Ebooks

The second edition of this book is renamed as Irrigation & Drainage Engineering, looking after the interest of the students and other users good numbers of numerical analysis particularly the GATE questions of Agricultural Engineering, a new chapter by the name Groundwater Hydraulics & Wells and the topic Grassed Waterways have been included. Hopefully, these will help to get the book more useful.

1.1 Introduction Irrigation is the artificial application of water needed to the crop field. It is the process of uniformly wetting the root zone depth. The wetting may be done over the land surface or below of it. The application of water in excess to the requirement or accumulation of water in crop field due to rain or unplanned seepage is not considered as irrigation. However, irrigation is supplemental to rainfall. Irrigation water is considered as single most important input for crop production.

Measurement of water used in irrigation ensures the application of required amount of water in crops. Knowing the rate of flow the adjustment to period of control at gates, etc. can be done to regulate the water at different branches of canals. To get the total control over the supply of water there should have the provision of water measurement at the critical points in irrigation systems. Units of Measurement Water is measured at rest or in motion. The water of reservoir, ponds, tanks, etc. at rest is measured in volumetric unit as liter, cubic meter, hectare-meter and hectare-centimeter. Water flowing in rivers, canals, channel, etc. is measured in litre/sec , liter per hour, meter per sec and hectare-meter per hour or day.

3.1 Introduction Irrigation efficiency evaluates how effectively the water has been distributed for the purpose of crop production. Water is conveyed to crop field through canal water courses and channels. During this process considerable amounts of water get lost through run off, deep percolation, evaporation, etc. The losses of water greatly depend on the skill of the irrigator, degree of land preparation, and design and planning of the system. The extent of water losses starting to diversion of water from its source to crop field though conveyance and ultimate application and distribution of water in crop field which can be estimated separately. The estimation of these efficiencies enables one to identify the step(s) where improvement is required. The method of measurement and evaluation for its efficiency is prerequisite for proper use of water. Irrigation efficiency in developing countries in general is poor particularly in gravity irrigation methods. However, water is become more and more scarce. Due to pressing need of it judicious and efficient management is one of the vital issues to be solved on priority basis.

Scheduling irrigation is the process of determining when to irrigate and how much to irrigate. Selecting the appropriate time span in between two irrigation and applying the required volume of water in each irrigation, ensures saving of water, minimize energy use, higher crop yield and lower production costs. Irrigation applied earlier than due date causes loss of water through evaporation or deep percolation and later causes water stress to plants and thereby affects the growth and yield of the plants. Scheduling of irrigation may be required in two field conditions: (i) when available of water is adequate, (ii) when availability of water is limited. In the places where water is adequate scheduling of irrigation follows full   irrigation with time sequence of minimum number of irrigation to ensure maximum yield and high water use efficiency. When water is limited its use is rationalized by using at the sensitive stage

5.1 Irrigation Channels Canal or channel is an artificially made conduit through which water is supplied to the field from river, tank, reservoir or wells. The open channel is the most common type of water conveyance system. If the pipes and drains are partially full behave like open channel. The irrigation channel may be lined or unlined and the flow is either uniform or varied. The canal may be classified in following ways. a) Classification based on the source of supply i) Permanent or perennial canal ii) Inundation canal A canal is said to be permanent when it is fed by a permanent source of supply and has permanent networks of distribution of supplies. Inundation canals draw their supplies from river usually when water stage exceeds a predetermined level. Headworks are not provided for diversion of river water to canal. However, canal head regulators are provided to regulate the canal head as required time to time to accommodate the changing pattern of river course.

The plants collect water by its root system available in the soil medium. A soil rich in nutrients cannot ensure success of crops unless the presence of satisfactory state of water and air, mechanical attributes and thermal regime. The physical properties of soil and plants greatly influence the movement, retention and use of water. The soil should provide sufficient pore spaces and size distribution and decomposition of rocks and for the movement and retention of water for plant needs. Rainwater, irrigation, groundwater, seepage, etc., contribute in replenishing the water in dried up soils. In all the cases thorough understanding of behavior of water is required to ensure the need of water to plants and optimize the use of irrigation water. 6.1 Soils Soil is initially formed by the disintegration and decomposition of rocks and minerals and accumulation of biological residues. Usually the outer layer of the land surface is referred

7.1 General The major portion of water in a crop field gets lost through evaporation and transpiration. Evaporation is the process by which water is changed from the liquid state to vapor state. It takes place from the adjacent soil, water surface or from the surfaces of leaves of the plants. Transpiration is the removal of water vapour through plant body. Evapotranspiration(ET) is the sum of evaporation and transpiration. The process of evaporation and transpiration goes on simultaneously and difficult to separate them in a crop field. The term consumptive use is used to refer evapotranspiration together with the water used in the metabolic activities of the crop plants. The actual amount of water used in metabolic activities is insignificant (less than 1% to ET). Therefore, the consumptive use and evapotranspiration are often used as synonymous term. It includes the water consumed by the plants

Hydraulics of wells means the laws that are applicable to understand the flow of water through the formation and certain aquifer properties that determine the discharge of a well. Darcy’s law is used in studying groundwater flow though Darcy’s law is not valid under certain cases. 8.1  Water Bearing Formation The earth’s crust is called the lithosphere which consists of hard rocks and disintegrated rock materials. The outer parts of the earth’s crust are porous and containing partially and fully filled up with water. The surface strata of the earth is partially filled with water and is called as zone of aeration and the zone below this which is completely filled with water is called the zone of saturation. The zone of aeration is divided into (i) the soil water zone (ii) the intermediate zone and (iii) the capillary fringe (Fig.8.1).

