Irrigation scheduling is a process by which the timing and amount of water applied are determined to meet the evapotranspiration demands of the crop. Both the water delivery system and the availability of water to the plant need to be considered in the scheduling process. Flood irrigation, which is the most common irrigation method for pecan production in the Mesilla Valley of New Mexico, is a type of irrigation practice in which a leveled orchard is divided by parallel soil ridges and water is successively delivered to each bordered plot from a well head or field ditch at its upper end. Depending on where the grower is located in the network of canals and ditches administered by the irrigation district, delivery of surface water may take several days to a week from the time water is ordered. Therefore, growers must be able to predict several days in advance when their soil moisture will be at a management allowable depletion (MAD) level.
New Mexico is one of the top three producers of improved variety pecans in the United States, and production has increased 5-fold in the last 30 years. More than 70% of New Mexico's pecans come from Doña Ana County (New Mexico Department of Agriculture, 2004). Compared with other crops grown in the Lower Rio Grande Basin, pecan trees have the highest consumptive water use (Blaney and Hanson, 1965; Sammis et al., 1979). While pecan production has grown in past decades, an increasing number of small to midsized orchards have been sold to larger farms or subdivided to accommodate rapidly expanding suburban development. As parcel sizes become smaller, existing surface water delivery systems become less efficient and more difficult to manage. This has led to great disparity in application efficiency among residential/lifestyle pecan farms with less than 10 acres, as documented in a recent study of irrigation duration stratified by pecan farm size (Skaggs and Samani, 2005).
In the interests of water conservation, the goals of the research community have been to help pecan growers maximize irrigation application efficiency through proper design and operation of irrigation systems, and at the same time, maximize water use efficiency and farm profitability through careful irrigation scheduling. The reduction of water stress with correct timing of irrigations can have a significant impact on yield, nut quality, and precocity (Stein et al., 1989). An incentive for pecan producers to monitor water inputs should come from the perception that the adoption of new soil moisture monitoring technologies will provide a means to increased profitability, which will in turn pay for the costs of those technologies many times over. However, in a limited study at five Mesilla Valley pecan orchards, growers were reluctant to adopt irrigation scheduling approaches that required measuring soil moisture with granular matrix sensors and data loggers, collecting biweekly tensiometer measurements, or tracking soil water balance with an internet-based consumptive water use model (Kallestad et al., 2006). According to the Farm and Ranch Irrigation Survey (U.S. Department of Agriculture, 2002) only 2% of farms in New Mexico use soil moisture-sensing devices, and less than 1% refer to daily crop evaporation reports or computer simulation models as methods in deciding when to irrigate; whereas 26% used a calendar, 23% use soil moisture “by feel,” and 62% of respondents said they use “crop condition” to schedule irrigation. Numerous recent articles and extension reports have concluded that instruments requiring high in-season labor input for field measurements are not likely to be used by farmers (Hill and Allen, 1996; Sanden et al., 2003; Thompson et al., 2002). Common criticisms include: excessive time required in learning equipment operation; too much time spent collecting and managing the data; difficulties accessing consultants for help with data interpretation; and technical problems associated with equipment malfunction or calibration. Up-front equipment costs can also be prohibitive, especially with small farming operations. Farmers are more likely to adopt technologies where the presentation of necessary information is easy to understand and can be accessed quickly, reliably, and cheaply.
Simplified irrigation calendars based on historic reference evapotranspiration (ETo), crop coefficients (kc), plant phenology, and average seasonal rainfall, with intervals derived from modeled soil water balance, have been developed for a variety of annual crops. The simplest calendars provide fixed irrigation intervals with respect to a planting date, and have been used in developing countries where access to soil- and climate-based scheduling technologies are limited (Hill and Allen, 1996). More flexible irrigation calendars account for the unreliability of rainfall and variability in seasonal temperature. Raes et al. (2000, 2002) devised calendars with irrigation intervals for specific crops using 15 to 25 years of historic climate data in a soil water balance model. Interval guidelines were recommended for 10-d increments over the growing season for four different weather conditions, which are based on probability levels for ETo and rainfall. Guidelines were also devised for delaying the irrigation intervals to account for rainfall. A delay factor is computed by the farmer by dividing the amount of accumulated rainfall by the typical irrigation depth. This factor is then multiplied by the recommended irrigation interval to determine the delay time in days.
Tabulated crop ET or reference ET for a specific region, typically presented in weekly intervals and based on averaged historic climate data, is a common tool for estimating soil water balance. ET calendars are primarily used in planning irrigation by using the “checkbook method.” Similar to balancing a checkbook, the previous day's adjusted soil water depletion level (current balance) is adjusted by adding irrigation and rainfall inputs (deposits) and subtracting crop water use from ET tables for that period (withdrawals). Using this information, a farmer can track daily soil water balance to a management allowable depletion based on crop root depth and soil water holding capacity. Many state and county extension offices have produced checkbook worksheets and guides to help farmers with this technique, but most also advocate the collection of real-time soil moisture and ET data to confirm the computed balance.
ET calendars are most appropriate for regions where climate is relatively consistent from year to year, and variability in seasonal rainfall and ETo are small. The advantage of this approach is that farmers can conveniently plan irrigations throughout the growing season without spending time collecting and processing climate and soil moisture data. In areas where weather conditions deviate considerably from an historical average, farmers scheduling with calendars run the risk of overirrigation or deficit irrigation, which could have economic consequences. Scheduling irrigation with historic ET has been advocated for some areas of California's semiarid Central Valley (Hanson et al., 1999). Weekly ET calendars have been made available for California almond (Prunus dulcis) growers through the University of California Cooperative Extension (Sanden, 2006).
Since rainfall contributes the greatest amount of variability to the soil water balance, our approach has been to produce a flexible irrigation scheduling calendar using irrigation intervals derived with a soil water volume balance model and historic climate data inputs, but without rainfall. Using this device, farmers that flood-irrigate mature orchards can determine the time interval between their previous and the next irrigation, for any time in the growing season, in a range of representative soil types. This calendar is also flexible in that it provides the user with a simple metric by which recommended irrigation intervals can be delayed, based on field-scale rainfall accumulations. Our hope is that farmers will use this tool as a scheduling guideline, be able to predict irrigations with moderately low risk, and better understand the relationship between seasonal climate variation and irrigation timing. Additionally, as some growers become more aware of the benefits of proper irrigation timing, they will become open to more sophisticated technologies such as real-time ET information currently available from local internet resources and electronic soil-based monitoring systems.
The objectives of this article are to describe the scheduling tool development and validation process and to elaborate on the potential for applying this process to other pecan-growing regions, as well as for a broader scope of crops and irrigation methods.
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