Use of Remote Sensing in Ground Water Modeling

Use of Remote Sensing in Ground Water Modeling

The depth of water table in the command of IGNP Stage I varied between 40m to 50m below ground level (bgl). However soon after the introduction of irrigation, water table started rising. During the period 1952-72, the average annual rate of rise of water table was 0.42 m/year. Waterlogging was first seen in the areas of Badopal & Manektheri in the year 1978. Photo 1 shows the standing water in the fields of Dabli Kalan near Matisawali Head. White patches are also seen indicating the severe conditions of soil salinity. Photo 2 indicates the similar conditions in Luna ki Dhani on the other side of the canal. Loss of trees and agriculture land is clearly visible in the photograph.

Photo 1: Waterlogged and saline waste land near Dabli Kalan.

Photo 2: Dead trees due to waterlogged area in Luna ki Dhani.
In period 1972-82, substantial rise in water table up to 1.17 m/year was noticed mainly due to return flow of irrigation & filling of depressions through Ghaggar diversion channel. In decade 1982-92, net rise in water table reduced in some of the area to 0.76 m/year due to increase in evaporation from already higher water table areas and larger horizontal spread of water built-up in the soil. In period 1992-94, water level reduced drastically in majority of the area due to scanty rainfall and meager flow in conveyance system. It has resulted in the significant reduction of waterlogged conditions in the area. However in the years 1995-97, water table again started rising at the rate of 0.30 to 0.70 m/year. Since 1998, Rajasthan is facing drought conditions year after year and so till 2002 there has been decline in water table and associated reduction in waterlogged area in the region. The status of waterlogging in IGNP Stage I from the year 90-91 to 2000-01 is given in Table 1 below.

Table 1: Status of waterlogged area in IGNP Stage I

S. No.	Type of area	Area in Hectares
S. No.	Type of area	90-91	92-93	94-95	96-97	98-99	00-01
1	Waterlogged area (Water table less than 1.0 m)	8600	13750	10192	17220	19492	12672
2	Critical area (Water table within 1.0 to 1.5 m)	17000	22000	18970	24140	27960	13425
3	Potentially sensitive area (Water table within 1.5 to 6 m)	198000	202960	198643	297820	298760	225153
	Total	223600	238710	227805	339180	346212	251250

Fig. 1: Study area (Major & minor canal network and towns)
The study area comprising parts of Hanumangarh and Sriganaganagar districts falls between 73°50' E to 74°33' E longitudes (about 80 kms) & 28°58' N to 29°20' N latitudes (about 47 kms) in IGNP Stage I phase I. The area, covering about 1772 square kms, lies between IGNP feeder, IGNP main canal, Ghaggar diversion channel, Suratgarh branch and Rawatsar distributory (Fig. 1).

Average annual rainfall in the region is about 240mm with temperature varying from 500 C during May-June to low of 50 C during December-January. The geology of the area is completely concealed under the thick blanket of dunal sand and alluvial deposits of quaternary age. No rock exposure at the ground surface occurs in the area. Sand dunes consist of silt & fine grained sand which are brownish, yellowish or buff in color. The generalized stratigraphic sequence as derived from lithologs of deep boreholes, dug cum bore wells & piezometers of State and Central Ground Water Boards and geological mapping carried out by Geological Survey of India is given below

Quaternary	Recent to Pleistocene	Blown sand, very fine, well sorted and rounded sand silty clays and kankar with frequent lenses of medium to coarse sand
----------------------	UNCONFIRMITY	-------------------------------
Paleozoic	Upper Vindhyan	Consolidate ferruginous, fine grained sandstone with shale intercalations.

Groundwater Mathematical Model
Groundwater mathematical model of the study area is being developed in with the following objectives:

Estimate the expected future groundwater levels and fluxes for the next 20 years considering that present conditions would prevail. This would also indicate the estimated increase/decrease in waterlogged and saline area in future.
Better insight into the reasons of waterlogging & soil salinity with the identification of the area which is further likely to be affected.
Methodologies to be adopted for its control and their comparative performance.

In the mathematical model, flow of groundwater is approximated by the partial differential equations, which are simultaneously solved by the computer code to predict future water levels and fluxes. U.S. Geological Survey modular finite-difference ground-water flow model, commonly known as MODFLOW is one of the most popular computer code. It can simulate ground-water flow in a three-dimensional heterogeneous and anisotropic medium.

Mathematical groundwater model can be developed either by using the grid approach or the conceptual modeling approach. Grid approach requires directly creating the finite difference three dimension grid covering the study area and specifying sources/sinks, soil properties, groundwater levels and other model input parameters directly for individual cells. The conceptual model approach involves using the geographical information (GIS) tools to develop a conceptual model of the site being modeled. Computer software GMS can then be used to convert conceptual model to grid model and post process the result of model simulations. In the present study AutoCAD Land Development Desktop (LDDT) software has been used to develop various GIS layers. IRS IC LISS III dataset was classified in ER Mapper software to utilize remote sensing data as input in the conceptual model. Various GIS layers were imported in GMS software to develop conceptual groundwater model which was then transferred into finite difference grid based model suitable for MODFLOW. Model is currently being validated by utilizing known groundwater levels of years 1991-1995, after which it would be run to predict future response of aquifer to the various stresses.

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