R.W. Hill, Professor, and R.G. Allen, Associate Professor, Department of Biological and Irrigation Engineering, Utah State University, Logan, USA
Simple calendars are described that express best dates of irrigation based on long-term weather data. The calendars are intended to promote easy and ready adoption of improved water management practices by farmers in both developed and developing countries by presenting simplified, non technical scheduling guidance. The calendars are developed using a daily soil water balance-crop yield model. Once developed, the calendars require no updating and no further input by technical personnel. Calendars are developed for several planting dates, soil types and initial moisture contents.
Developed calendars graphically show recommended irrigation occurrences expressed in days or weeks after sowing that correspond with the type of irrigation water delivery schedule utilized. In the example for Pakistan, the primary water delivery schedule is a fixed rotation type. In the example for Utah in the United States, the system was pressurized sprinkler. The calendars are generalized as much as possible to promote widespread public usability. The methodology demonstrated is transferable to any site where sufficient weather data are available.
Irrigation scheduling in both developed and developing counties is dependent on the irrigation system design, maintenance and operation and on the availability of water. In many regions of the world, the irrigation water supply is insufficient to irrigate all cultivated crops in one irrigation turn. Therefore, yields of crops that have a critical need for irrigation may be reduced because of the lack of water. There is a need for simple means for communicating the best irrigation timing to farmers to assist them in their decision-making processes.
Past research and practical experience have shown that irrigation management practices on the farm or at the water course level must be simplistic and understandable by farmers if they are to be adopted. Practices must have as much flexibility as is possible within the existing system design and maintenance constraints. Farmers need practical means for identifying which crops to irrigate when allocating water during the next turn.
Irrigation scheduling is defined as the process of determining when to irrigate and how much water to apply. For maximum flexibility, the irrigator should have control of the irrigation interval, water application flow rate and duration. Through proper irrigation scheduling, it should be possible to apply only the water which the crop needs in addition to unavoidable seepage and runoff losses and leaching requirements.
Common irrigation scheduling approaches include:
1. irrigating on fixed intervals or following a simple calendar, i.e., when a water turn occurs or according to a predetermined schedule;
2. irrigating when one's neighbour irrigates;
3. observation of visual plant stress indicators;
4. measuring (or estimating) soil water by use of instruments or sampling techniques such as feel, gravimetric, electrical resistance (gypsum) blocks, tensiometers or neutron probes;
5. by following a soil water budget based on weather data and/or pan evaporation; and
6. some combination of the above.
Previous experience has shown that in all countries, and in developing countries in particular, the use of 'gadget' methods (items 3 and 4) are not readily adopted by farmers. Reasons include: 1) equipment malfunction or miscalibration; 2) uncertainty in methods of operation or lack of understanding of principles of equipment function; and 3) time, financial and technical requirements. Item 5 (soil water budget) is not readily adopted, especially in developing countries, due to the technical and time requirements that include measuring or obtaining quality weather data, soil information, crop information, and soil moisture monitoring for updating schedules. It seems that farmers are reluctant to allocate the time required to monitor soil moisture and water application depths necessary to adjust computer estimated schedules. In countries with small landholdings and large numbers of farmers, communication requirements can be prohibitive. However, procedures used in item 5 can be applied on a one-time basis by trained technicians to produce irrigation calendars suggested in item 1 that can be applied for long-term usage.
Pakistan. In order to explore the needs and potential for simple, irrigation calendars or for other irrigation scheduling approaches, farmers were interviewed in Pakistan by Hill and Choudhary (1990) during a study on water management and irrigation scheduling. Questions solicited information on cropping patterns; planting dates, rooting depths; water sources; timing of irrigations for wheat, maize and other pertinent crops; adequacy and reliability of water supplies; and farmers' feelings of when crop water needs were critical. Generally, the farmers in wealthier parts of the country had a better knowledge of improved irrigation and other practices than did farmers in other areas. A summary of information obtained from interviews is presented in Hill and Allen (1996).
