The application of climatic data for planning and management of
sustainable rainfed and irrigated crop production
Martin Smith
∗Land and Water Development Division, Food and Agriculture Organization (FAO), Viale Terme di Caracalla, 00100 Rome, Italy
Abstract
Sustainable food production will depend on the judicious use of water resources as fresh water for human consumption and agriculture is becoming increasingly scarce. To meet future food demands and growing competition for clean water, a more effective use of water in both irrigated and rainfed agriculture will be essential. Options to increase water use efficiency include the conservation of rainfall, the reduction of irrigation water losses and the adoption of cultural practices that will increase production per unit of water.
Water use for crop production is depending on the interaction of climatic parameters that determine crop evapotranspiration and water supply from rain. The compilation, processing and analysis of meteorological information for crop water use and crop production will therefore constitute a key element in developing strategies to optimize the use of water for crop production and to introduce effective water management practices.
In the 1970s, FAO developed practical procedures to estimate crop water requirements and yield response to water stress which have become widely accepted standards in the planning and management of irrigated and rainfed agriculture. As a follow-up to recommendations of a panel of high-level experts convened in 1990, further studies have been carried out which have led to the development of revised procedures for reference evapotranspiration and crop evapotranspiration. The new procedures and guidelines have been recently published in the FAO Irrigation and Drainage series and include the adoption of the Penman–Monteith approach as the new standard for determining reference crop evapotranspiration (ETo) calculations.
Procedures have been developed to use the method also in conditions when no or limited data on humidity, radiation and wind speed are available.
Procedures for estimating crop evapotranspiration are revised with an update of the crop coefficients that allow more accurate estimates for a wide range of crops and for various crop, soil and water management practices. Daily ETocalculations are
included by separating soil evaporation and crop transpiration estimates through the dual crop coefficient.
The use of climatic data for the development of practical criteria in planning and management of irrigated and rainfed crop production is demonstrated at the hand of some examples using the FAO computer programmes and climatic database.
Agrometeorology needs to play a key role in the looming global water crisis. Appropriate strategies and policies need to be defined, including strengthening of national capacities in the use of climatic data for planning and management of sustainable agriculture and drought mitigation. Cooperation between FAO and WMO in this field may serve as an example of such efforts. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Crop evapotranspiration methods; Water use efficiency
∗Tel.:+39-06-57053818; fax:+39-06-57053818. E-mail address: martin.smith@fao.org (M. Smith)
1. Introduction
The great challenge for the coming decades will be the task of increasing food production to ensure food security for the steadily growing world population, par-ticularly in countries with limited water and land re-sources. While on a global scale water resources are still ample, serious water shortages are developing in the arid and semi-arid regions. An increasing number of countries face serious water deficiencies as exist-ing water resources are fully exploited. The depen-dency on water for future development has become a critical constraint for development. Over 30 arid and semi-arid countries are expected to be ‘water scarce’ by 2025, meaning an annual water availability of less than 1000 m3per capita annually, which will slow down de-velopment, threaten food supplies and aggravate rural poverty.
The situation is aggravated by the declining quality of water and soil resources, caused to a great extent by human activity. There is an urgent need to arrest this human-induced degradation of water and soil resources and reclaim those that have been already degraded in order to meet the present and future food requirements and other needs of the human population.
To focus attention on the growing problem of wa-ter scarcity in relation to food production, the World Food Summit of November 1996, drew attention to the importance of water as a vital resource for future de-velopment. Within the framework of the UN Agenda 21, FAO is closely cooperating with other agencies of the United Nations to address this important issue. A global initiative for concerted action is established in the Global Water Partnership where national and inter-national agencies and institutions from different sectors are working together to find solutions for a looming global water crisis.
The concepts and options that can lead to a more effective use of water for crop production are reviewed in this paper. Estimation of crop water use from cli-matic data is an essential element to achieve better wa-ter use efficiency. The new perceptions included in the revised FAO guidelines on crop evapotranspiration will contribute to this. Practical examples are presented to demonstrate in which way more effective planning and management of irrigated and rainfed agriculture can be achieved. The compilation, processing and analy-sis of agrometeorological information is a key element
in this. Further studies to obtain a better insight in the interaction between climate and water for crop pro-duction need to become a well defined priority area for agrometeorological activities.
2. Water for crop production
A major part of the developed global water resources is used for food production. The estimated minimum water requirement per capita is estimated at 1200 m3 annually of which 55 m3for domestic use and 1150 m3 for food production (FAO, 1994). In most countries 60–80% of the total volume of developed water re-sources is used for agriculture and may reach well over 80% for countries in the arid and semi-arid regions.
