Handbook of irrigation systems[1]
Collection "Technical Manuals" Manual directed by ISF with the support of the Directorate General for International Cooperation (DCIG)
Pierre-Emile Van Laere
Ingénieurs Assistance Internationale - Ingénieurs sans Frontières 2003 http://www.isf-iai.be mail@isf-iai.be Avenue du Marly, 48, 1120 Brussels - Belgium
We thank thank all the people without whom this document could have not seen the light of day; in particular Jerome Bindels and Emmanuel Grosjean
Introduction
Setting up an irrigation system needs to be done by holding on to certain guidelines. A series of elements need to be taken into account, from the project design process to long-term management of facilities. This handbook aims to describe the different steps to take. This handbook mentions but the basics, and is not inted to extensively answer all questions that might arise. If more precise information is necessary, you should consult this handbook's bibliography references.
When setting up an irrigation system, the logical steps to follow are:
- Evaluating the water requirements of the crops to be cultivated;
- Determination of dosage and frequency of watering;
- Selection, dimensioning and budget of the irrigation system.
A computerprogram to help with irrigation management has been developed by FAO. This software allows the calculation of waterrequirements and quantities of irrigation water necessary crops. It also offers the possibility of developing an irrigation schedule according to various farming practices, assess the effects of too little watering of the crops and the effectiveness of different practices irrigation. The CROPWAT software is freely available on the FAO website at following URL-adress: http://www.fao.org/ag/agl/aglw/cropwat.htm. Two versions exist. The first, which is very compact, runs under a DOS-environment and only takes the size of a single 3,5 inch floppy disk. The second, which is more user-friendly, works under the Windows operating system. This latter version asks nonetheless more memory. The two versions are respectively called CROPWAT 7.0 and CROPWAT for Windows. Both versions use the same methods of calculation and are simple to use. Their main advantage to the user is to avoid the manipulation of formulas, which are often difficult to use.
Evaluation of waterrequirements of crops to be cultivated
Firstly, determining the waterrequirements of a crop requires knowledge of various parameters on both the plant itself and the climate or soil of the region.
- Climate data provide the necessary information concerning water needs of the crop;
- Pedological parameters will estimate the usable water reserve available in the soil;
- The cultivation of data will specify the water reserve in the soil, usable by the crop.
With the help of the obtained results, it will be relatively easy to determine the quantities of irrigation water required for the proper development of the crop. These will be calculated using the CROPWAT-computersoftware.
Calculation of evapotranspiration
The water deficit, which we may refer to as waterrequirements (W), is defined as the difference between actual evapotranspiration (AET) of the crop and the effective precipitation (EP).
The actual evapotranspiration is calculated by multiplying standard evapotranspiration by a cultural coefficient.
ETo represents standard evapotranspiration, defined by Penman (1956) as the amount of water transpirated per unit of time by a short and green vegetation, which completely covers the soil, is of a uniform height and never has too little water. It is calculated by the Penman-Monteith formula and from regional climate data.
Cc is the cultural coefficient, function of the type of crop and its vegetative state.
Climate data (in monthly averages) to determine the evapotranspiration are listed below:
- Ta: average temperatures, expressed in degrees C.
- Ha: air humidity average, expressed in %.
- Vm: average wind speeds, expressed in m/s.
- Pa: air pressure, expressed in kPa.
- P: Precipitation expressed in mm.
- N: number of days of precipitation per month
- Expos. Duration of solar exposure, in hours.
- Eto: evapotranspiration calculated by reference method Penmann-Montheih expressed in mm/day.
The effective rain, Reff, represents the fraction of precipitation that is effectively used by the crop after deducting the losses by surface runoff and deep percolation. The choise of the most appropriate method for calculating the effective precipitation demands serious consideration. Several different methods have also been developed, each taking into account the climate of the region where the measurements must be made. CROPWAT proposes 4 methods:
- The first option proposes a fixed percentage: Peff = A * Pave in which A is a fraction given by the user. Generally, it lies between 0.7 and 0.9.
- The second formula was developed from data from arid and semi-arid areas:
- The third is an empirical formula developed locally. The coefficients used are determined by analyzing local climate data
The fourth option was developed by the U.S. Department of Agriculture (USDA): for Pave <250 mm / month for Pave> 250 mm / month
The waterrequirements (B) will be calculated for each crop using the CROPWAT-software by introducing the climate data and crop specifics. The waterrequirements are expressed in m³/ha.
