Rainwater Use By Cotton Under Different Irrigation Systems.

Rainwater Use by Cotton Under Different Irrigation Systems

To increase the efficiency by which agronomic crops use water input from both irrigation and rain during the growing season requires quantifying the proportion of rainfall used by the crop for any rain event. The rainfall pattern in the Texas High Plains (THP) is characterized by isolated thunderstorms of high rates and of short duration, where < 1 % of the storms produce rain events > 50 mm and 80 % of total rain events are < than 13 mm. The primary source of irrigation-water in the THP is pumped from the Ogallala Aquifer (OA), which generally has a different d¹⁸O signature compared to rainfall-captured water. Given this difference, it should be possible to quantify changes in the d¹⁸O signature of the plant water as the plant uptakes the rain- and irrigation-water stored in the soil. To this end, cotton was grown and watered with two commonly irrigation systems used in the THP, i.e., subsurface drip and center pivot, and under dryland conditions. The irrigation water was pumped from the OA and rainfall was gathered in a rain gauge with mineral oil to prevent evaporation. Additionally, plant and soil samples were collected before and after each rain event. Thereafter, water was extracted from the collected soil and plant samples using cryogenic vacuum distillation and analyzed for ¹⁸O/¹⁶O ratios using a Liquid-Water Isotope Analyzer. The difference in isotope concentrations in the extracts showed a change d¹⁸O of the cotton petiole water toward that of the rainwater signature of 13 % for sub-surface drip, 22 % for dryland, and 29 % for center pivot irrigation. These results imply that application of irrigation water to the surface of soils results in increased rainwater use in cotton compared to sub-surface drip or dry land management practices.