Principles and Practices for Irrigation Management with Limited Water Authors*: Troy Bauder, Joel Schneekloth, and James Bauder Water availability for irrigation in the Western United States is often limited, and Limited-irrigation situations can occur when in many cases, declining. Below-average any of the following situations exist: snow pack, drought, interstate conflicts, 1. Reduced surface water supplies or ground water pumping restrictions, and storage – in regions that rely upon declining ground water from non-renewable surface water to supply irrigation aquifers have all contributed to declining needs. water supplies for irrigation. These water 2. Restricted ground water pumping shortages have been occurring in many allocations from alluvial or western U.S. irrigated watersheds and designated aquifers. In some ground water basins to some degree for the instances, the allocations are past several years. Combined with water considerably less than what is transfers from agriculture to municipal and required to fully irrigate the crops industrial uses and increasing recreational typically grown. and environmental demands for water, the 3. Low capacity irrigation wells due to relevance of irrigation management with limited saturated depth of the limited water supplies has greatly increased. aquifer. Well yields are then This is the first in a series of six training insufficient to meet the peak ET modules intended to build upon concepts demands of the crop. and suggestions for limited-irrigation management, provide updates on research Under reduced irrigation water supplies, projects relevant to the topic of limited- using typical management practices, yields water irrigation, and suggest further and returns from irrigated crops will resources and techniques for managing generally be reduced compared to fully irrigated cropping systems under tighter irrigated crops. Management strategies can water supplies. help minimize yield loss and preserve net return. However, in order for irrigators to What is Limited-irrigation? implement effective management strategies, Full irrigation results when irrigation water an understanding of certain concepts and is applied to completely meet crop water principles is needed. These include: demand or evapotranspiration (ET) that is • an understanding of the not supplied by natural precipitation and soil relationships between crop yield water storage. In contrast, deficit irrigation and water use (ET) of the crops occurs when irrigation water is insufficient available; to fully satisfy the soil water deficiency in • knowledge about crop response the entire root zone and subsequently full to the magnitude, duration, and ET demands cannot be met for part of the timing of moisture stress, growing season. Limited-irrigation is a especially at critical growth form of deficit irrigation that seeks to stages; maximize water productivity through timing • options for and consequences of of irrigation applications at critical crop crop residue management for growth stages and through managed soil water conservation; depletions. 1 • plant population management in In this example, crops such as corn respond relation to available water; with more yield for every inch of ET than • crop rotations to balance or winter wheat or sunflowers with the same reduce water use; ET water use. However, corn requires more • and techniques and changes that water for development or maintenance than will result in improved irrigation winter wheat or sunflower before any yield efficiency. is produced, as indicated by where the yield- Limited-irrigation systems that incorporate ET line intersects the X-axis. Corn requires these practices coupled with input cost approximately 10 inches of ET to produce management can improve water use the first increment of harvestable yield, efficiency and help maintain overall farm compared to 4.5 and 7.5 inches of ET profitability. required by wheat or sunflower, respectively. Additionally, wheat and Yield, Evapotranspiration (ET) and Water sunflower also require less ET for maximum Timing production than the ET required by corn for Evapotranspiration (ET) is the sum of maximum production. evaporation of water from the soil or crop surface and transpiration by the crop. Knowledge of differences in crop response Evapotranspiration is the driving force to available water and ET, as in the case of behind crop yields when all other yield corn versus winter wheat or sunflower, can limiting factors, including soil fertility, be a useful tool in making decisions about pests, and agronomic practices, are the best timing of limited irrigation water minimized or taken into account (Figure 1). resources. Forage crops, such as alfalfa, Figure 1. Grain and forage yield as impacted by ET for alfalfa, corn, soybeans, sunflowers and winter wheat (from Schneekloth et al. 1991 and Nielsen, 2005). 2 produce harvestable forage yield with the crop such as winter wheat is also a rotation first increment of ET and thus are option for limited supplies. reasonable crop choices for many producers under limited water. Limited irrigation due to low capacity results when the rate of irrigation supply Crop response to water stress varies from a ground or surface water source is substantially among growth stages. When unable to fully meet the ET rate of crop good stand establishment is achieved, yields water demand for a given irrigated acreage. of most grain crops are not impacted as Low capacity most often occurs during peak much by water stress during the vegetative crop ET and the actual irrigation water growth stage or the late reproductive or capacity required will vary significantly by grain fill growth stages as they are by stress crop and region. Low capacity wells are during the flowering, pollination, and seed- those which have limited instantaneous development stages. When producers have water supply, either because of relatively limited water supplies, but have control over small well bore size, water being pumped when they can irrigate, limiting water during from a relatively thin aquifer, or rapidly the growth stages that are least sensitive to changing water level within the well cavity. water stress while saving water for the critical growth stages can be a valuable For irrigators with low capacity, planting strategy to maximize yield return from multiple crops with smaller acreages water. Saving that water for the provides some variability in crop water reproductive growth stages can be the most needs and allows for water to be applied at advantageous use of the water to maximize amounts and times when the various crops grain yield. Having some water available most need the water. On a whole-farm during grain filling will also enhance the scale, crop rotations which include a quality of the harvested grain. diversity of crops also spread the irrigation season over a greater time period, as Crops differ in their water use requirements compared to a single crop. When planting in both amount and critical timing. Crop multiple crops such as corn and winter rotations that include lower-water-use-crops wheat under irrigation, the irrigation season such as sunflower, spring small grain, is extended from May to early October, drybean or winter wheat, can reduce overall compared to continuous corn, which is irrigation water needs. Schneekloth et al. predominantly irrigated from June to early (1991) found that when limited to 6 inches September. Additionally, crops such as of irrigation water, a rotation of corn corn, soybean and wheat have different following winter wheat yielded 13 bu/acre timings for peak water use (Figure 2). The (8 percent) more than continuous corn. The net effect of irrigating fewer acres at any one increased corn grain yield in this rotation point in time is that ET demand of that crop was due to increased stored soil moisture can be better met and farm-wide efficiency during the non-growing season following of water use may be increased. Timing and wheat that was available for corn ET. amount of irrigation water applied can be on Following higher-value, fully irrigated crops an 'as needed' basis rather than in such as sugar beets, potatoes or vegetables anticipation of crop ET. with a low water requirement or dryland 3 Figure 2. Example of daily ET patterns of winter wheat, corn, and soybeans. Residue Management is a proven, effective occurred before the corn crop reached full management tool for capturing and storing canopy (Todd et al., 1991). Residue also rain, snow, and irrigation water in limited- reduces runoff of precipitation and irrigation irrigation cropping systems (Hatfield et al., water, causes longer opportunity time for 2001). Crop residues, whether standing or infiltration, increases infiltration and lying down, and reduced tillage, which decreases rainfall and irrigation impact. The minimizes surface disturbance and/or leaves net effect is generally a decrease in the soil surface rough, can significantly incidence of surface sealing, thereby increase the capture and storage of water by maintaining higher infiltration rates. As reducing evaporation and runoff and droplets impact the soil surface, they can increasing snow catch. Standing residue is destroy the surface structure, sealing the soil more effective than flat residue for snow surface and reducing infiltration rates catch. Studies in northeastern Colorado (Ramos et al., 2003). Residue also acts as found that standing sunflower residue small dams that slow water movement down increased the amount of snow