Effects of irrigation level and timing on profile soil water use by grain sorghum

Authors: BELL, JOURDAN - Texas A&M Agrilife; Schwartz, Robert; MCINNES, KEVIN - Texas A&M University; HOWELL, TERRY - Retired ARS Employee; MORGAN, CHRISTINE - Texas A&M University

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2020
Publication Date: 2/9/2020
Citation: Bell, J.M., Schwartz, R.C., McInnes, K.J., Howell, T.A., Morgan, C.L. 2020. Effects of irrigation level and timing on profile soil water use by grain sorghum. Agricultural Water Management. 232(2020):106030. https://doi.org/10.1016/j.agwat.2020.106030.
DOI: https://doi.org/10.1016/j.agwat.2020.106030

Interpretive Summary: Development of sustainable and efficient irrigation management is a priority for agricultural producers faced with water shortages. A promising management strategy for improving water use efficiency is managed deficit irrigation (MDI). Under MDI the crop is not fully irrigated (FI), but proportionately greater irrigation is applied during the reproductive stages. In contrast, deficit irrigation (DI) consists of irrigating at a fraction of the full irrigation requirement throughout the growing season. Scientists from ARS (Bushland, TX) and Texas A&M AgriLife Research and Extension Service evaluated soil water use, plant available water in the profile, and the distribution of plant available water within the profile on the High Plains of Texas, USA, for grain sorghum production under FI, DI, and MDI. The depth of soil water extraction and crop water use varied among the three years of the study. In 2010, a year with near normal precipitation, root water extraction extended to a depth of 1.4 m under FI and 1.6 m under DI and MDI. However in the two years with below average precipitation, soil water extraction was largely restricted to the first 0.6 m of the soil profile. Water extracted under DI and MDI deeper in the profile was negligible and did not offset reduced irrigation inputs. This resulted in severe water stress during flowering during the two years with below average precipitation. Irrigation scheduling based on managed allowed depletion may need to be modified to account for reduced water availability deeper in the profile where roots are sparse.

Technical Abstract: Deficit irrigation strategies have the potential to optimize crop yields, but increased variability in profile stored soil water in semi-arid regions or during periods of drought is often associated with deficit irrigation. Understanding variability in stored soil water and crop rooting depth is an important component of successfully managing a deficit irrigation strategy. A promising management strategy is managed deficit irrigation (MDI), which attempts to optimize yield and irrigation by synchronizing crop water use with the crop’s reproductive stages. In comparison, deficit irrigation (DI) is applied at a fraction of the full (FI) irrigation requirement; a managed allowable depletion or 50% of the potential plant available water (PPAW). The depth of the soil water use by grain sorghum [Sorghum bicolor (L.) Moench] was evaluated in the High Plains of Texas, USA, under three irrigation strategies: FI, DI, and MDI from 2010-2012. Water use during critical growth stages was year specific due to variable precipitation and antecedent soil moisture even though irrigation strategies were maintained as planned in all years. In 2010, soil water contents within the rooting zone (0 to 1.6 m) were > 50% PPAW at emergence under all treatments, and soil water contents remained > 50% PPAW at half-bloom for FI and MDI. Soil water contents were < 50% PPAW at half-bloom for both DI and MDI in 2011 and 2012. In 2011, water contents only remained > 50% PPAW under FI from 0- to 0.6-m through the growing season. Due to variability in the depth of the stored soil water, irrigation scheduling based on managed allowed depletion or PPAW may need to be modified by weighting the PPAW in the depths of greatest root densities more heavily to account for sparser root densities deeper in the profile.