Estimation of Surface Runoff using Rainfall – Runoff Modeling of Warasgaon Dam Catchment

Estimation of Surface Runoff using Rainfall – Runoff Modeling of Warasgaon Dam Catchment - a geospatial approach

A. A. Kulkarni, S. P.Aggarwal & K.K.Das
Indian Institute of Remote Sensing,
4, Kalidas Road, Dehradun.
E- Mail: amol1_k@rediffmail.com

Introduction:
The term ‘precipitaion’denotes all forms of the water that reach the earth from the atmosphere, and runoff means the draining or flowing off of precipitation from a catchment area though a surface channel after satisfying all surface and sub surface losses. (Dubayah, R., et al, 1997) Establishing a mathematical relationship between the rainfall and runoff events.

Prediction of surface runoff is one of the most useful hydrological capabilities of a GIS. system. The prediction may be used to assess or predict aspects of flooding, aid in reservoir operation, or be used in the prediction of the transport of water born contamination (Jain, M.K., 1996). The type of models that have been applied with a GIS will be classified as lumped parameters, Physical based or some combination of the two. There has been a growing need to study, understand and quantify the impact of major land use changes on hydrologic regime, both water quantity and quality (Engman, E.T., et al, 1991). This is necessary to anticipate and minimize potential environmental detriment and to satisfy water resources requirements. Hydrological modeling is a powerful technique of hydrologic system investigation for both the research hydrologists and the practicing water resources engineers involved in the planning and development of integrated approach for management of water resources (Schultz, G.A., 1993). Hydrologic models are symbolic or mathematical representation of known or assumed functions expressing the various components of a hydrologic cycle. (Beven, K.J. et al., 1979).

The modified SCS model enables the hydrologist to simulates various designs alternatives and compare the results. The expression used in Modified SCS method is given as following Eq.s 1,2 & 3. Using the Land use/Land cover and Hydrologic Soil Group (HSG) map Curve Number (weighted CN) value has been calculated and further CNI and CNII values are calculated by following equations as,

CNI = (4.2*CNII)/(10 – 0.058*CNII) ----------- Eq.1

CNIII = (23*CNII)/(10 + 0.13*CNII) -----------Eq.2

For, daily rainfall S values are derived from the CN values using the following formula as

S = (25400 /CN) – 254 -----------Eq.3

Where CN is function of watershed hydrologic land use/land cover units, hydrologic soil groups and antecedent moisture conditions (USDA, 1972).

Study Area:
The proposed Warasgaon Catchment is located in the Pune district of Maharashtra state in the scenic surrounding of the Western Ghats. There are three dams and back water lakes in the surrounding vicinity namely Panshet, Warasgaon and Temghar. The backwater helps in maintaining the surrounding pleasant and cool. The area is approximately 20 km in length and about 5 km wide forming a beautiful valley, through which Mose river traverse in East – West direction. One end of the valley borders the coastal area of Konkan and other end extends up to the Warasgaon dam. The study area lies between the geocoordinates 18°21'00" to 18°25'48" North latitude and 73°25'12" to 73°37'12" East longitude. (Fig.1).

Fig. 1. Location Map

The altitude varies from 2100 feet above msl to 3200 feet above msl. The Warasgaon backwater lake is about 18 km in length on the river Mose. The depth of water varies from 50 to 60 feet. The climate is humid tropical with moderately warm summer. Since, mountains form a barrier from all side and enough backwater surrounds the area, the entire valley has a microclimate quite different from adjacent areas.

Climate is subtropical monsoon with moderately warm to hot summer, high monsoon rains and of cold winter season. Most of the rainfall in the study area is received from the months of June to October. The fifty percent of the study area is covered with forests of varying density with Tropical Evergreen, Tropical Semi- evergreen and Tropical Moist deciduous forest. The terrain consists of low-lying valley to highly dissected ridges forming ridge–valley topography. There are 23 villages, with a combined population of three thousand only. The people are poor and marginal farmers, depending heavily on traditional farming techniques called shifting cultivation by burning and clearing the forest. The area is very closed by two metro cities viz. Mumbai, Commercial Capital on India and Pune, second city of Maharashtra State. The proposed land is purchased by The Lake City Development Corporation, Pune and they are developing this site as Model Hill Station known as “Lake Town”. The work on the project has already started in phased manner. In the first phase slope stabilization work has been taken up for ecological development. Three small dams are also under construction on tributaries to stop flow of sediment to Lake and for water conservation for sustainable development of Lake Town.

