Applying the proper amount of water at the correct time is critical for achieving   the optimum benefits from irrigation. The crop water requirement, termed evapotransporation, is equal to the quantity of water lost from the plant (transporation) plus the loss from the soil by surface evaporation. The evapotransporation rate is important in effectively scheduling irrigations. The amount of solar radiation, which provides the energy to evaporate moisture from the soil and plant surfaces, is the major factor. Other functions include day length, air temperature, wind speed, and humidity level.

Plant factors that effect the evapotransporation rate are crop species, canopy size and shape; leaf size, shape, and orientation; plant population; rooting depth; and stage of growth and development of the crop. The plant canopy size and shape influence light absorption, reflection, and the rate at which water evaporates from the soil. Leaf architecture affects the transporation rate from individual leaves. Rooting depths vary from crop species and may be effected by compaction or hardpans that may exist. Rooting depth determines the volume of soil from which the crop can draw water and is important when determining the depth to which the soil must be wetted when irrigating.

Cultural practices also influence evapotransporation. Cultivation, mulching, weed growth, and method of irrigation are factors to consider. Cultivation generally does not reduce evaporation significantly, but if crop roots are proving by cultivating to close, water uptake and thus transportation may be reduced. Shallow cultivation may help eliminate soil crusts and therefore improve water infiltration. Weeds compete for water and increase the amount lost through transportation. Sprinkler irrigation wets the entire crop area and thus results in a greater evaporation loss than does drip irrigation, which wets only the area in the immediate vicinity of plants.

Soil factors must also be considered. Soils having high levels of silt, clay, and organic matter have greater water holding capacities than do sandy soils or soils that are compacted. Soils with high water holding capacities require less frequent irrigation than those with low water holding capacities. However, when soils are irrigated less frequently, a greater amount of water must be applied per application.

Relationship Available Water Holding Capacities and Soil Texture.

Another soil factor that influences irrigation practices is the infiltration rate. Water should not be applied at the rate greater than the rate at which the soil can absorb it.

Table 3 lists typical infiltration rates of several soils.

Table 3. Relationship of Soil Infiltration Rates and Texture.

SOIL INFILTRATION RATE SOIL TEXTURE.
(inch/hour)

There is no to simple method for scheduling irrigation's accurately because all the above factors interact to determine water loss. Research is currently underway to develop improved methods of scheduling irrigations. The following factors should be kept in mind when deciding when and how much to irrigate.

Soils vary greatly in water holding capacity and infiltration rate, as previously discussed.

Water loss from plants is much greater on clear, hot, windy days than on cool, overcast days. During periods of hot, dry weather, ET rates money reach 0.35 inches per day or higher. The evapotransporation can be estimated by the use of a standard evaporation pan. Recently, researchers have found that maintaining soil moisture levels in a narrow range, just slightly below the field capacity (75 to 90 percent soil moisture), maximizes crop response. This may mean that more frequent application of smaller amounts of water is better than delaying irrigations. until the soil moisture reaches a low-level (40 to 50 percent soil moisture) and then applying a large amount of water.

Plastic mulches reduce evaporation from the soil but also a reduce the amount of water that can reach the root zone from rains. Thus much of the natural precipitation should be ignored when scheduling irrigations for crops grown under plastic mulch.

In general, for overhead irrigation apply 0.25 inches of water or more in each irrigation, except when establishing crops. This rule does not apply with drip irrigation. When using drip irrigation and plastic mulch, applying as little as 0.08 inches of water per day may be sufficient during periods of low evapotransporation.