GROUND WATER HYDROLOGY – Perspective of a common man

INTRODUCTION : Ground water plays a vital role to supplement water requirement for domestic, industrial and irrigation fields. Its use has gained an increasing popularity over the years, because of various factors like easy accessibility to this resource, its ubiquitous nature of occurrence and high use efficiency. Further the construction of ground water structure is mainly through individual’s investment and or with the credits from financial institutions. Presently, at about 52 percent of irrigation requirement is being met from ground water source.

Considering the above facts, central and different state organizations dealing with ground water have already undertaken assessment of ground water resource availability and also monitoring of ground water regime periodically both quantitatively and qualitatively. Along with these, efforts are also being taken up for augmentation of this resource through artificial recharge. But assessment procedure, quality monitoring scale and artificial recharge practices etc contain some flaws and these flaws are being discussed here briefly.

RESOURCE ESTIMATION : Ground water resource may be divided as dynamic and static resource. The resource, which occurs within the zone of water table fluctuation with respect to pre and post monsoon season is termed as dynamic resource. This is generally replenished in every year mainly through rainfall recharge. The static resource occurs below the zone of water table fluctuation. In the existing practice assessment of dynamic resource gets more importance than static resource and available dynamic resource is mainly taken into consideration for preparation of any development plan and also to determine irrigation potential to be created from ground water source. But as a matter fact, major part of dynamic resource cannot be used to fulfill any requirement. Because, like surface water, ground water, always flows sub - surficially to the down gradient i.e., towards lesser potentiometric head. This sub-surface flow of ground water either crops out as base flow of rivers and streams or flows directly into sea or lakes etc. The ground water maintains the flow of rivers and streams when there is no rain particularly in those areas where rivers are only rain-fed. This continuous discharge of ground water is reflected on the water table in the dynamic resource zone. The gradual depletion of ground water table from post-monsoon to pre-monsoon periods is due to natural discharge plus withdrawal of ground water through man made ground water structures for different purposes. Natural discharge creates normal fluctuation in dynamic zone and withdrawal of ground water takes place from below normal fluctuation zone i,e from static resource zone. As the depleted zone is replenished by monsoon rainfall, the entire zone appears to be dynamic zone, though as a matter of fact this zone contains normal zone of fluctuation plus part of static resource zone. This fact can be established if we study the water table fluctuation (pre & post monsoon) in such areas where stage of ground water development is negligible. It has been noted that in such areas also in spite of negligible draft water table fluctuation attains a considerable value. This fact indicates that ground water of natural/ normal dynamic zone either crops out as surface water resource in streams, rivers and simply occurs as seepage on ground surface or flows sub - surficially directly into the sea, lake and ocean depending upon the water table configuration in any river basin. Any withdrawal of ground water affects the resource of the zone, which occurs below normal fluctuation zone and withdrawal of ground water increases the downward extension of normal dynamic zone. In the existing resource assessment practice the resource of natural / normal fluctuation zone is also accounted as utilisable ground water resource. But as a matter of fact the utilisable ground water is available only from below natural / normal fluctuation zone. Part of the ground water of natural / normal fluctuation zone when cropped out as surface flow in rivers and streams is considered as surface water resource.

GROUND WATER REGIME MONITORING : The main objective of monitoring is to keep a vigil on the ground water regime and it is generally being done through permanent hydrograph stations, which are generally open or dug wells and also piezometers. The open or dug wells are being used for monitoring of ground water of phreatic / shallow zone and piezometers for deeper zone. As a part of monitoring, depth to water level data from the stations, are collected either four times (Jan, Apr / May, Aug, Nov) or tow times (Pre &Post monsoon) in a year. In some special cases the stations are being monitored more than four times in a year also. One of the main purposes of collection of water level data is to study the long-term trend of ground water table (rise / fall) with a view to keep a vigil on the ground water regime quantitatively. It has been noted that as a general practice, trends are determined mainly in following two ways to predict the water table behaviour,

1.Using all the water level data of some continuous years irrespective of months or periods, or
2.Using only pre-monsoon water level data of some continuous years.

