What is it about?

It has become necessary to calculate the quantities of surface runoff by knowing the amount of rain or precipitation that falls on an area. The SCS-CN method (Soil Conservation Services – Curve Number) has become one of the most accepted methods at the present time by all specialists and hydrological researchers for several reasons, the most important of which are its simplicity, ease of application, stability and efficiency in finding most of the hydrological parameters. It depends largely on soil type, land use/cover, and topography of the area, in addition to other hydrological factors. Recently, CN grid for any watershed could be done by using remote sense image (RS) and geographical information system (GIS used version 10.3) with its hydro tools and Geo-HMS extension (Hydrological Model System). The study area included the Diyala River Basin (DRB), which has an area of 32600 km2 located in the north-east of Iraq. It is very important watershed because of supplying the amount of water to the Diyala river which starts after Hemrin dam and meets to feed Tigris River. So, the goal of this study is to estimate the CN for DRB. It was used the DEM (Digital Elevation Model) Tiff file downloaded from USGS site of NASA with a cell size of 12.5m x 12.5m, land use/cover and soil type data from FAO. After re-classifications, the land use/cover of DRB was found in four classes by about 0.5% water, 76.66% vegetation, 5.15% forest, and 17.69% bare areas. Therefore, the land of DRB is covered for about 82% vegetation cover. The soil type of DRB was for about 87% of loam texture and 13% light clay texture. The estimated CN grid was between 60 to 100, the lowest value refers to low runoff and vice versa but 98 to 100 means the area covered by water. The ratios of estimated CN values were 60-70, 71-80, 81-90, and 91-100 were 52.8%, 10.3%, 6.3%, and 0.6% respectively.

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Why is it important?

The studied area is considered one of the important areas in Iraq in feeding the Tigris River, as well as all the areas it passes through, which they are very large ones. Therefore, calculating the expenses resulting from rain, whether in the watersheds in Iranian territory or Iraqi territory, is an important step to avoid probable droughts as well as expected floods as that flood happened in the first quarter of 2019. Because of the lack of field data related to the matters necessary to calculate the value of the required curve number, it was necessary to find an alternative in its calculation, and therefore the used methodology according to the research is considered an important tributary for all specialists, whether at the research level or at the level of making important decisions in water management in the country relating to the studied area. So, the goal of this study is to estimate the CN for Diyala River Basin (DRB).

Perspectives

The use of available information and data regarding the topography of the area, maps related to land use/cover, as well as the type of soil, gave good results. This fact was gotten where after re-classifications, the land use/cover of DRB was found four classes for about 0.5% water, 76.66% vegetation, 5.15% forest, and 17.69% bare areas. Therefore, the land of DRB is covered by about 82% vegetation cover. The soil type of DRB was for about 87% of loam texture and 13% light clay texture. The estimated CN grid was between 60 to 100, and as known, the lowest value refers to low runoff and vice versa but 98 to 100 means the area covered by water. The ratios of estimated CN values were 60-70, 71-80, 81-90, and 91-100 were 52.8%, 10.3%, 6.3%, and 0.6% respectively. The CN method with data from NASA and FAO is acceptable way. Although those were a good estimation, it was noticed during the estimation of the CN values that there were negative values, that have been omitted. This basically means the loss of some data, that is, the global maps did not give some accurate matters for all types of soil as well as its land use/cover, which requires treatment in the future so that the results are more appropriate for the cases studied.

Faris Alrammahi
Imam Kadhum College

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This page is a summary of: Creating SCS curve number grid in Diyala river basin using land cover and soil data, January 2023, American Institute of Physics,
DOI: 10.1063/5.0140733.
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