Crop data
The CROPWAT-software contains a file containing the specific characteristics of many crops. These data are:
- The crop coefficient, Cc, is used to calculate the actual evapotranspiration of the crop. This depends on the crop itself and its vegetative state;
- The allowed dehydration represents the critical level of soil humidity on which stress due to water shortage is being felt by the crop, affecting the evapotranspiration and crop production. The values are expressed as a fraction of the total moisture available in the soil;
- The coefficient of efficiency response, Ke, to estimate performance reductions due to stress caused by water unbalance.
Estimation of available usable waterreserves
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- UW is the height of the usable water available in the soil (mm / m). UW is the difference between water content to field capacity (θFC) and water content in point wilting (θWPW).
- Zr (m), the maximum rooting depth, determined for mature crops and grown in deep soil.
- RU (mm) is water available to crops in the volume of soil reachable by their roots.
- The readily available reserve (RRA) is the amount of water a plant can extract from the soil without its production being noticably affected. It is defined by the introduction of an empirical coefficient, f. This coefficient represents the potential risk of subjecting a crop water stress and is a charisteristic of the crop. It is generally accepted to provide a value of 2 or 3.
Estimated irrigation dosage and frequency by plot
When the waterrequirements of crops throughout their growth phase are known, the irrigation dosage per plot needs to be determined. To do this, it is necessary to know their pedological data. These will determine the storage capacity of water in the soil and hence determine the amount of irrigation that needs to be applied at a frequency defined by the farmer in order to cover the waterrequirements of the crops.
The parameters needed are:
- Soil type;
- The usable water content (UW);
- The rooting depth (Zr);
- The maximum speed of water infiltration into the soil (KSAT);
- The initial dry soil (initial q).
Qty Total (m³) = B (m³/ha) * Surface of irrigated plot (ha) The supplementary irrigation is distinguished from continious irrigation by the fact that it consists of supplying a small amount of water to crops to cope with insufficient precipitation in order to stabilize yields. It could not alone allow crops to mature, but it complements precipitation and classical irrigation. The effect of supplemental irrigation is greatest when it is practiced at a critical stage of crop development (flowering, maturation, etc.)..
Irrigation Systems
Irrigation systems are divided into 2 categories: gravity-fed systems and pressurised systems:
Gravity systems
- Irrigation pond (best known): Water is provided in the form of a tablecloth in a basin (which can be partitioned) built on a leveled ground (slope of 0.1 to 1%);
- Irrigation skate: water is made by runoff in separate paths from a distance of 0.6 m to 1.25 m; soil is leveled (slope of 0.2 to 3%);
- Irrigation siphon or bordered ramps : water is beamed down by siphons or railed ramps to allow a reduction of head erosion, better flowcontrol and consistency of waterdistribution.
Pressurised systems
- Sprinkler irrigation: distributing the water as rain with regulation and uniformity of the dosage given; only possible on the condition that the area does not suffer under wind with speeds over 4 m/s; systems sprinkler irrigation are either fixed or mobile;
- Localized irrigation: water circulates in flexbible, small diameter pipes, arranged on the surface and fitted with emitting devices providing water at the plant's foot; the most prevailent localised irrigation systems are drip-irrigation (targeted at domestic audience) and micro-jet (targeted at sylviculture-market).
Pressurized irrigation systems create, on average, a water savings of 30 to 60% compared to gravity-fed systems. Localized irrigation systems, in turn, can lead to water saving up to 50% compared to the sprinkler systems (limit maximum evaporation and percolation because water is delivered in a unhumidified, low dosage on a fraction of the soil). Localized irrigation systems also have following advantages:
- Preventing the development of weeds and the possibility of fertilisation. They are however unsuitable if crops are deep-rooted and if the water is too rich (with sand, silt, organic matter, iron, ... which can clog the pipes) or too salty (no leaching).
- Spray irrigation is recommended in case of low soil depth, light and permeable soil, where terrain is too uneven and when salt water is used.
Care and maintenance of irrigation systems
Proper maintenance of irrigation systems is essential if we want to maintain the potential for saving water and avoid waste. Among the main problems concerning the maintenance of the irrigation system, we remember:
- Defective pressure regulators or flow limiters;
- Leaks in the water pipes;
- Defective canals and borders.
These elements lead to a reduction in the life expectancy of the equipment, deregulation of uniformity of the spatial distribution of water, overconsumption of water, problems of water supply (which may penalize operators located at the end of the supply system) and userconflicts. To avoid these problems, we must:
- Maintain the irrigation system: replace joints; damaged concrete and/or sealing cracks; replacement and/or cleaning filters and gratings; clean and unblock or flush;
- Maintain the elevations/borders;
- Maintain storage openings (cleaning and flushing);
- Monitoring of water quality;
- Continuously observe the state of infrastructure and equipment;
- Plan the operations;
- Budget the cost of operations;
The cost of not budgeting and planning routine maintenance infrastructure and systems generally impedes the development of these techniques.
