 |  | Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.) | |||
 |  | Chapter 15 - Conjunctive use of surface and groundwater | |||
|  | Definitions | |||
| | Direct conjunctive use | |||
| | Indirect conjunctive use | |||
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In the narrower sense, conjunctive use refers to pumping from tubewells directly into a surface water canal system, supplementing the surface supply. In a broader sense, it refers to any use of groundwater within an area also supplied with surface water.
The term conjunctive use does not necessarily imply that the groundwater utilized originates from seepage from the surface supply in the area concerned, although it may often do so. The sources of the groundwater may be outside of that area some distance away. In the case where the source is recharge from the surface supply, the reason for groundwater development may partly be to control rising watertable or simply to utilize the recharge which would otherwise move outside the area as sub-surface flow and be lost to that area (although eventually appearing elsewhere as an increment to stream-How).
In general, where a source of groundwater recharge exists in an area supplied by surface water canals, technically there are two options for its use. One is to pump groundwater directly into the canal system (direct conjunctive use), and the other is to develop tubewell irrigation within the nominally canal-irrigated command (indirect conjunctive use). In the second case, the tubewell irrigation areas may be islands of purely tubewell irrigation within the nominal canal command, particularly in tailend areas which otherwise would depend on poor canal supply. Alternatively, the groundwater development may be in the form of private wells scattered throughout the canal command, supplementing canal supply in the low-Bow season or providing more frequent irrigation of specialty crops than is available from the rotational canal system (O'Mara 1980).
A key question in the design of direct conjunctive systems is the category of canal into which the groundwater should be discharged, primary, secondary, or tertiary. If the system is rotational at the secondary/tertiary level, there is an incentive to pump into the primary canal, permitting continuous use of the pumps. Alternatively, pondage capacity may be provided in the secondaries or tertiaries, or in small lateral storage, permitting pumping to be continued during the "off" period of the rotation. Pumping into secondaries (generally selected secondaries) has an advantage if the secondary/tertiary system is lined, as higher delivery efficiency may be obtained than if delivery is into an unlined primary canal. However, the proportion of groundwater to surface water is likely to be considerably higher with discharge into selected secondaries than with discharge into a primary canal. If power supply is unreliable, the operational consequences of interruption to pumping (the reduction in canal flow) are more serious in the case of discharge into secondaries than into primary canals.
Poor quality groundwater, requiring blending with canal supply, may be a reason for adoption of direct conjunctive use, also for delivery into the primary canal rather than selected secondaries due to the greater dilution in the primary. However, where indirect use is in the form of small tubewells scattered throughout the canal command, leaching with canal supply during the high-flow season may permit irrigation directly from wells, even with poor quality water, during the remainder of the year.
An advantage of direct conjunctive use is the greater degree of control on of groundwater extraction exercised by the irrigation agency as opposed to indirect conjunctive use by small privately owned wells. This may be of considerable importance where regulation of watertable is an important factor. Cultivators may or may not wish to install wells in an area where canal water is available for most of the time, at much lower cost. If wells are in fact installed, they are likely to be run only during periods of restricted canal supply, without regard to the need for consumptive extraction for watertable control They are even less likely to install wells if canal irrigation has already raised the watertable sufficiently to partially water-log much of the area, thereby reducing its productive capacity and the ability of the cultivator to recover the cost of the well.
On the other hand, direct conjunctive use implies a continuing budgetary demand on the irrigation agency for the cost of power, particularly if relatively large capacity wells are employed, with substantial draw-down and pumping head. It is noted that attempts to interest cultivator groups in taking over augmentation wells discharging into the tertiary canals, at no capital cost to the group, have not always been successful. The group then has to meet all energy charges. Technical problems in sharing the water pumped into a communal channel are also a deterrent.
As discussed above, the development of groundwater by small tubewells or dugwells within a canal irrigation command can be a very effective means of conjunctively using the recharge from canal irrigation. The use of such wells for dry-season irrigation of perennials such as citrus and bananas can permit closing canals at a time when canal delivery would be small in any case and inefficient. A degree of control on such development can be exercised by provision of credit, and particularly by electrification and favorable electrical tariffs, but otherwise the rate of installation of such wells is determined by market forces.
A further means of indirect conjunctive development, previously noted, is the installation of medium or major wells and their distribution systems in selected areas within the gross boundaries of a canal command, but separated from the canal system as far as supply is concerned. ("Islands"of purely groundwater impaction within the canal command). If utilization of recharge from the canal irrigation and watertable control are the purposes of the conjunctive installation, draw-down of the watertable below the critical level for small suction-mode wells by the installation of medium or major wells is unlikely to be a problem. Small privately owned wells and larger group-operated medium or major wells may then co-exist.
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