Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: November 14, 2000
Publication Date: N/A
Interpretive Summary: Irrigation scheduling goals may include maximizing yield and maximizing water use efficiency (how much yield is gained for a given amount of water applied) - two goals that usually conflict. Which goal is chosen depends on many factors. But, clearly, an automatic irrigation system that allowed either goal to be chosen would be a useful farming tool. We tested a system mthat measured crop leaf temperature and automatically applied irrigations when leaf temperature increases indicated that plants were stressed. Tests were conducted with corn (Pioneer hybrid 3162) in 1997 and 1999; and soybean (Pioneer variety 9481) in 1996 and 1998, with surface and subsurface drip irrigation. The automatic method was compared with a common manual weekly irrigation method that was 100% replenishment of water used by the crop as determined by soil water measurements, plus treatments of 67% and 33% of the 100% amount. Most or all of the automatic treatments out tyielded the 100% and other manual treatments in each year. Corn yields in the 240 bushel/acre range and soybean yields in the 70 bushel/acre range were achieved with the automatic systems. Manipulation of the leaf temperature threshold used to initiate irrigations allowed either yield or water use efficiency to be maximized for corn, but not for soybean. Water use efficiency was 3 to 4 times higher for corn than for soybean for all automatic treatments and the 100% irrigation treatment. This fact indicates why corn continues to be grown in the water-short Southern High Plains, even though corn uses more water than soybean. The automatic method shows promise for easing irrigation management decisions and improving yields and water use efficiency.
Technical Abstract: We tested a system using species-specific threshold temperatures and region-specific threshold times, compared against daily canopy temperature time above the threshold, to automatically schedule and apply irrigations to corn (Zea mays L., hybrid Pioneer 3162) in 1997 and 1999; and soybean (Glycine Max (L.), var. Pioneer 9481) in 1996 and 1998, with surface and subsurface drip irrigation. Four automatic treatments were implemented eac season using threshold temperatures of 28 and 30 deg C for corn and 27 and 29 deg C for soybean. For each threshold temperature, we used two threshold times. The control was a manual weekly irrigation that was 100% replenishment of water to field capacity as measured by neutron probe, plus treatments of 67% and 33% of the 100% amount. Most or all of the automatic treatments outyielded the 100% treatment in each year. Corn yields were 0.94 to 1.21 kg/m**2 for automatic treatments and 0.65 to 1.15 kg/m**2 for the 100% treatment. Soybean yields were 0.35 to 0.43 kg/m**2 for automatic treatments and 0.36 to 0.40 kg/m**2 for the 100% treatment. For corn, increasing threshold temperature or time shifted treatments away from maximum yields and closer to maximum water use efficiencies. As with corn yield, total water use (mm) and water use efficiency, WUE (kg/m**3), were stable over the 1997 and 1999 years for the automatic irrigation treatments (means of 752 mm and 1.48 kg/m**3, respectively), while they varied widely over the two years for the manual treatments (569 and 793 mm, and 0.65 and 1.45 kg/m**3 for 1997 and 1999, respectively, for the 100% treatment). For soybean, yields and WUE were not more stable for automatic treatments. Manipulation of temperature and time thresholds may allow a choice between maximum yields and maximum water use efficiencies to be achieved for corn.