Rainfall contributes to an estimated 65% of global food production, while water for irrigation provides the remaining 35% on 17% of the total agricultural area. Rainfall is, in most parts of the world, for at least part of the year, insufficient to grow crops and rainfed food production is heavily affected by the annual variations in precipitation.
Irrigation is an obvious option to increase and sta-bilize crop production. Major investments have been made in irrigation over the past 30 years by diverting surface water and extracting groundwater. The irrigated areas in the world have, over a period of 30 years, in-creased by 25% with, in particular, a period of accel-erated growth during the 1970s and early 1980s (FAO, 1993). The expansion rate has slowed down substan-tially because a major part of the reliable surface wa-ters have already been developed, while groundwater resources have become over-exploited at an alarming rate.
With water resources becoming scarce, waters of in-ferior quality are increasingly used. Excessive use and poor management of such irrigation water has had, in some cases, detrimental effects on soil quality, causing whole areas to be taken out of production or requiring the construction of expensive drainage works.
2.1. Options for effective water use
Considerable potential still exists to optimize the use of water for crop production but solutions for a more efficient use of water for crop production will be dif-ferent for rainfed and irrigated agriculture.
2.2. Water use efficiency in rainfed crop production
Rainfed crop production is subject to frequent fluc-tuations in precipitation. Failing rains will result in droughts and yield deficits, while excessive rains cause flooding and crop losses. Crop water use need to be op-timized through a more effective use and conservation of rainwater and by measures to increase crop produc-tion. Traditional cropping systems and genetic charac-teristics of the local crop types are adapted to minimize drought risks and to maintain minimum production lev-els under erratic rainfall supplies. Yields and water use efficiency will remain therefore low even in periods with ample water supply or increased fertility levels.
The strategy to increase crop production under a given water supply will require the introduction of better yielding varieties combined with a more secured water supply through improved water conservation. The introduction of measures to conserve rain water or sup-plemental irrigation combined with appropriate plant nutrition and cultural practices, will lead to increased
Fig. 1. Water use efficiency of rainfed and irrigated crops.
production levels per unit of water with equal water availability. The underlying concepts to improve water use efficiency are illustrated in Fig. 1.
To plan and manage the various options for increas-ing water use efficiency of rainfed crop production agri-culture, an analysis of the climatic conditions of the region is essential. Stochastically determined variabil-ity of rainfall and evapotranspiration is required for simulation and crop forecasting on expected yield im-provements and options for water storage. Examples of practical procedures to assess crop water use and crop production levels under restricted rainfall and water supply are given in Paragraph 4.
2.3. Water use efficiency in irrigation
evapotranspi-Fig. 2. Average irrigation water losses.
ration, loss of energy height and deterioration of water quality.
Most of the water loss (40%) occurs at farm and field level with a direct effect on crop production due to inadequate water supplies causing water stress or excessive water and resulting in reduced growth and leaching of plant nutrients. Considerable scope exists for a more accurate and efficient crop water application by improved field irrigation methods and better crop water management techniques through the introduction of irrigation scheduling and water supply control.
To introduce an effective crop water supply system adequate information is required on crop water require-ments as determined by crop and weather conditions. In Paragraph 4 below an example is given how to evaluate, plan and manage irrigation supplies and field irrigation practices based on crop and weather conditions for a given irrigation management system.
3. FAO methodologies on crop water requirements
The FAO Land and Water Development Division has been instrumental in developing guidelines for the pre-diction of crop water requirements, which have been widely introduced for the design and management of irrigation systems. The methodology, published first in 1974 as FAO Irrigation and Drainage Paper No. 24 and revised in 1977 (Doorenbos and Pruitt, 1977), has become an international standard, extensively used worldwide.
Advances in research and the more accurate assess-ment of crop water use have revealed weaknesses in
the original methodologies of FAO No. 24 (Batche-lor, 1984; Allen et al., 1989; Jensen et al., 1990), and have made a review necessary. In collaboration with the International Commission for Irrigation and Drainage (ICID) and the World Meteorological Organization (WMO), a consultation of experts and researchers was organized in May 1990, in Rome, to review the method-ologies and to advise on the revision and update of pro-cedures. Based on findings of comparative studies on the performance of various ETo estimation methods, the panel of experts recommended the adoption of the Penman–Monteith method and a revised definition and calculation procedures for estimating reference evapo-transpiration (Smith et al., 1991).
