April 7, 2003	Monday	Safar 4, 1424

Groundwater is hidden component of the hydrological cycle. Its occurrence and significance cannot be overlooked in the planning and management of water resources.

Groundwater represents an exploitable resource provided its quality and depth permit its beneficial and economical use. It provides an opportunity to farmers for timely application of water to crops. Groundwater has played major role in increasing the overall cropping intensity in Pakistan from about 63 per cent in 1947 to 120 per cent in 2000. Over 40 per cent of irrigation water at farm gate, and over 70 per cent of drinking water are now met from groundwater. Recent estimates regarding the availability and use of fresh groundwater indicate that the resource has been heavily exploited and is at the brink of exhaustion. Spatial distribution of its abstraction is highly variable. In some regions annual pumpage has surpassed the safe annual yields and watertables are declining.

There are however many areas where great potential for groundwater development still exists. This large-scale indiscriminate, uncontrolled and unregulated abstraction of groundwater has changed the policy approach from development to management. The major policy issues now relate to environmental sustainability and long-term availability. The standard approaches to manage groundwater often require monitoring of aquifers, establishment of formal water rights and regulation mechanisms to bring its abstraction within sustainable levels. This will require to control its pumpage in some areas, while in others it is to be encouraged. This can have tremendous economic and social impact. The potential areas are mostly located in the irrigated areas of Sindh and Punjab provinces.

The groundwater in the Indus basin is of marine origin. Seepage from conveyance system and deep percolation from field irrigation and precipitation has formed fresh water layers of varying thickness floating over deeper saline groundwater. The thickness of fresh groundwater is more near the recharging sources and decreases linearly away from the line sources. The areas with limited fresh groundwater thickness (45m) can be termed as critical areas where great care must be exercised so as to avoid the upconing of underlying saline groundwater, which otherwise would result in deteriorated quality of pumped water, thereby, destroying the productive lands. Sustainable development requires careful thinking in the selection, design and operation of irrigation wells. Wells designed to pump only the overlying fresh groundwater with minimum disturbance to the underlying saline groundwater are called skimming wells. The discharges of these wells are generally less than 1 cusec (28 lps). These low discharge shallow skimming wells can be effectively used by small and medium farmers as a source of dependable, flexible and demand-based irrigation water and to supplement the canal water supplies for boosting their agricultural production. The types of skimming wells are the conventional single strainer wells, multistrainer wells, scavenger wells, recirculation wells, radial collector wells, and dugwells.

The conventional wells are most commonly used by the farmers. Such wells, however, require a substantial gap between the bottom of the well and the fresh-saline water interface to avoid any mixing or upconing of the underlying saline water. Therefore, their installation is not advised in areas where thickness of fresh groundwater is 45m. Fresh water skimming in such areas can be accomplished by using multistrainer wells of smaller diameter. As discharge contribution of well diameter is very little as compared to length of the screen, therefore farmers are more inclined towards increasing screen lengths rather than well diameter. Total length of screen is enhanced by increasing the number of strainers while a substantial gap between the bottom of screens and fresh-saline water interface is provided. Performance of these wells depends on strainer diameter and penetration, distance among the strainers and fresh water cushion below the strainers. Distance among strainers may have a significant effect on pumped water quality end/or saline water upconing.

Usually the maximum possible distance among strainers should be brought in where multistrainer wells are to be installed. The Pakistan Agricultural Research Council (PARC) is conducting extensive research on the groundwater management and other water related aspects through its Water Resources Research Programme. A research project on skimming wells is under way in collaboration with Wapda and IWMI-Pakistan at Bhalwal, Sargodha. The research studies showed that multiple wells ensured abstraction of better quality groundwater than the conventional single strainer wells simply because an additional fresh water cushion is provided by limiting the screen length only in top fresh layer. This activity, however, is subjected to accelerated lateral as well as vertical components of flow velocity in the top layer resulting in greater upconing rate. This rate would be even more for larger discharges and greater operational factors.

Scavenger wells were thought to be the potential technology for sustainable groundwater development in shallow areas. However, the studies conducted so far revealed that these well could not control the mixing of two water bodies and also create the problem of saline water disposal. Moreover, these wells are not affordable by the common farmers.

Recirculation wells offer another alternative for fresh water skimming: These wells consist of two screens both placed within the fresh water zone. The fresh water is pumped through the upper screen, and a portion of which is injected back into the aquifer through the lower screen. The optimum position of lower screen and injection rate are, however, yet to be investigated. In the areas where thickness of fresh groundwater is less than 15m, radial collector wells are the attractive means for sustainable use of shallow fresh layers. These wells may operate under the gravity, as well as suction is created through pump. Subsurface tile drainage system is the most common example of radial collector wells under gravity flow. These wells are very suitable for Sindh province where watertable is generally high and most of the percolated fresh water is lost through non-beneficial evapotranspiration. The dugwells offer a potential alternative where fresh water thickness is too shallow and aquifer yield is too small to accommodate the other alternatives. These wells provide a simple, cost-effective and traditionally familiar option for fresh water skimming.

After selecting a suitable skimming technique, proper design and optimum efficiency are the key factors of a sustained skimming technology. The manageable factors are only design parameters but their proper management requires a thorough understanding of the hydro-geological properties of the aquifer at the specific location where skimming well facility is to be contemplated. The important design parameters are well discharge and fresh water cushion below well screen. The studies further revealed that pumped water quality deteriorated linearly with increase in well discharge, vertical hydraulic conductivity and hydrodynamic dispersion. Increase in fresh water cushion below well screen, horizontal hydraulic conductivity, effective porosity and recharge, on the other hand, resulted in improved quality of pumped water. The rate of improvement, however, was greater at their lower values and tended to stabilize at higher values. Vertical anisotropy ratio (Kh/Kv) was found to be the most dominant factor affecting the quality of pumped water and/or upconing followed by fresh water cushion below well screen, well discharge, effective porosity, hydrodynamic dispersion and recharge.

It was inferred that in the aquifers where salinity difference of fresh and saline water is small, upconing not only occurs at a rapid rate but also to a greater height. Hence, it was proved that pumped water quality and/or upconing of the underlying saline water are the functions of many parameters. Any excess or decline in the value of one parameter can be counter balanced by varying the values of other parameters. It is therefore imperative to manage the design parameters in accordance with the hydro-geological properties of the aquifer. As a thumb rule for a well having discharge of less than 15 lps, the fresh water thickness should be divided into four sections. The top section should be reserved for blind pipe followed by screen. The lower half must be provided as a cushion safeguarding against the upconing. For greater discharges this portion might be increased accordingly. The usual skimming well discharges for Pakistan as suggested by some learned researchers are within the range of 3-9 lps.

The last and, perhaps, the most important strategy for sustainable groundwater development is the operation of skimming wells keeping in view the design as well as aquifer parameters. Research trials indicated that for a shallow groundwater aquifer salinity rise was a linear function of pumping time. It was more pronounced for greater operational factors. Furthermore, it was observed that in case when the saline water cone has already intruded the fresh aquifer, any reduction in well discharge could neither ensure salt free water supply, nor any fall in the already raised saline water mound could be observed, however their rate of rise was slowed down. Intermittent pumping has been found to be effective in controlling the development of upconing.

During the period of well closure watertable maintains its original position and the generated streamlines are stabilized. At this stage the effect of saline water density becomes more prominent and saline water mound tends to recede. The rate of recession, however, is much slower than the rate of its rise. Usually the closure period should be in the range of 5 times of the pumping period.