The United States. English et al. (1980) interviewed farmers in the western United States who had been recipients of a computerized irrigation scheduling service that was based on computed daily soil water balances. The service was offered by the government at almost no cost to the farmers. In spite of this, farmers abandoned the program after only a few years and did not perceive any benefits of following the free service. Reasons for abandonment included difficulty in obtaining timely information on irrigation dates and depths, difficulty in converting forecast depths of water into hours of surface irrigation, uncertainty in growing conditions, uncertainty or lack of water measurement, lack of flexibility in water delivery, and lack of time to anticipate the irrigation schedules. Farmers often applied water earlier and in greater quantities than recommended by the scheduling service.
Wells and Allen (1988) eliminated the communication problem involved in centrally computerized irrigation scheduling programs by using an electronic voice synthesizer attached to a computer to speak to farmers and to deliver scheduling information to farmers over a telephone system 24 hours per day, whenever farmers called the computer. Irrigation depths for the predominately sprinkler irrigated fields were converted into hours of irrigation by the computer to facilitate application by the farmers. Knowledge of soil water content in the computer was updated using weekly measurements by the neutron probe. Farmers indicated satisfaction with the constant and instantaneous availability of scheduling information, with the 'talking computer' and with irrigation depths expressed in units of time so that they could easily implement them. However, utilization of the scheduling system died off after three years and the system was finally abandoned. Exit interviews with farmers indicated that they had 'learned enough' about how to schedule irrigations during the first few years of the program and had subsequently created 'optimized' irrigation schedules in their minds that they intended to follow without the continued assistance of the program. Therefore, the scheduling program was successful in training farmers how to follow a schedule, but farmers preferred adapting their own schedule that was fixed from year to year.
The following conclusions can be made from the Pakistani and American experiences:
· Farmers generally lack knowledge of important aspects of plant-soil-water relationships, including rooting depths, water flow rates and relationships between irrigation application time, flow rate, field area and needed depth of applied water.
· Farmers seem to relate crop growth and irrigation occurrences to days after sowing rather than to crop growth stage progress (phenologic development).
· The irrigation scheduling methods that are most likely to be used and adapted by farmers must be simple and applicable to wide areas and must require minimal subsequent advisory service.
IRRIGATION SCHEDULE CALENDAR DEVELOPMENT
Based on the interviews with farmers in Pakistan and the United States and experiences elsewhere, the simplified calendar irrigation scheduling method is recommended for general application. It is concluded that the developed calendars must show graphically the need for irrigation occurrences expressed in days or weeks after sowing and corresponding with the type of irrigation water delivery schedule utilized.
For simplicity and to promote adoption by farmers, the number of graphical irrigation scheduling calendars should kept to a minimum. This, therefore, requires some generalization. Irrigation calendars for each crop are normally determined for two, or in some cases, three planting dates, for the major soils (usually two per scheme) and perhaps for two different initial soil water contents at the beginning of the irrigation season.
The calendars are made to express irrigation dates in terms of days after sowing or 'green up' to provide for flexibility and variation in the beginning of the season. In some regions, dates can be translated into lunar month intervals or other local time accounting. Coding of sowing dates, soils, crops, position on watercourse, etc., using some sort of colour or alphabetic characters can be utilized to eliminate the need for technical numeric detail on calendars. Information should be presented in the local language of the farmers to enhance use.
Calendar generation software
A crop yield and soil water management simulation model (CRPSM) (Hill et al., 1984) has been used to determine scheduling calendars for Pakistan and for Utah in the United States. The CRPSM software combines the influence of weather conditions on plant growth stage progress with a model that predicts seasonal yields as influenced by soil water content during each stage of crop growth. The model requires daily weather data and calculates a soil water balance on daily time steps. Other daily soil-water balance models can be used to generate the irrigation scheduling calendars. However, CRPSM has the advantage of including algorithms for determining a series of 'best' dates and application depths, given a specified seasonal irrigation depth constraint. This option is useful in areas where a project or government ministry must apply deficit irrigation management due to regional water shortages. The option provides the best scheduling of water to maximize yields.