The review and update of the methodologies have been recently completed and are contained in FAO Irrigation and Drainage Paper No. 56 Crop Evapotran-spiration (Allen et al., 1998). The main elements of the review and revised procedures for crop evapotranspi-ration are presented further.
3.1. Review of reference evapotranspiration methods
A range of more or less empirical methods have been developed over the last 50 years by numerous scien-tists and specialists, worldwide, to estimate evapotran-spiration of a reference crop from different climatic variables. Relationships were often subject to rigorous local calibrations and proved to have limited global validity.
uniformity in calculated parameters. To evaluate the performance of some of the main EToestimation proce-dures, under different climatological conditions, stud-ies were undertaken under the auspices of the American Society of Civil Engineers (Jensen et al., 1990) and the European Community (Choisnel et al., 1992).
The studies demonstrated the superior performance of the Penman–Monteith approach, in both arid and humid climates, and convincingly confirmed the sound underlying concepts of the method. Based on these findings, the method was recommended by the FAO Panel of Experts, convened in 1990, to be adopted as a new standard for reference crop evapotranspiration estimates. The conclusions of the comparative studies are summarized in Table 1.
3.2. FAO Penman–Monteith equation
By introducing the aerodynamic and canopy resis-tance in the original combination method, a better
Table 1
Performance EToestimation method (Jensen et al., 1990)
Locations Humid. Arid
Performance indicator Rank No. Over-estimate (%) Standard error Rank No. Over-estimate (%) Standard error Combination methods
Penman–Monteith 1 +4 0.32 1 −1 0.49
FAO-24 Penman (c=1) 14 +29 0.93 6 +12 0.69
FAO-24 Penman (corrected) 19 +35 1.14 10 +18 1.1
FAO–PPP-17 Penman 4 +16 0.67 5 +6 0.68
Penman 1963 3 +14 0.60 7 −2 0.70
Penman 1963, VPD #3 6 +20 0.69 4 +6 0.67
1972 Kimberley Penman 8 +18 0.71 8 +6 0.73
1982 Kimberley Penman 7 +10 0.69 2 +3 0.54
Businger-van Bavel 16 +32 1.03 11 +11 1.12
Radiation methods
Priestley Taylor 5 −3 0.68 19 −27 1.89
FAO-Radiation 11 +22 0.79 3 +6 0.62
Temperature methods
Jensen-Haise 12 −18 0.84 12 −12 1.13
Hargreaves 10 +25 0.79 13 −9 1.17
Turc 2 +5 0.56 18 −26 1.88
SCS Blaney-Crddle 15 +17 1.01 15 −16 1.29
FAO Blaney-Criddle 9 +16 0.79 9 0 0.76
Thornwaite 13 −4 0.86 20 −37 2.4
Pan evaporation methods
Class A pan 20 +14 1.29 17 +21 1.54
Christiansen 18 −10 1.12 16 −6 1.41
FAO Class A 17 −5 1.09 14 +5 1.25
simulation of wind and turbulence effects and of the stomatal behavior of the crop canopy was achieved (Monteith, 1965). The earlier difficulties in the use of the method, related to the estimation of the resistance values, have been largely overcome by progress in re-search and reliable estimates of the two parameters for a range of crops, including the reference crops, grass and alfalfa.
actively growing, completely shading the ground and with adequate water.
Thus defined, the FAO Penman–Monteith equation can be further derived from standard crop parameters to the following relationship for daily reference crop evapotranspiration:
ETo=
0.4081(Rn−G)+γ (900/T +273)U2(ea−ed)
1+γ (1+0.34U2)
where ETo: reference crop evapotranspiration [mm per day]; Rn: net radiation at the crop surface [MJ m−2per day]; G: soil heat flux [MJ m−2per day]; T: average air temperature [C]; U2: wind speed measured at 2 m height [m s−1]; (e
a−ed): vapor pressure deficit [kPa];
1: slope of the vapor pressure curve [kPa C−1];γ: psy-chometric constant [kPa C−1]; 900: conversion factor. Full details of the FAO Penman–Monteith method, including procedures for determining hourly ETo val-ues and the various parameters, algorithms, recom-mended values and units, have been published in the proceedings of the consultation (Smith et al., 1991), in the ICID Bulletin Vol. 43, No. 2 (Allen et al., 1994a, b), and the new FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998).
