PRINCIPLES OF IRRIGATION SCHEDULING FOR THE DRY SEASON
Scheduling irrigation is attempting to apply water to plants at the appropriate amount for a specific stage in the plants development and growth. For instance potato and maize plant’s use of water is primarily for transpiration and tuber production and, therefore, irrigation is most important from emergence to vine senescence. Transpiration is the movement of water through the vine, from roots to leaves, to compensate for water loss at stomates (pores) that are open to allow gas exchange (oxygen and carbon dioxide) between the plant and the atmosphere thereby supporting plant photosynthesis and respiration.
The basic principles of irrigation scheduling from a soil-water view are:
1. Soil moisture. Sites for monitoring soil moisture should be chosen to be most representative of the field. The purpose is to limit under-watering of the heavier soils and over-watering lighter soils. For precision irrigation where watering can be controlled in smaller areas within the field, more monitors would be needed and both better and poorer soils would need to be monitored.
2. Root zone depth. Root zone depth is the zone where most of the root structure is found. This varies with different potato varieties but as a general rule, roots develop down to 18 inches below the seed piece.
3. Water holding parameters (Figure 1). Two measurements would be important. The “fill point” is the wettest a soil can be before water drains below the root zone. This would be near 100% field capacity (FC) or 100% holding capacity of the root zone and depends on soil texture. In general, sandy soils have the lowest FC while silt loams have the highest with clays being intermediate. The “refill point” is the driest a soil can be before daily water use is lowered due to too little water in the root zone. This begins to induce the shutting of stomates resulting in reduction of carbohydrate synthesis (photosynthesis) and respiration (metabolism), and leads to wilting. This has a direct relation to yield. The difference between field capacity and 40% depreciation is the “allowable depletion” (AD) amount of water and, for potato, is 20-25% FC or about half the total available water (about 40% FC). In sandy loam soils, the AD is three-quarters to one inch water up to a depth of 12 inches or one to one and a half inch for the root zone of a full-grown determinate potato variety (Curwen and Massie, 1994; Kranz et al., 1989; Yonts and Klocke, 1985). Soils that are compacted or tend to seal will lower water-holding capacity and reduce penetration of water into the soil.
4. Effective irrigation. Effective irrigation is the amount of water that actually gets into the root zone and is available to the plant. Some of the irrigation water (actual irrigation) is lost as run-off, evaporation or deep percolation.
5. Daily water use. Daily water usage by the potato is dependent on the growth stage of the plant and environmental conditions on that day. It is directly related to canopy development, mostly leaves which contain nearly all the stomates. Environmental conditions that affect daily water use are air temperature, relative humidity, wind, and solar radiation. An excellent guide to daily water usage is evapotranspiration (ET) data that is calculated from weather station data (Klocke et al., 1990). Most, if not all, land-grant universities like University of Nebraska calculate ETs for stations around the state and provide them on the web and for publication in local newspapers. ET is the total daily water use from both transpiration by the plant and evaporation from the soil and can be as high as a third of an inch in a day for potato. ET needs also to be adjusted for canopy size or row closure, canopy width divided by row spacing; 80% row closure is considered full canopy or 100% ET. Therefore, a potato crop that has row-closed on sandy loam at field capacity (three-quarters to one inch AD) can be carried for two and a half to three days under high ET conditions. From this, one can estimate a two to two and a half inches weekly irrigation requirement for potatoes during tuber bulking under high ET conditions — high temperature, low relative humidity, intense sunlight, high winds, and long days (July in the Nebraska Panhandle?). Seasonal ETs differ for crops due to duration at full canopy and growing season.
In summary, the key factors in managing irrigation are:
- how much water gets into the soil,
- how much water the soil can hold, and
- how much water is being used by the plant.