If the water level trend derived from above two methods shows any falling trend, consternations are raised, by declaring that ground water table is falling. But in reality it is not the fact. Because the trend which has been obtained by using all water level data irrespective of months or periods practically does not give any concrete information regarding the behaviour of water table. Trend should be prepared considering water level data of a particular period or season over the continuous years to get an idea on configuration of water table for that particular month / period over the years. The falling trend of pre-monsoon period also may not have much importance unless and until post-monsoon period water table show any falling trend during same years. No fall in water level trend in post-monsoon period and falling trend in pre-monsoon indicate that though there is a gradual increase in ground water withdrawal has taken place over the years but the aquifer is being fully recouped every year during post-monsoon period and for which the post–monsoon period water level remained unchanged .The falling of pre-monsoon trend may increase the cost of ground water withdrawal only .The falling trend of water table during post-monsoon period matters, when there is not much variation in monsoon rain fall over the years under consideration. This falling trend during post-monsoon period indicates that magnitude of withdrawal has gradually exceeded the quantity of available in an area and quantitatively the ground water regime of the area is under stress.

ARTIFICIAL RECHARGE : Now a days much stress is being given on artificial recharge of ground water to augment the ground water resource. The artificial recharge is being advocated to implement everywhere. But it is to be kept in mind that artificial recharge will not be effective in such areas where rainfall is fairly good and also terrain condition offers fair scope of infiltration. It is a well known fact that aquifer rejects excess rainfall recharge after it attains complete saturation. Moreover continuous recharge may create problem like water logging, which is a common phenomenon in major surface irrigation command areas. Hence this artificial recharge technique should be applied mainly in such areas where
1.Rainfall is low and or water table is deep either in pre or post-monsoon period or in both periods
2.Declining trend of water table has occurred during post-monsoon period

The roof top rain water harvesting, a new approach in artificial recharge technique is slowly gaining much importance. It is advocated that in towns and cities rain water falling on the roof of buildings should be used for artificial recharge through some recharge pits, bore holes and dug wells etc. In this case besides rejected recharge factor, another problem related to quality may also arise in future because of following reasons.

The rain water in general is acidic and acidic nature is modified during sub-surface infiltration of water through soil zone and unsaturated part of aquifer during normal infiltration process. It is a very well known fact that rain waters of cities, towns and industrial areas are more acidic than other areas due to so many factors. Hence, if these more acidic waters are injected directly through recharge pits or tube wells etc, the chances of modification of acidic nature of water becomes much less and this may lead to serious quality hazards in future resulting ground water unfit for use. Necessary treatment of rain water is needed for reduction of acidity before injection.

CHEMICAL QUALITY : Concentration of different chemical constituents in water not only determines its fitness for utilisation but also defines its fitness for different uses like domestic, agriculture and industrial etc purposes. In this process limit of concentration of different chemical constituents for drinking water has been fixed in different countries. To determine potability of ground water in India the BIS standard (BIS:10500,1991) is normally followed uniformly for the entire country, though the wide variation in geographical condition and also in food habit of habitants are found in different parts of the country. The variation in geographical conditions as well as differences in food habit certainly create different tolerance limit of the habitants of different parts. Hence, the regionalisation of drinking water standard is required instead of single uniform standard for the entire country. During fixation of standard, along with the upper limit of concentration the lower limit of concentration of the chemical constituents particularly which are vital for human health may also be prescribed, because excess fluoride is harmful but side by side complete absence of fluoride is not also good. There should not be both desirable limit and permissible limit, because limit should have single value when human health is concerned and as usual this limit may have some percent variation either on positive side or on negative side depending upon the health constitution of individual inhabitants of a particular area.