3.3. Use of FAO Penman–Monteith with limited climatic data
The limited availability of the full range of climatic data, in particular, data on sunshine, humidity and wind data, has often been the main restriction in the use of the combination methods and resulted in the use of empir-ical methods, which require only temperature, pan or radiation data. This has contributed to the confusing use of different ETomethods and conflicting evapotranspi-ration values. To overcome this constraint and to further standardize on the use of one single method, additional studies have been undertaken to provide recommenda-tions on the use of the FAO Penman–Monteith when no humidity, radiation or wind data are available. As a result, procedures are presented to estimate humid-ity and radiation from maximum/minimum tempera-ture data and to adopt global estimates for wind speed. The availability of world wide climatic databases fur-ther facilitates the adoption of values from nearby sta-tions. Such procedures have proved to perform better than any of the alternative empirical formulas and will
largely improve transparency of calculated evapotran-spiration values (Smith et al., 1996).
3.4. Crop evapotranspiration
Procedures for estimating crop evapotranspiration have been well established in FAO Irrigation and Drainage Paper No. 24 (Doorenbos and Pruitt, 1977), using a series of recommended crop coefficient values (Kc) to determine ETcropfrom reference evapotranspi-ration (ETo), as follows:
ETc=KcETo
Although the adoption of the Penman–Monteith approach would allow a direct estimate of crop evapo-transpiration by introducing the appropriate crop pa-rameters, research on crop canopy and aerodynamic resistance, for different crops, has so far not been suffi-ciently conclusive to allow the development of reliable global valid parameters. The crop coefficient approach is therefore maintained which integrates all those ef-fects that distinguish the concerned crop in the differ-ent growth stages from the standard crop reference, including soil evaporation.
A review of the crop coefficients has resulted in an update of Kcvalues to the FAO Penman–Monteith method and procedures to reach better estimates under various climatic conditions and crop height and ex-panding the range of crops and crop types (Allen et al., 1996). The procedure for adjusting crop-coefficients for non-standard climatic conditions is given below:
Kc mid=Kc mid(Tab)+[0.04(u2−2)
−0.004(RHmin−45)]
h
3 0.3
with the tabulated standard Kc values (Kcmid(Tab))
ad-justed for deviating wind (u<>2 m/s), humidity (RHmin
<>45%) and crop height (h<>0.33 m).
Fig. 3. Dual crop coefficient curve showing the basal Kcb(thick line), soil evaporation Ke(thin line) and the corresponding single crop coefficient curve Kc=Kcb+Kecurve (dashed line).
water management practices are included. Further de-tails on the revised Kc values and length of growing stages, are included in the new FAO publication (Allen et al., 1998).
4. Computerized crop water use simulations
Computerized procedures greatly facilitate the esti-mation of crop water requirements from climatic data
Table 2
CLIMWAT–Climatic data and calculated ETo(Monthly Reference Evapotranspiration Penman–Monteith)a Month Avg. Temp. (◦C) Humid. (%) Wind (km/day) Sunshine (h) Radiation (MJ/m2/day) ET
o-Penman–Monteith (mm/day)
January −9.2 69 156 4.2 6.3 0.4
February −5.4 72 181 3.7 8.0 0.7
March 1.8 71 199 3.5 10.5 1.3
April 18.8 64 190 5.5 15.7 3.4
May 16.0 59 190 8.0 20.9 4.0
June 21.9 45 199 9.6 23.8 5.6
July 27.2 35 181 10.7 24.8 6.6
August 22.3 42 164 8.2 19.6 4.8
September 19.4 64 181 9.1 17.7 3.4
October 8.3 57 173 6.6 11.4 1.7
November 3.7 70 164 6.1 8.2 0.8
December −17.6 72 156 4.5 5.9 0.2
Year 8.9 60 178 6.6 14.4 2.7
aMeteostation: Bishhket-7; Country: Uzbekistan; Altitude: 730 m; Coordinates: 42.87 N, 74.47 E.
and allow the development of standardized information and criteria for planning and management of rainfed and irrigated agriculture. The FAO CROPWAT program (Smith, 1992) incorporates procedures for ref-erence crop evapotranspiration and crop water require-ments and allow the simulation of crop water use under various climate, crop and soil conditions.
The CLIMWAT climatic database (Smith, 1993), allows a ready access from CROPWAT to 3262 sta-tions of 144 countries in Asia, Africa, Near East, South Europe, Central and South America, compiled by the FAO Agrometeorological Group. An example of the climatic data and calculated reference evapo-transpiration included in CLIMWAT is given in Table 2.
