Rain Water Management Using Remote Sensing and GIS



K.Ragul
B.E. (Geo Informatics)
I.R.S, Anna University Chennai.
raghulkk@rediffmail.com


V.Sathish Kumar
B.E. (Geo Informatics)
I.R.S, Anna University Chennai.
Satsdreams2003@yahoo.co.in

Abstract:
Rain is the main source of pure water in most regions of india. It is the cause for two extreme problems of drought and floods. Thus the need for proper management of rain water arises. Remote sensing and gis plays good role in this respect.

Remotely sensed data can be used to identify flooded areas and gis in analysing and diverting the flood water to the nearby lakes and reservoirs. The false colour composite for the flooded area is used to find the areas affected by floods. The locations of reservoirs and lakes are obtained from the toposheets of the area. Both these data combined in the gis platform is used to identify the possibility of diverting flood water to those reservoirs.

The areas with thick aquifers are located from the hydro geologic data of the region. GIS can be used to analyse the diversion of flood water to such regions by using temporary pipe lines. This would enable in recharging the aquifer faster. If such regions with thick aquifers lie along the coast, this would help in reducing seawater ingress.

The only way of recharging the aquifers in drought prone areas is rain water harvesting. In such regions, remote sensing can be well employed in identifying best locations for the construction of rainwater harvesting structures, such as check dams, bunds etc.

The slope map created from the contours in the toposheet of the region, and the rainfall characteristics of the region obtained from the irrigation department can be used to calculate the total runoff of water wasted by just letting them into drainage systems. GIS softwares such as Arcgis can be employed to identify those regions with heavy runoff and the possibility of constructing rainwater harvesting structures in those places.

Thus remote sensing and GIS plays good role in managing the extreme problems of floods and drought both caused by the rainfall pattern of a particular region.

Introduction:
Rain water is the main source of pure water to the world. The demand for water is increasing at a very faster rate due to various reasons such as population growth, industrial development and agricultural needs. Almost all the sources of water available are being utilized to overcome those demands. Rain water being the only source of recharging all sources of water, the need for managing it properly is very important. Remote sensing and GIS plays very good roles in this respect.

Need for Rain Water Management:
In India rainfall is the main reason for two extreme problems of floods and drought. More than 70% of annual rainfall occurs during the monsoon season. Due to the irregularity in onset and offset of monsoon, the whole agricultural pattern of India is affected. During the extreme drought conditions, the drinking water need of the people is also affected. On the other hand, the seasonal flood affects the habitat and livelihood of the people. During the recent floods in Tamilnadu more than 15 mcft of water has been let into the sea and there were more than 3600 tanks that were breached. These show the improper management of rain water. Thus there is a need for proper management of rain water.

Rain Water Management:
The main objective of rain water management is to prevent problems and to protect property and habitat. This not only considers the peak flow during heavy storms but also the water from lighter showers. This integrates the storm water management with other planning processes such as transportation networks, commercial buildings, residential areas, parks, recreation centers and all other places of interest.

Rainfall occurring at a place can broadly be divided into three categories based on their intensity and duration, namely light showers, heavy showers and storms. In all these situations rainwater has to be managed properly. When there is a light shower, the rain water has to be captured at the site where it falls and when there is heavy showers, runoff has to be prevented. When there is a heavy storm, we have to prevent flooding and protect the properties from being damaged by the flood water.

Methodology:

1) Heavy Showers:
The rain water has to be captured and made to infiltrate at the place where it falls and should not be allowed to generate runoff. But, this cannot be done at all places due to variations in soil type, geomorphology etc. Remote sensing and GIS are very much helpful in identifying areas which are suitable for rain water harvesting.

Soil type and Geomorphology:
Different soil types have different infiltration capacity. Based on their infiltration rate weights are given to them. The geomorphology of the region is also important in identifying suitable areas. There are different geomorphologic features with different ground water potentials. Based on their ground water holding capacity they are given suitable weights. These weights signify the suitability of various forms of the same layer.

Weightage:
Soil type and the geomorphology are the two main criteria involved in identifying suitable areas of recharge. Hence they are given equal weightage. These weights signify the importance of different layers in identifying the suitability. Then the suitability indices are then calculated for each area. The regions having higher suitability are then identified.

