Identification of Artificial Recharge Sites Using Remote Sensing Technology
Groundwater is a major source for all purposes of water requirements in India. More than 90% of rural and nearly 30% of urban population depend on it for drinking water. It accounts for nearly 60% of the total irrigation potential in the country, irrigating about 32.5 million hectares. The dependency on the ground water is expected to increase in future due to increase in population.
As per the estimation of Central Ground Water Board, the dynamic groundwater resource, i.e. utilizable ground water resource, which is meant for meeting the water requirements according to National Water Policy is about 43.2 million hectare meters. The static groundwater resource also known as fossil water available in the aquifer zones below the zone of water level fluctuation is about 1081.2 million hectare meters. The dynamic resource gets replenished every year through natural recharge, so that the balance is maintained.
However, the occurrence and distribution of the ground water is not uniform through out the country and varies significantly based on geology, rainfall and geomorphology. India is a vast country comprising of diversified geology, topography and climate. The prevalent rock formations range in age from Archaean to Recent and vary widely in composition and structure. Similarly, the variations in the landforms are also significant. They vary from the rugged mountainous terrains to the flat alluvial plains of the river valleys, coastal tracts and the Aeolian deserts. The rainfall pattern also shows similar region-wise variations. The topography and rainfall virtually control runoff and ground water recharge.
The safe development of groundwater resource primarily depends upon the groundwater recharge. Artificial groundwater recharge is essential in terrains with low natural groundwater recharge. Availability of non-committed runoff, hydro-geologically favourable areas for recharge and site specific design of artificial recharge structures are the major requirements of an artificial groundwater recharge system. A quantitative evaluation of spatial and temporal distribution of groundwater recharge is a prerequisite for operating groundwater resource management system in an optimal manner. The most suitable recharge structures suitable for this hard rock hilly terrain were suggested and suitable locations for the specific recharge measures were delineated.
Ground water being a hidden resource , is often developed without proper understanding of its occurrence in time and space. The total number of wells in the country has gone up from about 4 million in 1951 to more than 15 millions at present, and the number of energised pump sets in the same period have grown from initially negligible to about 12 millions (Press Information Bureau, Govt. of India). Most of these wells are drilled indiscriminately based on the requirement. As a result, many of the times, the wells have gone either unproductive or became failures causing financial loss to the users. In the over-exploitated zones, the wells are gradually getting dried up in due course of time. In some of the areas, the situation is so serious that there is a scarcity of water even for drinking purposes. In these areas, the ground water needs to be recharged artificially, as the natural recharge is not sufficient, to augment the resource and maintain the water table. In many areas in the country, however, the resource is not exploited to the optimum level.
Artificial recharge involves augmenting the natural movement of surface water into underground formations . Recharge can be either direct or indirect. In direct recharge, water is introduced into an aquifer via injection wells. The injected water is treated to ensure that it does not clog the area around the injection well. Indirect recharge involves spreading surface water on land so that the water infiltrates through the vadose zone, the unsaturated layer above the water table, down to the aquifer. Methods for spreading water include over-irrigation, creating basins, using construction methods, or making artificial changes to natural conditions such as modifying a stream channel. Artificial recharge techniques normally contribute to enhance the sustainable yield in areas where over-development has depleted the aquifer, and conserving the excess surface water for future requirements. Therefore, this study aimed at information generation for facilitating recharge zone site selections using integrated geospatial techniques. This was coupled with the consideration of hydro-geological factors for potential recharge site assessment and generating spatial information on suitability of potential recharge locations.
Remote Sensing technology, has been found a state of the art technology, which can provide information related to Geomorphology, drainage characters, Litho logy , Hydrological regime , Topographical details etc.
Selection of sites for artificial recharge structures:
In order to improve the ground water condition, particularly the sustainability of both drinking as well as irrigation wells in the hydrogeomorphic unit, sufficient recharge is essential. In case the natural recharge is not sufficient, it has to be met through artificial recharge. Many a times the sites for constructing recharge structures are selected based on administrative grounds. As a result proper recharge doesn’t take place leading to wastage of money. To provide the user a scientifically appropriate location for constructing artificial recharge structures, each hydro geomorphic unit is evaluated for its recharge potential. Accordingly, the types of recharge structures that are suitable in each hydro geomorphic unit are identified. The tentative locations for their construction are suggested. The prioritization of the hydro geomorphic units with respect to taking up for construction activity on different priorities is made.
