|TOLK J A|
|HOWELL T A|
|STEINER J L|
|KRIEG D R|
|SCHNEIDER A D|
Submitted to: Irrigation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/1995
Publication Date: N/A
Interpretive Summary: Irrigation increases and stabilizes crop yields in the semi-arid Great Plains. Irrigation efficiency can be defined as the ratio of total water stored in the root zone for plant use to the total amount of water applied. For overhead sprinkler irrigation, most losses are due to evaporation of water that is intercepted by and held on the foliage, or gross interception nloss, and of airborne droplets either inside or outside the irrigation application area. Sprinkler systems that apply water near the soil surface leave ponds that also evaporate before use by plants. Measured irrigation application losses averaged 19% for overhead sprinklers and 4% for those that apply water near the ground. However, these irrigation efficiency losses can also be expected to supply some of the atmospheric evaporative demands usually met by plant transpiration. Transpiration can be reduced both during and after an irrigation in response to the increased humidity created when evaporation occurs. In the altered microclimate, the plant benefits through a reduction in heat stress and maintenance of soil water reserves normally being depleted by transpiration. Any 'savings' in transpiration can be applied to the gross interception losses for a 'net' interception loss. In a study on corn using lysimeters, which are contained boxes of soil that can be weighed, measured transpiration reduction was shown to reduce overhead sprinkler application losses up to 6%. Evaporation of ponded water alone had little effect on transpiration and therefore on irrigation application efficiency of water applied near the soil surface.
Technical Abstract: Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (Igross), and airborne droplets evaporate before reaching the soil surface. However, this water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the wetted area as compared to a anon-irrigated area can be subtracted from Igross for a reduced, or net, interception (Inet) loss. This study was conducted to determine the extent in which Inet losses and/or microclimatic modification can affect total sprinkler irrigation application losses. Corn (Zea mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX, in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (T) was measured using heat balance sap flow gauges. Compared with evapotranspiration (ET) from a dry canopy, the ET from a wetted canopy only greatly exceeded dry canopy ET on days with high vapor pressure deficits, and then for periods less than 0.5 h. Measured amounts during this period, possibly associated with the evaporation of canopy-intercepted water and soil water, were 0.17 mm and 0.37 mm. Canopy wetting suppressed T 50% or more during an impact sprinkler irrigation, with predicted net irrigation efficiency increases of 2-3%. On days of high solar radiation, transpiration suppression following the irrigation contributed an additional 2-3%. Irrigation water applied only at the soil surface had little effect on the microclimate within the canopy and consequently in T or ET. Inet losses were less than 10% of the irrigation application.