Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 13, 2011
Publication Date: December 1, 2008
Citation: Tolk, J.A., Howell, T.A. 2008. Field water supply: Yield relationships of grain sorghum grown in three USA Southern Great Plains soils. Agricultural Water Management. 95(12):1303-1313. Interpretive Summary: Irrigation is often needed to produce grain in the dry climate of the Great Plains. However, as our water supplies drop, it is important that we use them wisely. Crops can use irrigation, rainfall, and water stored in the soil as sources of water for their growth, and together these sources are called field water supply. Sometimes we don’t wisely use the field water supply, especially the amount of water stored in the soil. This study looked at the relationship between grain sorghum grain yield, field water supply, and irrigation amounts in three Great Plains soils. It found that, if there is a large amount of water stored in the soil, it is a waste to add back the amount of irrigation that will fully replace the water used each week because the crop can’t use all of it. Much of it is lost through evaporation and drainage, and yields don’t increase. Reducing the amount of irrigation water resulted in less waste and produced the same amount of grain. We can best manage our water resources if we are aware of all the sources of water that are available to us.
Technical Abstract: Field water supply (FWS) represents the three sources of water used by a crop for evapotranspiration (ET), and consists of available soil water at planting (ASWP), rainfall, and irrigation. Since water is most productive when entirely consumed in ET, diversion of FWS into non-ET uses represents a loss of productivity. The objective of this experiment was to examine the FWS and grain yield (FWS:Yg) and ET:Yg relationships of grain sorghum grown under of a range of irrigation treatments (0, 25%, 50%, and 100% replacement of ET), beginning soil water contents, evaporative demands, and in the Amarillo, Pullman, and Ulysses soils of the Great Plains. The purpose was to determine the amount of FWS beyond which declines in water productivity (WP) and irrigation water productivity (IWP) began to occur due to non-ET losses as indicated by a change in the slope and intercept of the FWS:Yg relationship. Using piecewise regression analysis, the amount of FWS beyond which non-ET losses began to occur was identified to be at about 500 mm in both the Pullman and Ulysses soils. At this FWS, about 250 mm in available soil water at planting and irrigation applications averaging about 250 mm had been utilized, with about 60 mm to 130 mm remaining in the soil at harvest. The irrigation treatments at or near this point tended to have the largest WP and IWP. Beyond this FWS, a 60% or more increase in FWS due to irrigation increased yield only by about 20%. A similar response was not found for the crops in the Amarillo soil, which had a linear FWS:Yg response throughout the range of FWS. Irrigating without considering FWS can result in large non-ET losses with small gains in yield.