IMPROPER APPLICATION OF UNDERGROUND WATER POSES DANGER TO AGRONOMICS

DR.S.M.ALAM
May 11 - 17, 2009

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. For supplementing canal supplies about 48 MAF of groundwater is 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 Pakistan and in many other parts of the world is seldom comparable lo canal supplies in Pakistan.

Pakistan is an agricultural country and agriculture plays a pivotal role in its economy. Major part of agriculture depends on surface irrigation. However, in spite of having one of the largest irrigation systems, this water is not sufficient to meet the crop water requirement. To augment the inadequate water supplies the use of quality groundwater is imperative. However, the use of such water with out proper management and adoption of proper technologies will degrade the precious natural land resources.

The population of the country is increasing at an alarming rate and to feed this burgeoning population of the country, the increase in crops yield per unit area and bringing more area under cultivation is urgently needed, which will need additional water. This presently may only be available from the ground water resource, so the development of proper guidelines for the irrigation for the farmers 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. Poor soil drainage, improper irrigation method, poor water quality, and insufficient water supply for adequate leaching and insufficient disposal sites for saline drainable effluent aggravate the condition. Problems caused by soil salinity/sodicity are compounded, when a high water table impedes root development, and concentrates salts in the already limited root zone. This is the scenario in Pakistan, where salinity/sodicity is on the increase, though number of organizations is working and spending lot of the national resources to deal with the stated problems. Saline soils are not only rampant in Pakistan, but also occur in many arid and semi arid parts of the world. For a country like Pakistan, which relies heavily on agriculture, this is of particular significance.

At least 40% of total cultivated land throughout Pakistan and about half of Sindh province is saline to varying degree. Ever increasing demand of food, fodder, and fuel wood is pushing agriculture to marginal lands, thus increasing the need of finding ways to utilize them. This is possible by improving soil condition and/or finding plant, which could tolerate these stresses.

The existing irrigation system in Pakistan is handling about 130 billion cubic meters of water for irrigating approximately 17 million hectares of land. Due to increased needs of food and fiber and consequently the increased cropping intensity for the ever-increasing population, seemingly enormous amount of irrigation water has not been able to keep pace with increasing water requirement. This quality of groundwater is not as good as that of surface water and varies considerably in its composition and consequently its impact on physical and chemical properties of soil will vary under different climatic conditions. Therefore, the experience in use of such waters under different conditions, will give rise to the acceptability or rejection of any particular water for specific use however, with the increasing use of brackish water all over the world and in Pakistan the controversy has come up over water quality standards.

Pakistanis is a land of great contrasts. The northern part is wrapped in snow cold mountains, in the east lays the desert areas of Cholistan and Tharparkar while the central part comprises the fertile Indus Plain, the west and the North West cover 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 varies from 100 mm in southern areas to more than 1000 mm in northern mountain ranges. Fertile land of Indus plain and intermountain valleys of the NWFP and Balochistan and the water of River Indus, its tributaries and various other rivers and nallah flowing in the mountain area, are the sources of great natural wealth for the burgeoning population of the country.

Pakistan is an agricultural country and canal irrigation is its lifeblood. However, these canal water resources are not sufficient to meet crop water requirements. To overcome deficiencies, groundwater resources have been exploited extensively. About 700,000 public & private tube wells are working in various irrigated parts of the country. 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.

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 concentration in the Indus plain is variable and less than 500 ppm in good quality ground. The Indo-Gangetic plain is comprised of 300,000 sq miles and water is used in Pakistan for agriculture on a large scale 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, which the salts are left in the soil. Tube well irrigation mobilizes the salts lying down below the groundwater surface and spreads 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 changes from fresh to marginal and from marginal to hazardous with the passage of time.

Its application with heavy-laden salts has deteriorated the situation more. At present, it is estimated that about 120 tons of salts per acre is 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 the 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% of the total water available for agriculture.

Continuous additions of groundwater from an irrigation system, along with deep percolation from cropland, 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 because of irrigation with poor quality tube well water. The result is reduction or stagnant yields of crops, 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 then 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 unused.