August 13, 2007	Monday	Rajab 28, 1428

PAKISTAN is an agrarian country and its economy is based on agriculture. Most part of the agriculture here depends on surface irrigation.

But despite having the largest canal irrigation systems, the surface water is not sufficient to meet the requirement of the crops. To meet this shortage, use of underground water, which is of very poor quality, is imperative. However, such water without proper management and adoption of suitable technologies will harm the precious natural land resources.

Population of the country is increasing at an alarming rate, and to feed this ever- increasing population, there is an urgent need to increase crop yield per acre, and bring more area under cultivation. And for this additional water is needed. This requirement can be met through underground water resources. Therefore, development of proper guidelines for irrigation is essential for sustainable productivity.

Saline and sodic soils occur naturally in arid and semi-arid regions and as water development brings more land into irrigation, the salinity problem expands. The condition is aggravated by poor soil drainage, improper irrigation method, poor water quality and insufficient water supply for adequate leaching and insufficient disposal sites for saline drainable effluent.

Problems caused by soil salinity/sodicity are compounded, when a high water table impedes root development, and concentrates salts in the limited root zone. This is the scenario in the country where salinity/sodicity is on the increase, although a number of organisations are working and spending a lot of the national resources to deal with the problems.

At least 40 per cent of the cultivated land throughout the country and about half of Sindh is saline. Ever-increasing demand of food, fodder and fuel wood is pushing agriculture to marginal lands, thus increasing the need of finding ways to utilise them. This is possible by improving soil condition and/ or finding plant, which could tolerate these stresses.

Irrigation system: The existing irrigation system in the country is handling about 130 billion cubic meters of water for irrigating about 16 million hectares. Seemingly enormous amount of irrigation water has not been able to keep pace with the increasing water requirement. To augment the inadequate canal water supplies, the poor quality groundwater is used, which is creating a serious problem of secondary salinity and consequently reduction in crops yield.

The quality of underground water is not as good as that of surface water and varies considerably in its composition and impact on physical and chemical properties of soil under different climatic conditions. It is reported that 5.51 hectare-meter water is required for optimum growth of various crops while only 2.77 hectare-meter water is available at farm. Thus lack of scientific management is responsible for leaving 8.85 mha as the culturable waste. For supplementing canal supplies about 48 MAF of groundwater is being applied to grow crops. Unfortunately the major portion of this water is brackish/unfit for irrigation either due to high amount of total dissolved salts (0.75 dSm-1 ) or high residual sodium carbonate (RSC 71.25 mmol L-1). Quality of groundwater in the arid and semi-arid regions of the country is seldom comparable lo canal supplies.

The northern part of the country is wrapped in snow clad mountains, in the east lies the desert areas of Cholistan and Tharparkar; the central part comprises the fertile Indus Plain; the west and the northwest covers intermountain valleys of Balochistan and the NWFP and in the south lies the costal belt of Balochistan and Sindh. Climatically too, the country has great variation in temperature and rainfall. The temperature ranges from less than freezing to over 45oC, and rainfall varying from 100 mm in southern areas to more than 1000 mm in northern mountain ranges. Fertile land of the Indus plain and inter mountain valleys of the NWFP and Balochistan and water of River Indus, its tributaries and various other rivers and nullah flowing in the mountain area, are the sources of great natural wealth for the burgeoning population of the country.

To overcome shortage of surface water, groundwater resources have extensively been exploited. About 350,000 public and 350,000 private tube-wells are working in various parts of the country for irrigation. The water quality ranges from good, marginal to hazardous. Due to inefficient water management practices and brackish ground water, a good deal of agricultural land has gone out of production due to water logging and salinity.

Over 40 per cent of the irrigation supplies at farms are provided by the groundwater. As such groundwater is quite important for development. The preservation in terms of quantity and quality of water is important for sustained irrigated agriculture. The quality found in various aquifers is greatly variable which is due to the complex geology and the variable topographic, climatic and hydrological conditions. The salinity concentrations in the Indus plain vary from less than 500 ppm in good quality ground. The Indo-Gangetic plain comprised 300,000 sq miles and water being used in the country for agriculture on a large scale since the early sixties for irrigation purpose it is being opined not only from fresh areas, but also from marginally saline ground water areas.

When this water is applied to crops, water is taken up by the plants, while the salts are left in the soil. As such, tube well irrigation mobilizes the salts lying down below the groundwater surface and spread them on the surface, from where they are again leached to groundwater. This process when repeated for a long time, without the mechanism of export of the salts from the soil results in increasing the salinity of groundwater, which in time shall change from fresh to marginal and from marginal to hazardous with the passage of time.

Its application with heavy laden salts has deteriorated the situation even more. At present, it is estimated that about 120 tons of salts per acre is being added with irrigation every year to the agricultural land. Therefore, lands which are irrigated with brackish tube-well water develop hard pan/low permeability and accumulate salts on the surface. The Indus Basin represents an extensive ground water aquifer covering a gross command area of 16.4 million hectares. Water table was well below the surface and aquifer was in state of hydrological equilibrium before the development of canal irrigation system a century ago. When canal irrigation system was introduced, percolation to the aquifer was increased in irrigated areas of the Indus Basin resulting in the menace of water logging and salinity. Ground water contributed around 12BCM during the pre-storage period, which constituted 11 per cent of the total water available for agriculture. During the past storage period, the estimated recharge to the groundwater in the Indus Basin is 56BCM, of which 36BCM covers area of usable groundwater. Groundwater use has increased significantly in the last four decades.

The total annual groundwater potential in Pakistan is estimated at 68BCM about 80 percent of it in Punjab and 30 per cent in Sindh are suitable for agriculture. Since most of the easily exploitable surface water resources have already been tapped, the future demand of water will have to be met largely through additional storage, better conservation and efficient use of water.

Continuous additions of groundwater from an irrigation system, along with deep percolation from crop land have resulted in rising groundwater levels. Lowering of water table to some degree was achieved through public and private tube wells, but salinity developed in normal soils as a result of irrigation with poor quality tube well water.

The result is reduction or stagnant yields of crops. Other causes of low productivity include inadequate use of fertilisers and micronutrients, poor soil management and tillage practices, inadequate and shallow rooting depths. Water logging and salinity have adverse impact on crop yield. All the major crops such as cotton, rice, sugarcane, wheat, etc are badly affected by groundwater qualities.

Owing to the heat of Sun, water is drawn to the surface of the field by capillary action and them it evaporates, leaving behind in the top soil the soluble salts it contained. Without drainage, the water table can eventually rise to the ground surface or in the top 30 cm of the soil profile. Due to this, the profitable cropping becomes difficult and the land is abandoned or basically remain unused.