9.1 Introduction Water is a precious commodity. Necessity of conservation of water supply is increasingly felt all over the world as the demand of it continues to increase when the sources of supply are increasingly scarce. The conveyance and proportional distribution of water is one of the most important part of any irrigation project. Water available in the stream, reservoirs or wells is carried to the field of use through canals. The economy of any irrigation project largely depends on how efficiently the water is being transported and distributed in respect to water losses and cost. Water losses in irrigation channels are mainly due to evaporation from the water surface and seepage from the bed and side of the channels. Seepage losses some time become the single important factor which causes unavailability of water particularly in downstream of the channel in canal irrigation system and excess pumping of water in tube wells and river lift irrigation system.

Poor quality of water is one of the important factors causes gradual accumulation of salts on the ground or in the root zone of crops leads to saline and alkaline condition. This results in loss of permeability, toxicity and anaerobic conditions. In salt affected soils plants face constraint to derive their nutrients at the required rate due to high soil moisture tension, which affects adversely the crop growth and productivity. 10.1 Causes of Salt Problems The following are the reasons for causing the soils become saline or alkaline. 1. Rise in water table: A rise in water table and high evaporation and transpiration rates with poor drainage lead to build up dissolved salts. 2. Quality of irrigation water: Irrigation water contains the dissolved salts exceeding certain limit cause accumulation of salts in soil profile.

10.1 Land and Water Resource India has got the world’s 2.45% area and 16.87% of population with only the 4% of available fresh water (Table 11.1). This indicates the necessity of development of water resources and optimal use of it. India at macro level does not have short of water. But the shortage of water is due if not properly managed by conservation and sector wise utilization. The water resource potential of the country has been assessed by different agencies in different time (Table 11.1). It is observed that the latest estimate of 1869bcm (1993) are within the little variation of the estimates since 1954. The utilizable water resources of the country are assessed 1123bcm by CWC (1993) of which 690bcm from surface water and 433bcm from ground water sources. This 690bcm can be harnessed if matching storage structures are built. The surface water utilizable potential may increase by 220bcm if inter-basin transfer

12.1 Introduction Agricultural drainage is the removal of excess water from the farmland for the purpose of providing favorable soil air environment for plant growth. Removal of water may take place form the surface or below the surface of the land. The practice of lowering ground water table below the root zone depth to improve the plant growth or to reduce the accumulation of salts in soil is also called as drainage Drainage system is broadly classified as surface & subsurface drainage. Removal of excess water over the land surface is known as surface drainage & that of removing water below the surface is known as subsurface drainage. The flow of water in the form of evaporation or evapotranspiration also play important role in improving drainage situation. It gives the idea of the time required to get the wetland workable though it is not called a type of drainage as because this is very slow process & cannot be controlled by the human activities. However, using of certain tree plants e.g., Eucalyptus, Poplar, Casurina or similar species in wet fields, which transpires at high rate and improves the drainage problem by lowering the shallow water table is the new idea known as bio-drainage.

The purpose of a sub-surface drainage system is to lower the ground water table and removal of salts beyond the crop root zone. The ground water beyond table may rise due to commonly practiced flood irrigation method, seepage from unlined channels, percolation of rain and runoff water and sub-surface flow from higher areas. Sub-surface drainage consists of drains laid at a specified depth, spacing and grade below the ground surface. The size, spacing and depth of drains depend largely on the soil-water properties viz., infiltration, saturated hydraulic conductivity, drainable porosity; and the requirement of removal of water through drains in certain time period fixed on the basis of rainfall and crop cultivated. The drains are usually placed in a pervious stratum. Suitable filter of coarse sand and gravel may be used when it is placed in a low permeable stratum. Sub-surface drainage serves the important job of removal of concentrated salt solution from the crop root zone particularly in arid and semiarid region. Advantage of sub-surface drainage system 1. Since sub-surface drainage are underground, no loss of land for cultivation. Some part of the land goes out of use for making drains in surface drainage.

A Actual crop evapotranspiration 270, 334 Actual vapor pressure 281, 282, 283, 335 Advantage of sub-surface drainage system 547 Advantages of lining 395 Aeolian soils 193 Albedo 279, 284 Alkali or sodic soil 434 Alkaline soil 243, 465 Alkalinity hazard 465 Alkalisation 435 Alluvial soils 193