SPACING CRITERA:The norm of spacing between two energized ground water withdrawal structures has been fixed to regulate ground water withdrawal with a view to avoid mainly overexploitation and also to create scope for having legal/ equal share of ground water by all users. In this process, it is suggested that the distance between two energized irrigation dug wells may not be less than 100m. Side by side it has also been prescribed that the tentative command area of an energized irrigation dug well, on an average, is around one hectare i.e, 100m x 100m area. Considering this command area of a dug well, it may be conceived that, the above prescribed spacing norm for dug well may not have much practical importance. Because, size of agricultural land / plot under individual holding in an area is found to restrict within one hectare in the majority of the cases and agricultural plots having sizes varying from 0.4 to 1.0 acre which is much less than 1hectare, are very common .The water requirement is more or less same to irrigate an area of one hectare and 3 to 4 adjoining agricultural plots having cumulative total of one hectare area. Hence construction of more than one dug well to irrigate one hectare of land matters little so far the ground water draft is concerned. More over, field experiences indicate that the pumping effect in a dug well is hardly felt beyond 20 to 25m distance. The similar type of situation (command area: plot size) also exist in the case of other ground water structures, particularly for filter point and shallow (low duty) tube wells. This reality has to be given proper weightage while fixing the spacing norms, so that small plot holders may not be devoid of their rights to have own ground water structures on the plea of spacing norm.

The depth criterion is more important rather than horizontal spacing for structures like tube wells not only to minimize interference of pumping but also to minimize the chance of over exploitation of a particular aquifer. Fixation of depth range of aquifer zones (water bearing horizon) to be tapped by different type of tube wells like filter point, shallow, medium deep and deep tube wells will restrict tapping of same aquifer zone simultaneously by different types of tube well. Also tapping of same aquifer zone by same type of tube well within short distance should not be allowed and in this case horizontal spacing norm is required and this is to be fixed after considering the different technical and non technical aspects.

The revision of existing spacing norm in the light of above discussions is very much necessary because within next few years most of the State Governments may implement ground water law. In the absence of revised spacing norms ground water users may face problems in coming years for construction of ground water structures.

USE OF DIFFERENT TERMS : Now-a-days different terms like Watershed, Lineament, Schist, Gneiss, etc are used in different literatures in such a way that these may create confusions. The term ‘Watershed’ is very often mixed up with river basin and notion like ‘watershed management’, ‘watershed development’ etc, are being used. But the literal meaning of term watershed is the line dividing two rivers / rivulet / stream or it is the boundary line of a river basin and in any case the terms, ‘watershed’ and ‘river basin’ are not synonymous. Hence it is more appropriate to use the term river basin while using notations like, ‘river basin management’ or ‘river basin development’ etc and prefix like micro, macro, and mega etc may be used along with the word basin depending upon the size of the basin.

The term Lineament indicates any geographical or geological feature occurring in a line or in a linear fashion. But now a days the term lineament is used in such a fashion that it appears that lineament indicate only fracture zone, nothing else. Sometimes in same map or figure occurrences of lineaments (indicating fractures) and shear zones are shown separately though shear zone is also a lineament. The sand dunes occurring in coastal area, very often display linear fashion resulting a lineament and sand dunes definitely do not indicate any fracture zone.

Confusing nomenclatures are frequently used to describe different rock types of any area. The following type of statement is very often found in literatures now a days. “The most common rock types of the area are granites, gneiss, schist, and charnockites etc” The terms like gneiss and schist denote only the structural features of rocks and these terms may be used as only suffix, e.g Granite gneiss, Mica schist etc.

CONCLUSION: Ground water development plan for any area is formulated mainly based on availability of ground water resource in the area and for which precise estimation, as far as possible, of ground water resource is needed. Hence, such type of methodology is to be adopted which will lead to a correct assessment of resource as far as possible. Not only for resource estimation but also for monitoring of ground water regime both quantitatively and qualitatively and also for augmentation of ground resource such procedures and techniques may be adopted which will help to derive the maximum realistic situation.

DISCLAIMER : This is a compilation of the discussion and feedback received from people of various profession from agriculture to rocket scientists and I have merely contributed as a compiler. This article does not in any way reflect my personal opinion or the organization I work for.