of Irrigation and Drainage (DID) will provide rain gauge precipitation data, stream flow and land use maps. These data are fundamental backbone of creating hydrological modelling for example the land use map is essential in determining data required for sub-basin area and lag time in HEC-HMS. The land use is in digital elevation model format so that Arc-GIS-ArcMap can read the file, but when the data is in DEM format the data cannot be analysed because its compacted thus, the file needs to be converted excel format in order to categorize the land use activities such as, cemetery, recreational area, urban are grouped as concrete. Solid waste disposal site, tin mine, grassland, cleared land, quarry, floriculture and oil palm land area grouped as soil group. Aquaculture lake, pond, river and water ways are also labelled as water group, this categorization is done in order to find impervious and pervious of the ground of the study area in order to calculate correctly the infiltration rate. In this study only three rain gauge stations data are available namely, Ladang Dominion, Empangan Semenyih and Lalang Sungai Lui with their coordinates in decimal form are shown in the table below.
Another fundamental data for this study in the satellite data which will be obtained from NASA thus, this data will provide satellite precipitation data which is a significant factor in this study. Satellite precipitation data will be used in HEC-HMS simulation which the output result will be compared to the observed data from department of irrigation and drainage (DID). There is one satellite precipitation data used in this study, the Tropical Rainfall Measuring Mission (TRMM).
Tropical Rainfall Measuring Mission (TRMM) satellite was launched in 1997 with cooperation between the National Aeronautics and Space Administration (NASA) and Japan Aerospace Exploration Agency Satellite and its functions is to study precipitation for weather purposes and climate research shown in figure 3.2 Shows a picture of TRMM satellite.
FIGURE 3.2 TRMM Satellite [adapted from NASA]
FIGURE 3.3 TRMM Instruments Measurement Path [Adapted from NASA]
ArcMap is a software made by Environmental Systems Research Institute (ESRI). This software is used at the first stages of hydrological modelling and Digital Elevation Model (DEM) data is applied to create basin elements of the catchment, as stated in section 3.2, DEM data obtained contains elevation of whole Malaysia and this project only is interested on Semenyih basin. Thus, ArcMap tools are used to create the catchment needed for the project such as, table attributes, Arc Toolbox, Arc Catalog, and table of contents. This software also has identifying attributes features, this tool is very important because the researcher can determine points, lines and catchment area that make up vector map layers, this is because the map features in ArcMap have attributes which means all their data is stored in table of attributes and by opening the table of attributes the researcher can locate map location with its latitude and longitude in terms of (degree minutes and second) otherwise the program cannot read the location of the map plus shape length and description of the map can be found using this tool as well thus this is what makes ArcMap so unique in hydrological modelling.
Dem in ArcMap
The first stage or step in creating hydrological model is to have DEM Data. DEM data was obtained from DIVAS-GIS online website and once the data is acquired modelling was started. As started in 3.2.1 the DEM obtained was the elevation of whole Malaysia but in study it only needs the elevation of Semenyih basin thus, ArcMap tools will be used to create the required elevation model for the basin. ArcMap is the central core of this study as it will provide all maps required in this study.
Once the DEM is obtained its added into ArcMap using Add Data tool. The DEM data acquired has spatial reference which was set as default of Geographic Coordinate Systems (GCS) by NASA and needs to be converted to Projected Coordinate Systems (PCS) before the modelling begins. This is done because GCS uses three-dimension spherical surface to define locations on earth while PCS uses flat, two-dimensional surface. In this study PCS is adapted but DEM obtained was set on GCS thus, map projections require to be changed to the desired coordinates. The DEM data was set on WGS_1984_UTM_Zone_48N (PCS), this was done using Projections and Transformations toolset. This toolset contains tools to convert geographic data from one map projection to another. The DEM data obtained often needs to be transformed or projected this is because the data received is not always preprocessed thus, choosing the perfect coordinates will reduce errors occurring as the modelling progresses.
Once the coordinates issue is settled now modelling will be commenced using Arc Toolbox to create the Semenyih basin. Arc Toolbox in ArcMap is used to generate the Semenyih Basin and contains various tools which are used to create the catchment of the basin such tools include Fill, Flow Accumulation, Stream link and Snap Pour Point and many others. These tools are A tool and they also containing a geoprocessing tool. By applying those tools, they will require enormous data input and output data this is because every step in Arc Toolbox requires input and output file which will be saved with designated folders assigned by the researcher. As shown in FIGURE 3.4 word photo.
The required basin is created using a series of predetermined steps that have to be successfully executed step by step without skipping any step in the sequence they are arranged thus; each step and tool has its own function and its important in the process of creating the basin and they have to successfully executed to acquire the required result from ArcMap. Under Arc Toolbox is the Spatial Analysist tool. This is important tool which will be used to create the Semenyih basin because it has an extension which provides a rich set of spatial analysis and modeling tools for both raster and feature data. The capabilities of Spatial Analyst are broken down into categories or groups of related functionalities. Knowing the categories will help this study to identify which particular tool to be used because the Spatial Analyst tool requires input file which will produce an output file that requires to be saved so that it can be re-called as the modelling continues between different software. Under Spatial Analyst are various tools such as Distance, Raster Creation, Zonal, Groundwater and many others but in this study, it only interested in Hydrology tool. Furthermore, Hydrology tool contains various sub-tools which requires to be followed in order, otherwise skipping from one process will enforce the modelling to be re-stated from scratch. The functions under Hydrology tool will be explained in the following paragraphs. The first process under Hydrology is the Fill tool. This function is used to remove all errors in the DEM map and imperfections in the surface raster which is caused due to poor resolution of the data. Another reason for using the Fill Process is fill sinks and solve any errors in the data to ensure accurate delineation of the catchment. If the sinks and poor resolution problem are solved the delineation and streamflow of the basin might become discontinuous.
Once Fill function is carried out the file is saved and the modelling continues with Flow Direction tool. This tool creates flow direction of the catchment in raster format so that that each cell delineates to the cell below it, which has lower elevation to make sure perfect delineation of the catchment as a whole and its main function is to filter out singe cell sinks which the Fill tool could not able to.
Flow Accumulation is next step in the modelling because this tool calculates the flow into each cell and provide number of each cell and its accumulation result before the stream flow into the next cell downslope. This tool is used to make sure that at the output point will have accurate stream flow from highest point of the basin to the output point. By double right clicking the Flow Accumulation in ArcMap file the researcher can classify how many classes needed in this study under Symbology tool. A second class was chosen and this was done in order to inspect the results visually as seen in FIGURE 20 the threshold or stream flows of Peninsular Malaysia can be virtually seen thus, this is useful to detect any errors that might occurred in the previous steps of the modelling.
Figure 20 Flow accumulation
The Conditional tool will be used to control the catchment based on an output values which were placed as input values. The conditions used in this study can be categorized as two types, those that were set in the queries as manual input or conditional based on statement list. The queries tools identify all evaluated cells which are deemed as true thus, these cells will retain their original values whether they are set to another new value or if they are set to no data. The cells that are assessed as False can be set to values that different from the true condition. For instance, if the evaluated raster input is found out its greater ten its either returned to one or hundred. The main function of Con tool is to perform evaluation of each cell inputs as raster input. Conditional has two more functions but were not used in this study which are Pick and Set Null, respectively. The output cell value obtained from the raster position is used to determine which raster in the list is used as input while the Set Null tool sets no data to the cells which have specified criteria thus, the input conditional raster have no affect on floating and integer output type.
Furthermore, the hydrological modelling of Semenyih basin continues with usage of Stream Order tool. This tool is used for categorizing types of streams based on the number of stream network it has. It is also used because it assigns specific number to each segment of the catchment as a raster representation. If the input stream raster and input flow direction raster are derived from the same surface, the output of Stream Order will be of higher quality and If the stream raster is derived from a dataset of rasterized streams, the output may not be usable as the direction does not correspond to the location of stream cells on a cell-by-cell basis. The sixth procedure in ArcMap is the Stream to Feature. This tool is optimized to use direction raster to help vector intersecting and neighboring cells. Two adjacent linear characteristics of the same value can be vectorized as two parallel lines. This contrasts with the Raster to Polyline tool, which is usually more belligerent in regards with collapsing lines together. As shown in figure the Stream to Feature can be clearly visualized.
Figure of Stream to Feature.