As a decision support system CROPWAT’s main functions include: (1) the calculation of reference evap-otranspiration according to the FAO Penman–Monteith method; (2) crop water requirements using revised crop coefficients and crop growth periods; (3) effective rain-fall and irrigation requirements; (4) scheme irrigation water supply for a given cropping pattern; (5) daily water balance computations.
Table 3
:CROPWAT–Calculated crop evapotranspiration and irrigation requirements for spring wheat at Bishkent, Uzbekistana
Month Dec Stage Coefficient ETcrop ETcrop Effective Rain Irrigation require- Irrigation require-(Kc) (mm/day) (mm/dec) (mm/dec) ment (mm/day) ment (mm/dec)
April 1 Init 0.50 1.39 11.1 23.4 0.00 0.0
April 2 Init 0.50 1.75 17.5 40.0 0.00 0.0
April 3 Init 0.50 1.83 18.3 31.1 0.00 0.0
May 1 In/De 0.59 2.20 22.0 19.0 0.29 2.9
August 1 Late 0.70 3.76 37.6 3.4 3.42 34.2
Total 603.1 160.4 490.2
aRain climate station: Bishket-7; ET
oclimate station: Bishhket-7; Crop: spring wheat; Planting date: 3 April. spells on crop production due to water stress, based
on the methodologies presented in FAO Irrigation and Drainage Paper No. 33 ‘Yield response to water’ (Doorenbos and Kassam, 1979).
An printout of the standard CROPWAT calculations showing the crop evapotranspiration and irrigation re-quirements of spring wheat in Central Asia is included in Table 3.
Table 4 provides an example of the water balance calculations, with an assessment of the effects of rain-fall and irrigation on yield and water use efficiency. The results show that water stress occurring in the matur-ing phase causes a substantial yield reduction (21.7%). The irrigation application has been fully effective, but rainfall is exceeding actual requirements, resulting in losses.
5. WMO/FAO roving seminars
Adequate strategies and policies need to be defined in order to come to effective national programmes in drought management and effective water use for crop production. To assist member countries to strengthen the national capacity to plan and manage national grammes for sustainable rainfed and irrigated crop pro-duction, FAO in close cooperation with the WMO, developed a special training programme on the application of climatic data for effective planning and
management of sustainable irrigated agriculture. Two week roving seminars are organized on request of mem-ber countries to train staff from national meteorologi-cal services and irrigation and agricultural agencies on recommended methodologies for more effective and sustainable use of water in irrigated agriculture. Since 1991, roving seminars have been organized in over 25 countries.
The course introduces basic concepts on climate and soil–water–plant relationships, and provides practical training on computerized procedures to determine ef-fective rainfall and variability assessment, potential evapotranspiration, crop water requirements, and crop response to water stress. With practical examples, trainees are instructed in the use of specialized com-puter software and how to determine the yield level for rainfed crops under varying rainfall patterns and drought conditions, and the water supply for irriga-tion of various field crops under different management conditions.
Table 4
6. Conclusions
Serious water shortages are developing in many coun-tries and water for agriculture is becoming increasingly scarce in the light of growing water demands from dif-ferent sectors. To meet basic food requirements effec-tive strategies need to be developed to optimize crop production per unit of water both in rainfed and irri-gated agriculture. Options to increase water use effi-ciency include a more effective use and conservation of rain and irrigation water as well as an increase in production per unit of water used.
Water use for crop production is depending on the interaction of climatic parameters that determine crop evapotranspiration and water supply from rain. The analysis of climatic information for crop water use is therefore a key element in developing appropriate strategies to face the global water crisis and loom-ing food shortages. A better understandloom-ing of the in-tricate interactions between climate, water and crop growth needs to be a priority area in agrometeorological studies.
The revised FAO guidelines for estimating evapo-transpiration based on the Penman–Monteith approach may enhance the acceptance of a uniform and glob-ally accepted standard for determining crop evapotran-spiration, eliminating the need for maintaining other methods even when limited climatic data are available. Practical procedures and criteria need to be defined to enhance the introduction and application of effec-tive water use practices for crop production. The intro-duction of computerized procedures linked to digital data bases will greatly enhance the use of appropriate planning and management techniques for water use in irrigated and rainfed agriculture.
The formulation of national policies and strengthen-ing of the national capacities to implement effectively such national policies in better water use is essential. The FAO and WMO cooperation in this field will need to be further enhanced.
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