A dataset for the flood vulnerability assessment of the upper Cross River basin using morphometric analysis

The on-site collection of data is not only time consuming, but expensive and perhaps near impossible in restive communities within the upper Cross River basin (UCRB). Therefore, the importance of this data cannot be overemphasized. This article presents a Digital Elevation Model (DEM), land use and land cover (LULC) map, soil map, geology map and climatic datasets which enhance the understanding of the physical characteristics of the upper Cross River basin using morphometric analysis. The use of the LULC map, soil map and the DEM in conjunction with the climatic data enhance the creation of the Hydrologic Response Units (HRUs) and the water balance modelling. The simulation of the water balance at the HRU level enables the routing of the runoff to the reaches of the sub-basins and then to the channels. The geology map provides confirmatory information to the morphometric analysis. The compound factor computed from all the derived morphometric parameters enhance the determination of the overall flood potential of the congruent sub-basins.


Data description
The datasets in this article describe the land use and land cover, soil, geology, topography and the climatic condition of the upper Cross River basin. The delineation of the watershed offered by the Soil and Water Assessment Tool (SWAT) model was used to clip out the datasets. This was considered necessary in order to lay emphasis on the area under review. Fig. 1 describes the topography of the upper Cross River basin located in Southeastern Nigeria and Western Cameroon.  Fig. 4. The climatic data consist of precipitation, minimum/ maximum temperature, wind speed, relative humidity, and solar radiation datasets.

Study area
The upper Cross River basin (UCRB) is located in Southeastern Nigeria and Western Cameroon. Cross River originates from Cameroon, flows through Nigeria and drains into the Atlantic Ocean. The upper Cross River basin lies within Nigeria and Cameroon, between longitudes 7 33 0 and 10 06 0 East and latitudes 4 55 0 and 7 26 0 North. The UCRB has an area of 35,942.84 km 2 . The watershed experiences a Specifications Table   Subject Environmental Science (General) Specific subject area Flood vulnerability assessment Type of data   Table 1 gives the compositions of the soil by percentage of the watershed under review.
The predominant soil types are the Dystric Nitosols, Eutric Nitosols and Ferric Acrisols. Acrisols and Nitosols have significant amount of clay. The Dystric Nitosols within the UCRB consist of medium to fine texture particles, with gentle undulating relief.
The predominant lithological formations of the UCRB are sedimentary and basement rocks (Fig. 4). The varieties of the formations by percentage of watershed are as follows: volcanic (9.03%), basement complex (17.08%), igneous-volcanic (0.75%), igneous e younger granite (0.24), Precambrian basement Climatic data such as daily precipitation, minimum/maximum temperature, wind speed, relative humidity and solar radiation, were obtained for a period of 36 years, spanning the time frame between 1979 and 2014. The weather station (WGEN) file of the UCRB was created using the Weather Generator program and the daily climatic data as input. The Weather Generator program enables the simulation of missing weather stations statistics required by QSWAT for a complete cycle of modelling. The climatic data reveal that the UCRB has an average annual rainfall of 3049.5 mm.

Model input data source
The data were remotely obtained from different sources. The Digital Elevation Model (DEM) of the UCRB was obtained from the Shuttle Radar Topographic Mission (http://srtm.csi.cgiar.org/). The 90 m resolution DEM was used for the watershed delineation. The delineation was carried out considering an outlet at 8.25 E and 6.05 N (Hydrologic station). All the spatial analyses were carried out using QGIS (versions 2.6) set to WGS 84/UTM Zone 32 N Coordinate Reference System. The LULC map was obtained from the WaterBase website (http://www.waterbase.org/download_data.html) while the soil map was obtained from the Land and Water Development Division, Food and Agricultural Organization of the United Nations website http://www.fao.org/soils-portal/soil-survey/soil-mapsand-databases/en/. The climatic data was obtained from the National Centers for Environmental Prediction (NCEP), Texas, USA (http://globalweather.tamu.edu/). Also, the Weather Generator program was obtained from the Soil and Water Assessment Tool website (http://globalweather. tamu.edu/).

SWAT model
QSWAT installed as a plugin in QGIS is required for the geoprocessing operations. The geoprocessing operations are performed by QSWAT via a suite of programs called Terrain Analysis Using Digital Elevation Models [1,2]. QSWAT being a plugin, utilizes QGIS tools and functions including the Geospatial Data Abstraction Library (GDAL). The SWAT model utilizes the LULC map, soil map and DEM in conjunction with the climatic data in the creation of the Hydrologic Response Units (HRUs) and the water balance modelling. The water balance of the catchment area simulated at the HRU level enables the routing of the runoff to the reaches of the sub-basins and then to the channels.

Model setup
The discretization of the congruent sub-basins that makeup the UCRB can be carried out using a threshold area of 10 km 2 . The definition of slope classes is important for the HRU creation step. The specification of four slope classes such as: 0e5%, 5e10%, 10e25% and >25% is not out of order. The multiple HRU option of filtering by land use, soil and slope can be used for the creation of HRU. HRUs with less than 5% unique combination of land use, soil and slope range can be eliminated to reduce the model complexity, processing and simulation time.

Morphometric analysis
TauDEM of QSWAT1.2 enables the terrain analysis and the determination of the morphometric parameters. The drainage, flow direction, flow accumulation, slope, aspect, hill shade and the morphometric parameters are generated consequent upon the model input data, SWAT model and the model setup described in Subsections 2.2, 2.3 and 2.4 above. The requisite morphometric parameters include the stream length (Lu), stream number (Nu), stream order (Su), area of the sub-basins (A), minimum (h) and maximum (H) elevations. The use of the geospatial tools in QGIS aid the determination of the sub-basin length (L b ) and perimeter (P). The derived parameters can therefore be obtained using the mathematical expressions shown in Table 2.

Implications of the data
The geology, soil, topography and LULC of the watershed aid the understanding of the physical characteristics of the watershed. In respect of the influence of the sub-basins on the flooding of the main channel of the UCRB, the most flood vulnerable areas are underlain by hard rocks with high relief, hence, greater runoff, low permeability and infiltration capacity. A sparsely vegetated land, with  impermeable surface and high relief, has the potential to attain peak discharge in a short period of time. Consequently, the instantaneous high runoff contribution of their tributaries to the main channel.  Relief ratio (R r ) R r ¼ H r /L b Schumm [4] 15 Ruggedness number (R n ) R n ¼ H r x D d Melton [10] Where N u is the total number of stream of a given order, N u þ1 is the total number of stream of next higher order, L u is the total stream length of all orders (km), A is area of the sub-basin (Km 2 ), L b is the maximum basin length (Km), P is the perimeter of the basin (Km), D d is the drainage density (Km/Km 2 ).