Once the Stream to Feature process is completed the Basin tool is used. This function was applied to create accurate delineation of the basin based on the process described above and analysis of the drainage surrounding all the ridges of the DEM map. The stream flow and flow direction raster were analyzed by this tool in order to combine all cells that are connected and belong to the same delineation flow. The pour points are created around the edges of the DEM map and analyzed so that the outlet of the basin raster is located as well as sinks.
The DEM map is in raster format and need to be converted from raster to polygon thus, this is done through the Conversion Tool. This means the catchment does not only contain elevation but its fully developed map with flow direction, stream order and the catchment is divided into smaller polygons which their area can be calculated. The Conversion Tool usually convert integer raster to polygon thus, if the raster is floating type raster it needs to be changed to integer raster before using the Conversion Tool. To check whether the data is in integer or floating raster type, the Projections and Transformations tool is used which is under the Transformations Data Management Tools, this tool will enable the researcher to double confirm the data in integer or convert the floating type raster to integer raster.
Now the basin is created as polygon for the whole Malaysia map but this study was based on Semenyih basin, the student has to create the Semenyih basin from the polygon basin. This is done by creating a pouring point and saving it as a shape file with coordinates as the rest of project files, the pour point is then edited meaning the student can select any point of the polygon basin, once the selecting points is over the editing is stopped and the progress is saved. This editing causes permanent pour points located as specified location on the in project Semenyih basin.
After completing with Pour Points, the modelling of continues with Snap Pour Point, this tool is used to make ensure the pour point is within the required radius and converts the pour point into integer raster format which will be used in the next step of the model. Its also important to carry out this function because it ensures that the streamflow will be directed to the outlet so that later in the model the outlet results will be compared to the streamflow data provided by the Department of Irrigation and Drainage. In this study two Snap Pour Point were selected as seen in FIGURE BELOW with green and orang square boxes. This function will aid the next function in the modelling to delineate the streamflow to the lowest point in terms of elevation which the student had already selected it as an outlet or the sink of the basin.
The last step in ArcMap is Watershed tool. To execute this procedure, one needs two input files namely, input flow direction raster and input raster or Feature pour point data. These two were already used in previous steps of the model thus, can be easily recalled from Semenyih folder where they were already saved. Input flow direction raster will use the Flowdir_Fill while the input raster or Feature pour point data will definitely use snappou_shp. After filling the required input data, the software will automatically provide an outlet file named Output raster which contains the watershed flow data which in return is saved.
The watershed created is in a raster format and it needs to be converted to polygon. This is because polygon files are more useful when further geoprocessing and analyses are required thus, in this model the study, the created watershed will be further used in HEC-Geo-HMS for more geoprocessing. This conversion is executed and fully developed map of Semenyih basin was created. The purpose of using all above steps was to create Semenyih basin from DEM map which was given as the map of whole Malaysia while in this study was only interested on Semenyih basin as shown in FIGURE below.
RESULTS AND DISCUSSIONS
In this chapter, the hydrological modelling process of Semenyih dam from start to completion procedures will be explained in details aided with pictures, tables and formulas. This chapter will also explain further the processes stated in the methodology chapter. The guidelines used for the design and the creation of the Semenyih catchment was obtained from HEC-Geo-HMS user manual version 10.2 and HEC-HMS user manual version 4.2.
The data used in study was obtained from two agencies, NASA and DID, respectively. The Digital Elevation Data (DEM) was obtained from NASA. DID data is a core requirement in this study because DID data will be used in ArcMap, HEC-Geo-HMS until its exported to HEC-HMS model. NASA also provided the satellite precipitation data of TRMM satellite while DID agency provided rain gauge precipitation and streamflow observed data. These data are crucial for this study because the hydrographs of the simulated and observed will be compared to check the accuracy of the result obtained from HEC-HMS. DID also provided the Land-Use data which is used to determine the pervious and imperviousness of the created catchment and also to determine the Curve Number of the basin.
All the software, and data used in this study will explained in details throughout the next sub-chapters with the support of figures and tables. Three software will be used for this study namely, ArcMap, HEC-Geo-HMS and HEC-HMS. Every time one process is succeeded the software will show a green light which is a notification stating the process was successful and if it is not successful the user will receive a red alert that there some error in the data.