Water Bodies:
The water bodies act as the storage structures. Hence there is no need for the construction of rain water harvesting structures at these places. These areas are identified from the topographic sheet and are then eliminated from the highly suitable areas for harvesting. Thus the final areas of interest are obtained. In these identified areas the rain water harvesting structures have to be constructed in the lower most point of the region.

The Land Use pattern of the region is not taken into consideration, because rain water has to be harvested at all the possible and suitable areas. The roof top runoff from industries, residential and commercial areas should also be harvested. The transportation networks such as the road networks should be provide with separate storm water drains which carries the water to the nearby rain water harvesting structures. This will help in avoiding the wastage of pure water by not letting them into sewer lines.

Other areas:
At all other areas the rain water has to be diverted to the nearby storage structures. This will help in reducing the loss due to runoff. In these areas the lineaments have to be identified. If there is no good recharge structure nearby, they should be constructed across the lineaments.

Role of Remote Sensing and GIS:
The geomorphology and lineaments maps of the region can be identified from the remotely sensed data. The soil maps are obtained from the soil and land records. Overlaying these two layers and identifying the suitable areas by giving weights can be done in the GIS platform. The regions of the water bodies are identified from the topographical map and updated by using the latest imagery of the region. The above methodology would be more useful in the annual rainfall is very less. This technique would help in avoiding water scarcity during the drought season.

2) Heavy Storms:
When there is a heavy storm, the rain water falling in a basin would lead to a flooding situation at the outlet of the basin. The main aim of rain water management is to avoid flooding. This can be done by diverting the excess water to the regions with higher salinity (such as the coastal regions with higher salt water intrusion), or the water from a larger area to a comparatively smaller area of higher infiltration rate in the downstream side of the basin. When the canals have a lower carrying capacity, flood by-passing has to be done to prevent flooding along the sides of the canal. Remote sensing and GIS plays good roles in the above mentioned areas of interest.

Rainfall-runoff relationships:
This is the very important criteria required for the identification of areas of higher risk due to floods. From this relation the amount of excess water that is going to come out of a basin can be studied. This is also very useful in flood forecasting and taking preventive measures.

Land use:
The land use pattern of the region helps in identifying the regions which are more likely to be affected by floods. This is generally prepared from the satellite images of the region. This is also very useful in the canal alignment process.

Slope:
Slope is yet another criterion which has to be given importance in flood diversion, and canal alignment process. Slope map of the region is generally prepared from the DEM, which in turn is usually prepared from the contour details of the region. The contours available in the topographical maps are employed in the preparation of the slope map.

Soil Type and Geomorphology:
Soil type and geomorphology are two important criteria in identifying the regions of higher infiltration rate. The same methodology employed in the previous section is used here.

Salinity:
A number of investigations are being carried out in the recent past to determine the increase in the soil salinity. The results of these tests are alarming. Due to the over exploitation of the ground water the soil salinity increases gradually, especially in the coastal regions it has lead to the increase in salt water intrusion. The areas that are affected from increase salinity are obtained from these studies. These are can also be identified from the satellite imagery.

Role of Remote Sensing and GIS:
Remote sensing and GIS are very helpful in identifying the possibility of diverting the excess flood water to saline areas. This would help in using the flood water properly without wasting it. Taking the road networks into consideration, the best route for putting temporary pipelines to carry flood water from low lying inundated regions to other areas can be done.

Remote sensing can also be used in checking the encroachments in the water bodies periodically.

This is also very helpful in identifying areas of faster infiltration and identification of best route for this diversion process.

When a stream or a river passes through a city, remote sensing and GIS are helpful in identifying the flood by-pass canals.

Case Study:

Objective:
The objective of the study is to identify the regions of good recharge in the region and to suggest measures to improve the management of rain water mainly during heavy showers.

Study Area:
The area chosen is the ponnai minor basin, one of the sub-basins of the palar river basin. This region is in vellore district and covers an area of 208.19 sq.km. It extends from 79 deg10’ E to 79 deg 20’ E and from 12 deg 45’ N to 13 deg 15’ N. The river ponnai traverses through this sub-basin and joins with the palar river at the outlet of this basin. The mean maximum temperature varies from28.2 deg C to 36.5 deg C and the mean minimum temperature varies from 17.3 deg C to 27.4 deg C.