Materials and Methods:
Satellite images of IRS – IC LISS III (4th February 2002) & IRS – IC PAN (30th January 2002) were used for land use/land cover mapping. The satellite data was visually interpreted and accuracy was checked on the ground. Digital Elevation Model (DEM) was created using contour map for deriving slope map of Mose river catchment in GIS domain. Using the DEM percent slope has been calculated. Classifying soil map in to its hydrologic groups has created HSG Map overlaid with the Land use/Land cover map together to create CN map. To create CN map, AMC conditions has been considered. Modified SCS Model also considers the Slope while estimating the runoff. The following equation is used for calculation of Modified CN II. (Fig. 4.12)

Modified CN II =(1/3)*(CN3-CN2)*(1-2*EXP (-13.86*SLOPEPT))+CN2 -----Eq. 4

Finally, the runoff is estimated with help of following equation no.5 as

Q=(P – Ia)² / [(P – Ia) + S)] -------Eq.5

Where,
Q = Accumulated storm runoff, mm.
P = Accumulated storm rainfall, mm.
S = Potential Maximum Relation of Water by the soil.

Fig. 2 Methodology for Rainfall – Runoff Modeling

Using above equation no. 5, modified weighted CN map has been created.

Result and Discussion:
Rainfall Map: Daily rainfall data from Year 1966 to 1998 has been acquired from Indian Meteorological Department, Pune for nine rainfall stations in the study area. Using latest year data for each rainfall station Theissan Polygon has been generated and attributing the rainfall data for it R Map has been created. Fig. 3 shows the Rainfall values for each Theissan polygon.

Fig.3. Rainfall in mm for each Theissan Polygon

Land use/land cover map: After merging of the LISS & PAN data, visual interpretation of the study area has been done using image characteristics and prepared the land use/ land cover map. The accuracy of the land use/land cover was substantiated by correlating ground truth information. The map is shown in the Fig. 4 and land use/land cover statistics is given in Fig. 5.

Fig. 4 Land use/Land cover Map

Fig. 5. Area under various Land use classes

Soil Texture Map: Soil texture map has been acquired form the Soil Survey Department of Pune District, Maharashtra. In present study area four types of soil textures are present viz. Clay, Clay Loam, Sandy and Silty Soil. Soil Map is shown in Fig. 6.

Fig. 6. Soil Texture Map

Slope Map: The slope map has been used to Modified SCS Model i.e. it considers the slope for estimation of runoff. In present area, topography varies from Flat to Very steep slope i.e. 0 to 198 percent slope. And it is classified into three classes as Low, Medium and High as shown in (Fig. 7).

Fig. 7. Classified Slope Map

Surface Runoff Map: By above Eq. 4, using CN map and percent slope map, modified Curve Number map has been created. These values are valves of modified CNII for each Land use/Land cover. Again using the Eq.1 & 2, the modified CNI &CNIII values are calculated. For each Curve Number, S value has been calculated using the Eq.3. For this area Ia value is taken as 0.3S further runoff calculations ahs been done. Finally, the actual surface runoff has been calculated. Fig. 8 shows the Surface Runoff for each sub watershed. The Surface Runoff is varied from 320.93 mm to 3914.76 mm from East to West i.e. Runoff is goes increasing form Warasgaon Dam. Fig.9. shows the graphical representation of the runoff in mm for each sub watershed.

Fig. 8. Estimated Surface Runoff for each Sub watershed

Fig. 9. Surface Runoff for Each Sub watershed

Conclusion: The study shows that the western part of the study area having high Surface Runoff and most of the part is endowed with dense forests in medium slope and rich in floras. As these area may soil erosion prone due to high Rainfall and Runoff too. Therefore, this area can be developed as ecotourism destination by facilitating proper ecotourism infrastructure and services under policy guidelines. This will help to conserve and maintain the biological richness of the area as well as economic upliftment of the local people by providing employment and opportunities in the field of ecotourism management. Also, this runoff potential can be used for the Artificial Recharge by constructing the Nala Bundies and Farm ponds at suitable sites. Also, constructing the structures like check dams water can be stored and helpful for the dry summer days for drinking as well as agricultural purposes.

Acknowledgement: The authors are grateful to Dean, Indian Institute of Remote Sensing, Dehradun, India for providing guidance and extending all possible help for carrying out this task and we record our deep gratitude for inspiring and encouraging us to complete the work successfully. Thanks are also due to Mr. Anirudha Deshpande and Mr. Shekhar Vichare and team of The Lake City Corporation Ltd., for the help and encouragement given from time to time. Without all their support and inspiration it would not have been possible to complete the task in time.

References:

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