WATER AND PLANT GROWTH
The demand for irrigation water has increased and the supply reduced
By Dr. S.M. ALAM, Dr. M.H. NAQVI and M.A. KHAN, NIA
Mar 17 - 23, 2003
About 71% of the globe's surface is covered by water and less than 3% of it is fresh or sweet. Over 75% of fresh water is frozen. About 5% of worldwide consumption of water is for all domestic use, 75% for irrigation and 20% for industry. Pakistan has nearly 20 million hectares of cultivated irrigated area. Inspite of having the world's largest contiguous irrigation system, it has now become a water deficit country. The 77 years past inflow data of Indus River indicate that the watersheds of Indus River yield about 138 million acre feet (MAF) of water annually, of which nearly 105 MAF is diverted into the canal system. Nearly 70% of about 565,000 tubewells used for irrigation in the country are pumping brackish saline water with variable degree of salinity. Plant productivity is adversely affected by drought, salinity, temperature and heat.
Water is a basic element and its shortage results in famine. Many parts of the world are facing water shortage and some have reached at a crisis stage due to global climatic changes, increase in human population and land under crops, unfair distribution and the mismanagement of available resources. The water crisis in the country of the last three years resulted in big hue and cry, as the demand for irrigation water has increased and the supply reduced. Pakistan has abundant land and water resources. The prosperity of the nation to sustainable development of agriculture in various areas mainly depends on fair distribution of existing water resources and proper and scientific management.
Water is fast becoming an economically scare resource in many areas of the world, especially in arid and semi-arid regions of the world including the Mediterranean. The need for more efficient agricultural use of irrigation water arises out of increased competition for water resources and increasing environmental concern. Crop growth in arid and semi-arid regions usually depends on irrigation, but inappropriate irrigation practice can result in water stress. The reduction in growth yield and quality caused by water stress has been well documented, although different physiological processes have been put forward to account for this reduction in different species. Water deficit also affects photosynthesis and chlorophyll content by lowering concentrations of leaf nitrogen, in rhizosphere, pH and availability of some essential nutrients. Ammonium ion (NH4+) predominates in the bulk solution in waterlogged soils. Waterlogging leads to anaerobic conditions in soil, due to oxygen consumption by roots and micro-organisms, and a very low diffusion rate of O2 in water compared with the gas phase. Wetland plants possess a range of traits allowing growth in waterlogged soils. Formation of aerenchyma, which facilitates O2 movement from shoot to root tissues, is regarded as a crucial trait for wetland plants such as rice.
In roots, oxygen is required for respiration to provide sufficient energy for growth, maintenance and nutrient uptake processes. However, significant amounts of oxygen supplied via aerenchyma to roots in anaerobic substrates may diffuse into the' rhizosphere oxygen (O2) leakage from roots oxidizes compounds (e.g. Fe2+, Mn2+ and S2- ) that are potentially toxic to plants in their reduced state, also leading to change. Growth and distribution of plants are controlled chiefly by too little or too much water. Temporary or continuous flooding with fresh or salt water is very common throughout the world. Soil inundation is variously tracement to overflowing of rivers, storms, over irrigation, inadequate drainage and impoundment of water by dams. Flooding leads to rapid depletion of soil oxygen and changes in physiological process of plants that markedly influence their growth and survival. Flooding affects our well-being not only by extensive destruction and impairment of goods and services but also by restricting yield of food and fibres. Temporary or continuous flooding is common throughout the world, with about 72% of earth's surface covered by submerged soils. Much flooding is the result of overflowing of riverbanks and no continent except Antarctica, is free of such flooding. When a soil is flooded, gas exchange between the soil and air is drastically reduced. Shortly after a soil is inundated, micro-organisms and roots consume practically all of the oxygen present in the water and trapped in the soil and render submerged soil practically devoid of the gas.
The poor soil aeration associated with flooding induces a number of changes in the soil and in plants that usually adversely influence growth. A wide variety of toxic compounds accumulate in waterlogged soils. Shortly after they are flooded, plants exhibit sequential changes in metabolism and physiological processes. Reduced water absorption and closure of stomata leading to a lowered rate of photosynthesis are among the earliest plant responses to flooding. Subsequent changes include decreased permeability of roots, reduced mineral uptake, alterations in growth-hormone balances, leaf epinasty, chlorosis and abscission and arrested vegetative and reproductive growth. When flooding is severe and prolonged plants, often are killed. The flooding of soil by irrigation adversely affect plants by decreasing soil aeration and causing erosion and salt problems. Flooding adversely influences shoot growth by inhibiting internode elongation, leaf initiation, and leaf expansion and by inducing leaf senescence, injury and absission. Waterlogging of soil retards height growth of many species. Oxygen deficiency created by water logging stunts shoot and root growth, dry matter accumulation and find yield. Waterlogging can effect several physiological processes, such as absorption of water. Among various abiotic stresses, excessive soil moisture (water logging) caused by flooding, water stagnation or a high water table is one of the most important constraints for crop production and productivity in Asia Pacific region.
In Pakistan, waterlogging is of serious concern, where hundreds of million hectares of arable land is in the grip of waterlogging. The beneficial chemical effects of flooding on soil fertility include an influx of dissolved and suspended nutrients, accumulation of nitrogen, increase in solubility of phosphorus and silicon and an increase in concentration of potassium in the soil solution. Among the disadvantages are depletion of O2, accumulation of CO2, destruction of nitrate, and root and shoot harmone relations and decrease the uptake and transpart of ions through roots causing nutrient deficiencies. Negative effects associated with waterlogging are nitrogen deficiency by stimulating denitrification to leaching and accumulation of toxic substances. Waterlogging stress is one of the limiting factors influencing crop production.