The CRPSM software has the capacity to determine irrigation schedules for fixed time intervals when pre-set soil water depletions are reached. Irrigation amounts are set equal to actual depletions or to fixed amounts. This option was specifically developed to generate the calendars for Pakistan so that they were synchronized with the rotation (warabandi) water delivery system used in that country. An option to irrigate whenever readily available soil water was depleted was used for the demand types of delivery systems in the Utah application. The program computations proceed sequentially day by day, using an arithmetic accounting procedure to keep a running inventory of soil moisture by soil layer in addition to transpiration, evaporation, drainage, irrigation and rain. The phonologic stages bracketing growth periods are determined from the appropriate' phenology clock. Transpiration ratios are determined for each growth period and are used to estimate relative yield. Irrigations are scheduled as indicated by the particular option. CRPSM has been described in detail by Hill et al. (1984) and is distributed by Utah State University.
In practice, calendar irrigation dates should be determined using five or more years of recent weather data assuming either no rain or average rainfall conditions in order to approach long-term mean values of ET. Farmers can be provided with simple rules for adjusting irrigation dates when substantial rain occurs, such as re-initializing the counter for the number of days until the next irrigation (modularized if for rotation delivery).
Pakistan. Irrigation water in Pakistan is generally made available to watercourses in a continuous (but not necessarily constant) flow. The farmers receive their irrigation water on a fixed rotation basis known as warabandi. The warabandi is typically a seven-day rotation but may be 10½ days in some places. Trimmer (1988) concluded that deficit irrigation in Pakistan is prevalent but that much of the under irrigation was caused by overirrigation during irrigations spaced too far apart. Trimmer (1990) concluded that full-scale scientific irrigation scheduling was not practical in Pakistan, but that farmers would benefit from some simple water management rules giving the number of irrigations, the calendar dates or the stages of growth at which to irrigate, given normal precipitation.
Most farmers interviewed by Hill and Choudhary (1990) believed that wheat roots were only about 100 to 150 mm deep, based on pulling up plants or on plough depths. At the conclusion of an interview in Shahkot, a hole was dug to a depth of 450 mm in a farmer's wheat field (moist sandy soil). Roots were found throughout that depth. Thus, there was an education opportunity for soil-water-plant fundamentals with the farmers and a benefit of providing simple irrigation calendars that were based on real rooting depths.
In the example case for Punjab, Pakistan, two planting dates, one in mid-November and another in mid-December were representative for wheat. In areas where wheat follows paddy rice, the beginning soil moisture content may approach field capacity; whereas, beginning soil water content following cotton may be 50% or less of available moisture. Thus, two different initial soil water contents prior to the preplant irrigation were assumed, one following paddy and one being fairly dry. During development of the calendars for Pakistan, the infiltrated irrigation depth was assumed to average more than 50 mm for each irrigation event and to fully recharge the soil profile.
FIGURE 1 - Wheat irrigation calendars for Punjab, Pakistan, for mid-November (top) and mid-December (bottom) sowing dates following cotton (after Hill and Choudhary, 1990)
Example calendars for winter season wheat are shown in Figure 1 for two sowing dates for a loam soil that was dry initially. Wheat sown in mid-December and later needs relatively more irrigations, as compared to earlier mid-November planting dates, with irrigations concentrated in the latter part of the growing season. This is due to the increasing temperatures experienced in March - May. The best irrigation interval, correspondingly, reduced to two weeks (and occasionally one week) during the high temperature period. Figure 1 was developed using standard spreadsheet software with a laser printer (Hill and Choudhary, 1990).
FIGURE 2 - Alfalfa irrigation calendars for Cedar City, Utah, USA, for 12-hour (top) and 24-hour (bottom) sprinkler irrigation times (after Hill, 1994)
Estimated relative yields for the schedules shown in Figure 1 were computed by the CRPSM software to determine whether these schedules would produce near maximum yields. Projected yields varied from 91 to 100% for loam soils, suggesting that the restriction of irrigation to warabandi days in comparison with demand irrigation would cause yield losses in the neighbourhood of only 10%. It is doubtful that such a yield loss would be noticeable in comparison with other factors under Pakistani farm conditions.