The base map of the study area is shown below.


base map


Reason for choosing the Study Area:
There is no perennial river in this region. Flash floods occur in the ponnai river once in four or five years. The whole region depends on the rainfall for its water needs in agriculture, household purpose and industries.

The chemical industries and tanneries in this region have polluted the available ground water.
Finally, the availability of data for this region made us to choose this area for study.

Data used:

Soil map
Geomorphology map
Topographic map of vellore district in 1:250000 scale

Software used:
Arc GIS 9

Methodology:
First the sub-basin boundary of the ponnai river was taken from the basin map of the palar river. This was then overlaid over the topographic map of the region and important features such as road networks and locations of towns were traced out. This was used as the base map for the analysis.

Soil Map:
The soil map of the region was obtained from the tamil nadu Agriculture department. The predominant types of soil present in this region and their infiltration rates are as follows:

		Soil Types                               Infiltration Rates (in cm/hr)

		Sand and sandy loam  	10.00 – 16.90
		Red loam 			0.93 – 7.42
		Clay and clayey loam		0.22 – 1.98
The soil map was digitized and was taken as a layer. The soil map is shown below.

Fig:2

soil map


Geomorphology Map:
The geomorphology of the region was obtained from the state ground and surface water resources data centre. The major geomorphic units present in this region and their ground water potential are as listed below:

		Geomorphic Unit		Ground Water Potential

		Buried pediments		good
		Pedi plain			moderate
		Pediments			poor
		Residual hills		poor
The geomorphology map of the region is shown below.

Fig:3

geomorphology map


Weightage:
Since soil type and geomorphology of the region are the two main criteria in determining the suitable areas of recharge and they go hand in hand, they are given equal weightage. Based on the infiltration rate of the various soil types, they are given individual weightage as shown below:


		Soil Type 			Individual Weight

		Sand and sandy loam		3
		Red loam			2
		Clay and clayey loam		1
Based on the water holding capacity of the various geomorphic forms, they are given individual weightage as follows:

		Geomorphic Units		Individual Weight

		Buried pediments		3
		Pedi plains		2
		Pediments and residual hills	1
Overlay:
The soil and geomorphology maps of the region are overlaid over the base map in Arc GIS platform. The union operation is then performed.

Cumulative Suitability Index:

The cumulative suitability index (CSI) gives the suitability of the various parts for a particular purpose and is given by the sum of the product of weightage of the layer and individual weightage of the various types in the different layers. The cumulative suitability index of each portion of the overlaid map is calculated by using the formula

CSI = sum of ( weightage * individual weightage)

The mean and the standard deviation of the cumulative suitability index are then calculated and a normal distribution curve is drawn. The suitability of various parts are then identified by using the following criteria.

Highly suitable > (mean + S.D)
Moderately suitable > mean and < (mean + S.D)
Marginally suitable > (mean-S.D) and < mean
Poorly suitable < (mean – S.D)

Suitable Area:
Thus the suitable areas with higher and moderate suitability are identified. In all these identified areas proper rain water harvesting structures have to be constructed. Whatever may be the land use pattern of the area, if it has higher suitability for recharge proper rain water harvesting has to be carried out.

In the residential, commercial or industrial areas the rooftop runoff has to be harvested. In agricultural areas, the rain water should not be allowed to stagnate rather they have to be canalized to the nearby lakes or reservoirs such tanks and percolation ponds.

Fig:4


In all other non suitable areas, the construction of rain water harvesting structures is not advised as it is not economical. The rain water falling on these regions have to be diverted to the nearby ponds, wells and other storage structures.

Results and Conclusion:
This study shows that, only a part of the region is highly suitable for ground water recharge. In all these regions proper rain water harvesting structures have to be constructed.

In the areas which are not suitable for recharge, the rain water has to be diverted to the large rain water harvesting structures present in the regions with higher suitability. In the extreme case the water could be let into the river to flow out of the basin.

References: