Irrigation of Field Crops

IRRIGATION OF FIELD CROPS PRINCIPLES AND PRACTICES

S.S. PRIHAR Professor of Soil Physics and Head, Department of Soils, Punjab Agricultural University, Ludhiana and

B.S. SANDHU Senior Scientist (Irrigation), Department of Soils, Punjab Agricultural University, Ludhiana

ICAR

PUBLISHED BY

PUBLICATIONS AND INFORMATION DIVISION INDIAN COUNCIL OF AGRICULTURAL RESEARCH KRISHI ANUSANDHAN BHAVAN, PUSA, NEW DELHI

First Printed Reprinted Reprinted Reprinted

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November 1987 September 1994 February 2002 August 2012

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Price : Rs 200.00

Published by Dr D.K. Agarwal, Project Director, Directorate of Knowledge Management in Agriculture, Indian Council of Agricultural Research, Krishi Anusandhan Bhavan, Pusa, New Delhi and Printed at M/s Chandu Press, D-97, Shakarpur, Delhi-110092

CONTENTS Page Foreword

Preface

v vii

1. Introduction

1

2. Soil as a Water Reservoir

6

3. Soil-Moisture Measurement

24

4. Soil-Water Movement

32

5. Soil-Water-Plant Relationships

44

6. On-Farm Water Management

57

7. Criteria of Irrigation Scheduling

73

8. Optimum Irrigation Schedules for Crops Appendix Subject Index Author Index

84 136 139

141'

CHAPTER 1

INTRODUCTION THREE basic resources, viz. climate, soil and water, determine the nature of crops that can be grown successfully in a particular region. An efficient utilization of these resources is essential for optimum production of food and fibre for human population, feed for cattle and raw material for industry.

Climate determines the suitability of a region as a habitat for different flora and fauna as also the availability of water for production of crops and other uses. Under a given set of environmental conditions production of crops is limited by the availability of nutrients and water. Soil provides anchorage for the plants and also serves as a reservoir of water and nutrients required by them. While chemical fertilizers supplement the low nutrient-supplying capacity of the soil, there is no substitute of water for production of crops.

Efficient management of water, a limited resource as it is, is of utmost importance for sustaining and increasing agricultural production. Competing demands of water for domestic use, sanitation, industrial and recreational purposes make it all the more essential to maximize the efficiency of water for agricultural production.

Available Water Resources and Irrigation Potential Precipitation in the form of rain and snowfall is the main source of water country, although some water flows from across the borders. Annual the in entire geographical area rainfall of the country averages 112 cm over which amounts to 370 million hectare-metrfe of water. Twenty million hectare-metre water comes from catchments located outside the country.

Precipitation exhibits wide variations both in time and space. It varies from less than 30 cm in western Rajasthan to about 1,000 cm in Chirrapunji in Meghalaya. Besides, more than 75% of the annual rainfall comes during 2-4 months between June and September. This leaves many areas dry for most part of the year which necessitates the application of irrigation to help the crops grow. Provision of irrigation is possible by harnessing the surplus water of the high rainfall period in the high rainfall areas as also the water that seeps into the soil and forms the water-table. An estimated 215 million hectaremetre of precipitation enters the soil, of which 165 million hectare-metre is held within the root zone depth and 50 million hectare-metre is lost beyond the root zone as deep percolation and adds to the groundwater. According to the National Commission on Agriculture (1976), total water, resources of the country after accounting for soil moisture and evaporation are 185 million hectare-metre; comprising 135 million hectare-metre of surface water and 50 million,hectare-metre of groundwater (Swaminathan, 1982). Both the water

2

IRRIGATION OF FIELD CROPS

resources cannot be exploited in full for irrigation on account of topographic, climatic and soil limitations in the case of surface water and additional limitations of pumping depths and availability of power in the case of groundwater. Moreover, in some areas the groundwater is saline and unfit for irrigation. Altogether, it is estimated that 70 million hectare-metre of surface water and 35 million hectare-metre of groundwater can be utilized for irrigaÂŹ tion. This estimate is likely to increase in future if schemes on interbasm water transfer and recharge of groundwater with monsoon rain are implemented (Swaminathan, 1982). The ultimate irrigation potential of the country is estimated at 113.5 million hectares.

Development of Irrigation Potential The country has given priority to the development of irrigation potential and irrigated area has increased considerably during the last three decades.

Irrigation development schemes fall into three main categories (SwaminaÂŹ than, 1982).

(1) Major irrigation schemes which cover a culturable command area (CCA) of more than 10,000 ha.

(2) Medium irrigation schemes which cover a CCA of 2,000-10,000 ha.

(3) Minor irrigation schemes with a CCA of less than 2,000 ha.

At the end of 1984-85 the country had a gross irrigated area of 67.7 million hectares. Progressive increase in gross irrigated area from 1964-65 to 1984-85 is shown in Fig. 1.1. In some regions the irrigated area does not receive full irrigation and the limited water is used to supply one or two irrigations to crops in the command area. Many of the big projects are multipurpose and generate power in addition to supplying irrigation water.

Need for Scientific Management Since the irrigation potential has been created at a huge cost to the country, it is essential to derive the maximum benefit from the created potential and to utilize every unit of water for maximum returns. Huge amounts of water are lost during conveyance and distribution. Water from the main reservoir is led to the field through main canals, branch canals, distributories and minors. Conveyance losses of water in irrigation projects of the country range from 22 to 50% depending upon the location and level of management (Bos and Nugteren, 1974). Hence, there is a considerable scope to augment water supplies for irrigation of crops if these losses are cut down by using suitable sealants or lining of conveyance channels.

Water released from the outlet of a minor has to be led to the field through water course and applied through field channels. Here also considerable amount of water is lost as deep seepage and through breaches of the sides of water courses. To make the full use of the water supplied at the outlet, conveyance losses from the water courses must be kept at the minimum.

Having realised this several states in the country have undertaken lining of

3

INTRODUCTION

T

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Year Fig. 1 . 1. Progressive increase in gross irrigated area in India since 1962-63

water courses through government agencies. The cost of lining is recovered from the farmers in instalments spread over long periods. In Punjab and Haryana this programme has become quite popular.

On-farm water management, of which crop planning and irrigation scheduling are important components, is very crucial in determining water-use efficiencies. Depending upon the availability of water the objectives of irrigation scheduling can differ. Where land is limited in comparison to water, the irrigator’s concern is to increase production per unit land without wasting

4

IRRIGATION OF FIELD CROPS water. But where land is more plentiful in comparison to the irrigation water, the major objective becomes more production per unit water. These objecÂŹ tives can be realised by proper scheduling of irrigation to crops based on their requirements and their relative sensitivity to water deficits at different stages of growth. For high water-use efficiency, it is essential to adopt other compoÂŹ nents of technology such as improved agronomic practices, improved seeds, optimum fertilizer rates, weed control and control of pests and diseases.

Wastage of irrigation water in the field usually results from faulty irrigaÂŹ tion methods, lack of proper land levelling and grading, over-irrigation and untimely application of water. It is important to properly prepare the land for receiving water. Adoption of suitable methods of irrigation to'gether with appropriate size of plots in concert with the available stream size and water intake properties of soil, is essential for achieving high-water application efficiency. Newer methods of water application, such as drip irrigation have a greater control on rates and amounts of application. But, before they are recommended it is necessary to assess their benefits in terms of water saving and increased water-use efficiency vis-a-vis their cost of installation and operation for various situations of water availability, crops to be grown and other environmental factors. Sufficient local data are not available to permit genuine comparisons of these methods with other common methods of irrigation.

Adverse Effects of Irrigation Seepage from main and branch canals, distributories, water courses and field channels on the one hand and the deep drainage losses from the bare and cropped fields from heavy rains and over-irrigation on the other add to the groundwater and cause rise of water-table. If unchecked the water-table may rise close to the surface and cause waterlogging of soil. If the rising watertable encounters a salty layer in the soil, the salts may get dissolved in water and carried to the surface and render the soil saline. Location of water-table at shallow depths reduces the soil volume available for rooting and causes aeration problems for the respiration of roots.

To check the harmful effects water-table should not be allowed to rise beyond a threshold depth determined by soil type, crop and quality of groundwater. A good-quality groundwater located just below the root-zone may partly meet the water requirements of the crop and may be a boon. But shallow water-table of poor quality may increase salt concentration in the root zone and may adversely affect plant growth. Rise of water-table beyond the threshold depth can be prevented by providing requisite sub-surface drainage. Alternatively, in areas with good-quality groundwater, radial drainage through shallow-pumps and recycling the water for irrigation can be used with advantage to keep down the water-tables and stretch the irrigation supplies. Such a system is being used successfully in many districts in northern

India.

L

INTRODUCTION

5

Another aspect associated with injudicious irrigation is leaching of the mobile nutrients like nitrate below the root zone of crops which decreases the nutrient-use efficiency by crops. Our understanding about the displacement of nutrients as a function of applied water has increased considerably in the recent years and it is now possible to regulate irrigation and fertilizer applications so as to minimize leaching of nutrients beyond the root zone.

As is evident from the preceding sections, development and utilization of water resources is a vast subject and need for efficient management of water exists at every step in the chain. The present book, however, deals mainly with on-farm management of water, viz. after the water is delivered to the farmer.

This has been the most neglected aspect and there is a large gap between what is being accomplished at present and what could be achieved by following improved techniques of water management. Aspects related with the efficiency of on-farm use of water, viz. methods of water application, criteria for optimum irrigation scheduling for important crops and cultural practices conducive to increased water-use efficiency, are dealt with in greater details here.

REFERENCES Bos, M.G. and Nugteren, J. 1974. On irrigation efficiencies. Int. Inst, for Land Reclamation and Improvement. The Netherlands Pubis 19: 89.

National Commission on Agriculture 1976. Reports. Part II, V, VI and X. Swaminathan, M.S. 1982. Resources, planningÿand management for 2000 A.D. Souvenir I2th Int. Congr. Soil Sci. New Delhi, 8-16 Feb. 1982, pp 109-1 19.

CHAPTER 2

SOIL AS A WATER RESERVOIR RAIN or irrigation water is not used directly by plants. It has first to be converted to soil water and stored in the soil before it is utilized by the plants.

Therefore the properties of soil in respect of infiltration and retention of water are of great significance in the context of plant growth. Plant water deficits are likely to develop sooner, after rain or irrigation on soils with low waterretention capacity. The basic aspects of soil that determine the water storage properties of soil are discussed here.

—

Soil A Three-phase System Soil is a three-dimensional body and a three-phase system which supports plants by providing anchorage for their roots and supply of water and nutrients.

Medium of Plant Growth. Soil Is the three-dimensional natural and dynamic medium for the growth of terrestrial plants. It occurs as a crust over the earth surface except in the regions where bare rocks are exposed and which remain frozen throughout the year. It is the product of physical, chemical and biological weathering of rocks under the influence of climate, vegetation and topography.

Soil is biologically active and is a habitat for various micro-organisms. A vertical section of soil is called soil profile and shows different horizontal layers, called soil horizons. The influence of the natural soil-forming factors, viz. parent material, organisms, climate, topography and time and man’s interference, is reflected in the characteristics of soil profile. To assess soil productivity it is essential to take into consideration the whole profile rather than a shallow surface layer only. Although the cultivation of soil and application of fertilizers, manures and amendments is generally restricted to the surface layers, the plant roots go much deeper and obtain their water and nutrient requirements from sub-surface layers. Water storage, nutrientretaining properties and amiability to root growth of the sub-surface layers are significant in determining the production potential of soil and its response to crop, soil and water management practices.

Three Phases. In most of the cases soil has three component phases, viz. solid, liquid and gaseous. Exception to this may, however, be found in permanently water-logged soils or dry sands of the deserts where air and liquid phases are excluded respectively. The solid phase consists of mineral and organic components. In most soils, the mineral component constitutes the largest fraction. It exists in the form of particles of various sizes which provide the soil matrix and encompass certain amount of pore space.

SOIL AS A WATER RESERVOIR

7

The amount and configuration of the pore space depend upon the size, distribution and arrangement of the soil particles and are important parameters of soil physical condition. The organic fraction comprises only 0.2-5% by weight and includes plant and animal remains at various stages of decay. Although its content is small, it plays an important role in the physical and chemical properties of soil. Mineralization of soil organic matter releases certain plant nutrients and certain aggregation-promoting substances. It is also the mainstay of microbial activity.

The liquid phase of soil is mainly constituted by water which invariably contains some dissolved salts and nutrients. Although it seems more approÂŹ priate to call it soil solution we shall be referring to the liquid phase as soil water. The gaseous phase of soil comprises soil air which differs from atmosÂŹ pheric air in composition. Because of the respiration of soil micro-organisms and roots which consume oxygen and liberate carbon dioxide, soil air is poorer in oxygen content and richer in carbon dioxide than atmospheric air.

Also the soil air has a higher water vapour content than the atmospheric air.

Because of the differences in the partial pressure of oxygen and carbon dioxide in the atmospheric and soil air, they move into and out of the soil by diffusion. This geseous exchange is highly desirable for the renewal of soil air to maintain adequate supplies of oxygen for the respiration of plant roots and soil micro-organisms.

The pore space enclosed by the solid matrix is shared between soil water and soil air. As the amount of one increases, that of the other decreases. In order to serve as a favourable medium for plant growth, or more precisely root growth, soil must contain the three components in right proportions.

The relative proportion of the three component phases in a given volume of soil determines its compactness, wetness and aeration status.

Soil Texture. As already stated the mineral component, which constiÂŹ tutes the largest fraction of solid phase, exists as particles of various sizes.

According to the size, these particles are grouped into gravel, coarse and fine sand, silt and clay. Soil texture relates to the relative proportion of various size-groups of particles (soil separates) in a given soil. These separates (sand, silt and clay) are defined in terms of diameter of the particles (non-spherical particles are considered to have equivalent diameter somewhere between their maximum and minimum dimensions). Depending upon the system followed , sand fraction is further sub-divided into different groups. Size limits of particles for the two most widely adopted size-classification systems are given in Table 1. The percentages of coarse sand, fine sand, silt and clay are determined by mechanical analysis (Day, 1965).

Particles larger than 2 mm in diameter, termed as gravel, are excluded while determining soil texture. Although stones and gravel may influence the use and management of land because of tillage difficulties, they do not make any contribution to important properties such as cation-exchange capacity and water-holding capacity.

8

IRRIGATION OF FIELD CROPS

Table 1. Size limits of soil separates according to two systems Fraction

Soil separates

Sand

Very coarse sand Coarse sand Medium sand Fine sand Very fine sand Silt Clay

Silt Clay

Diameter limits (mm) USDA ISSS

2.0-1.0 1.0-0.5 0.5-0.25 0.25-0.10 0.10-0.05 0.05-0.002 <0.002

2.0-0.2 0.2-0.02 0.02-0.002

<0.002

Before attempting the textural classification on the basis of relative proportions of soil separates, it would be worthwhile to look into the properties of these separates.

SAND. Sand particles may be rounded or irregular in shape depending upon the abrasion they have received during their transport. These are inert particles and lack surface activity. Unless coated by finer particles of silt and clay the sand particles have no plasticity and stickiness. When dry they are loose and flow freely under the influence of pressure and gravity. They have a gritty feel. Owing to their large size they enclose large pore spaces which permit rapid movement of air and water through them. Soils dominated by sand have low water-holding and nutrient-supplying capacities, have open structure and good drainage. Such soils are easy 'to work and are generally referred to as ‘light soils’.

SILT. Powdery when dry and soapy when wet, silt is intermediate between sand and clay in respect of surface activity. It does not exhibit surface activity like clay. Silt is non-plastic and lacks cohesion. Soils rich in silt are usually more liable to crust on drying after wetting. Because of its finer size the silt can retain more water than sand but its nutrient-supplying capacity is low.

Both sand and silt act as skeletion of the soil. They usually contain primary minerals such as quartz, feldspar and mica.