Flooding (submerging) an air-dry soil in water sets in motion a series of physical, chemical and biological process that profoundly influence the quality of a soil as a medium for plant growth. The nature, pattern and extent of the processes depend on the physical and chemical properties of the soil and on duration of submergence. When rice plants are submerged by flash flood, they experience two different types of environmental changes, anaerobic conditions during submergence and aerobic conditions after submergence receded (hereafter referred as "desubmergence").
During submergence, limited gas diffusion and low light intensity are the two most important factors that affect plants growth adversely. In the plants under O2-depleted conditions in complete submergence alcoholic fermentation for energy production was enhanced. To keep high rates of alcoholic fermentation, a sufficient amount of carbohydrate supply is needed during submergence. Actually, the flash flood tolerant rice cultivator contained a higher amount of carbohydrates than the intolerant one even under limited CO2 supply and low light intensity during submergence. CO2 enrichment of flood water increased the photosynthesis and dry weight, which resulted in improved submergence tolerance. The flash flood-intolerant cultivars developed visual damage under aerobic conditions after desubmergence rather than during submergence.
Water availability of many developing countries are currently under tremendous pressure. Water is the basis of life. Nothing on earth, plant or animal can survive without water. We need water to grow crop plants and to drink. It is an universal solvent and cleanser. It is a unique gift of nature. The amount of fresh water in the world is more or less constant, but the population is increasing rapidly. According to an Food and Agriculture Organization (FAO) estimate, global demand for water is doubling every 20 years, and the renewable water resources available per person are today less than half of what they were in early sixties.
Now water is an important input to agricultural production and an essential requirement for many domestic, municipal and industrial activities. Increasing population and standards of living are contributing to steep rise in demand for fresh water. The consequent wastage, over-exploitation, pollution and depletion of available fresh water pose a serious threat to human being. According to UN report, there are about a billion people today who live in water stressed regions and as the population increases, it is estimated that upto 3.5 billion people will face severe water shortage by the year 2025. Water dissolves and carries nutrients to plants and living organisms. It provides chemical services, like oxygen production, carbon dioxide uptake and release of nutrients through biochemical processes. Thus, water not only has direct use for drinking, washing, etc. and is a vital input for food and energy production, but it has numerous other functions indispensable to life and human welfare.
Of all the water on earth, nearly 97.5% is in the oceans, thus the total fresh water is only 2.5%, most of which (68.7%) is in the form of snow and ice. About 31.1% of fresh water is underground. Lakes, reserviours and river contain only 0.26% of the total fresh water. Water, directly affects almost all sectors of economy. Historically, the high aridity index of the country is adding further to importance of water in any developmental activity in Pakistan. An amount of water of 1700 m3 per year is considered as nominal requirement. Below 1000 m3 per capita per year is considered as chronic water stress. The present annual per capita water availability in Pakistan is about 1050 m3, which indicates condition nearing chronic water stress.
According to an estimate, 51% of Pakistan's land is affected by desertification, which can be brought under cultivation, if the available water resources are developed and managed properly. The main deserts of the country are Thar (430,000 ha), Cholistan (2,580,000 ha), Thal (2,300,000 ha), Chagi-Kharan (600,000 ha) and other (1,220,000) with a total of 11,000,000 or 1.1 million hectares. The groundwater in almost all the desert areas of Pakistan is generally saline. The soils in such areas are sand in form of sand dunes or clayey in the form of dense calcareous. Rainfall is the only source of freshwater in desert. Due to high infiltration rates in sandy soils, high temperatures and low humidity, most of the rainwater is lost as seepage and evaporation. Moreover, a significant amount of rainfall is lost as runoff, which gets collected in calcareous clay pans and evaporates.
During the 20th century, largest contiguous irrigation system in the world was developed in the Indus Basin. The system includes Indus river and its major tributaries, 4 major reservoirs (Warsak, Tarbella, Manga and Chashman); 23 barrages/headworks; 12 link canals; 45 canal commands and some 90,000 watercourses. The total length of canal system is 60,800 km with watercourses, farm channels and field ditches running another 160,000 km in length. Surface water resources of Pakistan are mainly based on the flows of the Indus River and its tributaries. The Indus River with a total flow length of 2,900 km has a drainage capacity of 966,000 km of the agricultural land.
The total potential of water from the Indus River and its tributaries is about 138 MAF, however, the crops hardly use 31 NIAF and the rest is lost as conveyance and application losses. The main causes of these operational losses are: seepage, over flow, thin distorted, silt-loaded banks, vegetation, rodent holes, etc. The country also receives about 6 MAF water through rain and 38 MAF from groundwater through 562,000 private tubewells and about 10,000 public tubewells. On an annual basis, almost 97% of all river water is being used for agriculture and the remaining 3% for domestic, industrial and other purposes. Groundwater, in the irrigated areas is also mostly used for agriculture, while in the urban areas of the country this source provides water for municipal supplies to meet domestic requirements.
The current domestic need is about 5.2 MAF and industrial as 1.18 MAF. The barani agriculture contributes 10% of the total agricultural production and depends mainly on rainfall. More than 1200 kg/acre wheat is produced in these areas. Water is the only limiting factor for agriculture of these areas. The average seasonal requirements of water for wheat is 317 mm, sugarcane 1451 mm, cotton 631 mm, rice 960 and for maize 354 mm. Proper irrigation scheduling helps farmers in knowing when to irrigate and how much to apply. This issue is mostly related to awareness and education of the farmers.