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The quality of the underground water found in various aquifers in Pakistan is very variable
By Dr. S.M. ALAM
Oct 15 - 21, 2001
Nuclear Institute of Agriculture, Tandojam, Pakistan.
Pakistan is an agricultural country and agriculture plays a pivotal role in its economy. Major part of agriculture depends on surface irrigation. But, 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 poor quality groundwater is imperative. However, the use of such water without 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 ever increasing 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 groundwater resource, so the development of proper guidelines for the irrigation for the farmers are 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 waterable impedes root development, and concentrates salts in the already limited rootzone. This is the scenario in Pakistan, where salinity/sodicity is on the increase, although number of organizations are 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 semiarid parts of the world. For a country like Pakistan, which relies heavily on agriculture for its sustenance. 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 fuelwood 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 plants, which could tolerate these stresses.
The existing irrigation system in Pakistan is handling about 130 billion cubic meters of water for irrigating approximately 15 million hectares of land. Due to increased needs of food and fibre and consequently the increased cropping intensity for the ever increasing population. This seemingly enormous amount of irrigation water has not been able to keep pace with increasing water requirement. To augment this inadequate canal water supplies the poor quality groundwater is used, which is creating a serious problem of secondary salinization and consequently the reduction in crops yield. The 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 also vary under different climatic conditions. So 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 irrigation water quality standards. The standards, adopted for such purposes are for mulated for general application to these may not be applicable under all soil and climatic condition. So it is necessary to know the amount and nature by salts present in the irrigation water for sustainable productivity.
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 44 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 dS m-1) or high residual sodium carbonate (RSC 71.25 mmol L-l). Quality of groundwater in the arid and semi-arid regions of Pakistan and in many other parts of the world is seldom comparable to canal supplies in Pakistan. For supplementing canal supplies, about 47.5 MAF of water is being pumped to grow crops. According to many reports, this pumped groundwater in Pakistan is hazardous for irrigation and requires special management strategies. In Pakistan, over 40 per cent of the irrigation supplies at the farm are provided by groundwater. As such groundwater is very important for the economy of Pakistan and its development as well as preservation in terms of quantity, and quality is very important for sustained irrigated agriculture in the country.
The quality of the underground water found in various aquifers in Pakistan is very variable. The salinity concentration of such groundwater as measured in the Indus Plain vary from loss than 500 ppm in good quality groundwater to over 70,000 ppm in some areas. The purpose of groundwater management is to ensure the national and conservation of the groundwater resources. Unfortunately, it is often neglected until undesirable effects start to develop. One of the reasons for this is that the processes in a groundwater basin are very slow and groundwater, in contrast to surface water is invisible, so that the slow changes escape attention. Proper groundwater management as such as must for the sustainability of the precious resource.
Reclamation of such soils is not easy and may be costly. For instance, there may not be enough irrigation water available for leaching, the laying of tile drains and sinking of tube well may not be within the reach of every farmer, sufficient quantity of ameliorants may not be readily available or may be too costly to procure and transport and this may not be adviseable without expert technical assistance. Moreover, even under the most efficient system and management, it may not be possible to rid the soil completely of its excess salt, which may anyway, rebuild in the absence of proper precaution.
Saline agriculture usually involves some compromise on yields as even a very salt tolerant species is bound to suffer some yield losses under the adverse conditions. It is hence advisable to observe precautions in such ventures. Saline soils are usually reclaimed by leaching the salts out of the soils through irrigation and drainage systems. Leaching is the process in which extra water is added to a field and allowed to soak through the soil and drain away underground. A common method of leaching is to pond the water in basins over the entire field. Sometimes the excess water is removed by pumping from wells. Permissible depths for groundwater tables vary according to the type of soil being irrigated. The amount of leaching water that enters the soil by surface flooding determines how much salt is removed from the soil. For instance, when water is leached through the soil, a surface depth of 6 inches of water for every foot of plant root will leach out 50 percent of the salt. One foot of water for every foot of root zone leaches out 80 percent of the salt. One foot of water for every foot of root zone leaches out 90 percent of the salt. The upward movements of saline water from shallow water tables can cause salt build up in the plant root zone. A water table should be at least 41/2 to 5 feet below the surface during most of the crop growing season.
Modern research had identified more than 1500 plant species that have high levels of tolerance to saline soils. Some of these are able to withstand salt concentrations in excess of those found in seawater. These plants (trees, shrubs and salt tolerant grasses and herbs) are a major resource that can be used in the development of agricultural systems for salt affected soils. In addition, there are opportunities to increase the salt tolerance of existing crops using conventional plant breeding and molecular biological approaches.
The research that has been conducted in Pakistan over the last decade shown that there is a wide range of plant species with varying levels of salt tolerance that can be used in local agriculture. Furthermore, at least some of these plants are able to lower local water tables, improving the condition of the land, and acting as a form of "biological drainage". Crops vary widely in their tolerance to salts. Sugarbeet, cotton and barley can tolerate up to 10 times as much salt as most clover, beans and fruit trees.
Scientists have successfully grown a number or crops or marginal saline lands. It is pointed out that if the average electrical conductivity of a plot is 0-15 dS m-1(9600 ppm), which is fairly high and may have serious effects on the growth of plants, but the soil is loamy texture, underground water table is at 8-10 fit and some good quality irrigation water is also available, the conditions became less hostile for plant sustenance. A number of species may be grown in such conditions on given below. Good quality irrigation water should be used during the initial establishment phase and for periodical leaching of salts from soil surface. Under proper management, this can be a positive and cost-effective venture.
If however, the salinity of a plot ranges between 15-20 dS m-1 (9600-12800 ppm), with coarse textured soil and the only source of irrigation is had quality underground water EC 10-15 dS m-l present at a shallow depth, the growth of majority of the plants will be restricted under such highly adverse conditions and only some salt tolerant plants can be grown.