Types of recharge structures: The following types of recharge structures are considered for suggesting in each hydro geomorphic unit –
- Percolation Tank (PT)
- Check Dam (CD)
- Nala Bund (NB)
- Invert Well (i.e. Recharge Wells) (IW)
- Desilting of Tank (DT)
- Recharge Pit (RP)
- Subsurface Dyke (SD)
In hilly terrains, the following water harvesting/ recharge structures will also be included:
– Storage Tank (ST)
– Soil Conservation Measures (SCM)
In alluvial terrain, ‘Recharge Shaft (RS)’ is included as one of the ‘recharge structures.’ Locations for one or more than one type of the recharge structures are identified in each hydro geomorphic unit based on the following criteria. ‘In-situ water harvesting’ will be included as general recommendation in the footnote, wherever applicable.
Criteria for site selection: In general, the locations for recharge structures are identified about 200-300 m upstream of the problem habitations. They are located mainly on 1st to 3rd order streams and at the most up to the initial stages of 4th order stream. No recharge structure is located on major streams / rivers occupying large area and forming polygons. The locations of the recharge structures are shown in each hydro geomorphic unit with the respective symbols indicated in the classification system. The criteria for selection of tentative locations for various recharge structures are given below.
Check Dam: On the 1st and 2nd order streams along the foot hill zones and the areas with 0-5% slope.
Percolation Tank: On the 1st to 3rd order streams located in the plains and valleys having sufficient weathered zone / loose material / fractures.
Nala Bund: On the 1st to 4th order streams flowing through the plains and valleys where acquisition of land for inundation of large areas is not possible. In this case, limited water will be stored in the river bed for some time which increases recharge.
Invert Well / recharge Well: In the areas where transmissivity of the upper strata is poor, e.g. in shales underlain by sandstones, in buried pediplains with top soil having low permeability, in Deccan Traps where vesicular basalt is overlain by massive basalt or thick black cotton soil or impervious zone.
Desilting of Tanks: The desilting is recommended in small tanks which are partially silted up. Siltation in the tanks is found by study of the image and ground truth.
Recharge Pit: Around the habitations where drainage does not exist, e.g. water divide areas, hill / plateau tops, etc. The Recharge pits are preferred in the existing tanks also. Accordingly, in each hydro geomorphic unit, the suitable type of recharge structures are identified and shown in abbreviations as per the classification system.
Prioritization of units: As mentioned earlier, the priority in which the hydro geomorphic unit is to be considered for taking up construction of recharge structures also found to be an important issue based on the nature of the unit and cost – benefit analysis. In view of this, different priority categories are identified and all the hydro geomorphic units showing the recharge structures suitable in that unit are assessed in terms of priority.
The following categories of priorities are identified and considered for the prioritization.
- Very High Priority
- High priority
- Moderate Priority
- Low Priority
- No Priority
- Not required.
The prioritization of hydro geomorphic units for constructing recharge structures is based on the following criteria.
Criteria for prioritization
- Presence of villages with drinking water scarcity (mainly due to the decline in water table)
- Status of ground water development.
- Areas where ground water levels are declining fast.
- Areas where water quality problem exists.
- Where recharge is poor/limited due to unfavourable hydro geological conditions.
For example, in the hydro geomorphic units where drinking water sources have dried up or water levels are declining fast or more number of drinking water scarcity villages are located or percentage of ground water irrigated area is very high or quality problem is reported (which can be improved by dilution through recharge), ‘Very High Priority’ is indicated.
Similarly, the units, which are mainly covered under forests or inhabited or shallow water table having good to excellent recharge from canal commands and surface water bodies and rivers etc, are given ‘No Priority’. The remaining units are given the ‘High Priority’ / ‘Moderate Priority / Low Priority. For the zones, which are not suitable for recharge structures, it is indicated as ‘Not required’. The priority in which the hydro geomorphic unit is classified is indicated by mentioning as Low Priority, Moderate Priority, High Priority, Very High Priority and Not required.