CLAY. Clay particles constitute the most active fraction of soil since their fine state of sub-division provides large surface area for physical and electrochemical phenomena. Unlike sand and silt, clay contains secondary minerals derived from the weathering of primary minerals. These minerals are crystalline in nature and two basic units, namely silica and alumina, are identified which determine the structure of different clay minerals. In silica unit one silicon cation is surrounded by four oxygen anions. In alumina unit six oxygen or hydroxyl (OH) surround one aluminium cation. These units extend in lateral direction to form continuous sheets. In the clay minerals silica and alumina sheets are held together by mutual sharing of ions. These sheets have some unsatisfied negative electrical charges at their broken ends.

In addition to this some isomorphic replacement of ions imparts additional

9

SOIL AS A WATER RESERVOIR

negative charge to the units. Because of these unsatisfied negative charges cations are attracted towards the surface of clay minerals. Similarly, the dipolar water molecules also get adsorbed on the clay surface with the positive ends towards the negatively charged clay surface. The capacity to absorb water and attract exchangeable cations depends upon the type and amount of clay mineral. Three types of clay minerals are most common, viz. (i) kaolinite (with 1:1 lattice), (ii) illite, vermiculite, smectite (with 2:1 lattice), and (in) chlorite (with 2:1 lattice).

Of these clay minerals, smectite and vermiculite have the highest cationexchange capacity and kaolinite the lowest whereas illite and chlorite are intermediate. Smectite also exhibits considerable swelling on wetting and shrinkage on drying. Clay particles are plastic and sticky and because of fine pores retain large amounts of water. Soils rich in clay are slowly permeable to water, have poor internal drainage and are hard to work. Because of their high draught requirement they are known as ‘heavy soils’. When wet they are sticky and when dry they get hard and form clods on tillage.

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10

IRRIGATION OF FIELD CROPS

TEXTURAL CLASSIFICATION. The percentages of sand, silt and clay determined in the laboratory are utilized to determine the textural class using the triangular diagram (Fig. 2.1). The list below gives all possible textural classes.

Sandy soils

Loam soils

Clayey soils

Gravelly sands Sands Loamy sands

Stony-sandy loams Gravelly-sandy loams Loams, gravelly loams, and stony loams Silt loams and stonysilt loams Silty-clay loams Clay loams and stonyclay loams

Stony clays Gravelly clays Sandy clays Silty clays Clays

Textural class can also be determind by feel of the soil by hand. The plasticity and stickiness of the soil is governed by nature and amount of clay content.

Soils rich in clay get hard when dry, but plastic and sticky when wet. Sand particles impart a gritty feel to the soil while silt particles make its feel soapy.

Expert technicians evaluate the textural class by kneading the soil to form a thin ribbon. Soils rich in clay can be rolled into long thin and smooth ribbons while others in which sand and silt dominate fail to form such ribbons.

Soil Structure. All particles of sand, silt and clay do not exist as individual particles in natural soils. Variable fractions of them exist as clusters called aggregates. Because of certain cementing forces the aggregates behave as separate entities and are called ‘secondary particles’ in contrast to individual mechanical separates called ‘primary particles’. The term soil structure refers to the arrangement of primary and secondary particles in a soil mass. Indirectly, it refers to the pore spaces enclosed by such an arrangement of particles. Pores enclosed by structural units, viz. the inter-aggregate pore spaces comprise structural porosity . while the intra-aggregate porosity is called the textural porosity.

Inter-aggregate pores are comparatively large and permit rapid movement of water and air while the smaller pores help retain water against gravity. In an ideal growing medium porosity should be shared almost equally between small (micro) and large (macro) pores. Depending upon the forces binding the particles into an aggregate, the latter’s behaviour towards mechanical manipulation and wetting by rain or irrigation would differ. If aggregates are not water stable they slake and disperse on wetting. The presence of water stable aggregates in the top-soil helps maintain high rates of

water infiltration.

The state of aggregation of soil is affected by its contents of organic matter and the oxides of iron and aluminium, nature and amount of clay and the nature of cations on the exchange complex. Freezing and thawing in the cold climates, alternate wetting and drying, root action and mechanical operations

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