By Ghani Akbar

Introduction
Pakistan is a major irrigated agriculture country with more than 80 per cent of its cultivated land under irrigation. The annual influx of snowmelt and runoff into the Indus River system, one of the largest contiguous canal irrigation networks in the world, has a vital role in supporting national economic growth and the livelihoods of a large section of the population in the mainly rural nation. Unfortunately, Pakistan is located in a high-risk climate change zone (Shroder Jr. et al., 2007). More than 95 per cent of the country's agriculture is dependent on snowmelt-derived irrigation, which is highly prone to climate change-induced water shortages and flooding. The growing demand for irrigation water, climate-induced water shortages and population growth are leading to increased groundwater extraction to supplement the limited canal water in the Indus Basin. While a fresh water layer overlies the saline water (McWhorter, 1980; Sufi and Javed, 1988) due to seepage from rivers and irrigation systems, the extraction of low-quality deep groundwater has resulted in saline water infusion and up-coning in many regions (Hafeez et al., 1986). Both these processes have accelerated soil surface salinity (MREP, 1997), which is a challenge to agricultural sustainability in the Indus Basin. According to one estimate, around 0.247 million hectares of land cannot be cultivated in the Indus Basin due to soil salinity (GOP, 1992-93). Soil salinity and sodicity reduce soil infiltration, which increases the ponding period of flood water, exacerbating damage to standing crops. Therefore, controlling salinity is essential to mitigating climate change impacts and ensuring the agricultural sustainability of the Indus Basin.

To address these issues, many research studies have been conducted in the Indus Basin, focusing on ways to treat the saline water. However, these curative measures cannot be effective unless accompanied with preventive steps. Skimming well technology is becoming popular in the Indus Basin as it can extract from the shallow fresh groundwater layer without affecting the underlying saline water layer. A skimming well extracts less water than a conventional deep tube well and thus can either be used in conjunction with canal water or coupled with more efficient pressurized irrigation technologies (Qureshi et al., 2004) for irrigating larger areas with limited skimmed groundwater. Extraction of fresh groundwater by skimming wells and its efficient use through pressurized irrigation methods has been shown to have greater potential for controlling soil salinity (Akbar et al., 2001b; Saeed and Ashraf, 2005) and increasing productivity in the Indus Basin (Akbar et al., 2002), and is generally recommended.

Skimming well technology
A skimming well is designed to extract the thin surface fresh groundwater layer and control the up-coning of the underlying saline groundwater layer (Akbar, 2000). There are many types of skimming wells (Figure 1) which can be adopted for specific aquifer conditions (Saeed et al., 2003). A single strainer well can also be a skimming well if the gap between the lower end of the well and the fresh and saline water interface is sufficient to control the up-coning of saline water (McWhorter, 1980). A multi-strainer well is the most used skimming well in the Indus Basin (Saeed et al., 2002) and is capable of extracting 25 to 30 per cent more discharge than a single well (Hunting Technical Services Ltd, 1965) under the same aquifer conditions. A radial collector well can exploit a thin layer of fresh groundwater through radial collector drains (Bennet et al., 1968) which carry the freshwater horizontally from aquifer to the well. A compound or scavenger well comprises two wells located side by side. One well discharges water from the freshwater zone while the second operates at deeper depths and discharges saline water to control its up-coning. However, this well type has saline effluent disposal as well as environmental hazards (Stoner and Bakiewicz, 1992). A recirculation well works on the principle of injecting freshwater over saline water to control saline water encroachment. However, this technique, common in the petroleum industry, is new to the water industry. A dugwell can also be used as a skimming well (Akbar et al., 2001c) due to its small discharge, large diameter (Singh et al., 1992) and lower penetration depth.

Pressurized irrigation technology
Pressure-fed surface irrigation systems may either be sprinkler or drip irrigation types. The sprinkler system works at high pressure and the drip system works at low pressure. Sprinkler systems may either be set systems or continuous-move systems. Set systems are either periodic- move systems or fixed systems. Hand-move, end-tow, side-laterals, and gun-and-boom systems are examples of periodic-move systems. Fixed systems generally include small or big gun sprinklers mounted in stationary positions. Travelling gun or boom sprinklers, centre-pivot and linear-moving laterals are examples of continuous-move systems. The drip system is generally used for irrigating orchards or row crops by using different kinds of inline or online emitters such as microtubes, bubblers, key emitter, turbo-emitters, microsprinklers, pop-up sprinklers, overhead sprinklers and subsurface irrigation systems.

Pressurized irrigation systems usually consist of a prime mover, which can either be a diesel engine, electric motor or power take-off driven. A centrifugal pump with a single or multistage capacity is generally used. The distribution system consists of mainlines, submains, laterals, sprinklers or emitters, couplers, valves, bends, plugs, risers and debris removal equipment, among others. Additional accessories are required for tasks such as applying fertilizers, weedicide and pesticide. Similarly, a water metre is used for measuring discharge and a pressure gauge for measuring water pressure generated by the pumping unit.

The high cost of pressurized irrigation systems is the main constraint to their adoption, especially under Pakistani farming conditions (Akbar et al. 2001a). Therefore, the Climate Change, Alternate Energy, and Water Resources Institute (CAEWRI) of the National Agricultural Research Centre (NARC), formerly known as Water Resources Research Institute (WRRI) has developed a set of equipment and decision support guidelines for adoption of different sprinkler and drip irrigation systems (Ahmad et al., 1993) suited to local needs and farm sizes. For instance, a low-cost, weather-resistant and durable Low Density Poly Ethylene (LDPE) pipe was developed in collaboration with local industry. Similarly, different raingun sprinkler system designs (Table 1) were developed to support decision-making by farmers.

Options to mitigating climate change and soil salinity impacts in the Indus Basin
Irrigating crops with freshwater and restricting saline groundwater up-coning during climate-induced water shortages are key preventive measures for controlling salinity build-up in the Indus Basin. Skimming well and pressurized irrigation technologies can be instrumental in achieving this goal. For instance, skimmed groundwater quality from three different dugwells (Figure 2) under a 20-30m thick freshwater layer showed no temporal decline when used to sprinkle-irrigate five acres of land during two cropping seasons (Kharif: Maize and Rabi: Wheat) in the Indus Basin (Akbar et al., 2003).

 

Additionally, ensuring adequate leaching and flushing out of soluble salts from the root zone through optimized irrigation management, appropriate land grading, soil profile modification, installation of appropriate surface and subsurface drainage system, is essential for controlling soil salinity. Moreover, reclamation of saline and sodic soils and water through physical, chemical and biological treatment have emerged as potential curative measures, which are important for mitigating climate change impacts and ensuring agricultural sustainability in the Indus Basin.

(References available upon request)