Management Structure
Finally, the establishment of a irrigation infrastructure is essential to the establishment of a management structure, a representative body of all operators who manage the day-to-day functioning of the perimeter irrigation.
The establishment of such a structure with well-defined tasks allows to regulate conflict of users, the struggle between individual interests and the collective charisteristics of certain infrastructures, the monitoring strategies, and the maintenance of equipment.
Bibliography
- Allen R. G., Luis S. Pereira LS, Raes D., M. Smith (1998). Crop evapotranspiration
- Guidelines for computing crop water requirements. Rome, Italy, FAO. Available on the Internet, accessed 5 May 2002: http://www.fao.org/docrep/X0490E/x0490e00.htm
- CIRAD (2002), Mémento de l'agronome. Montpellier, France : CIRAD
- Grosjean E. (2002). Manuel de bonnes pratiques agricoles pour la région de Meknès-Tafilalet (Maroc).
- National School of Agriculture of Meknes - FUSAGx.
- Wallonie-Bruxelles/Maroc cooperation. Draft.
- Guidelines for designing and evaluating surface irrigation systems. Rome, Italy, FAO. Available on the Internet, accessed 5 May 2002: http://www.fao.org/docrep/T0231E/t0231e00.htm
Appendixes
Appendix 1: Questionnaire for identifying irrigation projects projects for crops
In the case of the existence of several plots and/or different crops, please use a single questionnaire for each plot and each crop. The requested information is highly technical. The requested climatological data can be obtained from the appropriate authority in your area. If this is not the case, please contact the Faculty of Agronomy of the your nearest university. For the pedological data, it is highly advisable to conduct a soil test in order to obtain information that is as accurate as possible. These studies can be ordered from universities or specialized firms.
I. Precise location of the place (Country, department, district, commune, ...) .......................................................................................................................................................................................................................................................................................... .......................................................................................................................................................................................................................................................................................... It is necessary to consult a topographical map of the region, on which the cultivated plot and water sources used (rivers, wells, water supply, groundwater tables, ...) are indicated.
II. Crop information
- Name of the crop:
.......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
- Size of the plot occupied by the crop (ha):
.......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
- Date when the culture commenced (months of the year):
.......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
III. Soil data: - Type of soil (sandy, clay, loam, sandy loam, ...): .......................................................................................................................................................................................................................................................................................... .......................................................................................................................................................................................................................................................................................... Indicate the soil's make up in percentage, if this data is available: % Clay: ......................................................................................................................................................................................................................................................................................... .......................................................................................................................................................................................................................................................................................... % Loam: .......................................................................................................................................................................................................................................................................................... .......................................................................................................................................................................................................................................................................................... % Sand: .......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
- Speed of water percolation into the soil:
KSAT (mm/j): .......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
- Initial soil moisture when planting:
θinitiale (cm³/cm³): .......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
- Presence of a water table: yes / no
If yes, what is its range: Maximum depth: ...........cm; minimum depth: ...........cm.
- What is the curve pf of the soil?
Otherwise, can you give capacity of the field? The wilting point?
V. Hydrological and climatographical situation of the region. Average monthly data, followed for one year to is required to determine the Crop Evapotranspiration (CET):
- Ta: average temperatures, expressed in degrees C.
- HRa: air humidity average, expressed in %.
- Wa: average wind speeds, expressed in m / s.
- Pa: air pressure, expressed in kPa.
- P: Precipitation expressed in mm.
- N: number of days of precipitation per month
- Insol. Duration of solar exposure, expressed in hours.
- ETo: reference evapotranspiration calculated by the method Penmann-Montheih expressed in mm/day.
If obtaining the required data to complete the picture above is too complicated, it is possible to ask the local authorities or the FAO for the data on the potential evapotranspiration (ETP):
Information needed for dimensionning of irrigation systems VI. It is essential to have accurate information about water supplies available for irrigation, including the waterflow of wells, the water movement of the closest rivers (see Section I). .......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................
VII. Choice of irrigation system (gravity-fed, sprinkler, drip irrigation, Other) Defined by the farmer: .......................................................................................................................................................................................................................................................................................... .......................................................................................................................................................................................................................................................................................... Still to be determined
VIII. Optional remarks: .......................................................................................................................................................................................................................................................................................... ..........................................................................................................................................................................................................................................................................................