Utah, the United States. Irrigation scheduling calendars were generated for locations in Utah by Hill (1994). These calendars were nearly identical to those created for Pakistan, except that the schedules were not tied to a fixed rotation schedule, but were varied according to rates of soil water depletion. Examples of irrigation calendars for irrigated alfalfa in central Utah are shown in Figure 2, where the first schedule is for sprinkler irrigations lasting 12 hours and the second schedule is for sprinkler irrigations lasting 24 hours. Computations assumed 70% irrigation efficiency and 7.7 mm h-1 sprinkler application rates. The longer irrigation sets correlated with longer time periods between irrigations.
Irrigation calendars were distributed to government ministry personnel in Pakistan with the goal of dissemination via extension personnel or water-user associations after translation into Urdu and Punjabi languages. However, the regional events of late 1990 and 1991 precluded follow-up work to accomplish this goal. Irrigation calendars for Utah are distributed to farmers through county extension offices and at workshops on irrigation water use. Utah farmers have expressed satisfaction and 'comfort' in having the calendars by which to evaluate their current management practices and to support experimentation.
CONCLUSIONS AND RECOMMENDATIONS
The generated cropping calendars are simple to read and provide farmers with important information pertaining to scheduling irrigations under both fixed and demand delivery schedules. Their adaption to various irrigation regions and promotion by national and international entities would be a step forward in global irrigation water management and sustained food production. The calendars can be generated by developed software using long-term weather information. Calendar graphs such as those shown in this article can be prepared for all crops in various regions of a country. Copies of calendars can be included in information packets for technical advisers to share with farmers and with water user associations during training and implementation processes. The simple calendars can be distributed to farmers directly by government ministries. Demonstration workshops by trained extension personnel are encouraged to train farmers in interpretation and to promote other water management and irrigation concepts.
English, M.J., Horner, G.L., Orlob, G.T., Erpenbeck, J., Moehlman, M., Cuenca, R.H. and Dudek, D.J. 1980. A regional assessment of the economic and environmental benefits of an irrigation scheduling service. US Environmental Protection Agency, EPA-600/2-80-063.
Hill, R.W. 1994. Consumptive use of irrigated crops in Utah. Final Report. Utah Agricultural Experiment Station Research Report 145. 361 p.
Hill, R.W., Hanks, R.J. and Wright, J.L. 1984. Crop Yield Models Adapted to Irrigation Scheduling Programs. Final Report USDA ARS Cooperative Research No. 58-9AHZ-9-440. Utah Agricultural Experiment Station Research Report No. 99. (User Manual is Research Report No. 100), Utah State University, Logan, Utah.
Hill, R.W. and Choudhary, R. 1990. Irrigation Scheduling Practices Review and Development of Recommended Improved Irrigation Management Programs. Final Report to the Command Water Management Project, United States Agency for International Development, CWM-ARD Inc., Lahore, Pakistan 160 p.
Hill, R.W. and Allen, R.G. 1996. Simple irrigation scheduling calendars. J. Irrig. and Drain. Engrg., ASCE 122(2): 107-111.
Trimmer, W.L. 1988. Partial Irrigation and Its Application in Pakistan. Res. Report. University of Idaho. Irrig. Systems Mgmt. Lahore, Pakistan.
Trimmer, W.L. 1990. Applying Partial Irrigation in Pakistan. J. Irrig. and Drain. Engrg., ASCE. 116 (3), 342-353.
Wells, R.D. and Allen, R.G. 1988. Irrigation scheduling using voice synthesis, the next logical step. Proceedings of the 1988 National Conference on Irrigation and Drainage Engineering, ASCE, Lincoln, NE. p. 740-747.
The authors express their appreciation to the Government of Pakistan Command Water Management Project personnel and to the CWM-ARD Inc team members who gave liberally of their time and resources in support of the site visits and analysis procedures. The financial support of USAID is acknowledged. The initial spreadsheet development of the calendar graphs was done by Marvin Lewis, Utah State University.